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Why Did Ford Decide to do a Flat-Plane Crank Differently?

Why Did Ford Decide to do a Flat-Plane Crank Differently?

(OP)
Most V8 engine layouts are Cross-Plane (sometimes called Cruciform) due to the 4 common crank pins being positioned in two planes, 90 degrees apart. Usually the two end pins are in one plane, 180 degrees apart and the two inner pins are on a plane perpendicular to the outer’s. Figure 1 shows a typical V8 cross-plane crankshaft.


## Figure 1 - V8 Crossplane Crankshaft ##

Flat-Plane V8s are commonly only used in high-performance engines, such as the likes of Ferrari. As its name suggests, a Flat plane V8 crankshaft has all its crank pins in a single plane. It looks very similar to an Inline-4 crankshaft, albeit with longer crank pins to accommodate two big ends. Figure 2 shows a typical Flat Plane V8 crankshaft.


## Figure 2 - V8 Flat-Plane Crankshaft ##

There are two main benefits of a Flat-Plane:

  1. Due to good inherent primary (1st order) balance (no primary shaking forces or couples) there is no requirement for large counterweights (a cross-plane has a rotating primary couple that must be balanced out with counterweights - hence the typical shape of the large end counterweights on cross-plane V8 cranks). This means reduced weight, reduced inertia, reduced package volume, which all equal increased engine acceleration and lower CoG possible.

  2. Exhaust Pulse Tuning - due to the layout, firing order is alternating from bank to bank, so each bank sees equally spaced pulses of exhaust gas pressure. This means exhaust tuning can be utilised to make the engine perform better. The cross-plane layout means each bank has unequal pulse distribution.

So the question I am puzzling over, why did Ford decide to make use of a flat-plane crank layout in the new 2016 Mustang GT350R but (seemingly) throw away all the benefits gained by doing it differently?


## Figure 3 - Ford GT350R V8 Flat-Plane Crankshaft ##

I’ll explain “differently”. The GT350R crankshaft is shown above in figure 3. The very first thing I noticed was that is had an “up - down - up - down” configuration of the crank pins instead of the usual “up - down - down - up” layout as illustrated in Figure 2. Straight away I wondered why they had done that as I suspected (before I had a chance to do any calcs) that it was going to introduce some imbalance. This suspicion was further strengthened by the obvious larger counterweights, opposing each other at either end of the crankshaft, giving away that there was some inherent unbalanced (primary) couple.

So I did some calcs and confirmed that the unusual layout of this flat-plane V8 crankshaft did indeed have some unbalance.

The typical U - D - D - U layout leaves only an unbalance secondary, horizontal shaking force and a relatively small secondary couple in the vertical plane, all due to the reciprocating components.

The U - D - U - D layout however, even just looking at the rotating masses alone, has a primary rotating couple. So before even considering the reciprocating masses you have to add 2 large, opposing counterweights at either end of the crank to just make the crank balance. Then when you consider the reciprocating masses, you get the same secondary imbalances as the U - D - D - U but also more primary couple imbalance.

Then end result is a flat-plane crankshaft with the mass/inertia penalty of the cross-plane crankshaft. So why did they do it?

That is actually my unanswered question...unless I haven’t considered some other great benefit, I can’t see why they did it...apart from perhaps, marketing? Maybe being able to say the GT350R is different from all the rest because it has an exotic “5.2l V8 with flat-plane crank” (quoted from the Ford website).

I would really like to know more behind the decision.

Regards,

Jon Reynolds

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

(OP)
p.s., why are my images not showing in the post?

edit: Sorted now.

Regards,

Jon Reynolds

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

(OP)
Sorry, is there really what, IRstuff?

Regards,

Jon Reynolds

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

What's the firing order on those engines?

In each bank, it could be 1-2-3-4 or it could have 1 and 3 swapped (3-2-1-4 i.e. 1-4-3-2 which is the exact same scenario flipped end to end) or it could have 2 and 4 swapped (1-4-3-2 which is equivalent) or it could have both swapped (3-4-1-2 which is 1-2-3-4). Interesting. Either way, each bank fires each cylinder in sequence from one end of the engine to the other evenly spaced, it's only a matter of whether that sequence goes front-to-back or back-to-front. (I know that's not the actual way the cylinders are numbered, we're just looking at each bank - bear with me.)

The other bank - let's suppose this is the one that trails by 90 degrees - either has its own firing order inserted 90 degrees after, which would put both power strokes on the same crank pin at the same time, or 450 degrees after, which completely separates the firings on each crankpin. I'd say that's more likely.

The first cylinder of the second bank in the firing sequence fires 90 degrees following the third cylinder in the first bank in the firing sequence in a nice progression from either back-to-front or front-to-back. The firings are nicely spaced between the front half of the engine and the back half of the engine and so are the intake suction strokes and there's never an overlapping power stroke on the same crank pin. (I don't think it is possible to achieve this with a cruciform crossplane-crank V8) Heck, the compression stroke on the second cylinder in the bank doesn't start until the first cylinder's power stroke is mostly finished.

I think Ford knew what they were doing.

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

There could be other reasons.
WAG1: Maybe their crank grinder automation deals with UDUD better than UDDU.
WAG2: Maybe it's used in a vehicle where you have to rotate the crank in increments in order to remove the oil pan axially, and two throws in a row wouldn't clear the rear seal face of the pan.

Example: The penultimate E-series van, where you had to remove the top half of the intake manifold and jack up the engine before you could begin to get the oil pan out, and you still had to rotate the crank to clear the oil pan, because the engine was installed in a tight fitting porthole in an otherwise very solid front crossmember.

Mike Halloran
Pembroke Pines, FL, USA

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

(OP)
IRStuff, I thought my OP highlighted what I saw as the disadvantages clearly enough.

FP UDDU (typical) is statically and dynamically balanced, hence no additional CWs required either end like on a cross plane V8.

Ford's UDUD FP reintroduces the dynamic imbalance similar to the cross plane hence the need for thicker, opposed CWs. So why go FP but loose the mass/inertia savings by going UDUD ?

Regards,

Jon Reynolds

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

They're only big if there are no big advantages. Inline engines that I have seen generally occupy more volume, by a substantial amount. The weight of the extra structure would seem to far outweigh the balance weights.

I do suggest you re-read your OP, since you wrote: "There are two main benefits of a Flat-Plane:"

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RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

BrianPeterson, I agree with your firing order analysis, but I don't see an advantage in terms of intake/exhaust pulse separation over a conventional flat crank. Either way, a 4 into 2 or 4 into 2 into 1 arrangement per bank will yield equal separation.
However, I do see an advantage in avoiding firing impulses on adjacent cylinders along the crankshaft (if that is in fact an advantage and one worth pursuing), which I don't see a way to do with a conventional flat crank.
The rotating inertia disadvantage remains.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

Did you answer your own inquiry? The UDUD is similar to the cross plane and thus perhaps can be machined on existing machine tools.

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

It wouldn't surprise me if the layout contributes towards brand sound. I can recall a European company many years ago wanting to rid their new (cross) V8 of its burble as it sounded like an old US muscle car, not something refined. Maybe this unusual flat V8 has the right perfomance characteristics, but still sounds "traditional"? I.e. the opposite situation to the European company.

Remember the TDM 850 parallel twin, with its unusual 270° firing interval? Some said that was all about making it sound like a V-Twin, not about performance - the extra hassle of balancing it was the price to be paid.

Steve

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

Making Ford's flat crank V8 sound "traditional" would take a lot of extra effort, as by design the firing impulses per bank are even (unlike a "traditional" V8).
Contrariwise, I vaguely remember pictures of exotic cylinder heads and exhaust system applied to the 351 Cleveland V8 as installed in the Ford Pantera that brought all the exhaust ports to the inside of the "Vee" and knitted them together into bundle of snakes for an even firing header system. I might have this wrong as it may have been a Ford smallblock applied to the original GT40, or a Pantera Cleveland with conventional cylinder heads where the individual exhaust pipes were extended rearward as needed to join them into even firing pairs and quartettes. But you get the idea, i.e. going to a lot of effort to get an even firing exhaust system from a "cross-plane" V8.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

(OP)
I am starting to suspect it was down to machining/production capabilities. Seems most plausible. Look at how many 90deg V6 engines are out there with the costly split-pin cranks just because of 90deg V8 production lines. Seems a shame...still a good discussion point.

Regards,

Jon Reynolds

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

Quote (SomptingGuy)

It wouldn't surprise me if the layout contributes towards brand sound. I can recall a European company many years ago wanting to rid their new (cross) V8 of its burble as it sounded like an old US muscle car, not something refined. Maybe this unusual flat V8 has the right perfomance characteristics, but still sounds "traditional"? I.e. the opposite situation to the European company.
I suspect that the details of the exhaust's primary pipe plumbing are having more to do with the 5.2 FPC sounding a bit different than expected for a FPC. Swap the stock header arrangement out for a set of aftermarket long tube headers and the character of the sound changes.

There's a link in http://forums.corner-carvers.com/showpost.php?p=10... to the "revised" sound. Pretty sure I've heard a stock GT350, and it's nothing like this.


Norm

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

(OP)

At last, some light shed on the exact issue I have been trying to explain/get to the bottom of:

http://www.edmunds.com/ford/mustang/2015/long-term-road-test/2015-ford-mustang-gt-coyote-voodoo-cross-plane-flat-plane.html

So...if I am reading into it right...its all about MARKETING! The sound it makes seems to have driven the decision making on the odd layout.

Annoyed I didn't find that in my searchings.

Regards,

Jon Reynolds

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

I strongly suspect the resulting sound is more a result of the unconventional "unequal-length 4-into-3-into-1 exhaust manifolds" than the crankpin arrangement itself. Perhaps the crankpin arrangement helped facilitate the desired manifold configuration, but I'd need to see the manifold layout and actual firing order to evaluate that.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

Does that oil filler cap really say 5W-50? No stinking energy conserving going on here!

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

Is the rotating inertia of that actually as great as Crossplane crank V8? Do their counter weights offer potential benefits for different crankshaft excitations?

Having more rotating inertia makes the power delivery more gentile. That pays off on race tracks: It delivers smoother power impulses to the drivetrain, and the engine has more inertial torque when you need to drop the clutch.

When I was racing motocross and XC, it was common to add flywheel weights to the engines to make them more ride-able.

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

On a road course or even an autocross layout, minimizing the torque "lost" to accelerating rotational inertias trumps all else. At least at the pointy ends of those sports.

I can understand why you might want to take some of the suddenness out of a wheelstand (I used to ride a 350cc 2-stroke triple that had light-switch behavior between being off the pipe and on it), but I doubt that to be a concern with the GT350. Nor will drag racing be all that important, though there will be those who will at least try it out there.


Norm

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

I've heard it argued, though not with back to back data or numerical analysis, that more rotating inertia can reduce 1/4 mile times, assuming a given launch rpm (and, I suppose, that the extra stored energy at launch goes into acceleration, rather than tire smoke or clutch heating). I remain skeptical until I see the data or analysis.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

Well, I would expect that the less torque pulsation there is at the wheels, the less likely the tires are to start slipping. I have no idea how important a factor it is in real life.

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

More flywheel makes a consistent launch easier but it would probably be a negative otherwise.

je suis charlie

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

Maybe so, but, by far the biggest torque impulse is the launch, unless the available energy (i.e. delta energy from increased rotating inertia) is converted to heat upstream of the tires, in the form of slip, clutch, e.g.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

Hello,

I saw a video on the GT350 over the weekend that mentions packaging constraints being a main driver for the crankshaft configuration. One constraint mentioned was not being able to use separate plenums for the cylinder banks. I'll see if I can find a link to the video and post it.

Kyle

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

For drag racing, the theory is that during launch and gear changes, the higher inertia engine has more energy to dump to the wheels when the engine rpm is pulled down. It's supposed to cause a surge of acceleration each time making you faster. It also ignores the time the engine has to accelerate the inertia.

For road racing, the theory is that less engine inertia allows the engine rpm to change quicker, which means quicker changes and faster acceleration/deceleration of the car.

The physics of acceleration don't change between the drag and road tracs, so one of the theories is wrong. I've run across a few people who reduced rotating engine inertia via a much lighter clutch/flywheel and then went down the 1/4 mile faster. So the reality is that having more engine inertia just makes it easier to consistently launch a clutched car.

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

The physics are the same, but you aren't chasing the same effects/benefits. Greater rotational inertia helps prevent bogging the engine down out of its powerband and is more useful in relatively heavy cars powered by smallish displacement engines. Especially so when the racing surface has been prepared for additional tire grip. With car weights commonly well in excess of 3000 lbs, a 5L engine is definitely "small". Since drag races between generally similar cars tend to be won or lost inside the first couple hundred feet and maybe two gears, it's more important to work with the launch side of the physics.

A closed-course racer, either road course or "oval" doesn't need to care nearly as much about 1st gear launch traction, either because the starts are rolling starts or because the wheels are allowed to freely spin (sacrificing a small amount there in exchange for better through-the-gears acceleration everywhere else). The classic illustration here would be the big NASCAR "sedans" leaving their pit space following a stop for fuel and fresh tires. The only "surface preparation" for closed course events is whatever tire rubber has been left on the pavement during practice, qualifying, and from what any supporting series leaves. Some of which may not even be on the line you need to be on to prevent being passed.


Norm

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

More inertia can only be a benefit at launch and gear changes when the clutch is slipping. Less inertia will accelerate the car faster the rest of the time when the clutch is engaged.

Hemi - more engine inertia would help the launches using the same launch rpm, but in reality you would launch at a higher rpm with less inertia.

This engine has to be kept above 3500rpm to stay in the powerband. The engine torque increases by over 100 ft-lbs going from 3000 to 3400 rpm.

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

The argument that I've understood, which I haven't analysed myself quantitatively to assess its validity, is that a drag racer can in essence, "invest" in stored energy in the flywheel prior to the start of the race, and draw down the investment during the race. Of course the counter argument of needing to continually accelerate the rotating masses during the race needs to be taken into account, when assessing the value of increased rotating mass in front of the clutch (or other coupling) between the engine and the remainder of the drivetrain.
Hence, I remain skeptical.
Of course, one can conceive of a high tech, ultra light, ultra high speed flywheel connected via a high ratio reduction gear to the drivetrain proper, that is spun up to the moon prior to the start of the race, and totally eclipses the engine in power delivery during the race; but that is not the configuration of a drag race vehicle as we know them today.
I guess the reduction gear needed for the task wouldn't be exactly lightweight, but maybe an electrical coupling could provide the needed speed reduction at a lighter weight. ponder

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

KERS.

je suis charlie

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

Slightly higher rotating inertia reduces amplitude of torque pulsations through the drivetrain. Torque spikes are smoothed out, which is good for clutch/transmission/axles/driveshaft/Ujoints. It also smooths out the powerband, which is good for traction.

As long as the crank is lighter than a cross plane crank, it is smart for them to do this.

I have also heard that balancing flat plane V8s with displacement over about 4.5L is harder to do. I do not understand exactly why that is said. Does anybody have ideas as to why that is said?

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

Looking at the pictures in the first post, it looks like the FP crank might have slightly smaller diameter bearing journals than the cruciform crank. This would reduce friction losses in the main/rod bearings.

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

Quote (hemi)

The argument that I've understood, which I haven't analysed myself quantitatively to assess its validity, is that a drag racer can in essence, "invest" in stored energy in the flywheel prior to the start of the race, and draw down the investment during the race. Of course the counter argument of needing to continually accelerate the rotating masses during the race needs to be taken into account, when assessing the value of increased rotating mass in front of the clutch (or other coupling) between the engine and the remainder of the drivetrain.

If you can't keep the revs up high enough on the launch - IOW, the engine bogs before adequate road speed is reached - it won't matter how much more advantageous the light-MOI setup is everywhere else. The race has already been lost. It at least used to be common for 40-lb flywheels to be fitted to small-block Chevy engines, and the numerically high axle gear ratios also frequently fitted would perhaps "cover for" the lower vehicle accelerations once the engine is operating in its best powerband and the tires fully "hooked up". It's really just a matter of optimizing the entire combination for a clearly and very narrowly defined performance measurement.

On a small time step level, rotational momentum converts to additional torque as the rpms drop, and how much this amounts to also depends on the rate of rotational deceleration. You might for a very brief period of time "gain" 100 ft-lbs this way, which might just be enough. Consider that clutch engagement isn't necessarily instantaneous from no grip to no slip, and that there likely is some torsional "wind-up" in driveshafts, axles/half shafts, and tires that provide some delay and (slightly later) energy returns of their own.


Norm

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

Quote (Panther140)

I have also heard that balancing flat plane V8s with displacement over about 4.5L is harder to do. I do not understand exactly why that is said. Does anybody have ideas as to why that is said?

Guessing the magnitudes of the forces and moments and the phase relationships among those components between banks that are still separated by 90° has something to do with it. Dividing the con-rods into two masses with no MOI of their own is at best an approximation.


Norm

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

"I have also heard that balancing flat plane V8s with displacement over about 4.5L is harder to do. I do not understand exactly why that is said. Does anybody have ideas as to why that is said?"

It used to be said that inline 4s with displacement over about 2 liters had too much secondary vibration to be acceptable in passenger cars, without resorting to the complication and expense of a pair of counter-rotating secondary balance shafts. The pistons etc got too heavy for the final result to be ignored or "hidden" with squishy engine mounts

As a flat crank V8 is somewhat equivalent to two inline 4s, the 4.5 l limit you describe may have been referring to secondary vibration.

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

I'm glad there was at least some merit to the idea that was put in my head. Are there any flat plane crank V8s which use a traditional flat plane crank without balancing weights?

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

That is true, but there are more factors at play here. Launching at that high of an engine speed would require an excellent and gradual clutch engagement, and the power impulses at that high of an RPM are not be staggered as well for maintaining traction.

Not only that, but you would have to launch the car when the engine is well past peak torque..

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

Hello,

I found the video. If you fast forward to around 7:30 in the video, there is an explanation as to why the GT350 flat plane crank is different than other flat plane cranks.

https://youtu.be/GbbNlkP-ZSM


Kyle

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

Sorry if my question is dumb, but why flat-plane V8 needs two different intake plenums?

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

It doesn't "Need" it ... but it is a tuning variable, akin to the difference between a "dual plane" manifold for a traditional cross-plane V8, and a "single plane" or "tunnel ram" manifold.

With a dual plane manifold on a cross-plane-crank V8, the intake strokes being drawn from each "plane" are evenly spaced 180 degrees apart. To do the same thing with a flat-plane-crank V8, it is simply the left bank kept separate from the right bank.

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

Quote (Panther140)

Not only that, but you would have to launch the car when the engine is well past peak torque..
Maybe not a bad thing, thinking about "torque backup" aka "torque rise"... ponder

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

Quote (Panther140)

Not only that, but you would have to launch the car when the engine is well past peak torque..

I suspect that's the rule rather than the exception, at least for semi-serious and up dragstrip cars still equipped with a manual transmission. You might be able to find out what launch rpms people are actually using in the discussions found in the dragstrip sections of the various automotive-enthusiast message boards.


Norm

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

"Stored energy is proportional to rpm^2 so after lowering the inertia of the engine, increase the launch rpm to get back to the same stored energy."

That's OK unless clutch slip is being used to regulate torque to the wheels. Higher rpm means higher energy loss in the clutch - reducing the flywheel energy available to propel the car.

je suis charlie

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?


In F1 and other high-buck series in the early to mid 1960s many engines were designed with "downdraft" intake ports with the intake coming in between the wide-spaced cams. Despite the breathing advantage of unshrouding the long side this trend ended with the coming of the narrow valve angle Cosworth DFV.

A highly beneficial feature of downdraft engines was the ability to put the exhausts in the V, allowing more compact packaging and, critically for V8s, the ability to conveniently cross exhaust tubes to the opposite bank. This allowed bent cranks to be used with no power penalty from exhaust pulse timing effects in the headers. Some engines took advantage, getting rid of the vibration problems of flat cranks.
There still remained the disadvantages of a heavier bent crank vs a flat crank and, especially then, a weaker bent crank. So, you saw some of these downdraft V8s with flat cranks as well as some with bent cranks. I think the most iconic of the downdraft, bent crank engines was the Ford 4-cam Indy engine.

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

The higher RPM you launch at, the faster all of the parts in the drivetrain will spin for that inertial torque to be transmitted. But spinning those parts up to that higher speed requires higher amounts of inertial torque. Not only that, but the energy required to accelerate the car to the speed which is needed to facilitate that higher rotational speed of the drivetrain would require an extreme amount of inertial torque AND extremely strong components to hold that much force. Even if you could get the force required, AND have the drivetrain durability to withstand it, you would STILL need traction.

The only way to actually implement your theory of using extremely high revs to get that inertial torque is to have an extremely low gear ratio for the gear that you are launching in, one which is impractical in real world racing.

And on the subject of engine torque backup: Given a specific amount of inertial torque, crossplane V8s attain that at an engine speed which is closer to their engine's peak torque. That means that their torque backup is used to maintain that rotational torque. Flatplane crank engines lose much higher amounts of rotational torque by the time the engine speed falls to peak torque RPM. That means that they then have to regain more that inertial torque, for which the speed was already poorly optimized to the physics of making wheels (with mass) spin and moving a car (with mass) forward to match the engines speed.

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

The torque curve shown in that video was interesting. It had an abrupt vertical jump around 3000rpm, and was fairly flat up to redline. The flat torque curve from 3000rpm to redline is nice for a high performance engine. But the lack of torque below 3000rpm, and the spike in torque at that rpm, would not be suitable for most other vehicles. Most auto engines spend most of the time operating below 3000rpm.

The GT350 V8 engine in the video did sound very nice though.

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

Quote (Panthe140)

The higher RPM you launch at, the faster all of the parts in the drivetrain will spin for that inertial torque to be transmitted. But spinning those parts up to that higher speed requires higher amounts of inertial torque. Not only that, but the energy required to accelerate the car to the speed which is needed to facilitate that higher rotational speed of the drivetrain would require an extreme amount of inertial torque AND extremely strong components to hold that much force. Even if you could get the force required, AND have the drivetrain durability to withstand it, you would STILL need traction.

The only way to actually implement your theory of using extremely high revs to get that inertial torque is to have an extremely low gear ratio for the gear that you are launching in, one which is impractical in real world racing.
That's as good a problem description for drag racing as any I've heard. Keep in mind that the initial inertial energy in the flywheel, balancer, and crank is acquired before the timing starts, where it does not adversely affect acceleration.


Quote:

And on the subject of engine torque backup: Given a specific amount of inertial torque, crossplane V8s attain that at an engine speed which is closer to their engine's peak torque. That means that their torque backup is used to maintain that rotational torque. Flatplane crank engines lose much higher amounts of rotational torque by the time the engine speed falls to peak torque RPM. That means that they then have to regain more that inertial torque, for which the speed was already poorly optimized to the physics of making wheels (with mass) spin and moving a car (with mass) forward to match the engines speed.
???

Seems to me that a lower MOI crankshaft would lose less stored energy for a given rpm drop. That it has less at any given rpm to begin with is a separate question, but circuit racing is going to be more interested in reducing MOI effects while the clutch is completely engaged (this being most of the time) than in developing brief torque spikes via a (harsh and potentially risky) no-lift upshift technique.


Norm

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

^ Right, it will lose less stored energy for a set RPM drop. However, the entire basis for what I am saying is that the RPM drops are not equal between those two engines. The flat plane engine has to be spinning faster to achieve that inertial torque. That inertial torque happens significantly less at the speed of max torque. That is bad. You want inertial torque and engine torque benefitting eachother. Launches are the period of time when engine speed, wheel speed, and vehicle speed are being coupled. The benefit of using inertial torque on launches is that it is constructive to the engines peak torque output, which means more energy to get the car up to speed, which means the car gets into the power band faster. Inertial torque + Engine torque

For launches, you want the inertial torque to be considerable around the speed where the engine makes peak torque. You also want that engine speed to be realistically close to what the car's speed can facilitate shortly after the launch is complete (when the engine is coupled to the drivetrain). Most guys don't let off much between shifts until going into 4th gear, and the gear ratio of that gear reduces how parasitic the MOI of the crank is to the car's overall acceleration.

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

I think lowest possible rotational inertia in wheels, clutch, flywheel and engine rotational components makes for the fastest acceleration in road racing.

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

Some engines have such broad, flat torque curves that "peak torque" is barely a blip on the torque curve.
Seems like Insistence on "hitting" the torque peak when shifting etc does not count for all that much.
Ed Iskenderian said it never did.
see attachment

==============

And then, specifically on the GT350 with flat crank, there are the on board confusers to be reckoned with, or do the reckoning.
"The performance software also includes launch control, which adjusts engine speed between 3000 and 4500 rpm and holds it there so you can simply drop the clutch and go."
http://www.caranddriver.com/flipbook/17-reasons-th...

It will be interesting to see who wins the 60 foot time competition. The launch control, or a keen driver au natural.

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

Yeah lowest moment of inertia of wheels and clutch is also important in drag racing. But, you can't sell a thoroughbread road race car to the general public very well, and/or a good chunk of Mustang customer base.

If you go to a car show and talk to the older guys that can actually afford new cars, its common to hear them appreciating cars that "drive like a big block". I think the GT350 is a great car. If I were to sell my Cobra, it would be so that I could buy a GT350. But I won't sell my cobra :p They don't make cars that simple anymore. Nothing makes you appreciate simple cars quite like owning a complex one for long periods of time.

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

Yeah launch control is popular in drag racing. Its better to have it installed from the factory, too. If somebody attempted to install it in their garage, the wiring might be intuitive, or worse, easy to work on :p

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

With questions like this about why a certain crankshaft layout is chosen and similar questions - why not just ask Ford? - it may not be a secret.

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

Big Clive, even if Ford gives a plausible answer there still may be a secret. And, Fords legitimate reasons are surely a compromise that other companies may differ with -since other companies do it differently.

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

Shall we start a "Torque War"?

"Seems like Insistence on "hitting" the torque peak when shifting etc does not count for all that much.
Ed Iskenderian said it never did."


Ed was right. The torque peak is totally irrelevant to shifting. Shifting should be performed so that the average power output is maximised for the period spent in each gear. (assuming negligible engine rotational inertia etc etc)

je suis charlie

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

Quote (Panther140)

Yeah lowest moment of inertia of wheels and clutch is also important in drag racing. But, you can't sell a thoroughbread road race car to the general public very well, and/or a good chunk of Mustang customer base.
Which makes it all the more remarkable that Ford has developed this engine and tweaked the chassis accordingly. But perhaps less surprising than it might seem on the surface, given that Ford has recently stepped back a bit from the drag racing scene, gotten more involved in 'drifting', built the strongly road course oriented Boss/Laguna Seca versions of the S197, and that they were mentioning the European makes as 'targets' for the S550 rather than its traditional rival the Camaro.

As a road course kind of enthusiast for longer than I've held a driver's license, it's been a long wait for U.S. domestic sporty cars to truly be about more than just a powerful engine and a rumble-y exhaust note.


Quote:

If you go to a car show and talk to the older guys that can actually afford new cars, its common to hear them appreciating cars that "drive like a big block". I think the GT350 is a great car. If I were to sell my Cobra, it would be so that I could buy a GT350. But I won't sell my cobra :p They don't make cars that simple anymore. Nothing makes you appreciate simple cars quite like owning a complex one for long periods of time.
I know what you're saying here - I'm likely as old as many of those guys, and either driven or ridden in some of the same cars. And I feel much the same about my '08 as you do about your Cobra. Wish it was a bit simpler in some respects, developed a bit further in others, some of which I'm slowly doing on my own (with a modicum of success - 1.2+ lateral g's on true street tires, datalogged) A complete Voodoo engine and 3160 transmission would be a great swap, but that would probably end up costing nearly as much as I paid for the whole car even if I did all of the conversion work myself.


To the matter of inertial torque + engine torque, maximum acceleration in a given gear, maximum acceleration at any given speed, maximum acceleration spike on upshift, and optimum dragstrip ET and MPH results probably won't all happen during the same run. That doesn't make the logic behind any of them wrong, perhaps just not applicable to every situation.


Norm

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

^I like the way you think. A voodoo swap is a highly advisable investment though. By swapping, you would have more freedom to tailor the build to your needs. Owning a simple car and developing it into what YOU want can have benefits.
You can implement ideas into the car in the exact fashion that suits your needs, as long as you're capable and know what you want.

Back on subject! I think there were other issues (non performance related) that lead to fords counter-weighted flatplane crank. I do think the main problem was that balancing a flat-plane V8 that's bigger than 4.5L is not easy. This one is 5.2L. As somebody mentioned earlier, second order vibrations might start to get vicious.


http://articles.sae.org/13709/
"Flat-plane-crank engines have limitations. First, the lack of counterweight balancing typically limits cylinder displacement to about 4.5-4.6-L due to greater second-order vibration. Ford has solved that in the 5.2-L application with a new crankshaft-mounted damper system and extraordinary attention to NVH abatement during the design and prototype phases. According to Nair, the engine program (which was concurrent with GT350 vehicle development) nearly wasn’t approved for production.

“This [vibration] was our biggest engineering challenge even after we had the first prototype,” Nair noted. “Things were breaking and the technical guys were worried. Whether or not we continued down the flat-plane-crank path for GT350 came down to a critical prototype drive we had scheduled. After that drive, we all went into a meeting room for the debriefing. And we unanimously concluded that we simply had to have this motor! We were determined to solve the issues.” "

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

Gruntguru, ".....Shifting should be performed so that the average power output is maximised for the period spent in each gear. (assuming negligible engine rotational inertia etc etc)...."

This is precisely why the torque peak is crucial, or more precisely, the torgue and hp peak spread. You specify gear ratios to be able to run the engine in that range AND if you have more gears, you can narrow that band to give higher maximums.

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

To elaborate a little, a purpose designed race engine optimized to give maximum power should have its power peak very close to or at the mechanically limited rpm. If its breathing is adequate for that then there is no reason to shoot for less hp. This means you cannot run in a range of speeds around the hp peak. You have to run from below up to the max hp. The breadth of the range is set by the available gearing. At the bottom of the range it should be making the maximum power possible. Due to the way the VE/friction combination works, the max BMEP/max torque occurs below the hp peak. You make that point the bottom of your operating range. You invariably want to end up with an engine/gearing system that is best used between the torgue and hp peaks.

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

"This is precisely why the torque peak is crucial, or more precisely, the torgue and hp peak spread. You specify gear ratios to be able to run the engine in that range AND if you have more gears, you can narrow that band to give higher maximums."

The rpm for peak torque bears no relationship to the rpm for peak power. While I don't disagree with what you "invariably want to end up with" you can't assume that is what you have actually got so rules about operating between the torque and power peaks are by no means universal - ie not a rule at all.

je suis charlie

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

To this assertion, "...The rpm for peak torque bears no relationship to the rpm for peak power...", I'll make this reply:
An auto IC engine is a tuned system that makes NO torque at zero rpm. The torque must follow a curve from some low value at some low rpm to a peak at some higher rpm where it will begin to diminish as rpms increase. I can support a claim that torque does not disappear suddenly as rpm rises past the peak, but diminishes gradually at a generally increasing rate of decline.

Since power is a product of torque and rpm, power will increase from what it is at the torque peak as rpm rises until the rise in rpm is matched by the rate of fall in torque. That point is the power peak.

The simplest thing you can say about the relationship between the torque peak and power peak is that the power peak must occur at an rpm above the torque peak. That fact alone completely negates your statement that there is no relationship.

The rest of what I said assumes you are able to fully design your engine/gearing system so that you are able to put your power peak at the mechanical limit and your torque peak at the lowest range determined by gearing. If you are not fully able to design the system because of rules or some existing starting design or constraints, then other effects can occur. For a common example, if the engine can't breath well up to its mechanical limit (restrictor?) such that there is a safe margin above the power peak, then revving somewhat past the power peak could be an advantage. But, then re-camming to raise the torque peak rpm will usually also increase max power. You would ideally still have the torque peak at the lowest rpm accommodated by gearing.

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

(OP)

Quote (Panther140)


Back on subject! I think there were other issues (non performance related) that lead to fords counter-weighted flatplane crank. I do think the main problem was that balancing a flat-plane V8 that's bigger than 4.5L is not easy. This one is 5.2L. As somebody mentioned earlier, second order vibrations might start to get vicious.

Same magnitude secondary horizontal shaking forces exist in both FP set ups. So I don't think this is purely connected to engine size as they didn't solve that problem.

Regards,

Jon Reynolds

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

It's a lot like 4 cylinder engines that do not feature balance shafts - typically pretty smooth up to about 2 liters but get progressively rougher as displacement is increased much beyond that. Just that with a FPC V8 you have two of them on a common crankshaft and can't fully use what's happening on one bank to offset what's happening on the other. The change from U-D-D-U to U-D-U-D isn't going to solve that, and might actually make it worse in some respects.


Norm

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

In essence, is this similar to having 4 V-twin engines coupled at their cranks, and putting them 180 degrees out of phase of their neighboring V twin?

In the future, that concept may be helpful as NWO emissions gestapo crack down on industry. Have the V-twins run and be balanced independently. The engine would basically be 4 separate V twins connected in line at their cranks. If you could do that, you could then connect them with clutches, instead of having them share a solid crank. The tricky part is adjusting phase appropriately so that the secondary forces don’t constructively interfere/overlap with each-other. That seems like a potentially beneficial platform for cylinder deactivation.

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

You are looking at finger instead of looking at tthe moon.
The cost of a cross plane crankshaft is 2 times more than a flat plane crankshaft.

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

I'd like to see the cost numbers on Ford's new crank vs the cross plane cranks from the current high end Mopar and GM V8s. I didn't think this market segment was in a cutthroat cost cutting mode. neutral

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

I tried to think of this from a cost-cutting perspective, but the money spent developing this rules that out unless they can start using it in their trucks as well..

I think they are early investors in the future market of where the Camaros and mustangs will be. T

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

If a flat plane is cheaper, it weight less, is strongher, is more racing oriented (look at all V8 racing engines Cosworth Ferrari etc, it will never reach 12.000 rpm and so no problem for destructive vibration, wich one is better?

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

Better for what purpose?


Norm

RE: Why Did Ford Decide to do a Flat-Plane Crank Differently?

I think that the production cost savings of a flat plane crank is much more than outweighed by the cost of isolating the vibrations from the passenger compartment and the cost of beefing up components that are adversely affected. Most components attached to the engine will suffer shorter service life if not modified.
Cosworth stated that when they shifted from V10s to V8s in F1, despite their long experience with FP cranks, they were surprised at how destructive the vibrations were to the engine accessories.

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