how much crankshaft twist?
how much crankshaft twist?
(OP)
We race a roots supercharged motor. After dynoing we noticed a small amount of timing(1 degree)made a huge difference in power. Now with the ignition signal triggered from the front of the crank and the load at the back of the crank and a firing order of 1-8-4-3-6-5-7-2, what cylinders would receive the greatest amount of twist and about how much? With individual cylinder timing there should be some power there(?) How is this twist related to the rpm? Thanks





RE: how much crankshaft twist?
After cyl. 2 and 1 fire, you can assume the timing for cyl 8 will be advanced with your setup. Think about it.
RE: how much crankshaft twist?
RE: how much crankshaft twist?
RE: how much crankshaft twist?
No matter where you drive it from, if there is say, 4 degrees twist in the crank then there will be 4 degrees variation in timeing wether the front is advanced or the rear is retarded.
Crank twist will change with torque output.
Cam twist will change with rpm.
You can eliminate losses from chain stretch, gear backlash, end floats tec by running a crank trigger, but you still get the twist, and it is variable.
The only accurate way would be to have 4 cam drives directly off the crank and 4 crank triggers for a cuniform crank on a V8, one trigger and drive at each big end journal.
I know, it cant be done without a complete redesign with this taken into consideration. i imagine the design would be very much like 4 V2s or 2 V4s strapped together.
Not exactly a viable solution
Regards
pat
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RE: how much crankshaft twist?
I have an old chart for a small block chev, that shows
a 12 order harmonic at about 1800 rpm that is at .2 degrees double amplitude. And a 4th order harmonic at about 5300 rpm that is at 2.0 degrees double amplitude. Both those figures are with out a damper. Now with a damper the 1800 rpm harmonic is about .1 degree double amplitude. And the 5300 rpm harmonic is at about .3 degrees double amplitude.
Which shows that if properly tuned you can reduce alot of that twist.
RE: how much crankshaft twist?
I'm trying to get a handle on the actual crank wind-up seen, but failing dismally.
I'd /guess/ of the order of some degrees. Sadly the only way to work it out accurately is to build an FE model of a crank and then apply the loads. An upper bound estimate of the torsional stiffness of the crank would be a cylindrical shaft of the same diameter as the mains, but that would be very high.
Cheers
Greg Locock
RE: how much crankshaft twist?
RE: how much crankshaft twist?
Thinking about it again, we rarely see torsional vibrations of more than one degree peak on a crank, so I guess THAT is a better guess for wind-up.
Cheers
Greg Locock
RE: how much crankshaft twist?
Sounds like a pretty good guess to me, even if it's nose-(ideal nose) instead of nose-tail (tail is close to a node anyway).
RE: how much crankshaft twist?
-Joest
RE: how much crankshaft twist?
RE: how much crankshaft twist?
It could probably be calculated using BMEP and the crank stiffness. I used to run a big tensile testing machine that could have been used to load and measure the front throw movement while the rear was fixed, but that was a long time ago.
RE: how much crankshaft twist?
RE: how much crankshaft twist?
RE: how much crankshaft twist?
I dont see the point of moveing the pick up point to another position on the crank as it just changes which cylinder is accurate.
for example if we got 4 deg consistent wind up, and we trigger of the middle of the crank, the front would be 2 deg adv and the rear 2 deg retarded, but this would detonate the front, so we would retard it untill the front stopped detonating, then the back is still 4 deg retarded.
You would need to individually time the cylinders by each crank journal location.
I expect how you could measure this is by inserting tranducers for a crank trigger system at each counterweight then reading on an oscilliscope under various conditions.
This would only measure wind up at the point where the trigger is triggering? and not account for torsional vibrations or variations at various points of rotation through the 720 deg cycle. Sorry if that statement was unclear, but I am having a bit of trouble clearly and concisely expressing my thoughts on this.
I like Gregs idea of programing it into the ECU for a road car. This might have benifits re emmissions and economy.
On further thought, if say 8 transducers were at 45 deg intervals on every counterweight, we could also get an idea about torsional vibrations.
I was a little involved in the promotion of plastic manifolds, and one major advantage is that plastics injection moulding is a more reproducable and precise process than aluminium sand casting, so plastic manifolds if designed correctly, have the potential to reduce the cylinder to cylinder charge variations.
Too bad about the ports and exhaust manifold though. I don't see to many manufacturers going to 5 axis cnc ports at this stage, nor precission jigged mandrel bent exhausts with uniform fireing order distribution in the collectors, at least for V8's. On a V8 this requires a flat plane crank or a few crossover pipes.
I think I have already done to much to hijack this thread, so I will butt out now. End of rave
Regards
pat
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RE: how much crankshaft twist?
This would only apply at part throttle, I'm pretty sure we just use a lookup table for full throttle.
Cheers
Greg Locock
RE: how much crankshaft twist?
Rod
RE: how much crankshaft twist?
Re crankshaft twist, yes we did get a bit accademic and pompous, for something that is insignificant, and probably unmeasurable.
Greg
As I understand it, the purpose of a knock sensor is to allow the ignition to be pushed to just into knock then retard a smidgen. This process is repeated at regular intervals to keep it at optimum.
I expect that one cylinder would always be the first to knock, so the entire engine would be optimised to the results from the worst cylinder, unless the ECU only advances one cylinder at a time up to knock, then does the test on the next etc.
Even though Rods experience is with the "A" series BMC and it's well known inherent design problems, Rods is a race prepared engine, so many production variations will be reduced or eliminated, appart from those designed in like the siame ports.
Several degrees of timeing can have a significant effect on power output, and I would think would be worth chasing on a modern production engine.
Regards
pat
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RE: how much crankshaft twist?
Re crank twist and roots blowers.
Just say, the motor is a SBC producing 1000 HP
To do this the parasytic losses from the blower might be 300 HP
Gross output 1300 HP, 300 being drawn from the front, 1000 being drawn from the back. journal 1/2 would be slightly retarded, 3/4 would be neutral, 5/6 slightly retarded and 7/8 more retarded.
The extra torque will produce more twist, but drawing some power from each end will at least partly compensate.
With the helix design of Roots blowers, the cylinder to cylinder variations in charge density and fuel distribution will be monumental vs the probably unmeasurable and insignificant effects of crank twist.
If you have an ignition system with individual tuneability from cyl to cyl, from what Ros says, I expect you could pick up some worthwhile HP by optimising on the dyno.
If you are running a points triggered maggy, then I expect this type of tuneing would be exceptionally difficult, and could only be achieved by running 8 single cyl maggys or making points cams with various timeing built in, then continually changing cams. Not what I would call a doable exercise
Regards
pat
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RE: how much crankshaft twist?
Also, the longer the distance between twist-input and twist-resistance the more the twist. Think of how much more you have to rotate the handle of a breaker-bar when you have a long extension vs. having the socket right on the breaker. Note-I've actually had people (Densans, not Mensans) tell me you lose torque with long extensions!
So, since the vast majority of "twist-resistance" is at the rear, throw 7/8 doesn't ever get adv. or ret. by much at all, but 1/2 could easily get adv., and also adv. the cam and spark for the next cyl. to fire, no.8
What I'd like to hear from Jstein is how the small timing change caused his big power loss-was it from being retarded or advanced?
I used to run a Mallory (I think) ignition trigger that used a light beam and a plastic rotor for timing. There were small slots in the OD of the rotor, which was ~2.5-3" dia. That big a dia. gave very even timing-spacing, but it could be easily modified to compensate for individual cyls. such as retarding no. 8, where I think the biggest variation would be.
But I agree the HP diff. would be tiny if even measureable.
RE: how much crankshaft twist?
The Electromotive system (and others I presume) can easily be converted to multiple pickups (four in our case) using single coils per cylinder. I am not very competent on how to modify/adjust the modern ECU's to do this same job but, I am told that "You can do anything with a PC these days, Dad!"
Rod
RE: how much crankshaft twist?
RE: how much crankshaft twist?
I don't know what a "state of the art" mag. is now-we ran a Cirello, with no advancer, period. Started and ran with 50 deg. lead, IIRC. But it sounds like it doesn't vary the timing for crank-twist, and you can probably pick up a little torque/HP if you can figure out a way to trigger it differently. The dyno will tell you if your changes are working. Cyl. 8 is the most affected in a front-drive setup, which means just about all BB and SBC's. If you can set up cyl. peak pressure or actual press. vs. time instrumentation, you'll see the results without the dyno. I think the next cyl. to be most affected is no. 6, following the power strokes of 4 and 3 on that throw just ahead of 6, but it won't be as retarded as no.8 because of the shorter distance involved.
Note that none of the above includes anything on crank harmonics/vibrations-I really have no idea how they affect timing, but when your pistons/rods are REALLY trying to turn that crank, it will twist elastically for sure. And it doesn't matter what kind of steel the crank is or how it's heat treated either!
RE: how much crankshaft twist?
Around 1968 FORD felt they could improve the perceived smoothness of their 302 V8 by changing the firing order.
I'm thinking the crank wind-up must have been significant at some street type rpm ( 3000 ?).
I'd fully expect A little time on the dyno with a timing light checking cylinder-to-cylinder timing variations and general timing scatter at various speeds and loads might be rather revealing. If a high speed timing light and a finely degreed hub or flywheel (not damper outer ring, which is supposed to dance, relatively, at resonance) could not make the changes obvious, I would concentrate on other areas, figuring the much harder to measure variations in valve timing events might be more harmful.
Then again, after the recent excess tempeature problems we've had at work with splash lube gearboxes it would be easy to convince me to start thinking looking at ways to "minimally lubricate" transmissions and rear ends as a way to preserve the horsepower the engine already makes.
RE: how much crankshaft twist?
Cheers
Greg Locock
RE: how much crankshaft twist?
Rod
RE: how much crankshaft twist?
I would think that driving a blower off the front of the crank would yeild less twist because the power is extracted from both ends of the crank.
I do agree however that the crank is not so rigid as I once thought. I once installed a 390 crank in a 370 Caddy, and there was about a sixteenth inch clearance between the rod bolts and the pan flange of the block. Once the deck height piston problem was solved I started it up to hear a real clatter in the bottom end. Pulled the pan and could see clearly where the bolts were bumping the block, but they still cleared nicely when turned by hand.
Thought for today (Not original but works for many things)
"When theory and measured results don't agree, invent a new theory."
Regards and Happy Holidays.
Pancholin
RE: how much crankshaft twist?
I really have no idea about the number of degrees of twist the crank would have-wouldn't expect it to be more than a few tho. But the orig. post noted a large drop in HP for a 1 deg. retard (all cylinders of course), so there should be a little "free" HP to be gained-and a racer wants every pony he can round up.
Merry Christmas to all!
RE: how much crankshaft twist?
The torsional vibration of a forged crankshaft can cause variation of up to 0.2 degree. Trust me, if we allow the crankshaft to vibrate up to 1 degree, the crankshaft will self destruct. The t.v. is at its worst at the crank web longest to the flywheel.
Anyway, you should check on how many teeth the trigger wheel has. The lesser it has, the bigger the variation may be.
AO
RE: how much crankshaft twist?
Cheers
Greg Locock
RE: how much crankshaft twist?
youve got to understand that a V motor with crank wind up and cam wind up (especially with heavy valve springs) is like a big rubber band.....especiallly at the first and second harmonic rpms. Now add to that spark scatter from the cam to distributor gear, the distributor shaft, vertical movement of the distributor shaft , and fore and aft movement of the camshaft.
What to do to help the situation....
1) live with it..as you have plenty of horsepower and you dont get on and off the throttle as its sounds like this is a drag race motor..... or...
2) try to minimize losses and maximize the power that can be gained thereby. This is all accepted practice. Take for example the BMC "A" motor cited above. The skew gear distributor drive was fine for tractors and MG Midgets....but terrible for performance....thats why a crank trigger system is used on these motors in race trim (rules allowing). Crank whip is so bad with the three main bearing design, that to run the motor sucessfully for prolonged periods of time at higher rpm (7 to 8.5K) you have to run LOTS of rod side clearance and crank end play to avoid pinching rod journals.
Back to your blower Chevy. A crank trigger system with the Electramotive box will allow you to be selective on firing each cylinder. You will definitely pick up some HP and the motor will be happier. NASCAR cup motors use the MSD distriibutor with adjustable firing capabilities also. Try a manual timing light when the motor is on the dyno or running in the car.....follow the bouncing timing mark. I am very surprised that you could even read the lead to + - one degree.
Good luck..
RE: how much crankshaft twist?
Interesting! How did you measure the difference? I know that some people measured the rotation at both ends. Is this similar to how you measured yours?
Anyway, I am also curious on how many teeth the trigger has?
AO
RE: how much crankshaft twist?
At over 2000 HP, he is running at least alky, possibly some nitro methane. I can't see where he says what fuel
For alky, and certainly for nitro methane, he will need a high amperage long duration spark, that to date has not been obtainable with computerised electronic engine management systems, unless you call MSD magnetos electronic.
I would love access to a true high amperage and duration maggy with crank trigger and long spark dwell time.
Regards
pat
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RE: how much crankshaft twist?
RE: how much crankshaft twist?
It may be possible to reduce the crankshaft stress due to a revised firing order by firing the cylinders as close as possible from one to another.
Torsional vibration is at its worse if the engine is first fired at the 8th cylinder and followed up by the first cylinder. Imagine how much twist the crankshaft will experience.
On the other hand, it will be great if we can fire the V8 engine in series. Perhaps I should put this in my wishlist.
AO
RE: how much crankshaft twist?
RE: how much crankshaft twist?
RE: how much crankshaft twist?