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Rotating Mass weight and fuel economy.

Rotating Mass weight and fuel economy.

Rotating Mass weight and fuel economy.

Hi.  I'm building a 1967 Mustang with a 289, multiple mapping Edelbrock Pro Flo EFI, RPM Heads, full length headers with 2.5" exhaust, a 5 speed (2.95 1st to .59 5th gear), and a 9 " rearend with 3.25 gear.  I'm trying to get as much mpg as I can and still have some fun performance with around 330 hp at the flywheel.  It's going to be a daily driver convertible.  I've been reading this forum for days now and think it's pretty cool.  And I have a question for the forum relating to my project:

I know a build trick for a quick car is to lighten the rotating mass.  It ramps up and down more quickly.  For economy I don't want this do I?  I want a stock crankshaft vs a lightened one to get more centrifical force so it will roll farther without ramping back down, correct?  Past the flywheel is where the weight should be shed (driveshaft, wheels, etc.), but the rotating mass within the engine will benefit from the added weight for adding mpg by allowing the motor to not ramp down so quickly.  Is this correct?


RE: Rotating Mass weight and fuel economy.

" Is this correct?" No

But whether it'll make a significant difference is more to the point and frankly I don't think it will.


Greg Locock

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RE: Rotating Mass weight and fuel economy.

You'll like the car more if you can snag a 6-speed for it.

The 0.5 sixth in my Camaro is great for mileage, but just a little too tall when there's traffic on the expressway.  The 0.75 fifth is about right for that.


Mike Halloran
Pembroke Pines, FL, USA

RE: Rotating Mass weight and fuel economy.

The lighter the rotating mas the better the fuel economy, but probably to a hardly measurably degree.

You would gain more by using a lighter differential than the 9". While 9" is great for 1000 HP or more drag racers, it wastes power on a 300 HP daily driver.

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RE: Rotating Mass weight and fuel economy.

The fuel you use to accelerate the rotating mass will not be replaced when you decelerate. It will go into waste heat.

However, a more massive flywheel may allow you to operate the engine in a higher gear at a lower rpm. This effect may be important if the cam allows it.

www.engtran.com  www.niswug.org

"Node news is good news."

RE: Rotating Mass weight and fuel economy.

Thanks Guys.  I bought the 5 speed as a full conversion kit and at that time there weren't 6 speed kits.  I planned to use it like 2 4 speeds, utilizing 1-4 for performance and 2-5 for economy.  It's an old trick we used to do with 4 speeds, drive around like a 3 speed using 2-4 then when you race you use 1st gear and they never see it coming!  I'm gonna have to live within my budget on that one for awhile.  

I appreciate the comment about the more massive flywheel and low rpm.  That's a target of mine.  I'd like to map the EFI's three mappings for all out performance, economy, and something more like a stock config (a bit of both).  When in economy the low rpms in high gear are what I'm looking for.

I realized the 9 " is more than I need now, but I bought it when I had the car built for more performance.  This project is taking the hp down considerably.  One thing I like about this rear is it has 11" drum brakes, which only came on the 428 Mustang in 1967 (and if you can believe it a guy in Atlanta was parting out a '67 428 Fastback!).  I might switch back to an 8" at some point.  Any idea if it's worth the effort in gas savings?

RE: Rotating Mass weight and fuel economy.

The 9" is in some ways stronger than the bigger heavier 9-3/8", and is a much better choice for 300+ HP than is the 8".  

I think you'd be hard pressed to measure a difference in fuel consumption at the same ratio.

Further, the 9" is very well supported in the aftermarket.


Mike Halloran
Pembroke Pines, FL, USA

RE: Rotating Mass weight and fuel economy.

If the 9" is a bit of overkill on torque capacity you could explore using a lower viscosity lubricant or a synthetic.  

www.engtran.com  www.niswug.org

"Node news is good news."

RE: Rotating Mass weight and fuel economy.

I think the 8.8" is slightly more efficient than the 9", it has had rather wide availability, and is still in volume production.  It's stout enough to be offered as OE behind the current supercharged 5.4L in the GT500.  One enterprising individual has even developed a kit for installing the 8.8 into certain GM vehicles as a replacement for those cars' 7.5".


RE: Rotating Mass weight and fuel economy.

In theory, more mass (rotating and non-rotating) means more power required to accelerate which means more fuel used to accelerate. The energy is stored in the moving mass but since most people use the brakes to stop it gets wasted as heat instead of recovered during a coast to stop.

Focusing on engine efficiency in the expected cruising operating conditions should net more fuel mileage gains than changing the rotating mass a little.

I don't understand why you feel the need for using multiple maps in the EFI system. Good for back to back comparing but I don't see any point for normal use unless you want to artificially restrict the engine. The cruise and power modes are in different areas of the map so both can be optimized at the same time. I could see it with an auto if the ECU is also controlling the transmission.

RE: Rotating Mass weight and fuel economy.

I do want to restrict the motor some in the first gears to max fuel economy.  The motor should make about 295 ftlbs of torque at 2000 rpms when mapped for performance.  That's more than I need for cruising down the road.  By restricting the motor I can get that down to around 200-225 for economy and it will easily push the 2925# car through the lower gears.  Then map for quickness/performance to max torque.

RE: Rotating Mass weight and fuel economy.

I don't understand what "restricting" the engine will achieve.  You already have a restrictor - the throttle plate.  It will only make full torque if you are at full throttle.

From my experience the "restriction" in the lower gears in an early Mustang is traction.

RE: Rotating Mass weight and fuel economy.

The big trick with 1st gear is to leave it as soon as possible (2-3mph). This means just get the vehicle rolling and upshift. Do the same with 2nd (5-8mph) and your economy will go up tremendously at the expense of rapid acceleration.

I have been experimenting with this on a manual and CVT vehicle and it makes a big difference.

On most modern vehicles it seems like the throttle to pedal mapping is such that you are putting a lot more throttle on in the first bit of pedal travel than you get with additional pedal travel. Changing that to a more linear mapping will also help. This is not necessarily a linear movement of the throttle plate, but rather a linear increase in flow which might be hard to calculate with some advanced techniques.

www.engtran.com  www.niswug.org

"Node news is good news."

RE: Rotating Mass weight and fuel economy.

The largest influence on fuel economy is the driver and the use of the right foot.

If FE gains are what you are seeking then altering your driving style is the most effective way to get them.

There are a couple of sites worth looking at listed below:




and a general search will bring up many more under the title: hypermiling.

Good luck with it all.


RE: Rotating Mass weight and fuel economy.

"The motor should make about 295 ftlbs of torque at 2000 rpms when mapped for performance.  That's more than I need for cruising down the road."

When you're cruising down the road the throttle will obviously be closed more than the wide open throttle position required to allow the motor to make 295ft-lbs of torque.....

You do realize that if the motor was producing that much torque when "cruising" that you would be accelerating???

By artificially restricting the motor I mean purposely cutting the power so that someone else driving it can't get into trouble, or at least can't get into as much trouble.

RE: Rotating Mass weight and fuel economy.

I'm afraid that I can't think of situations where you'd really need 'performance' mapping down at 2000 rpm, when you'd either already be in 1st or 2nd gear - or should be.  If you're at 2000 rpm in a slow corner and are driving to extract maximum performance, even an economy mapping ought to be giving you more torque at the drive wheels than they can cope with in combination with the cornering load.  Map the under 2500 region for economy, 3500 and up for performance, and play around with the transition in between.

I suspect that throttle mapping that's digressive is done to impress the customer by making the vehicle feel peppier than it really is.  Most of the time, new car customers appreciate this, though there are situations where too much 'throttle' too soon interferes with drivability.  I can see where this would also work at cross-purposes to increased fuel economy.


RE: Rotating Mass weight and fuel economy.

OK, I still don't get how some people think there a different "economy" vs "performance" mapping.

The fuel and spark tables you can program are a minimum 3-D. Typical is MAP vs rpm vs fuel and MAP vs rpm vs spark. There are other variations which are ofthen added-in as a correction factor or used instead of the MAP such as the throttle position and the airflow via MAF sensor.

So, you can set-up the "cruise" operating part of the tables for ecomony - typically this means more spark advance and a leaner mixture. You can also set the more agressive performance part of the tables to pull timing and add fuel for more power.

Both can be done in the same mapping and it doesn't require 2 different sets of tables that are changed depending on the purpose of your drive that day. You can get both good economy and 295 ft-lbs of torque at 2000rpm with the same mapping. Moving from economy mode to power mode is a simple as pushing down the go pedal.

RE: Rotating Mass weight and fuel economy.

OP's question may be tied to limitations in the specific tuning/calibration package, as not all aftermarket EFI systems require a laptop for making adjustments.

The throttle mapping that I referred to at the end of my previous post was in response to kellnerp's post with respect to drive-by-wire throttle plate vs pedal movement.  There is a mechanical equivalent.


RE: Rotating Mass weight and fuel economy.

That's true Norm, I have no idea about the Pro-Flow system but the new Pro-Flow 2 is laptop tunable so it should have the capability.

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