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Rotating mass

Dear Tom and Ray,

In a discussion with a few of my car enthusiast friends we discussed methods to improve performance and economy. Among those methods discussed was to eliminate rotating mass. This spring I intend to replace my OEM drive shaft with one made of some aluminum alloy, I will get rid of my clutch actuated cooling fan and install an electric one, and also replace the original pulleys with under-drive pulleys. Lastly, I have heard that a cars flywheel can rob both economy and performance. However, there is a difference between cars with standard versus automatic transmissions, flywheels in cars with standard transmissions are heavy while those in cars with automatic transmissions are already really light and are also called flex-plates. Moreover to manipulate the flywheel/flex-plate could make it difficult for the harmonic balancer to do its job. Can it do that? I wrote about these things because I want to know if you have any words of wisdom which may guide our priorities. I have a dodge Dakota with which I want to achieve 30 MPG, it has a 318 and I do not want to sacrifice performance. I now I will spend more money than I will ever save on gas doing this but this is what I like. I have two friends who are auto mechanics turned tool and die makers so anything is possible in my opinion, but sometimes I need to find ways to help them see that too! I’ve told them about Johnathan Goodwin in Kansas and would very much appreciate your input as well.



How can “words of wisdom” be applied to such a eccentric plan?

This is your baby,you make it grow.

The flex plate and ring gear for the starter are so together and so light that there is nothing to be done. Something has to connect the torque converter to the crankshaft. Nobody really cares if the harmonic balancer works; you could have lots of fun by grinding off the excess weight that’s on the balancer. Underdrive pulleys may add to the rotating mass but will make it easier for the engine to turn. Lightweight frames and suspension parts will help. The steering box can be replaced with a rack and pinion setup. Those truck magazines will be a lot of fun for the suggestions in them. Narrow tires will be good too.

This program will only save energy on stop and start driving. It will not help at all with highway mileage.

Underdrive pulleys and conversion to an electric fan can certainly help you gain some extra horsepower.

You’re going to have a serious issue if you eliminate too much weight from the flywheel. (and a flexplate won’t work - period)
Assuming you got rid of 90% of the flywheel weight, what you’re going to be left with is an engine that will hit high RPMs much quicker but will be totally gutless on pulling power.

If you intend to drive around with foot on the floor all of the time, no load in the bed, and don’t go uphill you’ll be fine.

Decreasing rotating mass will only help when you are accelerating. It will have no effect for steady-state cruising. Replacing your driveshaft will have little effect on inertia; the biggest gain here is overall weight reduction.

The flex plate inertia is lower than a manual transmission?s flywheel inertia due to the torque converter.

I don?t think you?ll be able to achieve 30 mpg with a 318 V8 unless you incorporate a cylinder shut down strategy of some sort.

Your best hope would be to swap in a small, turbo diesel. I bet you could hit 30 with that, as it was common to get 25 mpg in a full size Chevy with the 6.5L diesel back in the 80?s.

You’ve not mentioned the largest rotating masses on your truck, the wheels and tires. There’s much more to be gained, much more easily, by finding the lightest combination of wheels and tires. The other source of benefits is lightening the truck overall, removing every unneeded pound you can find. Don’t mess with the flywheel, little to be gained, much to be lost.

Remember, the rotational energy savings are proportional to the mass saved times the radius squared, so small mass savings at a small raius (as with the driveshaft) will have very little impact, while large mass savings at a large radius (as with tires and wheels) can have a much larger impact.

As the replies have noted, you need to be careful about ill effects of your proposed changes. Will you be doing any calculations ahead of time to understand the changed behavior? Or will it be trial and error?

By the way, don’t mess with the harmonic balance. Crankshafts have been known to break in half when the harmonic balance isn’t properly in place.

What about “dodgevan’s” statement “nobody really cares if the harmonic balancer works” did I miss a joke?

Highly refined driving techniques will gain most people MUCH more benefit than the benefits you mention. For example:

  • Watch highway speeds
  • Tire pressure
  • Approach a red light slowly, so that you don’t have to come to a full stop
  • Decelerate slowly (and obviously, accelerate slowly)
  • Learn about aerodynamics (putting your tailgate down will hurt you!)
  • Upshift early
  • Reduce electrical draw
  • Try the pulse & glide if possible

And no, don’t mess with your harmonic balancer.

Some people also like to improve the aerodynamics of their vehicle by making air dams, belly pans, and that sort of thing.

You may gain a barely perceptable improvement in acceleration by changing the driveshaft due to lower masses to get rotating, perhaps even a tiny mileage improvement due to the simple lower weight to cart around.

Do not mess with the flywheel or the harmonic balancer. Both go a long way toward smoothing rotational vibrations in your engine via the crankshaft, and compromising either without highly specialized equipment to measure the affect would be likely to result in real reductions in your engine’s lifespan, perhaps even make it impossible to get the engine to run smoothly again.

Mess with the automatic tranny’s connection between the engine and the torques converter would also be foolhearty. It’s designed that way for a reason, not by accident.

Replacing the clutch activated cooling fan with an electric one is a good idea, as the current fan adds load directly to the crankshaft.

I think your best bet is to focus on the wheels and tires and aerodynamics. Especially the tires. Just a change in tires can affect mileage 3 mpg. I’ve done it. It’s important to realize, however, that that comes with a compromise in poor weather traction. Harder compoundes and less agressive tread designs don’t do well in bad weather.

After many years of bicycle racing I cannot agree with you more. Frame weight, body weight are not as important as wheel weight and concur with the don’t mess with the flywheel!

Ryan, you mentioned the flex plate, yes it is lighter but the torque converter is heavy, besides you have to have the flywheel to attach the clutch to. I believe putting on an electric fan will help, I am running 2 fans on my '96 Dodge van 318, I don’t know how it is going to do in the summer. Also, as some have suggested, lighter wheels might help.

So a 3,000 pound car will get the same highway fuel economy as a 6,000 pound car? I would like to see evidence of that.

Weight is a factor in both city and highway fuel economy. One reason is that a vehicle that weighs more will require more fuel to maintain speed than a light vehicle.

Highway driving requires vehicles to speed up and slow down. No vehicle is capable of traveling at a true constant speed. Highway driving only lessens the acceleration and deceleration. It doesn’t eliminate it.

Rotating mass is not the problem…It’s RECIPROCATING mass, that force that is trying to rip your engine apart (and WILL rip it apart if you rev it high enough) that you should try to reduce. Honda built six cylinder 250cc race bikes for one reason. To reduce reciprocating mass and allow 13,000 rpm, a region where enough horsepower was being made to beat the 2-stroke bikes…

I’ve always heard that the biggest drag on gas mileage is vehicle wind resistance. I had a 69 Chevy pickup with a built 327. On good days, I got 17 mpg. I took that engine out and put in in a 49 Ford pickup that weighed more than 1,000 lbs less. The cab on the Ford stuck up like a phone booth, though. I never got better than 12 MPG.

And to boot, the Chevy had 456 gears and the Ford had 411 gears. I used the same tranny.

Look and see if you can find some tricked out effects to add to the vehicle. Cyclists put big plastic covers over their wheels that add weight but reduce drag enough to let them ride faster.

Also, try putting the skinniest tires you can get. I guess that’s another thing you can learn from cyclists.

You may want to see if you can get a standard transmission too. The hydrolics in an auto tran rob horse power just like running the air conditioning will. It stands to reason that if the motor has to use power to run the tranny it wastes gas. Not to mention something like a 5 speed tranny will probably get you better top end gear ratios and it will also let you run your vehicle at the power peaks more effectively with more gears.

Reducing weight is a great thing for performance, though. Old time racers used to say that 1,000 lbs is worth 100 horse power.

There you go again, whitey who in heck said “a 3,000 pound car will get the same highway fuel economy as a 6,000 pound car”?

Driving conservatively will do more towards increasing your mileage than anything related to anything you are asking about.

Yes, all else being equal reducing the weight of the car all improve the mileage, but all else will not be equal. Everything is connected and and making one change may cause several un-intended changes. The amount of weight you might be able to totally reduce by reducing flywheel weight will not be enough to measure.

melott said reducing weight would not help with highway fuel economy. He said it would only increase fuel economy in stop and go driving. I challenged that assumption and asked a question. Now if melott would respond, I would like to read her/his response. If you have anything useful to contribute, I would like to read your response too. However, if your only goal is to butt in and censor me, well, that isn’t going to work.

I never claimed melott said anything about a 3,000 pound car. That was my question related to the principle he described. Am I still allowed to ask questions here?

Elly, I find I am capable of having productive discourse with everyone here except you. I would like to exchange ideas with others without having to contend with your bitterness over an old argument.

mellott implied that the weight savings by replacing the crank shaft is trivial. You implied that the crank shaft could weigh up to 3000 pounds. Your response is an overreaction. If you want to dispute whether crank shaft mass is an issue, that’s fine. But inflating the mass difference by an order of magnitude or two isn’t appropriate.