Rotating mass

Rotating mass is not the problem

That depends on what your objectives are. If you’re trying to get from one spot to another as fast as possible, then reducing rotating mass is just as important as any other improvements like your example of getting the engine up to the power band as fast as possible. Anything with mass that must turn is cause for concern whether it reciprocates or just spins.

The opposite of this are high efficiency stationary engines that run low rpms but have very large rotating masses (flywheels) on them. They get up to speed slowly but run very efficiently once there.

BTW, none of the suggested modifications will have noticeable effect on efficiency IMO. The weight savings are negligible. There are plenty of people drag racing with stripped out cars- much more removed than suggested here- just to gain a few tenths.

I knew a friend of a friend who removed the harmonic balancer from his inboard marine engine- because it was hard to fit into the compartment with it on! Would not listen to any advice against it. Even after replacing THREE crankshafts, he insisted it must be something else wrong with the engine.

Is that what melott meant by “this program”? Are you sure?

My only point was that any kind of weight reduction would increase fuel economy in both stop and go driving and highway driving. I can’t think of a modification that would lead to increased fuel economy in stop and go driving but not highway driving. Can you?

My response may have been too big of an example, but I was just using an inflated example to demonstrate the principle. So please allow me to rephrase the question.

Let’s say you have two identical Honda Civics. One of them gets its suspension replaced with light weight components and gets the front and rear seats and spare tire removed. The driver’s seat is replaced with a light weight racing seat. It also gets light weight rims and a lighter drive shaft. Now the modified Civic weighs 80 pounds less than the other Civic. Will both Civics get identical highway fuel economy? I don’t believe so. What do you think?

OK, whitey, I’ll refrain from disputing you. You are right in the above post, any weight that decreases mileage in accelerating will also do the same cruising.

Whitey, the power required in steady-state includes:

  1. Energy to overcome resitance in engine, accessories, drivetrain.
  2. Rolling resistance.
  3. Aerodynamic resistance.

Of these, only 2 is weight-dependant (proportional to weight). AFAIK, 2. is a very small portion of the power required for highway driving; 3 is probably predominant. I would believe a 4% increase (+80# Civic) of something that’s already a small proportion of the total would be virtually unmeasurable. Also, there’s the chance that more rotating mass would act as a flywheel, damping out some of those deviations from the steady-state mentioned.

Excees weight would be a bigger deal for stop-and-go, beacuse: 1. More mass to bring up to speed 2. Lower average speeds means aero drag is smaller (proprotional to square of velocity) and thus everything else more important. 3. Downhills often steep enough to require braking (on interstate, one can usually coast or reduce power on downhills, thus recapturing much of the energy put in on the uphill). 4. Possible rougher/softer road surface, increasing rolling resistance.

Know this from bicycling: weight is an enemy mainly on hills and velocity changes. “Clydesdales” lose on hills, but (assuming equal fitness) have the edge on the flats.

“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.” Just curious, how would the first and third items reduce the weight?

“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 agree with the above quote. Also the fan on my van is/was on the waterpump so I believe that an electric fan will prolong the waterpump as well.

The only way you will get 30 MPG out of that truck with a 318 is to find routes that are downhill both ways.

Agreed. Although it does make for an interesting project if you have the money burning a hole in your pocket. If the OP is willing to spend two or three times what the truck is worth…

"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."
I’m afraid if you got rid of 90% of flywheel weight, it wouldn’t run at all, at least it would have to idle very fast to keep running.