Do lightweight flywheels make much difference?

[steffen707]

Well I have finally found a donor EK civic hatch with AC to swap my VX motor into. I have a fidanza 7lb flywheel from an old crx drag car i could use, or buy a stock 16-18lb flywheel. I was wondering if anybody else has experimented with this? I know it makes drivability much more difficult from stops, but if it makes a big gain in mpg it might be worth it "for me". I know it will reduce drag on the engine and that has to help efficiency but to what extent?

Please post any results you have found personally or have found from others.

[autoteach]

Heavy flywheel, better mpg. I dont have the patience to explain it right now, but believe me (7 lbs is really light, tough to drive)

[steffen707]

No offense but i'm not going to "just believe you". If you would like to explain later that would be great. The reason why I'm skeptical that heavier is better is because heavier could only be better to a point. Make a 100lb flywheel and the mpg would be horrible, so mpg could only possibly increase "to a point" and then begin to taper off. Sure 100lbs or 1000 lbs is extreme, BUT this does prove that heaver is only better to a certain weight. What is that weight, 2lbs, 7lbs, 18 lbs, 34 lbs...ect. ect. ect. Trouble is that ive only seen like 7, 11 and 18 lb civic flywheels.

One reason why racers use light flywheels is to reduce parasitic drag on the crank, just like deleting power steering and an alternator would reduce drag on the crank, thus "freeing up horsepower". Right? Freeing up horsepower should increase mpg because less load is on the motor.

[dcb]

I'm replacing my heavy dual mass flywheel in my tdi with a 10lb steel flywheel. I'm betting it will: bump start smoother, accelerate faster, shut down quicker, and save fuel from less energy lost accelerating the flywheel. That project is still a ways away though, can just synch shift for now. But some people have figured out how to drive a light flywheel just fine, takes a bit more clutch action on getting rolling.

[autoteach]

Fine, put it in and enjoy. Or you could do the research. This is exactly what I expected, an argument. Rather than always asking people and then telling them they dont know, do the research and tell us why.

[steffen707]

Hey i found a great link for this
Do Lightweight Flywheels Lose Torque? - Fact or Fiction - Import Tuner Magazine

my question still stands, How much of a mpg increase does 9lbs of drag reduction on the crank yield?

[steffen707]

Originally Posted by autoteach

Fine, put it in and enjoy. Or you could do the research. This is exactly what I expected, an argument. Rather than always asking people and then telling them they dont know, do the research and tell us why.

Well if somebody already has "real world-i put a light flywheel in and at 70mph with same gearing, engine, car, ect. I got worse gas mileage or better gas mileage" then i will have good proof if it will help or hurt.

Simply telling somebody on a forum "it won't work, but i can't explain why" doesn't give good confidence in the opinion, because its not backed by fact, example or theory.

[Ryland]

I'm sure if you look around there is a formula out there as to how heavy you need a flywheel of a given size for an engine of a given size at a given speed, having it hard to drive to me would be a giveaway that it's to light as it's not absorbing and giving back the energy between firings, this was part of the idea behind having multi piston engines and the V8 engine design is that you have the firing of pistons happens often enough that your power output over the rotation is high enough.
The only real disadvantage of a heavy fly wheel is that it takes more energy to get it moving, but as long as it's balanced it will maintain it's momentum, it's not like the rotating mass of a wheel and tire, because those are unsprung weight.
So yes, a 1,000 pound fly wheel would be nice, if the engine had enough power to get it moving and if you had a CVC transmision that could work with it, because the cranks shaft is always speeding up and slowing down depending on what part of the cycle the engine is in, slowing down on a compression stroke and speeding up when it fires.

[steffen707]

Originally Posted by dcb

I'm replacing my heavy dual mass flywheel in my tdi with a 10lb steel flywheel. I'm betting it will: bump start smoother, accelerate faster, shut down quicker, and save fuel from less energy lost accelerating the flywheel. That project is still a ways away though, can just synch shift for now. But some people have figured out how to drive a light flywheel just fine, takes a bit more clutch action on getting rolling.

Well if you could take some awesome stock baseline data, and then recreate the scenarios after the flywheel is the only thing you change i would appreciate your findings. As would others I bet.

[dcb]

from the link:
Dyno Comparison
Lightweight Flywheel
+8.35WHP and +8.10TQ

Win win with the lightweight flywheel, that extra rotational mass takes a big bite. The launching doesn't bother me, I'll get the hang of it.


re: baseline, can't do, the car is screwed and needs a clutch (isn't disengaging) not apples to apples after I get a new clutch in.