[Black-STi]

I am installing a lightweight flywheel and already had a lightweight crank pulley...my mechanic recommended I pick one and not run both because the rotational mass would be so low. So I opted to put a stock crank pulley back on...anyone running both? Wonder if you have any issues with revs dropping to fast and causing drivability issues.

I put the stock crank pulley back on becasue of the reduction in vibration as well. My flywheel is the Gruppe S unit FYI which I believe is about 15lbs...

[Interior Flaire]

I am running both-no issues.

CB

[n2oiroc]

i doubt the pully would make any difference. the flywheel is the big one.

[pj21086]

im running both but occasionally get a misfire code.

[modaddict]

talk to Ka mano on the other board.

the lightweightness of everything causes driveability issues. It makes the car harder to drive. but it rev's quick. it's a tradeoff.......and it will only be your preference once you drive one....

[n2oiroc]

Originally Posted by modaddict

talk to Ka mano on the other board.

the lightweightness of everything causes driveability issues. It makes the car harder to drive. but it rev's quick. it's a tradeoff.......and it will only be your preference once you drive one....

i agree. when i was getting a clutch installed in my supra, the shop thought i was nuts when i didnt want the flywheel lightened. its much easier to launch a turbo car with a heavier flywheel.

[V8Huntr]

The problem with lightening those components is that your rotational mass will actually be out of balance. The motor is actually balanced when the stock flywheel and crank pulley are on. When you swap one of those out you could technically shorten the life of your motor.

If you don't care about all of that stuff and still want one, I usually follow a quick rule:
Lighter Flywheel for track use, Heavier flywheel for drag use

Like n20iroc said, it's a lot easier launching the car with a heavier flywheel. But if you're on the track then you want your rpms to move faster due to frequent shifting, so that's where the lighter flywheel comes in

[imprezarsx]

Subaru engines are internally balanced.

Black STi has an entire built motor that was fully balanced and weight matched without a pulley or flywheel on it. The flywheel he is using is balanced seperately also.

Balance will not be an issue for you, Ken. However, vibration can become one. Not everyone understands the necessity for a little weight on our engines.

Think of it as a gyroscope. IT's harder to change direction on a heavy quickly rotating gyroscope. When combustion occurs, there are forces placed on the bearings... of course. It's how the force is distributed that you have to worry about.

If the rotating assembly is very light, it will undergo slight speed changes every compression and combustion stroke. This constant acceleration and deceleration is bad for the engine. It creates a sort of stress frequency. A heavier rotating assembly has the required mass to dampen the accelleration and deceleration of the crank shaft. Going too light does not yield this dampening effect.
*This is shown very very often by misfire codes. Misfire codes are a product of the crank shaft not showing the proper acceleration and speed after a combustion event. That's how the ECU knows which cylinder the misfire occurs on.

Another effect is when the combustion event occurs, energy is transmitted through the rods to the crank. Instead of the energy being applied angularly to spin the crankshaft, load is applied in a way that jams the crank against the bearings. It's a very harsh and sudden force. This banging at the bearings (of both rod and main) will eventually lead to bearing failure. If there was more rotating mass, the crankshaft would not be slammed against the bearings. The gyroscopic effect of the rotation would overcome that force and almost all the energy would be applied to ROTATING the crank.

Weight IS REQUIRED to properly pull the engine through it's combustion process.
*This is apparent to drivers that pay attention to their ride. For example, before a lightweight flywheel, you can lug the engine at 1500rpm and accelerate normally in a high gear. However, after a lightweight flywheel, you get the large vibration/noise/lugging engine that requires a downshift in order to run smoothly. The lighter the mass, the more apparent this is.
If someone would like a demo, come on over. I'll whip out the RS with the 12lb flywheel and lightweight forged internals then we'll drive the STi with stock flywheel and internals.

So, my advice is this:
If making big power (which we know you will be) stick with one or the other. Lower cylinder pressures from stock engines and turbos won't show the error with running both. However, when we add the boost to your monster build, we will be asking a lot of everything.

[Black-STi]

excellent post imprezarsx! thanks again, this is why I listen to you...so helpful at every step of my build

[Gami004pilot]

WOW! Now I'm freaking nervous! I have an ACT street light flywheel. Is that a good thing or a bad thing?

[flatthump]

Originally Posted by Gami004pilot

WOW! Now I'm freaking nervous! I have an ACT street light flywheel. Is that a good thing or a bad thing?

The only downside you'll probably notice is a strange sensation when cruising and difficulty launching the car.

[VSGTS14]

i run both, no problems and yes a boxer motor is internally balanced due to the the design.

[neverendingmods]

Look into the ATI crank damper. Pricey but it takes care of the issues of running light weight internals and flywheel.

here's the link to discussed thread...
ATI Super Dampener