What are the effects of a lightened flywheel

[pbs911]

I hear a lot about driveability problems with lightened flywheels on a street car. Has anyone noticed any differences with the l/f?

The only effect I can see is there is immediate power when depressing the throttle, probably from engine not having to load up with the flywheel mass.

[old_skul]

I have a friend with one.

Your assumption is correct. There is less energy required to accelerate the flywheel and therefore the drivetrain. Lessening mass anywhere in the drivetrain results in a gain in RWHP, even if incrementally. The real benefit is essentially in response time.

[Joe Bob]

Problems are....low idle problems....stalling with fuel injection chips not programmed to deal with the light load. Not neccesarily a good thing for a street car....

[Jim Richards]

Dr. Paul, I'm still kind'a new to working on my 993; however, I seem to remember reading on the Rennlist BBS that switching to the LWF caused idle /stalling problems. The solution that some folks found was to adjust the idle stabilizer valve. Check out the Rennlist 993 forum for more on this.

I'm hoping to replace my dual mass flywheel with a LWF later this year.

[widebody911]

You're also more prone to stalling the car when taking off from a dead stop, as you don't have the mass of the flywheel to help you out. This is a user education issue, and it's funny to watch your mechanic try to drive your car.

[speeder]

A couple of things; #1, it depends on how light you go, any reduction in rotating mass has the same general effect, but you can get away w/ a little bit of weight loss w/o affecting idle much. On my SC, and all cars '89 and older, the flywheel is pretty light to begin with. It is the P.P. that is heavy. I am switching to the Sachs aluminum P.P., apparently these are a good compromise on CIS cars, no idle problems. Not sure about motec, (3.2), cars. Maybe someone who has done this can chime in.

Also, it is not true that a lighter F.W. "results in a gain in RWHP". They have exactly zero effect on H.P., but they cause motor to spin up into peak torque zone more quickly, therefore possibly making acceleration better w/o actually changing output of motor. There are also disadvantages to lightened flywheels that have nothing to do w/ idle, but that is a whole 'nother technical discussion.

[JonT]

Paul--from my experience with the lightened flywheel on my 3.6 conversion, there is little noticeable difference in drivability compared to the original 3.0 and stock flywheel. It does rev up a little quicker and someone mentioned the SC flywheel is light to begin with which is true. It is slightly noticeable during shifts when the engine spools down a little quicker as well. I really have to shift quick to keep the engine revs up in the powerband (which is very wide with the 3.6). I also have an Autothority chip that has been programmed for the lighter flywheel and I have no problems with stalling. Good luck with your project!

[pbs911]

Thanks for the responses. Here is what I have found after driving my car for about 3 weeks with the 3.6 w/ l/f:
The engine will stall if a chip to compensate is not installed. I tried two chips one stalled, the Centex chip from Timmins "saves" the motor from stalling, but the rpms still dip a little when lifting off the accelerator with the clutch in.
Acceleration time is greatly reduced to the point that I can no longer coast as far as I did in first gear when lifting off the gas. I guess the motor spools down quicker with the l/w which results is less coasting time since the car is now resisting the compression of the motor. I think this is a bad thing for the motor so some education is required on the part of the driver not to lift as soon when entering driveways and the like.
Acceleration time is greatly reduced in stop and go traffic. Even slight acceleration may cause the car to jump b/c the l/f absorbes less energy from the motor and the car is ready to move quicker - the result is the car moves quicker almost to the point it will jump forward. Again not a good think for the trans, but can be learned by the driver. This is the most annoying thing as far as I can tell.
It is essential to heal to toe in down shifitng. The rpms drop so quickly.
I can tell that there is definatly a increase in time of acceleration throughout the gears. It is amazing.

[69 Soft Targa]

Originally posted by widebody911
You're also more prone to stalling the car when taking off from a dead stop, as you don't have the mass of the flywheel to help you out. This is a user education issue, and it's funny to watch your mechanic try to drive your car.

Avoiding that stalling from a dead stop results in the clutch wearing out sooner. The clutch gets a little more of a workout from a stop without the mass of the flywheel.

[Jack Olsen]

Some of the hair-trigger acceleration issues you're describing might be attributable to the whole 2300-pounds-3.6-liter-motor thing.

I have the LWF and -- horror of horrors -- air conditioning, and my idle is pretty good with the Cyntex chip from Timmins. If yours is dropping off too quickly, there are a few adjustments to correct it. Tyson did some things on mine about 6 months ago which made it better; I just can't remember what they were right now. More recently, he reconnected the throttle lever by the e-brake. It's both a poor man's cruise control, now, and it'll up the idle to a higher baseline if I ever get the hunting and stalling again.

Windward Performance makes a modified idle stabilization valve for the LWF, also. If it keeps bugging you, you might want to look into that.

[jrb964]

I just installed a LWF in my '80SC - stock engine - along with a Sach's Power Clutch with light PP. Accerlation is better as the mass of flywheel is reduced dramatically. (New Flywheel is 6lbs) Do notice that the engine winds down faster between shifts - so don't back off the accerlator as much. Stalling does not seem to be a factor. The engine will get to 5000 rpm in an instant - but the reverse is also true. It only been in about 6 weeks and so far it's excellent.

Still having shifting problems. Installed solid engine mounts, have the sport mounts for the tranny but have not installed yet. When the engine was out I check the linkage and it was in really good shape, may go back in and check it after driving with new LWF to check for wear.

My thoughts.

[KTL]

I'll second Denis' comment about the pressure plate. A lightweight pressure plate is going to cut more weight that a lightweight flywheel.

Anybody who's picked up a stock G50 pressure plate will know what i'm talking about. This hunk of metal could anchor a battleship if necessary!!!!

Yes, the motor revs more quickly because there is less rotating mass to move. The downside is that that reduction of rotating mass also reduces momentum stored in the crankshaft. The result is a motor that comes down in revs more quickly and needs more throttle input to get you going from a dead stop. Quick rev drop requires you to shift quicker (not always an option with a 915) or match revs with more throttle input during the shift (increased disc wear). Also, the quick rev drop causes the stalling everyone mentions because the motor returns to the idle circuit faster than the ECU is setup to manage this condition. Hence, the remapped chip.

[old_skul]

Uh, note that I said RWHP, not BHP. Rear Wheel horsepower is the net result of the engine horsepower at the flywheel minus drivetrain drag and inertia. Lightening the flywheel will slightly reduce the inertial drag of the 'train, and you wind up with more RWHP. Engine horsepower doesn't change.

And wouldn't "lightened flywheel" abbreviate to LFW, not LWF?

[Chuck Moreland]

Light Weight Flywheel

[rdane]

"I just installed a LWF in my SC along with a Sach's Power Clutch. Accerlation is much better. Do notice that the engine winds down faster between shifts."

Stalling is NOT a factor on my '79 with the same mods. I should have done these mods much earlier. In another thread most said LWF wasn't worth the effort or expense...I would disagree. I think it is one of the best things to do to an SC for seat of the pants improvements. YMMV

[NYSCAR]

Lightweight flywheels reduce the rotating mass of your drivetrain, thus reducing the inertia. I've read magazine articles that show before/after effects of lightweight flywheels on a dyno with all other parameters remaining the same. The effect is increased 'power' (perceived by the dynojet) in the lower gears and slightly decreased 'power' in the higher gears.

A) The increase in the lower gears is from having less rotational mass to accelerate. In lower gears, the flywheel is having to be accelerated over a range of RPM very quickly, and mass makes a big difference. In higher gears, say 5th gear, it takes much longer to accelerate over a certain RPM range, so relatively the flywheel mass makes very little difference.

B) Heavier flywheels can actually contribute to percieved 'power' in the higher rpm range where rpm changes are slower. In that case, the inertia of the spinning flywheel (the heavier, the greater the inertia) actually stores energy and helps the car accelerate on the upper end.

The difference in the top end with a heavy flywheel is not much vs the difference in the 1st couple of gears with a light flywheel (quite noticable). They're always great for high performance applications as long as you can deal with the noise and finicky-ness.

Brad

[pbs911]

Some of the hair-trigger acceleration issues you're describing might be attributable to the whole 2300-pounds-3.6-liter-motor thing.

Don't you hate when that happens.

[rick conrath]

The lightened flywheel is not necessarily a good thing for those of us running on boost. The reduction in rotating mass makes it difficult to keep the rpm's and boost up between shifts, especially in the 4 speed cars.

[zanick]

So, this is close. There have been a lot of good posts here, but here is the deal.

The effect of taking weight off the flywheel will not effect rear wheel hp, except in two circumstances.
1. quick reving the engine in idle to match gears, etc
2. in 1st and 2nd gear.

In 3rd gear and beyond, even a 10lbs on a 1 foot radius, would add non-measureable HP to the driven wheels, and of course none measureable on an engine dyno at any time. (engine dynos are usually brake dynos of some sort.

as was mentioned, one of the reasons you would want a heavy flywheel is that it can store energy before a clutch dump, and add energy on speed shifts in a drag racing situation.

One of the main reasons you want a lighter flywheel is to reduce the overall weight of the car, thus improving HP to weight ratios, and braking/handling performance.

In no way does the heavy flywheel help with HP at the high end of the RPM. (this is the only true incorrect statement below) The stored energy in the flywheel has to be put in, and after it is accelerated to a high rpm, the energy is there to be released as heat or hp(ie a clutch dump would release some of this energy at a drag start)

The reasons for this are simple. the HP needed to acclerate a rotating load is based on its form and rate of acceleration. the slower it accelerates the less HP it takes (or takes away ) to accelerate it. so, in real life situations, a high performance car, with 0-60 performance of sub 5 seconds, will take over 7 seconds in 3rd gear go from 60mph to 100mph. 7-10 seconds is usually the time it takes for a dyno run too, (3rd or more commonly 4th gear) so, over 7 seconds, 3000rpm to 6500rpm, 10lbs, 1 foot diameter, will be less than 1/2 of a hp. Now, blip the throttle in 1-2 seconds and you may find 10-20hp. In first gear , its not uncommon to see a 20 hp advantage for 10lbs off the flywheel, but for all the other reasons, it may not be the best thing to do.

Bottomline, an engine dyno and a chassis dyno will not detect the 1/2 hp savings of a 10lb lighter flywheel or for that matter 10lbs lighter rims and tires on the driven wheels.

Mk

Originally posted by NYSCAR
Lightweight flywheels reduce the rotating mass of your drivetrain, thus reducing the inertia. I've read magazine articles that show before/after effects of lightweight flywheels on a dyno with all other parameters remaining the same. The effect is increased 'power' (perceived by the dynojet) in the lower gears and slightly decreased 'power' in the higher gears.

A) The increase in the lower gears is from having less rotational mass to accelerate. In lower gears, the flywheel is having to be accelerated over a range of RPM very quickly, and mass makes a big difference. In higher gears, say 5th gear, it takes much longer to accelerate over a certain RPM range, so relatively the flywheel mass makes very little difference.

B) Heavier flywheels can actually contribute to percieved 'power' in the higher rpm range where rpm changes are slower. In that case, the inertia of the spinning flywheel (the heavier, the greater the inertia) actually stores energy and helps the car accelerate on the upper end.

The difference in the top end with a heavy flywheel is not much vs the difference in the 1st couple of gears with a light flywheel (quite noticable). They're always great for high performance applications as long as you can deal with the noise and finicky-ness.

Brad

[NYSCAR]

Ok Mark you're right, I was talking out my ass on that last bit -Sorry, I get carried away with bs sometimes ....but I have seen changes on a dyno from flywheel changes. You can go to europeancarweb.com and look at their project M3 and one section shows dyno before and after of a standard and lightened flywheel. You're not changing the HP, but you are changing the inertial 'cost' of your drivetrain. Incidentally, they show that the lightweight flywheel 'reduces' effective power on the dyno at lower rpms, and 'adds' at the upper rpm range - strange. Here's a link to help ya find it:

http://www.europeancarweb.com/projectcars/0109ec_projbmw/

go down the right side to find the dyno charts.

Brad