Performance increase from a lighter flywheel?

[Mad Mike]

I will have my engine apart soon enough. Replacing a piston & cylinder on my Euro 1983 SC. Engine has 3.2 JE pistons, 21/20 (20/21) cam, stock CIS injection, headers, dansk sport muffler, etc. Still in the process of diagnosing the problem, but that will all get worked out as well.

I have thought about adding carbs and changing the cam again; however, will not be able to pass emissions in Arizona. So I am going to keep it the same as it is right now...Unless it is going to need a full rebuild. If that is the case, I will get a 3.6 and go from there.

I was wondering if there are recommendations for a light weight flywheel, and what performance gains one should expect? Or is it even worth the investment?

Thanks,

[randywebb]

They are fun. There is no performance gain except that you have lost some wt. from the car and can more easily rev the motor.

[lateapex911]

Well, there are no measurable gains on an engine dyno, but the lowered rotating mass does help the engine spin up faster, and that can equate to faster shifts, better downshifts, and better acceleration. It is slight though, and the better setups, like WEVOs', aren't cheap, so the gains aren't good "value".

[Mad Mike]

I would think the revs would be easier both up and down. Besides the money, are there any downsides to going with a lighter flywheel?

[MotoSook]

You're best off just getting an aluminum pressure plate (versus the cast iron PP) if you don't already have it.

[2.7RACER]

There is a very good article regarding light weight flywheels at
UUC Motorwerks.com. Look into the flywheels.
Bottom line, there is a reason that every performance oriented motor sport routinely uses light flywheels.
It is part of reducing weight of the entire rotating mass in the powerplant.
These folks pay big money for these upgrades.
Now is it something you want for a typical street engine?
Maybe, maybe not.
Read the article, "How light weight flywheels work"

[masraum]

I'd be surprised if a slight difference didn't show on a dyno. Some of the engine power must be used to spin the reciprocating mass, if the reciprocating mass is reduced then that power is free to accelerate the vehicle.

[Jeff]

"I will get a 3.6 and go from there."


Great, just when I had talked myself out of throwing money at suspension upgrades. I'm going out to the garage right now to cut my shock towers, DA$NIT.

.3 seconds.

[jluetjen]

Yes. But...

A flywheel is not an energy consumer -- it is an energy storage device. Car manufacturers (including Porsche) actually do apply some thought to the flywheel weight, and once again it's not as simple as "lighter is better".

True:
- A lighter flywheel will allow an engine to spin up faster when disengaged from the transaxle. So sitting in your driveway and blipping the throttle, your engine will spin up faster.
- A lighter flywheel will allow an engine to spin down faster, so when you shift (up or down) the engine will adjust to the transmission speed faster.
- Under straight acceleration, a car with a lighter flywheel will accelerate faster since less energy is required to accelerate the flywheel. If this is improvement is even measureable is debateable.

But...
- A car with a heavier flywheel will be easier to start from idle. Why? Because the flywheel will have developed inertia while spinning that will help to keep the car from stalling when you release the clutch at low revs. In the absence of an adequate flywheel, you'll need to rev the motor more and slip the clutch more which will shorten clutch life.
- A heavier flywheel will tend to absorb driveline vibrations from the engine before they get to the transaxle. So cars with lightweight flywheels will tend to have noisier gearboxes, and potentially a shorter gearbox life.

So...
While race cars (often light-weight cars, or at least lightened compared to street cars) often use lightweight flywheels, they also tend to have fewer starts from idle, and more frequent rebuilds. These factors will skew the compromise towards using lightweight flywheels. Street cars on the other often start from idle at slow speeds, and are rarely rebuilt -- which swings the compromise back to using the stock flywheel.

[jluetjen]

Originally posted by masraum
I'd be surprised if a slight difference didn't show on a dyno. Some of the engine power must be used to spin the reciprocating mass, if the reciprocating mass is reduced then that power is free to accelerate the vehicle.

Uh... Masraum,
You might want to re-read this.

be used to spin the reciprocating mass

Don't reciprocating masses .. reciprocate, as in move back and forth while rotating masses spin? I think that a flywheel is a rotating mass, while pistons and rods (at least the small ends) reciprocate.

[Rick Lee]

I know a LWF is one of the best bangs for the buck you can do on a 993. But that's also because the stock dual-mass flywheel is crazy heavily, problem-prone and expensive to replace.

[Mad Mike]

The car is used on the street only to drive to the track and the occasional blast to the grocery store. Not used regularly as a daily driver.

Living in Arizona, with the AC removed, no sunroof and only windows to roll down, makes for an uncomfortably warm ride. Yesterday was around 112.

Plus with the 9.3:1 compression pistons, when the ambient temperature get above 85 or so, I only use race gas. Too expensive to use on the street on a regular basis.

What I am trying to say is that the car is pretty much a dedicated track car. I understand that the intention is not to tear it down every weekend (race team) and keep the maintenance to a proactive but not overwhelming level.

Are there any testimonials about SC's or Carrera with a lighten flywheel. I appreciate Rick Lee's 993 comments, but that might be apples and oranges.

Thanks for all the replies.

[oliver88]

I had a Patrick Motorsports LWF on my Targa and it was a great upgrade. The engine will response will definitely improve. i would do it.

[Superman]

With all due respect to John's comments, I think Mike should continue to discuss and consider using a LWF. Sure, it stores less interia and therefore is more challenging to drive from a full stop, but there are substantial benefits.

On the track, Mike will need to spool his engine up and down many times. If this engine spools more quickly both up and down, shifts are quicker. This saves time and keeps power on the road.

Also, the energy savings, even when the car is moving and in gear and the clutch pedal is not depressed, is substantial. Perhaps not hugely different from the stock setup, but substantial nonetheless. At 6000 rpm, the engine is turning 100 times per second. If the flywheel's circumference is two feet, then its outer edge travels at 200 fps. A gram turning in a two foot diameter circle at 200 fps is effectively "heavy."

[911pcars]

In the future, it would help provide more succinct answers to understand the intended purpose of the modification. Track use can tolerate more compromises than a street-based car.

For track use, yes, install a light flywheel. No stop lights (usually) on the track.

Sherwood

[wholberg]

Assuming that you reduce your flywheel weight 10%, if the engine torque remains the same, the angular acceleration of your flywheel will increase about 11%. The real number will be less than this due to internal engine friction changing at higher acceleration rates, and by the time this increase in flywheel acceleration filters to the ground through the transaxle, wheel bearings, etc, I think your overall gain will be pretty small.

My experience with street cars with light flywheels was that they were ridiculously easy to stall when starting out. And what really concerns me is that the 11% increase in angular acceleration mentioned above will be most apparent when you miss a shift and the engine exceeds safe RPM's 10% quicker.

[David]

I've got a light flywheel and PP in my 930, but since I haven't driven with them I can't comment.

I replaced the 27lb stock flywheel on my 78 280Z with an 11lb HKS flywheel and I think it was the best upgrade I ever did. And I upgraded everything on the car.

You may stall leaving a light once or twice at first and maybe later if you're not thinking about it, but for the other 99% of the time I think you'll enjoy it.

It'll make the biggest difference in 1st and 2nd gear, after that it feels about the same.

[gerard vaglio]

I've got one on my street car.

Yes the engine spins up faster but it also spins down faster when changing gears which, once you get use to it, is not a problem.

No troubles with stalling after the innitial akward stage, I'd say a day or so.

Yes, it transmits more vibration which I suspect is one of the reasons Porsche didn't include them in their street cars, feels less refined.

Overall, it makes the car a little more entertaining to drive. Certainly more fun but not a night and day difference in acceleration.

I enjoy it but try to drive a car with one if you can. I don't think you'll disslike it I just don't know if it will be enough of a transformation given the money you'll spend. Obveously, the best time to do it is when the tranny is out of the car to minimise the cost.

[autobonrun]

I have a LWF in my 96 C4S and it does rev faster. After driving other 993's, I feel it is also quicker than the equivalent stock 993.

But my question is if acceleration is a function of automobile weight and torque. Would not moving up the torque curve quicker with the LWF allow the car to utilize a higher torque value faster, making the car quicker? Granted it's the exact same curve LWF or not, but how quick the engine can move up that curve should translate to faster acceleration. My buttmeter seems to think that's the case.

I've heard the argument about less inertia and that is true, but my thought is how quickly you can apply the torque would be the biggest benefit.

On the negative side, I do have some rattle at idle and the car does idle a bit lower. Some 993's have a problem with stalling with the LWF but I think that's primarily in the OBD 1 cars (1995).

[Mad Mike]

I still am undecided... Seems like a clear cut 50/50 on what I should do.

Patrick Motor sports is located in my city, perhaps I should inquire with them? Anyone use a Patrick Motor sports products?

- Track car 95% of the time
- Engine is apart for repairs
- Ez installation of the part

Still undecided.

If you were in the same situation, would you consider it or would you go ahead and make it happen?

Thanks for the replies.