Originally Posted by dprantl

When I swapped out the single-disc clutch in my '91 for a dual-disc one, I could tell the difference in the speed at which the RPM dropped in neutral. With the DD sometimes the engine would die when pressing the clutch in certain situations, whereas it did not do this with the SD clutch. Nothing I couldn't tune out with the ST2. The difference could easily be logged. I'm not sure I would be able to tell a difference by just changing out the flywheel for a lighter one...

Dan
'91 928GT S/C 475hp/460lb.ft

Originally Posted by Courtshark

I'd really like to see dyno results to resolve this. All of the math is hard to dispute because, well, it's math. But my experience in my Saab with the lightened flywheel tells me there's a power benefit to it; definitely noticed a seat-of-the-pants difference, plus my basically stock clutch didn't slip as much (IIRC, I changed out the pressure plate and disc for a Saab 9000 clutch kit; this was many moons ago). Maybe I was getting a lot more loss from clutch slippage than I realized? Sadly I didn't do a before and after dyno with the change (because I was on a shoestring budget during that project!).

Would it make sense that a clutch could better grip a lighter flywheel, and thus generate the sense of more power while not actually being anymore powerful?

Originally Posted by Carl Fausett

I think if you focus on a light flywheel as a power-adder, you will be disappointed. Saving 3 pounds on a 200 pound rotating assembly (crank, rods, pistons, harmonic balancer, flywheel, clutch, pressure plate, driveshaft, input shaft, primary gear shaft, a pair of half shafts, a pair of wheels and a pair of tires and a pair of rear brake rotors - all of which must be rotated before the tires can move) - it is an reduction of 1.5% of the entire rotating mass when the clutch is OUT. Measured at the tire (chassis dyno) the improvement will be there, but it is slight.

However, if you focus on the improvement in throttle response for shifting, the clutch is IN, and the rotating assembly is now just crank, rods, pistons, harmonic balancer, flywheel, pressure plate. 3 pounds out of maybe 50 pounds. Much better.

I would buy an aluminum flywheel because I want faster throttle response, especially useful if you are an old-school down-shiftter like me. True heel-and-toe stuff. I don't know that I'd buy one as a power-adder. Its there mathematically, but too slight to feel in the seat of your pants.

Originally Posted by Courtshark

If not thickness, how about the "freshness" of the surface? https://rennlist.com/forums/928-foru...ng-clutch.html

Sorry to be a noob. Thought I understood how clutches, flywheels, and pressure plates work, but it's certainly possible I'm missing something.

Originally Posted by Carl Fausett

Kibort,

I agree that link I posted makes poor use of the word "power" and thats the most confusing element always in these conversations.

"Power" can be used as a verb, a noun, and an adjective.

As an engineer, you know that Torque is not power. Torque is work. Power is work over TIME. (I know you know this, just talking to the larger audience.) Hence, torque has no timeline, while power (common HP) does have a time element.

In that link, the author is speaking to laymen, not engineers, and the word "power" and "powerful" are being used like laymen do - to describe both torque and HP. I am not a degreed engineer myself, and I still find it very difficult to write to a broad audience of both academics and lay persons and get it right.

If I write for a laymens understanding, the engineers will have a field day with it. If I write in engineer-speak, the laymen can't understand it and I have failed to communicate an idea.

Its hard to please both readers at the same time, and the topic of HP vs Torque is one of the most common examples I see.

Originally Posted by mark kibort

Carl, if ithe car is in gear, then yes its everthing you said, AND THE CAR ITSELF! you left that out its 3lbs out of 3000lbs!!! (only that the 3lbs is spinning) on the dyno, add the dyno drums, they are all connected and part of the equation. you are really missing a lot in your discriptions!
if you have it in neutral, its all you said, right up before the halfshafts.

with the clutch in, its the 60lb crank, rods , pistons, flywheel clutch sans the discs and the short shaft. more like 125lbs, for which the 3lbs is near 2%

Originally Posted by GregBBRD

I'm glad someone understands this!

Originally Posted by GregBBRD

The "freshness" of the surface fools most people.

When hot spots form in the flywheels, they are actually impossible to get rid of. That is why flyweels get "ground" instead of "turned" when they are reconditoned. "Grinding" flywheels to make them flat and true "smears" everything together, so it all looks the same, for a short period of time. But since those "hot spots" are still there, they reappear virtually as soon as the new clutch disc hits them.

The "replaceable" surface that people put into their aluminum flywheels is generally very handy, since a "new surface" can be simply installed, when the old surface gets "cooked". However, you have to be careful that the entire aluminum flywheel is not distorted from the heat.

Originally Posted by mark kibort

unless you are drag racing or living in san francisco, you should never have a hot spot.

Originally Posted by Lizard931

If they get heat cracks tell the owner to get an automatic.

If the have some bluing then simply reuse or replace. They may need a slight turning to true them but if they need much then they should just be replaced.

Originally Posted by Lizard931

Again Greg, tell the owner to sell the car and buy an auto.

Originally Posted by blown 87

Not every one is as perfect as you are Mark.