Jeremy Clarkson

I have always wondered what the cons to lightening a flywheel are. In my head I think that lightening the flywheel will lower the torque and the potential energy which is carried by the flywheel. I think a of it like one of the carnival games (High Striker) where you get a hammer and have to hit a weight that is shot up vertically in a column and hits a bell at the top…then you win a big teddy bear. Now assuming that the handle lengths of the hammer were the same if you were to hit the weight with a 5lb hammer you would move the weight less than if you were to hit it with the same force and a 10lb hammer. Now if I am revving the engine and I drop the clutch wouldn't I accelerate slower with a lighter flywheel than I would with a heavier one which holds more potential energy and therefore transfer more energy to the wheels?

ack5252

Lower down in the RPM band you'll have less torque. In your given scenario however, you will be swinging the 5 lbs. hammer faster than the 10 lbs. one don't forget, and the same applies to the flywheel. At high RPM, its easier for the engine to spin faster because of the reduced weight. I think the biggest noticeable difference is in part-throttle to full throttle transition. If you're at part throttle and slam it down, the increased weight of the heavier flywheel will help with more immediate acceleration that tapers off, where the lighter flywheel offers the opposite. But also, the decreased weight will make it easier for the engine to rev down faster offering quicker shifts.

zeroptzero

I went from a stock ap1 fly to the ultralite ACT fly (approx. 8.6 lbs). Contrary to popular belief it is very easy to launch, it only took me one launch to get used to it. I have a stock p.p, and clutch disc, which helps.

It revs up faster when accelerating, and the revs drop down quicker with clutch disengaged (between shifts). Response during acceleration from a cruise situation seems a bit slower for a slight moment with less momentum to push you forward. I like the upgrade, not sure what it translates to in terms of overall performance, but it feels good IMO.

Jeremy Clarkson

This is something that always confuses me, people say "it revs faster" but wouldn't that mean you accelerating faster? Or does it just "feel" different?

zeroptzero

yes accelerating from a dig to say 100 mph should result in a faster time , hopefully the difference is measurable, but it could be small. But accelerating from a cruise condition could be slightly slower for initial throttle response compared to a heavier fly setup, but after that it should accelerate quicker if you stay in the throttle long enough.

baZurk

I lightweight flywheel only helps with transient engine response when the clutch pedal is engaged or the engine is in neutral. The only increase in acceleration would be due to decreased rotational mass in the drivetrain or decreased times in between shifting. It is unlikely that it is measurable on its own but it does help a skilled driver modulate engine revolutions better than he would have with a heavier flywheel.

spdracerut

Read this:

http://www.motoiq.com/MagazineArticl...rivetrain.aspx

s2kreeper

When people say "it revs faster" they're talking about with the car in neutral. Less mass means the engine has less work to spin up (and down). The weight of the flywheel with the car fully in gear and the clutch engaged makes the difference in weight negligible (think about how many lbs you're spinning at that point of rotating mass and proportionately how much 10-15 lbs is of that, then add in statics of the car's full weight and you get close to nothing in performance gains). What you do gain is feeling and response for shifting up and down faster via better immediate response when trying to rev match.

It's touchy to launch for some people because they don't normally control the clutch as carefully as they should and they have to adjust to it, which is fine, and this is due to the rotating mass and therefor inertia of the engine being less, which makes it easier to stop. You compensate the same way you do on hill starts. 1) blend smoother, 2) add more power, or 3) get hopping in the drivetrain when you let out too fast

grubinski

In low gears (1st and 2nd) a light flywheel makes a *noticeable* difference in acceleration. This is because the flywheel is spinning at engine RPM, so is a huge contributor to rotational inertia. You're not only trying to accelerate your car, you're spinning up everything that's rotating (wheels, axles, driveshaft, transmission), and things that are rotating really, really fast, and have a large radius (hint: flywheel on high RPM engine) have a much larger effect.

When I put a light flywheel in my Supercharged MR2, I did some calculations based on the weight of the flywheel and its geometry. Going from a ~22 lb stock flywheel to an ~11 lb Fidanza flywheel penciled out to be worth 6 hp in second gear and a bit more in first gear. Since I autocrossed the car quite a bit, it was worth it to improve low gear performance.

My AP2 is eventually going to get an AP1 flywheel for this reason.