clutches and flywheels - Honda Prelude Forum

merlindavis · 08-28-2002, 04:12 AM

okay, sometime in the next few weeks, a JDM b20a is going into my car... i have everything lined up, except i have just one question:

do i need a new flywheel if i have a b20a5?

i have heard that the b20a5 flywheel will not bolt up with the b20a... but that a b21a1 flywheel would...

i really need to know this because it would help to not have this problem later on... if i need the new flywheel, i'll get one, otherwise, i'm all set...

Jump · 08-28-2002, 10:07 PM

If you have an automatic, then yes you will need to swap out your fly. Or, if you had a 2.1 to the 2.0 yoiu'd have to swap the fly. Otherwise you are in good shape.
I'd consider, if yoiur present fly looks to be in good shape, having it lightened my 2.5 to 3 pounds. This is not so much weight that it will affect your hill climbing performance, tec. but it will dramatically improve the bottom end take off speeds. Trust me, you'd be foolish not to take the time to lighten the fly now while you have the opportunity.

If you haven't already thought this out, replace the intake and exhaust manifolds from your old engine to the JDM. Either way, change out the distributor, timing belt, plugs, wires, etc on the new engine. everything will interchange. Also, for the small price and comfort, put in a new water pump. This is also a good time to put in the new street cam gears, replacing the stock ones.

Hope this wasn't too confusing.

91prelude2se · 08-28-2002, 10:34 PM

i well have to say i lighten my fly wheel, ACT clutch, and b20a all the same time.... and the revs are much better... had the flywheel shaved and rebalanced to about 14lbs(where stock is about 24lbs) and the needle jumps up the revs. but i only noticed the diffrence after 3000 rpms and highway speeds...

Jump · 08-28-2002, 11:58 PM

That's curious. No wait. Really? maybe it's a bit more pronounced on the B21A1 motor - 'cause I totally noticed better take offs - but hell - 3000 RPMs comes on pretty fast anyway.

Here's a bit of discussion you might find of interest:

So how does a lighten flywheel work? Amongst the majority, there are two schools of thought concerning light flywheels. The first is that they do not contribute to power output. The second is that they do. Which thought is correct? In fact both, in a way, are correct.

If we measured the power output of an engine first with light flywheel and then again with the standard part on an engine dyno, no change in power will be seen to occur. At first it appears that the light flywheel has done nothing and was a total waste of cash. This is not the case. A dyno that shows max power at constant revs does not demonstrate what occurs to an engine's power output in real life situations - like acceleration. If an engine is accelerated on a dyno ( we are talking about a rate of around 2000rpm a second ) it would show a power output of around 20%-25% less than at the constant rev state.

The reason for this is that when accelerating a vehicle the engine not only has to push the total mass of the car but the internal components of the engine need to be accelerated also. This tends to absorb more power as the extra power is used accelerating the internal mass of the engine components and is why a motor accelerating on a dyno will produce less power than at constant revs. Also it must be remembered that the rate of acceleration on the engine internals is much greater that the rest of the car. This would then suggest that by lightening the flywheel, less power would be required to accelerate it and therefore more power would be available to push the car along.

Now, it may seen unbelievable that by removing a few kilos from the flywheel a noticeable difference to a 1000+ kg cars acceleration will be made. In fact the difference is quite noticeable and the secrete behind this is hidden within the gearbox. Everyone knows that cars accelerate at a greater rate in low gears, this is because a cars gear box basically a mechanical leaver and just like when using a leaver to lift a heavy object, the gearbox reduces the mass of the car that the engine sees. For example, in first gear an engine will see the cars mass as only around say 100kg but the engine internal mass would still remain around 20+kg .

It is now easier to see were the extra performance comes from when you lighten a flywheel. In some cases, and not necessarily extreme ones, you could effectively " lighten " a car by 20% in first gear just by removing mass from the flywheel. As the gear used increases this "lightening" effect is reduced. This is why cares acceleration reduces in higher gears, to almost no effect in top gear. Great for drags and tight race tracks but will not increase a cars top speed one bit.

Just one final point. If contemplating installing a lightened flywheel, it is much more important to remove mass which is close to the flywheels radius of gyration ( the flywheels rotating center on mass ) which is mostly found close to the flywheels outer edge rather than just all over. Due to this, it is possible to receive greater benefit from removing 1 kg from a flywheels outer edge than 4kg from the flywheels center. If in doubt, see an experts advice.

merlindavis · 08-29-2002, 02:40 AM

bah, i know all about lightening the flywheel and how it works and stuff, thanks for the responses on it though... my car is a manual, and like i said before, everything else is set to go... from the responses i got, it seems that the flywheel in there now will bolt right up... if i have the funds though, i will lighten the current one...

as for the perfomance mods going in... they will have to wait... i have barely scraped up enough money to get the engine purchased and put in, so everything else, like the cams, will have to wait... i don't have the time nor the energy right now to put the engine in myself, so the shop is doing it... once the engine is in, i'll be all set to do everything else on my own...