A lightened Flywheel is a very fun, interesting, yet risky item to play with. I bought a stock flywheel for $50 and took it to a Porsche Speed Shop here around the Dallas, TX area. The guy that serviced me was Wes. Wes ownes his own shop, and LOVES to lighten stock flywheels. If you do this, the flywheel SHOULD look brand new. No rust, stress cracks, imperfections, chips, scratches, etc.
Before you condone this unorthodox move of mine, let me say this. If you are unaware, ACT is a great company. Everyone in the performance world knows who they are, and for the most part have no problem with their products. However, just like plastic/glass/paper recyclers these days in the "environmentalistic" world, ACT does the same thing. By purchasing one of the ACT Fly's your in actuality purchasing a stock Fly that has been lightened by them. To help prove this point, call up ACT. They produce fly's for about every manual tranny car known to man. I had a friend that called them once, and they told him that it's not smart to lighten stock fly's. It's the buisness world, and you have to learn to see past a few things.
After sayin all that, it's importaint to also look at the amount of weight you'd like to drop. For instance, my stock fly for an 88 Prelude runs 21.5 lbs. I went ahead and droped the fly's weight to 16 lbs. Significant you ask? YES. That's one heck of a drop in weight on a fly. I believe that anything over 2 lbs. is noticable on a launch, or just in a plain "gun the gas" accell. By lightening the stock wheel however, you'll want to keep in mind that more than 6 lbs. is pretty much a "no no". Yes, you can lighten the fly, but it does have to retain some of it's strength.
Most flywheels are thick, with several raised sections on one side. The machienest will more than likely shave off most of those ridges. They are put there in the first place to give a normal drive feel to a clutch driven car/truck. By removing them, and taking off just a few more lbs., think of how much faster that wheel will turn. Does it produce more hp? Well, the answer is only on a dyno. In all actuality, by lightening the flywheel, your doing nothing more than freeing up lost power. In stead of that extra 10 hp going into pushing the fly, it's now turning your wheels. You didn't gain any more, instead you relocated it.
Relocating hp is the MAIN reason for performing this kind of a mod. Another might include someone telling a friend that they spent money on a lightened flywheel. Silly, but I know people that have done this. Don't go overboard with this mod. The more you lighten, the faster the revs (RPM'S) are going to take place. As you take off, you'll have to adjust by shifting a bit faster the more you lighten the fly. By doing too much, you might have to re-learn how to drive. Don't get in stop and go traffic either with too light a fly wheel. Mine is okay for me, but then again, I'll have a turbo with insane Power for a Prelude in a few months.
Here's some of the serious information to also concider. First of all, when replacing the Fly, use NEW BOLTS. There should be 8 of them(12 point), and this step is very importaint. Second of all, obtain the correct Torque #'s for the Fly bolts. Too loose, and the fly could come right though the floor board and you MIGHT lose a leg, foot, or whatever else is down there. Not to scare you, but I've SEEN a car on a 1/4 strip loose a fly, and have it get stuck about 3 inches from his foot on the gas pedal. That happened though b/c he used the previously used fly bolts, which as stated above, is a bad idea. The bolts will stretch or loose their strenght over time(ie. get weaker). The Flywheel is not an item to play around with when it comes to reusning bolts. Finally, don't bother looking into "counter balances". Some engines require them, but some are also Internally Ballanced. Our Preludes are internally ballanced, and we don't have to compensate for the loss of the weight from the fly.
I know that this is prety long, but I hope that your getting some use out of the info. From here, you could go into many things such as the clutch and the Pressure Plate. The Pressure plate is a good way to help add a bit of weight to the launch so that you don't kill yourself trying to get around. The clutch is actually in my opinion, not as importaint as the PP and the Fly. As long as a clutch has damper springs on it, you should be good. I'm running a 16 lb. fly, street clutch, and an ACT $300 Extreem Pressure Plate. Feel? Great! I don't have wheel hop, the Clutch doesn't Slip due to the extreem plate, and my throtle response is excelent. Eveything depends on what you want as the driver. No one can decide for you, and you are the one that has the final yea or nay on the whole matter.
Ps. My flywheel lightening job ran me $150, and was done in three days.