Can someone help explain the mechanics of proper flywheel weight (Page 1 of 2)

1. How do we mathematically figure out the perfect flywheel weight?
     I haven't done the math but the flywheels are part  of the rotating mass of the engine and more weight = bad. Rotating mass is much more detrimental to HP.

2. What is the advantage of having a light or heavy flywheel?

Flywheel weight robs horsepower from the engine. The heavier the flywheel the more HP lost.  In general, drag racers prefer a heavier flywheel because it helps maintain the momentum of the engine between shifts even though it takes a more HP to spin it. Road racing favors lighter flywheel because the engine gets into the power band faster, more HP and easier to decelerate the engine for cornering.

3.  What happens when you go too light or too heavy?
   Too heavy and you rob lots of HP from the engine, too light and it is hard to modulate speed at lower RPM.

I’m pretty dumb (my wife says I’m ugly dumb), but it would surprise me if a light fly wheel was helpful for what we do. Here’s my poorly thought out ramblings with no tangible evidence or math to support. Consider this opinion, not fact:

It takes a bunch of power to accelerate a FW from 600-6000 RPM in say 2 seconds, like when gunning the throttle in first gear. The acceleration of the FW would require a big power investment relative to total output. Spinning up same FW from 3500-5500 in third gear over say 10 seconds represents a much less significant investment considering the rate of acceleration and total output.  Flywheel weight would be a big deal if we raced around a tight track from hairpin to hairpin with speeds ranging from 0-20mph, but reality is we are on tracks with speeds ranging from 25-90, usually in 3rd and 4th gears where RPM increase is slow because of drag and increasing ratios as we shift through gears.

In summary: flywheel weight matters if you accelerate from 0-10mph, doesn’t really matter when you are accelerating from 50-60, and even less so from 90-100.

I’m happy to be schooled.

Can't modified flywheels explode sending high speed shrapnel through firewalls, floorboards, etc. right about where your feet or legs are? 

Trick question, yes they can.

I’ve experienced really light flywheels as being about impossible to drive slowly (endemic stalling trying to drive onto an alignment rack) and I suspect that zakP is right; there’s not a lot of difference to be felt when you’re winding 60 mph in 3rd. Probably there are better places to spend money on your Lemons car.

I didn't mean to imply that all lightweight flywheels will explode, but I have seen evidence of stock flywheels that were lightened come apart, especially with an increase in power and RPM in Volvos.

Only personal anecdote I can offer is I was using an aftermarket race only "ultralight" flywheel in a 4 cyl inline engine and the design was rumored to be too light and gave off weird harmonics as it no longer dampened crank pulsing. Supposedly the crank would whip against itself. Granted, this is on only a 3-bearing main setup. And yes, I used 2 different crank dampers with that flywheel.

It wasn't until after I broke my 3rd crank in the same block (same rods same pistons, too, with 1 re-ring) that I finally swapped out the flywheel.  That engine lasted 3 more years before the piston ring lands got too worn out.

Buy from a reputable Manufacturer who won't build an engine killing flywheel just to have the lowest number.

By the time you have a flexplate with a ring gear on it to start the thing, the splined hub to match up to your DCT, and whatever parts you use to connect those it will probably be heavy enough.

https://supermiata.com/miata-twin-disc-clutch.aspx

That's 15 pounds total. Probably about as light as you can get with a conventional setup. The designer says the following comparing the light one to the heavy one:

"Simple choice between the two. The Street flywheel is still fast revving, holds torque, is quieter than the Race and very driveable on the street. The Race flywheel sucks everywhere, makes your drivetrain sound broken except exiting pit lane, revs like a superbike and is godlike on track."

Shoot for 15 pounds and let us know how it goes

NOT true that a flywheel doesn't dampen vibrations or provide some sort of external balance. some flywheels are dual-mass stock (Audi UrS4) specifically to address NVH issues.

that being said, switching to a single mass flywheel in most of these applications seems to not have any adverse affect (other than increased NVH).

in my experience with lightweight flywheels, they allow the engine to spin up slightly quicker (and down slightly quicker). in a road racing application, I think this will be most noticed in rev drop or rev hang between shifts (and have negligible affect on overall acceleration).

All this is way beyond my expert-ease....
But a friend helped me rebuild my windowed engine on 1987 Honda Prelude a few years back
I was looking for an aftermarket lightweight flywheel, just cuz, well, seemed like thats what all the fast cars, big boys use....
But none were available....
So someone suggested just cutting the flywheel down...
I got a shop to take off , maybe 25%....I don't remember exactly....
But I love the feel of the clutch right now...
Not too light, not too heavy.
Seems to me there is some benefit of investing some energy into spinning the flywheel when driving/racing and shifting
Since some of that energy would then be used in pushing the car forward once the clutch is engaged...
But I was never really good at physics....
Anyways, I just bought a 2002 toy Miata (new engine, lowered, coil overs, etc.), and it has a light weight flywheel...for my wife, of course...
And I don't really like the flywheel/clutch feel...I feel like I'm just learning to drive a manual transmission
Embarrassing trying to start going uphill on slick,wet, surface from a red light...driving around town...