Lightened flywheel

[Bodhi]

Gang -

Are any of the coldside folks running a lightened flywheel? If so, I'm curious to hear about your experience. Specifically, are you able to get the car to idle properly? Thanks in advance...

[Tom @ Fast Forward]

I have the 9# Fidanza. No problems with idle. However, next time the tranny is out the stock flywheel is going back in. If you are building a track car, the light flywheel is great. If it is a daily driver and you drive in stop/go traffic, it can be a pain. Especially with a race clutch.


Gang -

Are any of the coldside folks running a lightened flywheel? If so, I'm curious to hear about your experience. Specifically, are you able to get the car to idle properly? Thanks in advance...

[Bodhi]

Thanks Tom. My car is a daily driver, so the stock flywheel stays.

[Tom @ Fast Forward]

I should almost make this thread a sticky. I think it is one of the most asked questions.

[chuckerants]

I should almost make this thread a sticky. I think it is one of the most asked questions.

No, I think the most asked question is, "do you think I should go UP a pulley size?" :D

[Tom @ Fast Forward]

That is an easy answer now as well. Install Timing Card and 130mm pulley unless it is your hotside. :)

[MiataMTF]

I have the 9# Fidanza. No problems with idle. However, next time the tranny is out the stock flywheel is going back in. If you are building a track car, the light flywheel is great. If it is a daily driver and you drive in stop/go traffic, it can be a pain. Especially with a race clutch.
I thought it might be informative to describe my experience. I recently resurrected my '99 (it's NA for now) but took the opportunity to replace the original clutch with a used '04 MSM clutch kit and a lightened OEM flywheel.

I don't know exactly how much was taken off the flywheel, the mechanic wasn't sure either, although I suspect it was at least three pounds. If the original weight is 18-lbs, then the lightened flywheel is between 13-to-15-lbs.

I believe that the MSM clutch is good for about 250 bHP (that was a number bandied about during MSM development) and the PP appears to be a Exedy Stage-2, but I could be mistaken.

Overall, the feel is "tighter" than my original clutch at 90K miles; this is welcome because I thought the OEM clutch was a little loose/weak. It's a little heavier than the original but hardly worth mentioning since I got used to it within the first few drives. The engagement is clean and quick but nowhere near as abrupt as the ACT clutches I've driven.

Lastly, I come from a VW which has gobs of torque and no trouble idling around at low RPMs…no gas, no bogging. My problem with a light flywheel was that it bogged and made it difficult and inconvenient to drive at part throttle/low speeds…it was too much work, meaning lots of clutching and declutching or riding/slipping the clutch to keep from stalling or over-revving. The modified OEM flywheel is much better in this regard. I still have to pay attention to avoid bogging in a parking lot but as long as the car is moving and is in first gear, it doesn't exhibit unwanted behaviour.

I plan to reinstall the modified JRSC this weekend which will raise the available torque across the operating range. I expect this setup to be perfect for a daily driver.

Mike.

[FormerDatsun510Man]

I thought I would add my 2 cents too :). I have an ACT HD coupled with a lightened steel flywheel (8lb). It works well and actually I like this clutch the best. However, like Tom I intend to put a stock flywheel back on when this tranny comes out. The torque of the Coldside makes a lightened flywheel sort of redundant. Additionally, the lightened flywheel makes it a bit more touchy off the line and creates a buzz in the transmission. Actually, it is the buzz that I find annoying. With the lightened flywheel I did notice my shifts, particularly downshifts, were a little bit quicker... but I don't think this benefit is strong enough to outweight the drawbacks. Either way it is pretty minor and I would recommend those considering it with their Coldside to save their money and just resurface the stock flywheel.

Bill

[tann3r]

I guess I'll comment too since I've gone to the lightness extreme. I'm running a 1.6 7# Fidanza along with 1.6 clutchnet disk and PP. It is 14# total and holds the torque just fine. It does shutter and chatter a little when cold, but a small change in driving style got rid of it.

Since mine is mostly a track rat, i dont see any drivability problem and really enjoy the responsiveness it creates. I have no idle problems even with the ac running.

the lighter the better =)

[lds2002black]

I don't remember off the top of my head the parts I used (could look them up if needed) but the combined weight of the "new" flywheel, clutch & pressure plate came out 3# less than the stock combination. Flywheel lighter but clutch heavier. Most of my street cars have run lightweight flywheels so I guess I am used to it and is not an issue for me.

[oldtimer02]

I have run light flywheels in cars both street, strip, & autocross in several different cars over the years, so it is natural to me.
As far as clutch set ups go the best I have found for both track & street is the Centerforce dual friction. It feels stock as far as petal pressure, but locks up like a bank vault! Try it you will be impressed!

[Toby Weir-Jones]

Resurrecting an older thread...

There seems to be some "conventional wisdom" that a light flywheel is Good For Turbos but Bad For Superchargers. But I haven't found any specific explanation of where that thinking originated.

Reading through this thread, I see lots of chatter about the drawbacks of a lightweight flywheel in general, but nothing about supercharger-specific issues beyond Bill's comment that the torque of the coldside renders it unnecessary.

So assuming one is OK with being a bit more ginger with the clutch pedal, why would the reduced rotational inertia and improved throttle response be a bad thing?

I always had light flywheels on my other cars and the clutch work didn't bother me, but since I'm now older, wiser, and lazier, I'm looking for a fresh perspective.

twj

[FormerDatsun510Man]

The benefits of a lightened flywheel are still there. I.e., I did notice slightly quicker upshifts and downshifts with the lightened flywheel. I suppose there may have been a slight acceleration improvement in the lower gears... however, I didn't notice this... probably because you have enough excess torque that traction is the limiting factor more than net power to the ground in 1st and sometimes 2nd gear, which is where a lightened flywheel will primarily make a noticeable acceleration improvement in a normally aspirated car. For a racecar, sure, a lightened flywheel would still be not only beneficial, but part of the package for maximum performance. However, on the street, the lightened flywheels disadvantages (transmission buzziness and annoyingly touchy off the line) outweighed its advantages, IMO. Of course you know that is just an opinion :biggrin:

I think that the lightened flywheel touchiness off the line is exaggerated by the supercharger. That is one situation where the immediate torque at all rpm of the supercharger could be seen as a disadvantage to some I suppose. A turbo probably doesn't make this quite as noticeable because they do not make the constant torque increase irregardless of load like a blower. They depend on exhaust gas inertia for their torque addition.

[Tom @ Fast Forward]

The Flywheel's job in life is to make it easy to get the car going from a dead stop. It helps the engine to keep rotating as the clutch is engaged and 2400 pounds of dead mass starts moving. The heavier thye flywheel the easier this is. As we start increasing torque, we tend to put stronger (read heavier pedal) and stronger clutches in. With a heavy clutch and a light pedal a majority of people will have problems getting away from a stop sign, burning/slipping the clutch to compensate.

Light flywheels are a boon to turbos as they need help spooling up from zero. Superchargers, especially the Intake manifold mounted Coldside, make torque so easy that the light flywheel is unnecessary. The heavier flywheel makes the Miata more streetable and easier to drive.

I guess the way it should have been stated is that with a Coldside, the light flywheel is unnecessary. When I touch/blip the throttle to down shift, the RPMs are there instantly. Evidently that isn't true with a turbo. Stock Miatas have so little low end torque and power in general that, when on a track, a light flywheel is also a help like the turbo.

I'm sure you will say that YOU have no problems with the light flywheel in traffic. Neither did I after I got used to it. However, People who drove my old car with the light flywheel stalled it repeatably when they first drove it. When I had the stock flywheel, nobody stalled it.

[Toby Weir-Jones]

When I touch/blip the throttle to down shift, the RPMs are there instantly. Evidently that isn't true with a turbo.

See this is the crux of it I think. Blipping the throttle, wither N/A, super'd, or turbo'd, should produce a no-load/high-vacuum type response. All else being equal, the rate of change of engine RPM should be the same regardless of mode of induction. Strictly speaking, the parasitic drag of the supercharger might negatively impact that rate.

On my turbo car that was certainly the case. It only made boost under load, same as the MP62. Unloaded, it behaved just like a stock N/A engine, so the light flywheel improved the visceral and mechanical response of accelerating the reciprocating assembly. That made the engine feel more responsive, and you also saw it with the rate at which revs dropped off during upshifts.

I absolutely agree that, once in gear, the immediacy with which the MP62 produces positive manifold pressure vastly outweighs any mechanical advantage from lower reciprocating mass. The % difference between the two is enormous, with the torque contribution easily winning. So I wouldn't expect to measure any statistically valid improvement in vehicle acceleration, for example, since that's a loaded condition.

I therefore put the flywheel in much the same bucket as the muffler. It's done because of subjective issues of feel, and personal preferences about how touchy the engine should be, rather than absolute performance improvements measured net of everything else.

But I also fear I'm missing a trick. FM specifically highlights the fact that they sell a less-light flywheel for supercharged cars, and a more-light version for turbos. I wonder whether the real issue is that the stalling risk is increased because of the lower flywheel mass plus the increased parasitic losses, thereby compounding the overall difficulty of getting the car moving from a standstill without slipping the clutch.

Somebody cleverer than me can probably calculate an equivalency ratio which states that the addition of the MP62, with pulley ratio of x:y, is comparable to changing the reciprocating mass of the crank/flywheel/clutch assembly by [z] pounds. That would be an interesting figure to know, because you could see whether the predicted values fall within the same range of the common aftermarket lightweight flywheels' mass reductions.

twj

[Tom @ Fast Forward]

I agree that a N/A Miata will not produce boost when the throttle is blipped.

I agree that a turbo Miata will not produce boost when the throttle is blipped.

Please do me a favor and start your engine, let it warm up, stare at the boost gauge and blip the throttle. I think you will see boost. The supercharger will not maintain boost without load BUT, there is load when you blip the throttle. Add a couple PSI to what you read on the gauge as the gauge is not that fast. That is the difference between the three systems.

Moreover, not only dos the turbo not make instant boost like that but it has to overcome the load of the backpressure created by the blower (ypically 1.5X boost or more) to create the inertia to get the turbo rotating to create the boost. A supercharged Miata is ready at any RPM to make boost. When you lift, the SC is tracking the engine. With a turbo, when you lift, exhaust basically stops and so does the turbo.

Let me know what you see on that gauge.

[Toby Weir-Jones]

Let me know what you see on that gauge.

Max of ~10psi momentarily.

Very interesting. I wonder what the rpm rate of change looks like compared to a n/a engine with stock and light flywheels?

[Tom @ Fast Forward]

Curious, do You have stock or light?

[Toby Weir-Jones]

Curious, do You have stock or light?

Stock. Stock clutch too. The car only had 14k miles when I bought it in June.

[FormerDatsun510Man]

The rate of change of rpm, or angular acceleration, is proportional to the torque for a given moment of inertia. For torque, you are seeing at no load in neutral the torque that the engine will make on a dyno with load for a supercharged or normally aspirated engine. As long as your foot is down :). Thus, with the FFS kit making roughly 50% more torque than stock, that translates into the rpm rise being about 1.5x as fast as stock unboosted. This is different than a turbo in that a turbo does need load in addition to time to make the extra torque.

The other side of the equation is the moment of inertia. The acceleration is inversely proportional to the moment of inertia of the entire rotating assembly. Entire rotating assembly is the key, not just the flywheel, but the entire system of engine rotating parts, supercharger, accessories, flywheel, pressure plate, clutch. So, going from a 16lb flywheel to an 8lb flywheel wouldn't double the observed rate of change in rpm. Since the pressure plate and clutch are still the same weight, that alone would probably make 1.5x as fast rpm change in neutral the maximum. Maybe more like 1.2-1.3x would be my guess. One thing the lowering of the moment of inertia has going for it is that it affects both rpm rise and rpm fall rates in neutral. This is where it helps the shift speed. The extra torque of a blown engine (or say a normally aspirated engine with higher output) only helps in the rpm rise rate... which would be great for rev matching when downshifting, but not for upshifting quickly.

[Tom @ Fast Forward]

Bill,

I thought we had to include the weight of the crank and pistons too?

[FormerDatsun510Man]

Yep, that is what I meant with "entire engine assembly". To be precise it would include everything that is moving or rotates that is connected in some way to the flywheel when the clutch is disengaged. This would also include not only the pistons and crank, but also the timing belt, cams, even the valves and valve springs, water pump, oil pump, etc. etc. And I didn't mean to insinuate it would be as simple as adding up the weights. One would have to figure the equivalent moment of inertia for rotating and linearly moving parts. Gets pretty involved in the engine internals...

[Toby Weir-Jones]

The extra torque of a blown engine (or say a normally aspirated engine with higher output) only helps in the rpm rise rate... which would be great for rev matching when downshifting, but not for upshifting quickly.

So the moral of the story seems to be:

If you tend to shift faster than the engine wants you to, and you're OK with the potential tradeoffs in getting rolling from a standstill, possible transmission noise, etc, then shedding flywheel weight is not a bad thing to do.

BUT there is no reason to believe that the 'revvyness' will improve when the FFS kit is also installed because the blower is increasing the rate of acceleration far more than the reduction of flywheel mass would do anyway.

Fair summary?

[Tom @ Fast Forward]

Yep. Turbos suck. Blowers rule. What else can I say.

PS, ECool doesn't work. You can't run more than 5 PSI without an intercooler.