Lightening the Flywheel....

[DDtB]

Pros?? Cons?? ... Discuss....

[Tom]

Pretty pointless I reckon. I'd have thought that whatever you gain somewhere you'll lose somewhere else (see 52mm throttle body). That and the fact that I reckon the K revs up quite nicely as it is. I'd be keen to hear what Robiniho reckons though.

HTH

[tut]

I put a lightened flywheel on when I had the K series engine, and I thought that it was one of the best upgrades that you could do.

Never found a drawback, and the engine spins up much quicker and feels so much better.

tut

[philthy]

not if you are gonna do it yourself

[BiggestNizzy]

If i'm right it would make it idle worse as you don't have the energy stored in it.

However it would allow the engine to rev up quicker for the same reason, don't DIY I work in a machine shop and wouldn't tackle it myself as if it goes bang it will come throughthe bulkhead into your passengers back.

[tut]

Agreed, buy a lightweight one if you are going that route Dave.

Mine did not affect idling at all, and that was with 190hp.

tut

[roadboy]

I've got a PTP one lying around that I could maybe be persuaded to part with.

Dan

[robin]

I don't believe it's a sensible thing unless you've got the gearbox and/or engine split for some other reason?

After that, it's personal choice. Clearly it does not make any difference to power output, however it does change the way that power is applied. The engine will be slightly more prone to stalling when pulling away and also when lifting off the throttle & dipping the clutch (like stopping at a junction). This is less of an issue on the elise as the flywheel is designed for a heavier car to start with; it could be an issue for you if you already have a slightly lumpy/erratic idle caused by running non-standard cams on the standard ECU, for example.

I could do the maths for you if you like ...

rotational kinetic energy is 1/2 * I * w^2 (where I is the moment of inertia - this is what you're altering by lightening the flywheel, and w is angular velocity - like rpm but in different units).

The moment of inertia for your solid flywheel is 0.5 * m * r^2 (where m is the mass in kg, r is the radius in m).

Rough guess, flywheel is 0.50m diameter, radius 0.25. PTP say their flywheel weighs in at 4.5kg - Eliseparts is 3.5kg, original is 7kg.

Let's assume that the mass is evenly distributed across the flywheel (of course it isn't, but I don't have any better data to work with).

I(oem) = 0.5 * m * 0.25^2 = 0.5 * 7 * 0.25^2 = 0.22 kg/m^2
I(ep) = 0.5 * m * 0.25^2 = 0.5 * 3.5 * 0.25^2 = 0.11 kg/m^2

OK, now let's see how much power is diverted to the flywheel during an acceleration from 3,500RPM to 7,000RPM.

We need to convert those into radians/second - 3,500 RPM is 3,500 * 2 * pi / 60 = 366 radians/second. 7,000RPM is then 733 radians/second. Remember rotational kinetic energy is 0.5 * I * w^2:

K(oem@3.5K) = 0.5 * 0.22 * 366^2 = 14,700J
K(oem@7.0K) = 0.5 * 0.22 * 733^2 = 59,100J
Total energy absorbed during acceleration = 44,400J.

K(ep@3.5K) = 0.5 * 0.11 * 366^2 = 7,400J
K(ep@7.0K) = 0.5 * 0.11 * 733^2 = 29,600J
Total energy absorbed during acceleration = 22,200J.

Not surprising it's half ... now, it's pretty tricky to apply this result directly to get a performance difference - all other things being equal, one will obviously get to the 7K mark quicker than the other.

Let's assume we're in second gear for a moment and that 3,500 is 30mph and 7,000 is 60mph and that your car and you weigh in at 850kg.

30mph is 13.3m/s, 60mph is 26.6m/s.

At 30mph your car has kinetic energy 0.5*m*v^2 = 0.5 * 850 * 13.3^2 = 75,200J.
At 60mph your car has kinetic energy 0.5*m*v^2 = 0.5 * 850 * 26.6^2 = 300,700J.

So your engine has to deliver 225,500J to the car mass in order to accelerate from 30 to 60; in the OEM case it must also deliver 44,400J to the flywheel; in the EP case 22,200J to the flywheel.

So the difference is 269,900 with OEM and 247,700 with the EP. A difference of 22,200J or 8%.

So a pretty respectable improvement in second gear - probably something like 0.1 to 0.2s off the 0-60 time.

Do the same sums in 3rd gear - call it 45 to 90 and the result is more like 4%, but then aerodynamics start to come into play and will further dampen the benefits.

There are also other effects which you may or may not like - the engine revs drop much quicker during downshift, heel and toe needs to be a bit more precise, there's less engine braking, etc.

Cheers,
Robin

[r10crw]

A big difference there then!
What about the VHPD, does anyone know the mass of the flywheel used ( Im sure its lighter but dont know by how much)?

[mwmackenzie]

Had mine lightened by Pollock Precision Engineering (Stu160 on here) in Livi when my clutch was getting replaced.. Not as light as a Lotus lightweight one but lost a couple of KG and revs and idles nicely!

[robin]

A big difference there then!
What about the VHPD, does anyone know the mass of the flywheel used ( Im sure its lighter but dont know by how much)?

Actually, I think the lightweight flywheel is only fitted to the 340R, not the Exige, 190, etc. No idea what the mass is though.

Cheers,
Robin

[mwmackenzie]

VHPD is 4.2 KG STD is over 7 mine is about 5

[Sanjøy]

Did Uldis get a lightened or balanced one ? Just searched his posts and must have been SE1 pics of it.

[shortie2]

on a road car you might have issues with driveability but for competition use its very important to reduce flywheel and engine mass as much as possible , in lower gears if you reduce flywheel mass by something like 3kgs thats the equivalent of removing 100 kgs of the weight of the car ,this obviously reduces as you go up the gears

[mwmackenzie]

on a road car you might have issues with driveability but for competition use its very important to reduce flywheel and engine mass as much as possible , in lower gears if you reduce flywheel mass by something like 3kgs thats the equivalent of removing 100 kgs of the weight of the car ,this obviously reduces as you go up the gears

That's why I did it.... 2+Kg off my flywheel allows me more KFC