88 325ix suggestions

[berlow94]

I call BS. A lightweight flywheel really only affects gears 1 and 2 in acceleration. As the multiplier of the gears go down, so does the effect of the flywheel.

I want to see some hard data here. I'm willing to be wrong, but I want to see the data why I'm wrong because I don't believe it.

I think you might be forgetting that the flywheel spins on the engine side of the clutch. Not transmission side.
The effects of a lightweight flywheel in 1st gear at 4k rpm are no different than in 4th gear at 4k rpm. In both scenarios, the flywheel is spinning at 4k rpm. You might get more audible "rattle" in 1st and 2nd gear but that is a harmonics thing, not a flywheel weight thing. A 7lb aluminum flywheel will make very different noises than a 7lb steel flywheel.
Think about it, why would race cars be built with lightweight flywheel and clutch setups? They spend maybe 30 second total in 1st and 2nd gear all day!

And by harmonics, im not talking about audible "rattle" or how quickly the engine revs or sounds.
The factory rotating assembly is balanced to a certain spec. (Pistons, rods, crank, flywheel & clutch, harmonic damper) If you drastically change the weight of one of those components then things are no longer in proper balance. Any harmonic issues that these motors would have from the factory (cyl. #5 in most L6's) will be magnified. Again, why most properly built S54 race cars use a un-modified factory flywheel. The harmonics are crazy on that thing with its long rods and stroke. The only way to put a lightweight flywheel on an S54 is to blueprint and balance the motor with the lighter flywheel from the start. Otherwise you literally need to run cooling ducts to your harmonic damper because it's working so hard to keep the motor in balance.

Here is some hard evidence about crank harmonics from the leader in aftermarket harmonic dampers for BMW engines: http://www.atiracing.com/products/da...mper_dinan.htm

[Kershaw]

That's my point. Race cars are largely unaffected because the only difference they make is in 1st or 2nd gear, from a performance standpoint.

You said it negatively affects the engines harmonics. I disagree. It might negatively affect the harmonics entering your ear (the chatter) but to actually affect the engine negatively... I don't see any proof of that statement.

[berlow94]

That's my point. Race cars are largely unaffected because the only difference they make is in 1st or 2nd gear, from a performance standpoint.

You said it negatively affects the engines harmonics. I disagree. It might negatively affect the harmonics entering your ear (the chatter) but to actually affect the engine negatively... I don't see any proof of that statement.

I just edited my post. Read the link. It provides the hard evidence you asked for.

If race cars were unaffected by lightweight flywheels, then trust me they wouldn't use them. Have you ever seen a factory race car without a lightweight aluminum flywheel and some sort of solid twin-disc clutch? In my car, my lightweight flywheel assists in quicker heel-toe downshifts. Other than that i have no reason to use one. My motor was also blueprinted and balanced from the ground up with my specific flywheel and clutch.
My blueprinted S52 provides less vibration at idle with aluminum motor mounts than my last stock M50 did with E28 M5 mounts. Harmonic resonant motion baby!

[Kershaw]

That link doesn't say anything detrimental about lightweight flywheels. Only that flywheels and vibration dampers experience different load at different RPMs.

Also, you claim they are bad but use one yourself.

[berlow94]

That link doesn't say anything detrimental about lightweight flywheels. Only that flywheels and vibration dampers experience different load at different RPMs.



Also, you claim they are bad but use one yourself.

My motor's entire rotating assembly was balanced with the lightweight flywheel. The rods, crank counterweights and damper were modified to be balanced with the lighter weight flywheel.
I'm stating that simply bolting on a lighter weight flywheel to a stock motor will upset harmonics.

Here is more harmonics info from the machine shop that built my motor: http://www.rlengines.com/Web_Pages/C...Balancing.html

These engines are externally balanced. The flywheel and damper work together to keep the motor balanced. Changing one without the other WILL upset things.
If you must put a lightweight flywheel onto a stock engine without balancing the rotating assembly, then atleast upgrade to a lighter damper at the same time.


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[Kershaw]

If you replaced your stock flywheel with one that your buddy "lightened" with an angle grinder, I think you have a case.

But to replace one rotationally balanced item for another, no I still don't agree that it is detrimental. But hey, we all have different ideas on how things should be done. So I'm glad we both can discuss them.

[Nisse Järnet]

Lighter flywheel, less mass that can "absorb" vibration?

[berlow94]

Lighter flywheel, less mass that can "absorb" vibration?

Something like this, yes.
Both the damper and flywheel work together to absorb the harmonics of the engine. You lighten the flywheel and now the damper has to take up the slack.


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[jaime88325ix]

this is good stuff! but man you all are making this hard.. hahah

Thanks for yalls opinions

[The Dark Side of Will]

I think you might be forgetting that the flywheel spins on the engine side of the clutch. Not transmission side.
The effects of a lightweight flywheel in 1st gear at 4k rpm are no different than in 4th gear at 4k rpm. In both scenarios, the flywheel is spinning at 4k rpm.

The gain in first gear acceleration from a lightweight flywheel is far greater than the gain in 4th gear acceleration. The acceleration gain a flywheel frees up is related to the square of the overall gear ratio.

And by harmonics, im not talking about audible "rattle" or how quickly the engine revs or sounds.
The factory rotating assembly is balanced to a certain spec. (Pistons, rods, crank, flywheel & clutch, harmonic damper) If you drastically change the weight of one of those components then things are no longer in proper balance. Any harmonic issues that these motors would have from the factory (cyl. #5 in most L6's) will be magnified. Again, why most properly built S54 race cars use a un-modified factory flywheel. The harmonics are crazy on that thing with its long rods and stroke. The only way to put a lightweight flywheel on an S54 is to blueprint and balance the motor with the lighter flywheel from the start. Otherwise you literally need to run cooling ducts to your harmonic damper because it's working so hard to keep the motor in balance.

Here is some hard evidence about crank harmonics from the leader in aftermarket harmonic dampers for BMW engines: http://www.atiracing.com/products/da...mper_dinan.htm

Harmonics and balance are two different things.

Did you ever stop to think that the factory S54 flywheel is DUAL MASS? The flywheel is a second harmonic damper on those engines.
The M20 doesn't need that, as evidenced by the fact that BMW built them with single mass flywheels for years.

If BMW didn't put a balance weight on the flywheel or flexplate, then the engine is internally balanced. This means you can use any neutrally balanced flywheel without hurting anything.
Balancing the crank and flywheel together is just an excuse a lazy shop makes because they don't want to balance those components separately. Since the tolerance of the balancing equipment is what it is, balancing them together doesn't give a better result... it's just less work for the shop. It also means that you may experience increased vibration if you replace the flywheel at some point.

[The Dark Side of Will]

Lighter flywheel, less mass that can "absorb" vibration?

Vibration from power pulses and crank harmonics are also two different things.
The weight of a heavy flywheel reduces the variation in torque between a cylinder firing and the lull in torque delivery between cylinders firing. This makes the engine "feel" smoother to the driver.

This is completely different from how well the engine and balancer combo deals with crankshaft torsional harmonics.

[The Dark Side of Will]

These engines are externally balanced.

Source for this info?
This is the first I've ever heard anyone say that about an M20.

[jaime88325ix]

The gain in first gear acceleration from a lightweight flywheel is far greater than the gain in 4th gear acceleration. The acceleration gain a flywheel frees up is related to the square of the overall gear ratio.



Harmonics and balance are two different things.

Did you ever stop to think that the factory S54 flywheel is DUAL MASS? The flywheel is a second harmonic damper on those engines.
The M20 doesn't need that, as evidenced by the fact that BMW built them with single mass flywheels for years.

If BMW didn't put a balance weight on the flywheel or flexplate, then the engine is internally balanced. This means you can use any neutrally balanced flywheel without hurting anything.
Balancing the crank and flywheel together is just an excuse a lazy shop makes because they don't want to balance those components separately. Since the tolerance of the balancing equipment is what it is, balancing them together doesn't give a better result... it's just less work for the shop. It also means that you may experience increased vibration if you replace the flywheel at some point.

So you're saying its okay to slap a lightweight flywheel on to a stock 325ix?
Has no one ever done this..

[Kershaw]

It's totally fine dude.

[Chrisbike]

It's totally fine dude.

Cool; just bought one :)