Hi Guys,
This is a two part post, a specific product review of the new RHD LTW Flywheel
specifically for the BMW
M42/4 and M50/2 BMW engines (no more sourcing rare M20 starter motors) and a general explanation for
anyone who would want to run a LTW Flywheel in any car.
First off, I would like to disclose that I'm friends with the owner of RHD, I've worked closely with him bringing an ITB kit for the M42/4 engine to life ( more info here
http://www.m42club.com/forum/index.php?topic=18442.0). That said, I did pay for all the parts I have from RHD.
Should I get a LTW Flywheel?Yes.I recently needed a new clutch for my car, and when I looked at options I looked at a full DMF, a Valeo DMF replacement (a solid flywheel of equal mass) and a LTW option (M20 Flywheel and the UUC LTW option). I ended up getting a Full DMF as I saw no benefit getting a heavy solid flywheel, the difficulty in sourcing an M20 starter (at least in AU was too high to justify) and the UUC option was paired with a heavy duty clutch.
My previous experiences with LTW options were always terrible. People tend to pair a lighter flywheel with a heavier clutch. When you do this your clutch becomes a button. It is terrible, and this is what deterred me from the UUC option. Having the clutch instantly engage on a track may be fine, but if you intend to live with your car it is a
terrible idea. Over engineering your clutch is probably the biggest mistake anyone makes with setting up their car.
There is an persistent
urban myth that lightening your flywheel will make your car harder to drive and this is categorically false. A hard clutch is hard to drive, a lightened flywheel is easy provided the clutch matches the engines power output.
The M42/4 Drive-train consumes ~25% of the engine power (and this is similar with most modern cars) and lightening your flywheel will reduce this loss
while under acceleration. Basically the lower the gear you're in the more power will reach the wheels while accelerating. This is due to the simple law of Newtonian Physics
F=ma. The lower the flywheel mass, the higher the acceleration for the same force - your wheels spin up sooner - and I
guaranteeit will be enough of a change for you to feel it in most cars up through 3rd gear.
So this begs the question
Why was the flywheel so heavy to begin with and why a DMF from most manufacturers? The common statement is that it is due to noise - it isn't. There is extra noise associated with a solid flywheel (which I'll get to soon) but that isn't
why they're used in almost all modern cars. The other myth is that it is easier to take off - which it isn't (this has probably been facilitated by silly clutch choices).
- Air-conditioning
- Shift time
That is why they use heavy DMF options. Air-conditioning in modern cars tends to range from 1/4hp-1hp. At idle in traffic your car may be making <10bhp. The air-conditioning compressor engaging can be enough to stall the engine between its power demand and the amount of energy required to spin it from zero RPM to its operating range. Think of a heavy flywheel as a battery for inertia, when the air-con engages it needs to spin up, and this sudden power demand can be partially supplied by the energy in the flywheel. It smooths out the engine at idle, that is the main goal of the DMF. The bigger the engine the less this becomes an issue as well as there can be enough mass in most engines above 2.0L to negate any risk of cutting out.
The heavier mass takes longer to spin down, so it allows for 'lazy rev matching'. Basically you have time to find your gear and get it in place - and if you match the mass correctly with the shift length the average time taken to shift will match well with the next gear RPM. It has nothing to do with taking off - in fact it is usually easier to take off with a LTW Flywheel because the power is so much more directly available in first gear. The extra stored energy of the heavier flywheel isn't used here since you 'ride' the clutch to take off for a smooth engagement anyway - your gradually coupling the drive line which makes no use of the stored energy.
So what is this extra noise?A DMF is a tuned mass-damper system - like your suspension. They're typically matched to expected engine behaviour to dampen vibrations. They're considerably more effective than a sprung clutch.
BUT most modern cars with have enough under-body insulation to completely wipe out the noise (which
only happens when sitting in neutral at idle) so that it doesn't intrude the cabin. I can't hear it at all from inside my car. The noise is similar to a coffee grinder but a bit quieter, and I can only hear it from outside the car. A thicker transmission fluid will almost always reduce the noise here as well.
So to summarise:
- More power available during acceleration
- Easier to take off when paired with a sensible clutch
- You will need to rev match - but if you're driving manual you should be doing this anyway
- Yes it is louder but you won't hear it
The RHD FlywheelIt may be less of an issue overseas, but here the M20 starter motor is impossible to find so an M20 conversion is basically impossible so a native option is the way to go.
The flywheel only weighs 3.6kgs, with the total assembly being ~8kgs down from ~18kgs. A common remark about the LTW options is that going too light makes it harder to drive - and it really doesn't This flywheel is prefect for these cars, and I would recommend this over any standard clutch replacement for sure. Pairing this with an M20 clutch makes for a nice progressive transition.
Fitment was perfectly standard. The only issue encountered was with the flywheel bolts - with the flywheel being
so thin the standard DMF bolts are too long. The solution was to use the Auto flex-plate bolts which were a perfect fit. No other adjustments were required.
That is pretty much my review - it fits perfectly and behaves exactly as stated above about why you should get a LTW flywheel. The only reason you would stick with the OEM option is if you are trying to get the softest most comfortable car imaginable. To be honest though, any increase in harshness is imperceptible - I really can't think of a justifiable reason for BMW to have used such a massive DMF for these cars. I notice the RPM dip in traffic when the air cuts in, but that is it.
Don't get a DMF
I can't say this enough match your clutch to your engine power and the car will be perfect to drive!
