E30 Zone

How much torque can a standard M20 clutch take before an upgrade should be considered?

May I suggest that it depends how much traction you have at your wheels Rich. I learned this the hard way when I first fitted semi-slicks to my track car. At such point as the tyres no longer wanted to let go I found out that there was indeed a limit to how much torque my clutch could handle. Weakest link in the chain and all that. I wouldn't think that a stock M20 clutch would be good for holding on much beyond 450 to 500 Nm. Given that the friction coefficient is temperature dependant this will get worse as your clutch takes abuse around the track.

If you're not keen to go the route of a proper race clutch (I assume this is for one of your track cars) then step #1 would be to at least have the fingers stiffened/doubled up so as to clamp/bite the clutch-plate harder between the flywheel and pressure plate. I assume you have calf muscles You can also have a stock plate relined with a more aggresive friction material (copper-ceramic type) if you wanted.

Cheers Geoff

Reason I ask is I'm looking at the 'stages' of clutches. The higher the stage (1,2,3) the higher the %-age over OEM they're supposed to be able to handle. So I am after a rough starting point so I've got some idea

M20 front crankbolt is torqued up at the factory to 400ft/lb.

I was unable to release this bolt using 'normal' methods,the clutch would slip.
Upon re-building,I was able to torque to about 250ft/lb before the clutch slipped...this was with a 320 clutch,which I understand has a lighter spring compared to 325.

daimlerman wrote:M20 front crankbolt is torqued up at the factory to 400ft/lb.

I was unable to release this bolt using 'normal' methods,the clutch would slip.
Upon re-building,I was able to torque to about 250ft/lb before the clutch slipped...this was with a 320 clutch,which I understand has a lighter spring compared to 325.

Nicely illustrated Daimlerman, great way to measure! 250 POUNDS-FEET is 345Nm in my language, which sounds about right to me for a 320. 400 lbs.ft is 553Nm. I would certainly expect a 325 clutch to slip somewhere before this value, as observed by your good self. That figure will, however, decline somewhat on track as the friction material gets hot.

Rich, dare I suggest 450Nm (325 lbs.ft) as a reasonable upper limit for a 325 clutch?

All with a pinch of salt

rix313 wrote:All with a pinch of salt

Not at all Rich, I am quite serious about 450Nm as a reasonable upper limit. No doubt a myriad of tests performed as Daimlerman did will yield a statistical variation, but I suspect you will genuinely find the mean torque at which a 325 clutch lets go to be around 450Nm. And this is in line with Daimlerman’s observations.

TBH, I suspected quite a few to suggest a much higher value than this given that a clutch is often observed or perceived to withstand a much higher torque during standing starts with say a super or turbocharged M20B25 under the bonnet. In reality, however, the gearbox functions as a torque multiplier and the tyres often let go before the clutch does, leading to the conclusion that the clutch is adequately specified for the task. Gain sufficient traction and that assumption is proved wrong, as I found out for myself the hard way.

Geoff is absolutely on the nail with this...

Another thing to consider is what do you want as the weakest link in your drivetrain...? Speccing the clutch 'right' and using this as the 1st weakest link is no bad thing if everything else in your driveline stacks up - clutches are easy to change and not too expensive - I would rather change a clutch than have to rebuild a gearbox / worry about breaking driveshafts / knackering diffs etc etc...

Enjoy...
Mike

Fair point Mike, but TBH I'd rather not have have ANY of my drive-train components let go. If my clutch slips beyond a certain torque then there is effectively no point to my engine being able to produce that much torque in the first place, so I might as well down grade my engine. If my engine can deliver it, I'd ideally like all my components to be able to handle it. But then we all know what happens when you put too big an engine in too small a car, eh

Good to see that you are still around Mike.

Of course Geoff, you are right once again - everything should be spec'd right otherwise there is no point beefing it up in the first place!

This was something done in DTM in the 90's, where clutches were juuuussssssst the weakest link to save all the other (far more expensive) gubbins going bang - sometimes the calcs worked, sometimes they didn't!

It has been a while - I have been cooking up some S14 bits and pieces for my trackday car - hopefully some juicey threads coming over the (UK) winter...!

@rix313 - sorry to hijack the thread mate! but... Geoff is the man!

Not at all. Interesting stuff