M90 "L block" peak power question

gazzol

gazzol

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Hi all, My coupe has a L block from a 81 M535 and it produces peak power at 5200 (ish) RPM. I installed a 270 cam and it still produces peak power at roughly the same RPM my question is this, why does this engine produce maximum power so low in the rev range? The later 3.5 litre M30 produce peak power at over 6K RPM. It can't be the head or the valve timing which is the limiting factor as I've ported the head and changed the cam (Cam timing set with a dti guage and vernier cam wheel).
Granted this engine has a wonderful torque curve which when plotted on a graph after a rolling road session looked like a dinner table drawn from the side but I want to use that extra 1000 RPM on the road without feeling like I'm flogging a dead horse as once peak power has been reached it just stops pulling.
As this engine uses the same bore/stroke as the S38 which produced peak power at over 6K (admitedly with a 24valve head) I know that what I want to achieve is possible but what is holding it back? The only thing I can think of is it might be the length of the con-rods as this can dramatically effect where peak power is produced, do any of you chaps have any thoughts on this or even any info on the legth of the con rod in the various engines?
Help this is driving me nuts.....Gazzol
 
effective duration......

In talking to Jim Rowe of Metric Mechanic, he mentioned that they measured the stock CSi cam at 272º, rather than 264º per BMW, which would explain your situation. It depends on what clearance they measure at - the hot rodders started using .050 for comparing duration back in the 70's, much better since the milder stock cams (especially in pushrod engines) have very gradual acceleration ramps.

Generally the power curve moves 100 rpm for each 2º duration, assuming the lobe centers are the same and you are not making other drastic changes. So by that score a near-stock 3.5 with a Schrick 282º should peak at 5700 to 5800 - anyone had theirs on a dyno that can confirm that?

It would be very usefull if Korman or MM would publish dyno sheets of various M30 configurations, or better yet conduct one of those gradual engine build-ups like they do for small-block Chevys in the hot rod mags. If I should happen to strike oil in the back yard........
 
Bert-Ola Thanks for that information it was interesting but still leaves me confused. As I've allready stated when driving my car hard through the gears it "hits a wall" at about 5200 RPM and whilst it will rev further to make progress I'm better off changing up a gear. I've had numerous BMW's over the years of all engine sizes and this is the only engine I've ever come accross that produces its power in this fasion.
Any one want to buy an M90 rebuilt 10K miles ago??? :lol:
 
I think it has a lot to do with the cam profile, as is suggested by others, as well as the limitations imposed by the SOHC nature of the head. Once you get that high in the RPMs, the VE of the old motors takes a nosedive.
 
My 3.5 with a Schrick 282 will spin strongly to the rev limiter, which cuts in at about 6500 rpm. The head is stock m30b34 spec aside from the cam and Schrick springs (i.e. no reason for it to breath better than any other pre-m30b35), and the engine is 10:1 compression and has the 3.2 flywheel (which is a little lighter than the 3.5 flywheel). I have not dyno'd it, however my "butt dyno" says the HP probably peaks and levels at about 6000.

A 270 cam is mild, the equivalent of the stock cam in the 3.2 engines.
 
Thanks chaps, Veloce I know 270 deg is mild, try telling Piper cams in the UK! A few years ago when I rebuilt this motor I ordered a Piper 285 (292 deg) cam, I installed it using a dti guage, re-installed the motor fired it up and it ran like a complete pig! After much head scratching and re-checking of the valve timing I eventually decided to check the exhaust valve timing (I set it up using the inlet valve only) I descovered that the lobe centre line angle was to tight so not only had I efectivley timed the cam asymetricly (silly me) but if I'd evened it up then there would have been too much valve overlap and the car would have been crap to drive with the stock manifold/plenum chamber (high overlaps don't work well with plenum chambers). I contacted Piper who appologised and duly sent me a replacement which I fitted and which was also wrong! This time they sent me another replacement and refunded my money, the second replacement was fitted, timed up and descovered to be a 270 deg cam which they'd sent by mistake! As you can imagine by now I was fed up with taking the head off to change the cam and thats how it's stayed ever since althogh in fairness to Piper they did say that if I ever decided to change it they would still honour their mistake.
I think it's time to give them another call!!
 
I dunno, I think I'd take the money and order a Schrick. I've never heard of anyone having that kind of problems with the good Doctor.
 
Thing is I've allready had my money back and as they said they'd change it I might as well take advantage of the oportunity (if they can remember that far back). I have tried doing an internet search for Schrick and I've never had much luck finding a retailer here in the UK, any ideas?

Regards....Gazzol
 
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