Jump to content

E92 enine and possibilities!


Recommended Posts

Thanks to Steve d over on BM3W for finding this post:


There's been so much talk about the S65's torque number that this needed to be discussed.

Everyone relating torque to displacement is essentially correct.

However, there are five other big design variables that affect torque output besides displacement.......volumetric efficiency, bore/stroke ratio, rod ratio, compression ratio and electronic engine management.

Often when we were given design criteria, we would increase the bore/stroke ratio (oversquare) to increase valve area, unshroud port flow, increase maximum engine speed limitations, shorten the height of the engine and reduce the engine's weight. But there are a couple of tradeoffs....one being slightly less torque output, primarily due to the reduced lever arm on the crank. Other negatives of an oversquare engine are increased cooling requirements and an increased tendency to exhibit pre-ignition and detonation for a given fuel and compression ratio and increased exhaust emissions.

The reason I'm bringing up the bore/stroke ratio is that the S54 engine design was at the opposite end of the spectrum from the S65 engine design. The bore/stroke ratio for the S54 is 0.945 or undersquare while the S65 ratio is 1.223. This is one of the reasons why the S54 had such an impressive torque number for its displacement of 12.39 cc/ft.lb. while the S65's number is 13.56 cc/ft.lb.

But the S54 was really a one off oddball design because you almost never see high RPM, high specific output, undersquare engines. You only see them when a manufacturer wants to put a small displacement engine in a fairly heavy car. By shedding the six cylinder historical requirement and going with a larger displacement V8, BMW could use a proper high RPM oversquare engine.

You have to understand that the engineers were considering many other factors in addition to peak torque when deciding on a bore/stroke ratio. These are the possible reasons to use a more oversquare design like the S65's:

-Lower hood height for reduced frontal area (reduced drag for efficiency)

-Reduced weight

-Higher RPM limit

-Improved crankshaft and rod life under high RPM operation

-Reduced vibration

-Reduced stresses on the engine block

-Reduced fuel consumption

-Improved volumetric efficiency through larger valve area and unshrouding of the valves/ports

-Now that they had 3995cc to use, absolute maximum torque was less critical than in the small 3246cc S54 in the 3400lb. E46 M3.

Believe me, if we were dropping the S65 into a lightened M coupe weighing 2700lb., we wouldn't be having this discussion. A big part of the impression of torque is weight. It is frankly amazing to me that the 3400lb. E46 M3 felt as torquey as it did for having a 3.2L engine. And that was the bore/stroke ratio coming into play. (in addition to its excellent volumetric efficiency and compression ratio) My E46 M3 is 3172lb., and it really feels fairly torquey now compared to the loaded 3400+ versions.

So my point here is that when the engineers were no longer constrained with using the smaller six cylinder design (which was maxed out due to block length), they were able to use a more naturally high RPM, oversquare, short, lightweight, fuel efficient, small displacement V8 engine with excellent power characteristics at the expense of a little peak torque.

Another interesting point is that the S65 is supposed to just be 4/5 of the V10.....so the specific hp and tq should be approx. the same between the two engines, yet they're off a little. The V10 puts out 13.05cc/ft.lb at 6100rpm while the S65 has 13.56cc/ft.lb. from 3500 to 6300rpm, yet at the power peak the V10 puts out 10.00cc/hp at 7750rpm while the S65 puts out 9.51cc/hp. at 8300rpm......this suggests a couple things may be going on.....the S65 either has different cam duration, cam overlap, vanos cam timing settings, port/valve design or a combination of them. This is because they both have the same bore, stroke (thus ratio), compression, intakes, ECU design etc. And the reason for the V10-V8 similarities was certainly provided by a financial incentive for BMW to not re-design the heads and ports. The shift of the power upward in the S65's operating speed range then primarily suggests a cam duration/timing/overlap difference when all else between the two engines is essentially of the same design. This S65 torque number also makes it easier for BMW to show a torque and power gain later on when the CSL version comes out.....something they don't have to do with the V10. I wouldn't be surprised if there is a little sandbagging going on and the real torque number is closer to 305-307 ft.lb.(approx. 13.04cc/ft.lb).....just because the two engines are so similar along with the financial incentive to keep them similar. This can also be seen in how the S65's torque curve rises rapidly to 3500rpm and then just stops.....LOL....8300rpm power peak engines don't do that naturally.....it should have kept rising slowly to about where the V10's peak was around 6100rpm, obviously, since the S65 is actually showing better VE or volumetric efficiency at 7500rpm+ than the V10 is. So what am I saying? Either BMW is sandbagging and underestimating the S65 or it is being electronically limited by some combination of vanos timing and spark timing. Why would they do this? To enable a simple bump for the CSL. Just allow the optimum settings on the CSL and you get the curve like it should look and the improved numbers....Keep in mind this is exactly what BMW did with the differences between the E90 330i and the E90 325i.....same 3.0L engine, just vanos/valvetronic/timing/ignition electronic changes......

so what should those numbers be on the M3 when not limited? Somewhere around 445bhp. and 312ft.lb.

[/ QUOTE ]

So as said on topic over there this means:

The engine isnt wound up to full so little chance of it going pop.

This maybe one of the first n/a 100bhp/ltr engines which can benefit well from a remap

If they do bring out a new CSL it will have little difficulty reaching 450bhp.

Apparently according to Steve BMW have said they will release a CSL version of ALL M coupes.

An M6 CSL may be released at Frankfurt.

If they do release the above I think there is every chance there will be a CSL.

I for one will be very happy holding out for that. 169144-ok.gif

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


  • Create New...