Water cooled, twin cam, 4 valves, and with 4 pistons there is less mass (minus 2 pistons). I don't want to assume too much without seeing all the comparable numbers (ie: weights), Ferry himself said something to the tune of "If I'd known that the 2.0L 911 engine platform would evolve to 3.8L, I would have had my engineers build a much smaller platform"...

Something else to keep in mind -- when Porsche made the 906's, it's not clear to me that they ran them in the full 220 HP (MFI version) state of tune at Daytona, Sebring and Le Mans. Even to this day, most engines at those races are detuned slightly from their "Sprint" level of tune. I'm not sure of the specifics, but I suspect that the CR may be reduced slightly, the cam timing retarded slightly, the cam timing adjusted to provide a broader torque spread at a slightly lower peak RPM's, and the drivers will shift at a lower maximum engine speed.

As far as the Honda S2000 motor compared to a 2 liter 911 motor, there are numerous differences, but I think that the key points in regards to the ability to rev to a high level, with a long service life, consider the following comparision:

Factor:..............................911 2.0.................Honda S2000
Peak HP engine speed.........6700 RPM.................8300 RPM
Bore:....................................3.15".... ....................3.42"
Stroke:................................2.6"....... ....................3.3"
Piston Speed (peak RPM).....2902 Ft/Min..............4565
Corrected Piston Speed........3195........................4647

So given the significantly higher piston speeds, I would expect that S2000 incorporates the following design differences that allow it to do what it does:

1) Pistons: Both engines share "slipper" style pistons which are not full-skirted, which saves weight. On one hand, I would expect the Honda pistons to weigh a little less given that they are essentially "flat topped" pistons with some valve reliefs cut into them, while the 911 pistons have a very substantial dome. On the other hand, as a result of there being 6 pistons as opposed to 4, the diameter of the 911 pistons is less which will help them to weigh less. So I guess the piston weight is most likely a wash.

2) Rod geometry and the bottom end: Surprisingly, the Honda motor has a 6.02" (center to center) rod, which is longer then a 911's rod, but in the end the rod/stroke ratio is not as high asthe 911's (S2000 R/S ratio equals 1.824 versus a 911's (1.965). Because the 911 is opposed, and thus already very wide, fitting a "long" rod into the engine I would have thought would be a problem -- but I was wrong. The 911 2.0's rod length is 5.11 inches, given the R/S ratio given earlier. This means that the piston will be accelerated in a longer period of time compared to the Honda motor, and the block will have to withstand fewer stresses across the cylinder's bore. This chart might help:
http://forums.pelicanparts.com/uploa...1096733439.jpg

Note that with the longer R/S ratio, the overall acceleration forces are reduced. Also notice the crank angle where the peak acceleration occurs, at about 84 degrees (0 is TDC), so the bottom of the rod is being accelerated laterally quite strongly at that moment. So it would appear that at the bottom end, at least geometrically, the 911 is at least as well suited to high rev's as the S2000. The magnesium block on the 911 may on the other hand be a weakness, even in spite of the cross bolted design to the bearing carriers. I believe that the S2000 has it's bearing carriers fully supported by the sides of the block, as well as by an additional frame underneath -- which is definitely a racing inspired design.

3) Which brings us to the heads. The 911 motor has 6 big 45mm intake valves. I have'nt been able to find the 2 liter S2000's valve size, but I suspect that it is about 34mm or 35 mm across, so substantially smaller and lighter then the 911's intake valve. This means less valve to accelerate as the rev's increase, and is most likely one of the primary reasons that S2000 can rev so high, because there won't be any valve float. I suspect that the S2000 valve size (x2) also results in more flow, which makes cam design easier. Finally, the S2000 uses Vtech technology to provide mild, street-car valve timing at lower rev's, while allowing high-lift and long duration race-car valve timing at higher RPM's.

Interesting question, and one whose answers are not as straight forward as I would have thought when I started writing this.

John, interesting analysis; thanks for that!

I was wondering myself about how these 911 engines fare in endurance races, since they are after all more or less the same engines. Just detuned then.

Do you know if the engineers leave it up to the drivers to shift lower, or do they impose a limiter at what would be considered safe for 12-24 hours of operation?

I just did a quick search on the S2000 intake valve size (I bought an S2000 two weeks ago!). The best information I've been able to find so far is 36mm.

If that's correct, then that's pretty much in line with your suspicion.

Thanks for the replies!


KT

Why not just go here:

FS: Engine '77 2.7L (freshly rebuilt) 250hp

http://forums.pelicanparts.com/porsche-911-used-parts-sale-wanted/319900-fs-engine-77-2-7l-freshly-rebuilt-250hp.html?=&threadid=319900

S2000 2.0 or 2.4?

As far as the detuning for endurance racing, that's pretty common knowledge. The race engines (often in retrospect) are often quoted with a lower HP level in "endurance" trim compared to "sprint" trim. I suspect that the engineers changed the rev limiter -- at least prior to computer controls. I doubt that the drivers needed it at that level of sport. Unfortunately I don't have any first-hand experience since I've never even been to either of the three tracks that I mentioned. There are others though ( Grady, BK911 and "Tony a2z Racer") who could tell you first-hand if the rev limiter was set lower. If they driver is consistent enough, it's most likely better to have a little margin in performance at the top of the rev range in case it's need towards the end of the race.

I believe it was in reference to the 2.0, not the 2.2.

I guess I was thinking that a given car might have different motors for different races, but I suppose it really is the same thing. Yeah, detune. Makes perfect sense!

That makes sense too. I'd imagine that at that level, yeah, drivers can really keep a lid on the revs without being forced to.

i don't know if it is necessarily fair to compare an early MFI setup with points ignition to the technology of today. for that matter, you might as well look at Honda's RC51 motorcycle, which delivers 118hp out of a 1000cc engine. these engineers were the same braintank that applied their motorcycle technology to the S2000 motor...

what will they think of next?... turbo diesel race cars?... ha, ha... uh, oh!...