The Low Comp 2.5L

[CurtEgerer]

There's something to be said for low compression, I think. Intentional low compression. The 2.5L engine in my '71 is basically the hotrod engine described in Bruce Anderson's Porsche 911 Performance Handbook (basically, RS pistons/cylinders, upgraded cams (Solex in my case), and Webers on a 2.2 case). It results in an estimated compression ratio of only 7.5:1. Mine was built in 1979 by Stoddard. I've always run it on 87 octane (only because I can't find anything lower - the lower the octane, the more volatile the fuel).

So here we are some 33 years & 110,000 miles later. Although it was a garage queen in the original owner's hands, I've driven the car the last 13 years 'the way it was intended', so to speak. Current engine status:

Leakdown: 2%-2%-2%-2%-2%-2%
Compression: 150PSI-157PSI-150PSI-145PSI-149PSI-152PSI
HP at Wheels: 183!

[letsrollbabe]

Curt:

Good feedback on a neglected 2.5L variant which duplicates the performance of an engine I had in a 72T including the Solex cams. Mine was built on a 70.4 crank, and your dyno results shows that sometimes the expected data from BA doesn't always work out that way in the real world. I suspect your CR is higher than 7.5, and not much different from the 2.687L version. BA admits that the theoretical CR when substituting the 2.2S or E pistons with a 70.4 crank doesn't produce the higher calculated values in the real world. Variations in how the engines are assembled can affect results of course, and sometimes things done are not reported.

Not many R Gruppe or stock factory cars get driven the miles yours has, and it is great to see one actually getting used with gusto.

[t6dpilot]

Lookin' good Curt. Nice to see you have the Wolf back home and out for a run.

[Chuck Miller]

Lookin' good Curt. Nice to see you have the Wolf back home and out for a run.

I guess I missed something Curt........ thought you had sold the Wolf awhile ago?

If it's back, I'm very happy..... always thought you and the Wolf were one of the same......

[CurtEgerer]

Gib - I suspect you may be onto something. Based upon the basic engine specification, it doesn't seem to add up to either the 183WHP or the seat of the pants performance. There's something in there we don't know about, or I got lucky and the build ended up on the very high end of the margin of error. It was dyno-tuned for the 1st time ever just recently and more HP was found via jetting changes. But my point with this thread is this: is a low-compression engine 'less stressed' than a high compression engine, everything else equal? It would seem to be the case. But on the other hand, any engine producing a certain amount of HP is doing the same amount of work to get there, right?

Chuck - I did put it up for sale briefly a couple years ago, but it never sold. It just doesn't get driven much anymore (which is why I was selling it).

[pu911rsr]

I had a car with a very similar motor from the best I could piece together. Had 180K miles on and still ran very well.
Phil

[304065]

Didn't we conclude that the Mahle spec was for ZERO deck height? This would make sense given that it's up to the builder to establish the deck height. This way, when the pistons are actually installed, the compression in practice works out lower than specified by Mahle, because you're using positive deck height (the theoretical deck of the piston is below the cylinder edge).

182 BHP at the wheels with a 15% drivetrain loss is about 217 BHP at the crank, or 86.8 BHP per liter. Maybe Stoddard flycut the heads, or offset bored the rods-- one wouldn't typically see this kind of efficiency with Solex cams timed straight up. It would be great to know!

[Harvey Weidman]

We did many 90mm conversions in the 70s. The redily available aftermarket Mahle P/C sets that we used were marked 235hp. They were the 9.5 to 1 (about). We used them in both crank situations. They still had a respectible compression ratio with the 66 stroke. Some of the builds we did the other compression mods. Most all had some head work and cam timing fudging. The crank dyno ranged from low 200s to 250+, one customer build was 275+, depending on the level of mods. I couldn't imagine using the low compression (8.5) even with the long stroke. Also they all used S or S mod cams.I suspect that Stoddards was using the same thinking. We had a frendly,small amount of banter during that time
H

[CurtEgerer]

Good info Harvey. I need to double check the service bill from Stoddard. I think it just says 90mm P&C set. I always assumed they would just use the RS set which would have been readily available back then, particularly to a dealer. Maybe I need to increase my octane