John Gregory decided that the time was right to put the car on the dyno, so we did -- on Friday.
It wasn't long before it became apparent that the engine was getting too hot too quickly. Friction, and lots of it. It's still warm down here, and while the engine has hit 100C (during spirited runs) on a few occasions since it was put back together with the 10.5:1 JE pistons (the actual CR is closer to 11.1:1 than 10.5:1) and the GE-60 cams, I was pretty sure that the high engine temps had more to do with ambient temperature and a still-fresh engine than a compression ratio that's too high.
In a fortnight we'll be taking the heads off so that we can reduce the compression ratio. Thankfully, John is considering this "part of the job", as it's the first time he's rebuilt an engine with non-German, high-domed pistons. He wanted to see if he could get away with that compression ratio, and to his credit, he came close -- very close. We could leave the engine as it is, and it would be fine for spirited street use. However, since the engine is going to be raced, we have to reduce the compression ratio (by modifying the pistons) down to 9.8:1 to make sure it can be raced for a few seasons.
In case some of you are wondering what sort of power the engine is currently developing (I know there's a few of you!) we were able to get one reading (at only 6,000rpm) before the engine reached 100C:
220bhp, at the flywheel.
Not brilliant, I know. We're a long way from our lofty claims of 260-270bhp, but the engine was only going to develop that sort of power with modified stacks, a special exhaust system, and after 8-10,000kms of use. With a compression ratio of 9.8:1, 260-270bhp is now impossible without modifying the heads. But I'll be happy with 240-250bhp, and John won't be satisfied until the engine is developing that sort of power after the necessary modifications are made.
Matt Holcomb
1974 911 Carrera 2.7 RoW
1975 Mercedes-Benz 280SE
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