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Posted by: nine14cats Dec 23 2003, 10:53 PM

Anyone know what the factory valve train is good for at the upper rev limit in terms of reliability? I've got a pumped 2.7L in my 914 with carbs and 10.5:1 compression, mod heads, big cam...the works. Last year I ran without a rev limiter (i'm a knucklehead) and used 7500 rpm as my limit. Towards the end of the year I over-reved the car with my right foot to ~8K a few times and broke a few rings. I had the engine dis-assembled and it's all prettied up again (including more mods!), but I was curous as to what the motor's hardware was originally designed to do. I've heard alot of folks recommend 7200 rpm as safe and reliable.

I purchased a new MSD ignition with the adjustable modules. I can program the rev limit in 200 rpm increments. I'll start at 7K for awhile.

Anyone know what the design limit is? The car has stock valves, springs, crank. Heads have been chamfered for clearance and the pistons are JE. Camshaft from webcam. Weber triple throats on each side. The motor will rev easily to 8K with the tap of the foot.

Thanks in advance for the opinions!

Bill P.

Posted by: ArtechnikA Dec 24 2003, 08:49 AM

what -kind- of stock valves and springs ?

911S had dual valve springs, T and E didn't.

the (factory Spec Book) redline on my 2,2E is 6700, 500 rpm over max power. the 2,2S revs to 7300 - the difference is the valve springs. the 2,7RS engines had 7300 rpm rev limiters but it'll take more research for me to figure out what kind of valve train they had...

for power, more than 500 rpm over peak power doesn't seem to buy you much...

Posted by: drew365 Dec 24 2003, 09:24 AM

My engine is similar to yours. It's a 2.7, twin plug, S cam 10.5:1 pistons, electromotive crankfire ignition. The adjustment pods on my electromotive units apparantly need cleaning because they don't match my tach which I had rebuilt and calibrated. But I set my rev limit at 7400 or 7500 rmp by the tach. I've been running it hard that way for two seasons and the engine has continually run and sound beautiful. Because the set pods don't match the tach, I've run it to 8k several times before I realized I had a problem with not matching the tach, and the engine never complained.

Posted by: Brad Roberts Dec 24 2003, 03:46 PM

Rich nailed it.

Dyno it and find out where peak torque is. Then drive it 500 RPM over that. Your wasting parts/time spinning it for no reason. The factory GT3RS' can be spun to 9k rpm but the restricted engine only makes power to 8500. Most owners set the rev limit at 7800 or so and get 20 more hours of use out of the engine before it needs the recommended freshen from Porsche.

B

Posted by: nine14cats Dec 24 2003, 07:22 PM

Thanks for the input guys....I'll break in the engine and dyno it. Jerry Woods is down the block....

BTW, how do you break in a "track only" engine? Drive around the neighborhood?

Bill P.

Posted by: Brad Roberts Dec 24 2003, 07:24 PM

YOU dont. They break it in on the dyno. Let it run for 20+ minutes with a small load on it at 3k RPM or so.

B

Posted by: nine14cats Dec 24 2003, 07:45 PM

B,

After I have JWE break it in...am I good to go? Or do I hold back from the dyno rev limit until I put a few hours on it?

I was going to Sears this weekend, but I ran out of time on the car....I'll try for Sears again on February 6th with Trackmasters....

Thanks,

Bill P.

Posted by: Brad Roberts Dec 24 2003, 07:59 PM

I cant really say whether you are good to go or not. It depends on what tolerances your machine shop used on the crank/case/rods/pistons. Most race engines tend to be a little on the loose side. If they built it with stock tolerances... then you will need to go easy on it for a few hours.

We break engines in at the track in normally 2 sessions on a Friday and race on Saturday and Sunday.

I would certainly use a open track day to break it in after the dyno session.


B

Posted by: drew365 Dec 24 2003, 10:03 PM

I assume your talking about peak horsepower? My car dyno'd with hp peaking at 196.5 @ 6900rpm. So using 7400 to 7500 for the limiter falls right in there. I found that on the track I usually run, if I lower the limit I get caught having to shift where I don't want to.