Compression, Leakdown, and HP

[autobonrun]

I was considering doing a compression test on my engine to try to establish it's current condition. It has never felt better in terms of pull, but I'm concerned after driving it for so many years, I may not have even have noticed a gradual drop in HP over stock. Also, the SSI's I've added could be masking a drop in available HP.

I'm wondering what are normal/good figures I should see from these tests and is there a way to roughly figure the impact that low numbers would have on HP? For example, if my compression figures were the same across all cylinders but 5% below spec, would that necessarily mean HP is down 5% from the stock figure?

[jluetjen]

What year model is your car?

[cowtown]

My Carrera has compression numbers in the low 170s, BUT I wouldn't get too hung up on the actual compression numbers. They are affected by the strength of your battery, temperature, oil viscosity, etc. You have the right idea of looking at variance between the numbers. If it's a CIS car, you could also get your mixture checked out.

As far as I know, the only way you can find out whether power is where is "should" be is on a dyno, and even then it's going to be open to interpretation (again temperature, dyno type, drivetrain loss estimate, etc.).

[autobonrun]

Originally posted by jluetjen
What year model is your car?

Sorry, that would help. My car is a 79SC. The engine has never been opened. I thought there might be some rough rule of thumb that associated compression loss with loss of power. I'll keep the issue of battery power and oil in mind if I perform the test. Doing a dyno is an option but I was hoping I could make some generalization from the compression figures. Anyway, do you know if the SC figures should be in the same 170 range as the Carrera?

[rdane]

Here are mine from a '79 USA

Compression checks and leak down
10/0/95 @160,000 160,160,165, 160,160,165
4/01/01 @174,000 145,150,145,150,150, 90
6% 4% 6% 5% 5% 60% (call the 60 carbon as it was never touched)
8/31/02 @ 179,000 145,145,145,145,145,145
10/27/03 @ 193,000
8% 6% 12% 6% 12% 10%

I was 182 hp at the rear wheels on dyno 10/27 but burning 1 quart of oil every 300 to 400 miles. That 182hp was better than all the SCs we dynoed that day and the most mileage. I ended up rebulding my engine but in retrospect a valve job would have done it after looking at the P&Cs.

Look at oil consumpsion first, a dyno second and compression and leak down third to get a realistic and whole picture of the condition of your engine. Any one of the three is never going to tell you much.

[autobonrun]

Thanks Rdane, at least I have a baseline to use. My 79 has similar mileage.

[rdane]

No problem. Stock SCs will dyno closer to the 160/170 range at the rear wheel. Anything at 180 and up with CIS has a performance cam and a set of SSIs or headers at the least. Here are some of the dynos I have compiled on 3.0s and their mods. There are 10 total for the CIS 3.0s there now.

http://69.55.224.199/quelle/phpBB2/viewtopic.php?t=12