Compression Results and a question

kstar · 12-10-2001, 10:39 PM

I got a good news/bad news call from Black Forest today:

1) Good News: Compression across all cylinders is 155-165 - I'll get the cylinder to cylinder break down tomorrow.

Question: How does the compression ratio one is running relate to 'spec' compression in the cylinder? I.E., If my car is actually set-up with pistons and cylinders to run 10:1, should compression results be higher or lower in 'spec' vs. a motor set-up with 8:1? I'd guess higher, and is 155-165 in 'spec' for 10:1?

2) Bad News: Oil leak source at rockers fixed a couple of weeks ago, BUT a more serious leak is now apparent on the driver's side where the heads meet the cam tower! The rocker leak was masking this leak, but now it has reared its ugly head.

I am going to have the top end torn down and 'investigated' to see what other surprises this motor may be hiding . . . and have the leaks fixed.

I'll be documenting the process and taking plenty of pictures.

So it was a great day and a shi##y day. Hope your day was better than great and not less than shi##ty!

Best to all,

Kurt

Superman · 12-10-2001, 10:48 PM

I believe that higher compression engines give higher compression readings. It seems to me that 160 is typical for the early SC engines with 8.6:1 (I think). Mine (9.3:1) tested 190-195.

but I also think there is considerable variation in testing gauges and in test conditions. Warm engine versus cold makes a big difference. Throttle open, plugs out, battery strong, etc.

Tyson Schmidt · 12-10-2001, 11:09 PM

Lots of other factors effect this, like Jim said. But all things being equal, higher compression ratio=higher compression readings.

My 10/1 CR 2.4 motor has read 165 on all cyls since day one. (Cams and cam timing have a big effect)

BK911 · BK911's Garage

One other thing to keep in mind is the compression ratio isn't a pressure ratio, but a volume ratio. So a 10:1 compression ratio doesn't increase pressure 10x's, it reduces the volume 10x's. Swept volume, final volume, temperature difference and other things contribute to the final pressure. So a 2.2 litre 10:1 could (would?) have different compression results than a 2.4 litre 10:1 engine.

I hope I didn't confuse the issue more!

BK