Compression Ratios

[wastintime]

My mechanic suggested that I check the compression ratios on all of my cylinders so I just did. The problem is that I don't really know where the values should be. These are the results that I got:

Cyl 1: 160 psi
Cyl 2: 157 psi
Cyl 3: 160 psi
Cyl 4: 155 psi

Are these numbers good, bad, or indicative of anything?

Thanks,
Andrew

[Steve F]

Your #'s should be no more than a 10% variance from each other and yours not only reflect that ,but are "Happy numbers " to boot!

[cgarr]

with those numbers it makes cents that in your "sticking valve" post that it could be spark knock, give the high test petro a try, my numbers are all rite at 100# but the most important one is 128 over 75!!!

Craig

[wastintime]

I've never run anything but 93 octane gas. That's the highest you can get where I live. Octane boosters don't seem to help at all.

[cgarr]

did you ever say what #distributor you have on that engine? is it a vacuum or mech advance? could be advancing too much thru that rpm range

cg

[wastintime]

It's an .009 - mechanical advance. That actually would make a lot of sense if the distributor was advancing too much, but I've checked the distributor though... a couple of times. The timing seems correct, I cleaned the contacts, and checked the points.

[mein12]

Hi,
I just wanted to point out that those numbers are pressure readings and not compression ratios. They are very high though. I can only guess that you have a fairly high compression ratio.
I am curious about what pistons you used and what your combustion chamber CC's measure out to.
Did you use B heads or pistons with a high dome?
Just curious, JMB

[wastintime]

I did know the numbers were pressure readings, I realized I'd typed Compression Ratio after I posted. I figured everybody would know what I meant though.

Honestly I have no idea what's in that engine... I bought it as it was being converted into a race car so who knows what was done to it. I know it has a fairly high-lift cam, other than that I really don't know too much. If I ever end up tearing the engine apart and finding out what it has in it I'll let you know.

As for the strange noise.... well I had even more go wrong with the car tonight so I'm seriously debating whether to fix it or just part it out right now... on the one hand if I part it out I cut my losses and don't lose anymore money. On the other hand... I have an awfully large amount of money in the car, and I'd really hate to just throw it all away. I guess I'll wait and see how much it'll cost to fix it.

Andrew

[mein12]

Hey Andrew,

Don't give up yet. You have an engine and that's the main thing. The engine isn't broken, so it is just a matter of making it right. My current motor was a race motor that I did a piston swap on. It also has too much cam, but I am used to it.
You could do the same thing if that is the case. If you do it yourself the parts would cost about $1300. That's for forged pistons,new 86mm cylinders and misc materials. If you added shop labor it would cost around $1800. There are cheaper ways around it, but the Shasta pistons are hard to beat at any price.
So. If the motor is a low mile motor, this may be an option. You could even sell the old pistons and cylinders to recover some of the loss.
The cheapest solution would be to take it apart and measure the parts to calculate the compression ratio. Then select a shim for the cylinders that will get you back down to a reasonable 9.3/ 9.5 to1 ratio. The cost for this is around $60.00 in parts.
Feel free to contact me via e-mail. I would be glad to help you.

Good Luck! JMB

[wastintime]

Mein,
Thanks for the advice and pep talk, but it's time to get rid of the car. I just can't justify spending any more money to fix it. I've spent enough money on the car to buy 3 or 4 nice 912s, and it is barely ever driveable... it's time to cut my losses.

I really appreciate all of the advice everyone on this forum has given though.

Thanks,
Andrew