Compression question
Thread Starter
Advanced
Joined: Sep 2001
Posts: 54
Likes: 0
From: Middlefield CT
Compression question
How can I convert my compression gage reading to a ratio? I'm not getting the power I expect from my 383 so I was checking the compression. I was checking it cold and I get 210 psi accross all cylinders (+/- 3 psi). Can that be converted to Compression ratio?
I did find during my test a shorting spark plug wire ( on the header), most likely the culprit to my skipping engine.
I did find during my test a shorting spark plug wire ( on the header), most likely the culprit to my skipping engine.
Racer
Joined: Dec 2005
Posts: 368
Likes: 4
From: . New Zealand
How can I convert my compression gage reading to a ratio? I'm not getting the power I expect from my 383 so I was checking the compression. I was checking it cold and I get 210 psi accross all cylinders (+/- 3 psi). Can that be converted to Compression ratio?
I did find during my test a shorting spark plug wire ( on the header), most likely the culprit to my skipping engine.
I did find during my test a shorting spark plug wire ( on the header), most likely the culprit to my skipping engine.
To answer your query, no you can't use cranking pressure to obtain the compression ratio, because it depends upon the valve opening timing, which affects the cylinder filling opportunity, therefore the volume of air/fuel within the available compressible volume. What you have measured at 210psi is the amount of compressed air/fuel available.
It's usually around 150-180psi
What head cc have you got?
What pistons (flat top, dish cc?)?
Deck height clearance?
With all of that data, you can get the comp ratio.
It sounds as though you have either way too much compression, or too little cam, early cam timing or all of these.
Remember, there is very little power difference, really between say 9.5:1 and 10.5:1, but a whole lot of power penalties if you are getting detonation.
Thread Starter
Advanced
Joined: Sep 2001
Posts: 54
Likes: 0
From: Middlefield CT
Thmprr,
Thanks for the info. I'm not sure what all the measurements actually are as I purchased the complete engine from a race engine builder. I only run 93 octane in it because I fear detonation. It has iron heads which doesn't help reassure me that my 383 isn't a time bomb. I'm thinking of pulling off the heads to see what I got. I have a MSD ignition with 36* of timing. I just purchased a new timing light that can be adjusted for RPM and advance. The old light suffered a quick and painless death when I went to hook it up to a running engine.... not good.
I have a lot of tuning work to do. (and learning)
PS does it matter that when I measured the compression it was cold and didn't loosen the other plugs as I was testing? Also, I didn't cut off the fuel, only spark.
Thanks for the info. I'm not sure what all the measurements actually are as I purchased the complete engine from a race engine builder. I only run 93 octane in it because I fear detonation. It has iron heads which doesn't help reassure me that my 383 isn't a time bomb. I'm thinking of pulling off the heads to see what I got. I have a MSD ignition with 36* of timing. I just purchased a new timing light that can be adjusted for RPM and advance. The old light suffered a quick and painless death when I went to hook it up to a running engine.... not good.
I have a lot of tuning work to do. (and learning)
PS does it matter that when I measured the compression it was cold and didn't loosen the other plugs as I was testing? Also, I didn't cut off the fuel, only spark.
Le Mans Master
Joined: Jan 2006
Posts: 6,506
Likes: 139
From: Butler Pa
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Thread Starter
Advanced
Joined: Sep 2001
Posts: 54
Likes: 0
From: Middlefield CT
I am running Vac advance. I don't have any pinging so far. I went with the medium springs in the dist so I'm not all in too soon.
I have had one plug, the rear pass side that fouls. The pressure is high so no ring or valve problem. Could that be because the gap is to large on the spark plug (.045) and with the high pressure (210psi) I'm blowing out the flame? Should I tighten up the gap?
I recently went a little hotter on the plug because I was trying to stop the plug from foulling. (Was autolite 23 now I'm using 24)
Thank again for everyones input. If your not to careful you can actually learn something here.
I have had one plug, the rear pass side that fouls. The pressure is high so no ring or valve problem. Could that be because the gap is to large on the spark plug (.045) and with the high pressure (210psi) I'm blowing out the flame? Should I tighten up the gap?
I recently went a little hotter on the plug because I was trying to stop the plug from foulling. (Was autolite 23 now I'm using 24)
Thank again for everyones input. If your not to careful you can actually learn something here.
