Compression question
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Joined: Sep 2001
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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.
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Joined: Dec 2005
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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.
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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.
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Thread Starter
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Joined: Sep 2001
Posts: 54
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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.
