Engine compression adjustment question - Team Camaro Tech
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post #1 of 7 (permalink) Old Dec 11th, 05, 08:52 AM Thread Starter
 
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Engine compression adjustment question

I decided to post a related topic to my last post on grey/white milky oil since it is related but in case someone might have done this and wasnt interested in the other topic...

I bought this motor and I am taking it in to have it checked out to determine why I have the white/grey milky oil. The motor has 11:1 compression a larger cam, etc. I was told I have to use 106 octane gas or race fuel in the car since the compression is so high...my intent is to drive the car maybe two or three times a week just around town. My question is since I am taking the motor in to be checked out, what would be the best way to lower the compression down to something that I can use premium gas from a gas station? At $5.50 a gallon, I want to avoid using race fuel...is there an easy or inexpensive fix to drop the compression down to maybe 10.3:1 or something like that?
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post #2 of 7 (permalink) Old Dec 11th, 05, 09:08 AM
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Re: Engine compression adjustment question

engine octane requirements depend on a ton of variables, including static compression ratio, intake valve closing point (dynamic comp) combustion chamber design, even fuel system mixture control and distribution accuracy.
without experience with a particular combo, it's impossible to say that 11:1 won't work right on pump gas... one would assume the people who told you this did have that experience, but if all they ran it on is race gas, maybe not.
On top of that, unless everything is checked and measured, no one really knows what the compression is, just a guess as to what it should be.
a .030 thicker head gasket will lower the comp. about half a point (10.5:1)
Depending on the piston, some of the dome may be able to be milled off.
The piston could be replaced.
The heads could be replaced.





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post #3 of 7 (permalink) Old Dec 11th, 05, 09:23 AM Thread Starter
 
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Re: Engine compression adjustment question

Thanks...actually I am not positive on what it is and it may even be a bit lower than 11:1. The motor was running on premium gas probably like 92 octane which is a good possibility of why I have a water leak, but as you have said the leak could be from a lot of other things too. I guess I am assuming that it was or is the cause of the leak if there is a leak.

Is there a relitivley easy way to test the motor to find out what octane you should be using? The motor is out of the car and is not running at this point. Should an engine rebuilding place be able to do some checks and give me an idea of whether i should be running race fuel or can run regular premium gas? I know the guy that had this motor before said there were no bubbles in the radiator, the motor seemed to run good on premium gas, and the exhaust ports are all dark black, no plugs fouled, etc and he was running premium gas not race gas. Is this just a trial and error thing that if it dosent run good on regular gas you go with race fuel? How can you tell you need one over the other?
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post #4 of 7 (permalink) Old Dec 11th, 05, 10:25 AM
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Re: Engine compression adjustment question

With the engine out of the car and being opened up the shop should be able to take some measurments (how far in the hole the pistons are, the head chambers, head gasket thickness, stroke etc and compute the engines static compression. Then with the cam spec's they can determin the dynamic compression (ratio taken when both valves are closed on the compression stroke) and quench (distance from top of piston at tdc to head surface) and make an assessment of what fuel to burn.

...Dennis

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post #5 of 7 (permalink) Old Dec 11th, 05, 12:09 PM
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Re: Engine compression adjustment question

Easiest way to check dynamic compression is with a compression gauge. This will tell you how much compression your engine is actually "seeing". You might have 13:1 static compression based on calculations of all parts, but if you are using a huge cam that's closing the intake valve real late, then your effective (dynamic) compression will be much lower.

Just take the spark plugs out and do a cranking compression test of all the cylinders. If you see under 200 psi, then you can probably just run 93 octane with no trouble. I have 185-190 psi cranking compression in my 454 and I run 93 octane with just a touch of octane boost, usually one can per tank of gas. Even without the octane boost, I don't hear any pinging. Of course, pinging (aka: detonation) is sometimes difficult to hear.

Dave F. in Rhode Island
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post #6 of 7 (permalink) Old Dec 11th, 05, 01:58 PM
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Re: Engine compression adjustment question

No disrespect ment by this SS, I don't think cranking compression is a real good indicator of dynamic compression. My iron head 383 has a static of 9.87:1, a dynamic of 7.82:1 and cranking compression was above 200lbs in all cyls, closer to 220lbs. I run on 91 oct pump gas (no 93 out here) with no additives just fine.

I'm not an engine guru and can't throw out more than what I have seen. My heads are advertised as 76cc and with a quench just below .040". The cam is a hyd roller 224degs @.050 with .525 lift. My guess is if someone had more static and the same dynamic their cranking compression would not workout the same as as mine. My cranking compression being higher because the static and dynamic are closer together. Again that's just my guess...

...Dennis

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post #7 of 7 (permalink) Old Dec 11th, 05, 03:05 PM
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Re: Engine compression adjustment question

I'm no professional engine builder, but I realize that dynamic compression is just static compression, with intake valve closing calculated in. Dynamic compression will always be lower than static. I know I am not 100% correct to interchange "cylinder pressure" with "dynamic compression", but they are certainly related.

DjD, the initial post said the guy was told to use 106 octane. Well, if he's got cranking compression of only 160 or 170 psi, then why in the world would he ever want to use 106 octane? He doesn't need it. Variations in chamber design, quench, gaskets, pistons, etc., as well as rear gearing and vehicle weight will affect detonation, and maybe that's what's allowing you to run 220 psi on 91 octane with no detonation. I'm no expert in that area, but that number sounds very high for detonation-free running on 91 octane.

All I'm saying is; if a guy has an engine in his car (built with unknown components) and he's just trying to get a quick feel for cylinder pressure and what gas to use, then a cranking compression test is a good first step.

------------------

Dave F. in Rhode Island
'68 Camaro SS Accel DFI 454 (SOLD - I'm a boring Corvette owner now)
Bought my first big-block Chevy in 1970

Last edited by Chevy-SS; Dec 11th, 05 at 03:43 PM.
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