It’s my understanding that these cars came from the factory with around 180 psi compression. And yet it seems very common even after a good rebuild to find the compression around 150. Why is 180 so elusive.
How would you define “good rebuild”? The main issue for small shops and especially the home restorer is machining. The first time mine was done (by me), I just wasn’t up to putting 10 grand into it, so I replaced the things that could easily be replaced, bearings, chains, timing, oil pump etc, had the head rebuild and just did a hone and re-ringed it. I knew it couldn’t be perfect and it wasn’t. It yielded 160-165.
After that build blew because an aftermarket chain guide delaminated and caused a seizure, I gave it to CJ for the works. As they always do, they pressed out the liners and installed top hat liners and new pistons. I have well over 180 psi now. I haven’t driven enough to know whether I’ll be happy with that. I may decide to install a thicker gasket.
IMO the biggest reasons for less than than ideal compression are worn pistons and liners. After that would be less than stellar valve honing, failure to install new valve seats, and general sloppiness.
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In a nutshell, that’s very well-stated.
Another consideration is the altitude at which the car lives: at my altitude of 1600m, Tweety (9:1) would only crank over about 165 psi.
The S3 8:1 engine I have in the Jeepster will only do about 125.
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Pete as a general rule the problem with rebuilding E type engines is keeping the compression down rather than the opposite.
A total of 40 thou/1mm skimmed off the block and head decreases the combustion chamber volume by 6ccs. As the combustion chamber is around 90cc this has a significant effect on compression. It doesn’t take too many rebuilds to end up with compression numbers that lead to pinging, which is a hard problem to fix.
We routinely used 8:1 pistons for our rebuilds for exactly this reason, aiming for around 165/170 psi.
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Yes I live at 1700m & get 150, all is good.
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I live at 1000ft and with an older engine, Im getting 170 pounds. Js
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Which engine/ CR?
64’ coupe 9:1, original mileage at 59k, never been rebuilt
There are factors to compensate for the different altitudes and the corresponding temperature differences. These are as follows:
1,000 feet = 305 meters = .9711
2,000 feet = 609 meters = .9428
3,000 feet = 914 meters = .9151
4,000 feet = 1,524 meters = .8881
5,000 feet = 1,219 meters = .8617
6,000 feet = 1.829 meters = .8359
7,000 feet = 2,134 meters = .8106
8,000 feet = 2,439 meters = .7860.
The equivalent compression reading for a cylinder that should be 135 psi by the data at 5,000 feet would be 135 x .8617 = 116.33.
Its a 3.8 w/ 9 to one pistons
Are you holding the throttles wide open when checking compression?
As others have mentioned, your altitude could be a factor.
See if it improves when you squirt a small amount of oil into the cylinder.
What is the ring gap supposed to be in general
What was done to your engine? As I said, unless you got new pistons and bores (whether new liners or simply an overbore), you’re probably going to lose 15psi right off the top. If the the head rebuild wasn’t comprehensive, you’d lose some more. Rings not run in correctly, a bit more.
Another possibility is that a thicker head gasket was used. The 3.8 originally used a very thin tin gasket, but it’s more prone to leaks so a composite one is often used. That’s at least 2, maybe 3 times thicker and will lower your compression.
So long as it pulls strong, I wouldn’t worry too much
0.004" per inch of bore diameter.
I did get new + 20 piston and comteck 054 gasket…it runs strong…not going to worry
Engine is also supposed to be hot when tested. Hot engine will increase pressure. Also what does your compression gauge look like? Mine has a long hose that screws into the spark plug hole. That added volume from the hose reduces the peak pressure. I just worry about the uniformity of the readings. Not the absolute value.
Is the new engine fully run in yet Pete? This will make a difference as well - other comments all valid.
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as long as in the psi range…uniformity is what is important. All the atmospheric and altitude factors others have listed may or may not affect the value–which would affect all cylinders the same.
So what does uniformity mean–depends on how fresh the engine is…fresh maybe 5-10 psi…Older engine may see a bit more variation–but if 20 or 30…it is gonna affect even running. Then ya have to do a wet test…and some head scratching. More than that–yer gonna be taking it apart.
Nick
Two K miles seems to be fine otherwise
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