Compression Test Question

[Strype]

I need to change the subject of my original thread a bit...

93 LX 66k original
Just installed B cam, GT40X aluminum heads, intake, 1.6 roller rockers, roller lifters

On a cold motor:
1=158
2=155
3=150
4=140
5=120
6=130
7=130
8=unknown

If cylinders 1-4 (passenger side) is reading an average of 151, and cylinders 5-8 are reading an average of 127, then what can cause this?

I have read that low compression amongst all cylinders issues, and I had gas in the oil and exhaust that we figured were from timing and a hung injector.

But, I have also read that not degreeing a cam can cause one side to be lower than the other. Evidently if the cam is a few degrees off, even though you have the marks lined up nose to nose on the crank and cam, they can still be a bit off. I guess the theory is that the valves either open or shut too early on the low compression side.

Let me iterate that all cylinders are withing 25% of each other which is fine by the Haynes manual, but not by me. I have 58cc combustion chambers on these new aluminum heads so this car should be pushing more than 127 average on one side right?


Key notes:
1) The car supposedly had a head gasket blow on the low compression side years ago
2) The cam and the crank were positioned nose to nose
3) Battery is really low

I have not finished shimming the rocker arms, but would that affect compression on just one side? None of them were shimmed when I did the test.

 

[Strype]

Will leaving some spark plugs in on one side make that side read higher???

 

[Chuckman]

Will leaving some spark plugs in on one side make that side read higher???

shouldnt, its on a per-cylinder basis, it would just make it harder to turn.

i dont see how degreeing a cam will make a difference in compression, but im no expert, so.......

if it blew a gasket on that side before, i'd bet that side of the block is a bit warped, especially being that you have new heads on. dont think it'll make a huge impact right now though, but it may rear its head later on. if it does, you'll need to have your block decked.

honestly, right now i wouldnt worry about it. still sounds like the intake/fuelrail problem is to blame for alot, get that and your valve adjustment down and see what it does.

 

[Strype]

sweet
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[PJC Racing]

Yes degreeing the cam can make a difference, the cam effects cranking compression. The alphabet cams are proven to be off on their grinding. I had to advance my "B" 2deg to get it close to where it should be. Could it be on just one side like this? With an alphabet cam, possibly.

But in this case I am willing to bet it is not cam related. Did you measure for correct pushrod length? And how did you adjust your lifter preload?