[jpg69bird]

I compression tested my 455. Stock 76 with 6H heads; 7.6 CR. Idle vacuum is about 16".

All but two cylinders were between 125 and 135 psi. Cylinders 4 & 6 were both down about 15 psi; 110 psi (#4) and 115 psi (#6).

Since these two are adjacent, is this a sign of a bad head gasket bleeding between the two cylinders?

I don’t see any water or oil smoke in the exhaust, and I am not losing any coolant.

[77 TRASHCAN]

If you are able, you might squirter some oil in all the cylinders and see if the readings change any, especially on low cylinders.

[steve25]

Take out all the plugs and then do a test on 4 or 6, if you still get the same pressure readings then the head gasket is intact, real simple!

[steve25]

Take out all the plugs and then do a test on 4 or 6, if you still get the same pressure readings then the head gasket is intact, real simple!

[steve25]

A leak down test is really the way yo go.
This will tell you is it’s rings or if the valve job is going away in either of those low cylinders.

If it’s valve related I would try a get a set of 6X-8 heads to freshen up and bolt on.
These would reduce the chamber volume by some 20 CCs and pick you up 15 Hp and likely over 20 ft lbs of torque.

[Half-Inch Stud]

Re-TQ them headbolts. Re-measure, then go for a test-beat.

[steve25]

If a head gasket is burnt through enough to make for a compression loss do you really think that retorting the bolts is going to solve that issue?

If it does then you have magic going on there!

Let’s use some logic here.

Here’s the symptoms of a bad head gasket between two cylinders.

With only one plug in ( your choice) when you crank the motor for a compression test you are then asking one cylinder to pump up twice the volume of CID.

So if one of your known good cylinders is at say 140, then a one cylinder trying to pump up itself and another is only going to make at best 70 PSi during a compression test.

[jpg69bird]

Thanks for the help. @steve25, I had all the plugs out. I have a set of 6X-4's I am planning on putting on. I was testing to decide whether I have to machine the block before starting with any upgrades.

@77trashcan, I'll probably do the oil trick next weekend.

[steve25]

Dropping on a set of -4s if your motor is still running its stock 067 cam might make for a pin monster if you don’t stuff in a bigger cam.

[jpg69bird]

Not keeping the stock cam. Still undecided though. Maybe an 068 with 1.65's or a 2802. Want it very streetable.

[Half-Inch Stud]

Originally Posted by steve25

If a head gasket is burnt through enough to make for a compression loss do you really think that retorting the bolts is going to solve that issue?
..........

A stock 455 like that might have fairly loose headbolts. Ran across a PMD engine where that was the solution.

Otherwise you would be darn correct.

[Schurkey]

Originally Posted by jpg69bird

I compression tested my 455...
...All but two cylinders were between 125 and 135 psi. Cylinders 4 & 6 were both down about 15 psi; 110 psi (#4) and 115 psi (#6).

Around here, 135 would be low. Not so low as to be out-of-spec, but low enough to be disappointing.

There was a time when GM spec was "worst cylinder at 75% of best cylinder, minimum 100 psi" or something similar...but that's more to avoid warranty claims than to actually have a properly-running engine. It's like saying a quart to 700 miles is "normal" oil consumption.

I'm used to seeing ~150-ish on stock V-8 older engines. This is very altitude-dependent (higher altitude means lower compression pressure) as well as being dependent on the speed the engine cranks, camshaft, and a dozen other variables.

Have you verified the gauge on your compression tester for accuracy/repeatability? Nobody seems to check their test equipment for accuracy.

Originally Posted by jpg69bird

Since these two are adjacent, is this a sign of a bad head gasket bleeding between the two cylinders?

Possible, but not likely. If the gasket were wounded, I'd expect the compression pressure on those two to be much lower. (Like "20 psi") You'd have to have a very small defect/very small leakage in the gasket to have pressures that high.

Originally Posted by 77 TRASHCAN

If you are able, you might squirter some oil in all the cylinders and see if the readings change any, especially on low cylinders.

Yeah, that's the common wisdom that wasn't new when I heard it fifty years ago. I think that may have been true when engines were upright--inline 6, inline 8, inline 4. Once engines got lazy enough that the cylinder banks leaned over--V8, V6, Slant 6, Slant 4, etc., I don't see how the oil you squirt in through the plug hole can climb "up" the rings to seal closest to the intake manifold side of the cylinder.

Compression pressure goes up because you added incompressible oil, not because the rings are actually sealing better.

That's my experiences, anyway.

Originally Posted by Half-Inch Stud

A stock 455 like that might have fairly loose headbolts. Ran across a PMD engine where that was the solution.

Otherwise you would be darn correct.

Yeah, I can imagine "seepage" across the fire-rings to an adjacent cylinder...but that seems to be a stretch. "Possible, not likely".



As said--a cylinder leakdown test is in order.

[jpg69bird]

Thanks Schurkey. It's a brand new tester, first time use. I haven't calibrated it.

I was also a little disappointed at the low psi numbers. Though, considering the stock heads are 124cc and the engine had a 7.6 cr when new, I wasn't too surprised.

A leakdown will definitely answer more questions, but I'm thinking I should still clean up the block before putting the new heads on. Much better to do it now, then find out after doing the upgrades. Either way, the budget is going to get blown.

[Schurkey]

Originally Posted by jpg69bird

It's a brand new tester, first time use. I haven't calibrated it.

Then you also have no history with it in terms of "known good" engines. Less a matter of "calibration" and more a matter of merely verifying the accuracy. It's possible to adjust pressure gauges, at least the older ones, but I've never done it. If the gauge isn't accurate, you're more-likely going to get a different gauge.

Given the state of Chinese tool-making, I'd plug it into an air compressor and see if the tool pressure gauge matches the compressor regulator pressure gauge. Or test a couple of compression testers against each other and the regulator gauge.

MOST (Not all) compression testers with a quick-coupler in the hose use an Industrial Interchange coupler and plug (Milton "M"). A union (or tee) with a coupler plug suitable for your air compressor hose, and an Industrial Interchange (Milton "M") coupler plug or two is all you need.

[wolf]

Originally Posted by Schurkey

Around here, 135 would be low. Not so low as to be out-of-spec, but low enough to be disappointing.

I'm used to seeing ~150-ish on stock V-8 older engines. This is very altitude-dependent (higher altitude means lower compression pressure) as well as being dependent on the speed the engine cranks, camshaft, and a dozen other variables.

Have you verified the gauge on your compression tester for accuracy/repeatability? Nobody seems to check their test equipment for accuracy.


As said--a cylinder leakdown test is in order.

A worn out timing chain and resultant retarded cam timing will also show low compression pressure.

I also suggest a leak down test.

[Bruce Meyer]

Was the engine fully warmed up before you did the test? If not, warm it up and try it again.