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Compression Test

69 R/T

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I decided to do a cylinder pressure test on my 69 440 just to see what it looks like. The number 5 and number 7 cylinders are pretty low, everything else is between 145 to 155. The number 5 is only 100 psi, and number 7 is 130. I was losing light and I needed to put the car away, so I put it all back together and drove it in the garage. Now my question is, do I need to remove all the spark plugs again to test those two cylinders or can I just pull those two plugs and do the oil squirt test? The car has got the factory manifolds on it and it's a real bastard to get those plugs out and screw the hose in. So what do you say guys?
 
If all spark plugs are removed the cranking speed will be higher then when the other 6 are still installed, to me this could make a difference to the compression as with high speed cranking there is less time for the pressure to escape through the leaking valve, worn piston rings or were it might be going.

I assume you performed the test with a warm engine right?

I would stick to the same method you used before, just to avoid any deviations that cause a false readings.
 
^^^^ Right on. All plugs out.

Also, if you already did a static test skip the oil in my opinion. You need to put compressed air to the cylinder and do a leak test. That will pinpoint exactly what's leaking.
 
It does take some luck to spot the improvement after the oiling test.
As above, the leak test with compressed air will give you a very clear indication straight away.
 
Thanks guys. My buddy offered to do one for me sometime but he said just do the oil test for now. I told him I just bottled up the car, so I was just wondering if I could just do a quickie and just pull the two affected plugs.
 
If all spark plugs are removed the cranking speed will be higher then when the other 6 are still installed, to me this could make a difference to the compression as with high speed cranking there is less time for the pressure to escape through the leaking valve, worn piston rings or were it might be going.

I assume you performed the test with a warm engine right?

I would stick to the same method you used before, just to avoid any deviations that cause a false readings.
Yes, warm engine....ouch, and carb at WOT.
 
I'm thinking it is either exhaust valve seats or head gasket? Either way, sounds like you will be pulling the head?
 
That isn't exactly the way I wanted to spruce up the engine, but hey, if I pull the heads I can change the springs to go with a little bit more cam, repaint the exhaust manifolds, repaint everything and make it pretty. The engine was rebuilt back in 2000 or 2001 when the guy restored the car, but it was rarely driven. I've had the car since 2009 and maybe put 4,000 miles if that on it. The oil pressure is tight, plugs are ckean and even in appearance, and thing runs like a champ.
 
That isn't exactly the way I wanted to spruce up the engine, but hey, if I pull the heads I can change the springs to go with a little bit more cam, repaint the exhaust manifolds, repaint everything and make it pretty. The engine was rebuilt back in 2000 or 2001 when the guy restored the car, but it was rarely driven. I've had the car since 2009 and maybe put 4,000 miles if that on it. The oil pressure is tight, plugs are ckean and even in appearance, and thing runs like a champ.
if the plugs are burning clean and here's no oil lose just drive it until your ready to get serious.
 
5 and 7 would make me suspicious of the head gasket between the two.
 
my experience with blown head gaskets between two cylinders is that the pressure is nearly identical .
 
I've had the car since 2009 and maybe put 4,000 miles if that on it. The oil pressure is tight, plugs are clean and even in appearance, and thing runs like a champ.

If you are happy with the way it runs, maybe leave it alone and drive it.
 
If you are happy with the way it runs, maybe leave it alone and drive it.
It runs flawlessly. I had no idea I had a compression loss. I'm considering maybe putting in a different cam, it's bone stock, so I was just curious about my cylinder pressure. I guess I opened up a can of whoop-*** on myself
 
If it was rebuild and barely run, I would think more likely of corrosion or carbon build up that causes a valve to slightly leak.
The head gasket will be something if the rebuild was not done right and the deck and head were not resurfaced causing a leak.
To me if you rebuild an engine you will never skip that process.

I would put some more miles on it, high way driving with some power on it to burn it clean and after test again.
Maybe throw a known brand of fuel additive for cleaning the internals in the tank. (not sure if that is any good to these engines but on modern engines it does make a difference with a "lazy" engine full of carbon build up.)
 
I thought it was a high mileage unit. If it has that low miles, put some miles on it first.
 
It may be those two holes the rings didn't seat well, or the ring gaps lined up or something. I'd drive it and try the test after another 500-1000 miles.
 
Yeah, it will be your best bet before ripping the engine to bits.
500-1000 miles is quite a lot, but 100-200 miles would do I guess.
Bit of highway cruising, best is constant load at some decent rpm's. (so not WOT at 6000 rpm :) )
Let us know what happened after checking again.
 
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