Compression Test

[Todd L.]

When I bought my car I was told there was a rebuild done ~500 miles ago. It was running real rough and so my friend and I did a compression test on it. 5 cylinders had ~150psi and 3 had 0psi. I took the valve covers off and found a push rod missing for one intake and two push rods bent/broke for two other intake valves. I contacted summit thinking I would need a new cam, valves and push rods. I was told to just purchase three push rods and get the car running, which is what I want to do until I gather my parts for the frame off restoration. I was also told that I have a more performance oriented cam since the cylinder pressure is at 150psi. I was told it should be more at 200psi. Is this accurate? Why would three of my intake ports be bad? I am waiting for my fuel pump and sending unit to arrive, so I can get the car running. It did run on only 5 good cylinders. Where did the broken pieces of the push rods go? I bought a new gasket for the oil pan, thinking pieces may have made there way to it. I do not wish to do a full tear down yet, but I do not know what the previous owner considered a "rebuild", if it even had one. The push rods were not even the same, there was an odd ball shape for one of them.

Help

Todd

[Duke W.]

Re: Compression Test

Year? Engine option? Do you have any documentation on the rebuild including mfg/part number of new parts installed?

Duke

[Martin N.]

Re: Compression Test

TODD-
Like Duke said, can you provide us with the specific's on your engine / car combo?

Marty

[Todd L.]

Re: Compression Test

Whoops,

I have a 74 BB, it has A/C, Pwr steering/bakes, 4-speed. I have no information whatsoever for the rebuild. Right now it has what looks to be hooker headers, a Holley carb with an adapter plate mating it to the stock intake manifold and it has the original heads. Other than that I do not know what has been done.
I can not afford the frame off I want to eventually do, so I am acquiring parts now. I want to drive it for a bit, (as rough as it is) I do not have the equipment or money right now to do the complete rebuild I think it needs/deserves.
I would like to be as minimalistic as possible.
I am in the Denver area, and will be looking for a good motor shop, powder coater, and painter. Looking for suggestions there.

Todd

[Jamie F.]

Re: Compression Test

The actual compression pressure number is not as important as all cylinders being even. So much can affect the psi number that you should not try for a target number. Things like the temprature, cranking speed, throttle position, air cleaner, how recently the engine ran, and cam type all effect the number. Usually anything over 120 if they are all even is good, but I've seen perfectly good HiPo engine crank less, but of course all even.

[Joe L.]

Re: Compression Test

Whoops,

I have a 74 BB, it has A/C, Pwr steering/bakes, 4-speed. I have no information whatsoever for the rebuild. Right now it has what looks to be hooker headers, a Holley carb with an adapter plate mating it to the stock intake manifold and it has the original heads. Other than that I do not know what has been done.
I can not afford the frame off I want to eventually do, so I am acquiring parts now. I want to drive it for a bit, (as rough as it is) I do not have the equipment or money right now to do the complete rebuild I think it needs/deserves.
I would like to be as minimalistic as possible.
I am in the Denver area, and will be looking for a good motor shop, powder coater, and painter. Looking for suggestions there.

Todd

Todd-----


The pushrods that failed may have done so because they were not hardened or, at least, not hardened properly. All big blocks use guide plates and require pushrods that have hardened tube sections. I've never seen any big block pushrods that were not hardened (i.e. why make non-hardened pushrods for an engine that always requires them?). However, someone may have found a way to come up with some, perhaps similar length pushrods from another application.

I would replace ALL the pushrods with known, high quality pushrods. They just aren't that expensive.

The 150 PSI compression sounds about right to me for your engine; I would not expect 200 PSI.

What happened to the broken pushrod pieces? They're somewhere in your engine. Will they cause problems? It's possible, but not certain. I'd install a magnetic drain plug and place magnets on the oil pan as a possibly effective precaution.

[Todd L.]

Re: Compression Test

I purchased 3 new push rods. I received my sending unit and fuel pump I will install those and start where I left off. My other question:
Why did this happen to 3 of the intake ports and no exhaust?
Should I remove the oil pan? I purchased new gaskets in case I would need to pull it.

Thanks

[Joe L.]

Re: Compression Test

I purchased 3 new push rods. I received my sending unit and fuel pump I will install those and start where I left off. My other question:
Why did this happen to 3 of the intake ports and no exhaust?
Should I remove the oil pan? I purchased new gaskets in case I would need to pull it.

Thanks

Tom-----


It's very hard to say, for sure. However, my expectation is that the non-hardened push-rods that were used (likely for another non Chevrolet big block application which used the same length pushrods) were only of the length used for intakes (big block intake and exhaust pushrods are different lengths). I would also be concerned about the other intake pushrods. You can do a rough check for hardness of the pushrod stems by running a file across them. The file will not easily cut into a hardened stem; it will easily cut into a non-hardened or improperly hardened stem.

It might not be a bad idea to remove the oil pan and see if you can "recover" any of the failed pushrod debris.