Cylinder Compression

[peterinman]

My dad brought home a cylinder compression guage from work today to test the psi. in his Corvair. I decieded to pull out the Chiltons and check the EB. Chiltons, said that I need 150 psi. When I tested each individual cylinder I was getting 80 to 100 psi. I am hopping that the discrepancy is in the guage. It isn't a screw in type, but rather a rubber end that you press up into the spark plug holes. I pulled all the spark plugs out and pressed the guage up into the hole with as much force as I could and some air would escape out the sides anyways. Am I using the right guage or is my psi. that low. As a side note, the guage worked just fine on the corvair and the psi's were right on the money.

 

[tabascom16]

As far as I can remember a psi of 80 to 100 is something like 8 to 9 to 1 compression. 150 is probably upwards of 10.5:1. What year is your bronco and is it a stock engine? Starting in 70 or 71 I beleive the started dropping thier compression ratios. I could be wrong but I thinks thats something like it goes.

 

[2badrotties]

camshaft will change compression also. If you have a bigger cam with more overlap the compression will be lower than you would think. Try squirting just a few drops of oil in the spark plug hole and see if your psi gets higher. This has nothing to do with the cam thing but will tell you if the engine is worn out.

 

[broncnaz]

when checking all cylinders the lowest should be within 20% of the highest I wouldnt worry to much sounds like your ok. someone could have rebuilt the engine and used low comp pistons or as stated changed the cam

 

[wildbill]

And a push in gage is not 100% if you want a good reading get a real gage those are only close but the # you showed arent bad. When you hit one at 25lbs then you worry good luck.%) %) %) Bill %)

 

[peterinman]

Thanks fella's, I'm sure you all know how disappointing it can be when you check something just for fun and find out that it isn't what you think that it should be. They first thing that came across my mind was......Great, more stuff to fix!

 

[Broncobowsher]

There is another factor to consider, timing chain wear. This effects cam timing and thus cylinder compression.

The RV style cam I am running helps build low speed cylinder compression. I have 190 to 200 PSI compression on my 10.4:1 engine. yes that means that I have to run premium fuel.

Worst one was a friends mustang that had early 289 small chamber heads and some oddball cam. Compression was running about 285 PSI That poor bastard rattled so bad it wasn't funny. OK it wasn't my car and I could hear the detonation 2 blocks away, that was scary.

As for the 80 to 100 I consider it a little low, but nothing to worry about. definatly don't need premium fuel for that engine. But I would check the timing chain and replace it if it is highly worn.

 

[Mr Joe]

When I start seeing 80 psi, I start saving for an overhaul. Tells me that the engine is showing wear, but at least it's pretty even.

 

[scotch]

Another thing to consider is altitude. I don't know how high you are in MI, but as you go up the readings will go down.