911F1

I'm looking at a car that has the following compression numbers:
1) 145psi 2)140psi 3)120psi and 4) 150psi

#3 is 10% lower than the others. Is this a problem?

I was told this could be the potential problem-
"The cooling air to #3cylinder is blocked by the oil cooler so it tends to run hotter and as a result it tends to wear faster than the other cylinders."




Any help is much appreciated.
Thanks

Grease Monkey

I had similar numbers on my 68 on my #3, drove it a all last Spring and Summer and burned about a quart of oil every 500 miles, opted to overhaul the engine this Winter. The #3 piston and cylinder were pretty ugly when I pulled them, hope to have it back on the road in the next few weeks.

911F1

Hmm, seems like a potential big job issue. Can you share how extensive a repair and bill you had? Did the car smoke on start up? or just eat oil?
Thanks

Grease Monkey

Yes it a pretty big job. I am doing most of the work myself, new pistons, rings, cylinders, bearings, and seals. I have not totaled all of my receipts (afraid to know).

It did smoke a little at start up. What I found when I disassembled was that the piston ridge between the top 2 piston rings had broken loose in a 2 inch section on the #3, all the other pistons were good.

911F1

Very cool your doing the work yourself. Do you remember your exact compression numbers?

Grease Monkey

I don't remember the numbers. I should have written them down. I do remember all the cylinders except #3 where with in specs. but #3 was slightly below the specs. The car had sat for almost 20 years so I was hoping it was a stuck ring. I squirted marves mystery oil into the cylinder and let it set overnight.

Mr Michael B

For compression numbers I take the highest number & look for the other numbers to be close to within 15% or so / for what can be considered acceptable street engine use & performance (this does not apply to race engines).

So say 150 is your highest... With my 15% range I would find a reading of 127 as your lowest to be within acceptable parameters. That is not saying that your 120 low reading wont work for you, but it does exceed my acceptable mechanical range for normal use & performance.

Now as Monkey said you could have a stuck ring in that cylinder, or the car may just benefit from a spirited drive or two that could perhaps improve the low cylinders readings. These engine are that simple.

Test it twice to confirm. Follow up with a leak down test to pinpoint the potential weak area.

911F1

Mr Michael B, I sent you a PM. in regards to PPI's in St Louis. Much appreciated!