Compression Test

[65 Mustang]

Hey Guys! Ran a compression test and found cylinder 1 - 25 psi , 2- 35, and 3-6 were 125. I changed the head gasket and got the following values: 1-110, 2-120, 3-115, 4-120, 5-110, 6-120. I was happy that the first 2 cylinders significantly came up, but I was hoping for the pressures to be higher. The shop manual gives a range of 155 to 195 psi. Are these numbers acceptable? Should I possibly get the cylinder head reconditioned or should I just leave well enough alone? Thanks

 

[69.5Mav]

How was the test conducted, warm up engine, wide open throttle, Did you try a teaspon of oil down each spark plug hole to see the effect.

What type of head gasket was on the engine? These engines were built with a thin steel head gasket, no longer available. If you replace it with a composit gaskit that it thicker the compression will drop. The head is usually milled to compesate.

What do you use this engine for, daily driver or something else?

The variation is a little bigger then I'd like but how is it running?

 

[65 Mustang]

Thanks for responding. The engine was warm during the test. I did not try putting oil down the spark plug hole. The head gasket I put on was similar to the one that I took off. It must have been replaced once before. The car is running fine. I use her more for a weekend driver.

I took the rocker arm off of the cylinder head before removing the head from the engine block. In retrospect I probably should have left it on. When I was reattaching it, I torqued it to specification (30 to 35 psi). I noticed, though, that the valve springs were being compressed. Should that have happened. I was thinking if they were compressed too much it could cause the valves to stay open a little thereby decreasing the pressure.

Do the bolts in the rocker arm need to be torqued in a specific sequence like the bolts on the cylinder head?

 

[MPGmustang]

IIRC that looks close the same number I got when I had my stock engine when it was cold and throttle was closed and I didn't use oil...

65mav is right, to do a proper Compression test, get the engine to temp, then pull all spark plugs (so none make compression affecting the cyl your testing) and keep the trottle open (or take off the carb so u don't flood the engine) and compression test each cyl, then add oil and compression each cyl again. the difference should be the same ish ratio accross each cyl. those are the numbers you want to look at and determine in the engine is tired or not.

the good thing about your current numbers shows that your rings are sealing properly, the head gasket is now sealed properly, and your valves are consistent on on all cyl's. so I would say you have a healthy enough engine. granted it is low, but a thicker head gasket won't lower it that much, I think it's just showing it's age IMO, which isn't too old...


as for the racker arms, it does not matter the order, and compressed springs show what stroke the piston is on for that cyl so it's normal and shows your camshaft is working properly , and 30-35 is plenty enough torque. I'm glad you took the rocker arms off, you could have dmged the pushrods when putting it back together , so you did right by unbolting them.

 

[Route--66--]

Yep, agree with MPGmustang on that.
Just done the test myself and gor 175-175-180-180-175-175 dry .. tried oil just in number 2 to see if there was a difference and it took it up to nearly 200! ..
but.. reading this forum they say a teaspoon of oil - I used more like 10 teaspoons .. so maybe that's why it read 200 as I may have decreased the size of the compression space thus bumping the figure up..?
..BUT saying that .. surely if you just put 1 teaspoon's worth it, the oil you put in would just sit in the dimple in indent in the top of the piston and not around the edges where you need it to be to make any difference ..
any comments ??

 

[bubba22349]

If you use a pump type oil can all you need is 3 or 4 squirts of oil you are just looking for a change in compression and it dose not take much oil to do that.

 

[69.5Mav]

The car is running fine. I use her more for a weekend driver.

If the engine oil consumption is exceptable and the power and economy suits you then I would leave it alone.

If it ain't broke don't fix it.

However if you want alittle more you could have the head milded done to restore the stock compression ratio and I would seriously consider a holley/webber carb swap.

 

[Route--66--]

sounds good to me
yeh , mine uses no oil at all that I can tell, and none out of the back..
and yeh mpg I can get mid 20's and power is fine too .. may be a good reason to leave it then !
thanks again all P