Increasing Cylinder Compression

[65 Mustang]

Hello Everyone,

Looking to increase the compression in my cylinders. Ran a hot engine compression test not too long ago and found all the cylinders to be somewhere between 120-130 psi. The service manual says the compression should be between 155-190 psi. While the 190 may be wishful thinking, I was hoping to at least get close to the 155. My first thought is to have my cylinder head reconditioned and shaved down. I am not sure if it ever was. How do you determine how much should be taken off? While the cylinder head is at the machine shop, is there anything else that can be done to enhance compression? Thanks

 

[First Fox]

It is likely that the head gasket was replaced at some point and the OE type steel shim gasket was not available and was instead replaced with a multilayer gasket. I believe the steel shim gaskets were only about .010" inch compressed and the composite gaskets are all around .040". A .030" cut on the head and a valve job will raise your static CR but I would guess if the engine has any real miles on it that the timing chain is stretched also and that would keep your cranking pressure lower than it should be as well. If the rings are in good shape, then a GOOD timing set and cam degreeing, and the head milling should get you much close to the stock numbers you mentioned.

Good luck!

 

[65 Mustang]

Ok thanks. I didn't even consider that the timing chain could be stretched. Nor did I realize that could affect compression. As far as I know it is the original chain. You mentioned getting a Good one, any suggestions? I'm running a 200 ci engine.

 

[First Fox]

If I had to do it again, I would get one of Mikes timing sets at Classic inlines. I had all kinds of problems with cheap replacement sets as they all seemed to place the cam in a retarded position and without my degreeing the cam to verify, the engine would have been a total dog just installed "on the dots". Mike is the only supplier of timing sets that allows for timing adjustment and while they are pricy, it is really the only option.

It is pretty easy to degree the cam while the engine is in the car and should really be considered if you do indeed replace the timing set.

 

[65 Mustang]

Ok. Thanks for the timing set suggestion. I'll go to their website and take a look at them. I am a bit new to all of this and I have to admit I never heard about degreeing a cam before. What is the purpose of that and what's involved? Once again, thanks for the help.

 

[CZLN6]

Howdy Back:

I seem to recall that you have already replaced the head gasket once? Correct me If I'm wrong, but what head gasket did you use?

As to what else to do to increase cylinder head efficiency, along with milling to regain a higher CR, here are my suggestions; On a C8 casting (am I recalling correctly) The stock steel shim head gasket is .025" thick. Composite replacement gaskets range from .045" to .055", so on the average you will need to mill approximately .025" to regain stock OEM compression rating of 9:1. You can mill an additional .025" with no risk, for a total cut of .050". Also, consider replacing the exhaust valves with 1.5" head diameter valves from Classic Inline. The hydraulic lifters will easily accommodate this new tolerance.

Assuming you'll be doing a valve job while the head is in the machine shop for milling, a low cost improvement can be had by adding a back-cut of 30 degrees on the intake valves along with, at least, a three angle valve seat.

While not a biggy, I would have the machine shop bore the intake hole/carb hole from its stock 1.5" out to a 1.75". then I'd spend some time rounding the bend into the log in both directions. This will improve air flow slightly, increase volume and have you ready to accept a later, bigger cfm one barrel carb at some point in the future.

While the timing chain suggestion is a good one, the time to do that will be when you're rebuilding the short block. My guess is that your loss of cylinder pressure is due to a thicker than stock replacement gasket.

I hope that helps you.

Adios, David

 

[rbohm]

to determine whether or not you need to replace the timing chain set, pull the distributor cap off, and put a breaker bar with socket on the bolt that holds the crank pully/dampner in place, and rotate the engine in one direction until the rotor starts to move. then rotate the crank in the other direction again until the rotor starts to move. how far you have to rotate the engine will tell you how loose the timing chain is. if when you rotate the engine in the opposite direction, and the rotor starts to move almost immediately, say 3-4 degrees, then the chain is fine. if you have to move in the opposite direction more than about 5 degrees or so, then replace the chain with the aforementioned full roller chain listed here;

http://classicinlines.com/proddetail.as ... PP-200-DRC

 

[65 Mustang]

You are correct. I did replace the head gasket a few years back. I don't recall off hand the type but I do remember it having an orange outline if that helps. Anyway, it turned out not to help anyway. The reason I replaced it was because my first two cylinders were reading 40 and 70 psi so I figured it was a blown head gasket. To my disappointment I got the same readings after replacing the head gasket. It turns out it was the rocker arm assembly causing me problems. I shimmed it a little and the compression shot up.

I'll first start with getting the head reconditioned and see how it goes from there. Then I'll start playing with the timing belt if need be. Thanks for all the advice on what I should do to the cylinder head. I'll bring it up with the machinist. Are hardened seat valves to compensate for unleaded gasoline worth it?

Just out of curiosity what type of cylinder compression are you guys getting? Thanks again

 

[65 Mustang]

Thanks Rbohm for the info on testing timing belt. Had no idea. I'll give that a shot as well.

 

[chad]

"...Are hardened seat valves to compensate for unleaded gasoline worth it?..."
I understand that was a concern immediately after the lead was dropped out &
is no longer of import in engine wear worry (EWW). I had added a bottled product
for a few yrs. they said replaced the lead, no longer do so.

 

[rbohm]

You are correct. I did replace the head gasket a few years back. I don't recall off hand the type but I do remember it having an orange outline if that helps.

if it was black, it sounds like a fel pro permatorque head gasket. a good gasket to be sure. since your compression pressures are even across the board, i would leave the head in place until you have exhausted all other leads.