compression values

[mustangman65_79]

Here are my (dry) compression values:

#1 119
#2 92
#3 118
#4 117
#5 121
#6 118
#7 115
#8 91

These are really low. Now I know where my horse power isn't coming from. What would cause low #'s all across the board? Bad cam?

[Rev]

Is this the '65 or the '79?

Rev

[PKRWUD]

Was the engine warmed up? Was the throttle blocked open?

Forgetting either one will result in false, lower than normal results.

Take care,
-Chris

[mustangman65_79]

damn, there is more to this then I thought, lol. I take it then I need to block open the throttle? Also, no, it's wasen't really warm, but now it's cold, and I'm going to do the wet check.

[PKRWUD]

STOP

You're wasting your time. The results won't mean anything. For accuracy, you MUST perform the compression tests with the engine as close to operating temp as possible, and have the throttle blocked open. You should also do both tests at the same time, dry first.

In short:
1) Warm up the engine
2) Disconnect the coil
3) Block the throttle so that it is wide open
4) Remove all 8 spark plugs
5) Install the tester and crank the engine through 5 revolutions
6) Read and write down the results
7) Repeat steps 5 & 6 for the remaining cylinders
8) Add 3 squirts of engine oil into the spark plug hole of the first cylinder you tested, and test it again
9) Read and write down the results
10) Repeat step 8 & 9 for each of the remaining cylinders, but do not add the oil until you are about to test that specific cylinder. Adding oil to all of the cylinders at the same time won't work correctly.

Post your results.

Take care,
-Chris

[mustangman65_79]

It takes too long to remove the plugs, it will be cold by then. Same thing as last time. I got it warm, but it cooled before I got them out. They are hard to get out. The headers are in the way.

[PKRWUD]

Do it anyway. The metal inside retains heat longer than you think. If you do it cold, the numbers are meaningless because the conditions are different. It would be like performing a dyno on an engine with the ignition turned off. The info you would get would have little or no effect on the engine when it was running.

Take care,
-Chris

[mustangman65_79]

Crap, well, my day tommorrow is all planned out now-lol

[PKRWUD]

Think of it as experience!!!

Take care,
-Chris

[mustangman65_79]

Is the motor being warm really going to bring the #'s back up to 150?

Hey, can a cast iron crank)stock 2bbl crank) handle 200 psi? I'm thinking of shaving the heads to this. Or do I have to buy a new forged steel crank?

[PKRWUD]

Originally posted by mustangman65_79
Is the motor being warm really going to bring the #'s back up to 150?

Not if it's junk. I can't guarantee what your numbers will be, but I can tell you this, if the engine is cold, the numbers won't be accurate. Metal expands when it's warm, and it contracts when it's cold. As a result, an engine that would have great compression when it's running may have lousy compression when it's cold, because the piston and rings haven't expanded to their running size, and are allowing more blow-by than they do when driving. This could generate compression readings that are lower than they should be. This could mean performing unnecessary repairs.

Look, it's your car, time, and money. Do it however you want to. But don't ask my advice, and then act like I have nothing better to do than tell you to do unnecessary procedures on your car. You followed my advice about doing a compression test, but you did it wrong, and the numbers you came up with are meaningless, and don't tell anyone anything. This wasn't your fault, because no one ever taught you the right way to do a compression test, so I did. I'm not mad at you, Kaspar, but please understand that I don't have any reason to instruct you to do things that aren't necessary. If you want my help, you need to supply me with accurate information.

Originally posted by mustangman65_79
Hey, can a cast iron crank)stock 2bbl crank) handle 200 psi? I'm thinking of shaving the heads to this. Or do I have to buy a new forged steel crank?

Okay, first off, forget the cranking compression numbers. You don't use them to build an engine, you use them to diagnose an engine. It's cylinder compression that you would consider modifying, and it is that number that you need to be concerned with. There are too many factors involved with cranking compression, and they won't remain constant. Cylinder compression, however, will.

What kind of compression are you thinking about? Keep in mind that more compression means more octane will be needed to control the compressed fuel mixture's stability, which means running a more expensive gas. If you go much beyond 10.5:1, you should plan on having to use a mixture containing at least 20% AV fuel.

Take care,
-Chris