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How to check compression on a 360 V8

L

lottanerv

Member
My friend and I had a debate. I say one should remove all spark plugs and then check compression on each cylinder. He says no, one should leave all plugs in except of course the one you are checking. Who is right?
Second question. Can you check compression on one side of the engine after you have removed the intake manifold and head from the other?

Lottanerv
1981 Trojan F32
 
you are right, throttle must be open, coil disabled, and engine warm(if possible). a full battery is also required for a true test.

will
 
I cant think of any reason that is not possible. it will make a oily mess on the side with no cylinder head because it feeds the oil to the rockers through the head.

will
 
lottanerv said:
My friend and I had a debate.

  1. Who is right?

    A..... I say one should remove all spark plugs and then check compression on each cylinder.

    B..... He says no, one should leave all plugs in except of course the one you are checking.

  2. Second question. Can you check compression on one side of the engine after you have removed the intake manifold and head from the other?
Click to expand...

  1. It's not quite that simple.
    In all honesty, either method will work, but one is certainly a more correct and more commonly practiced method, IMO.

    A..... If you do as you suggest, your starter motor will have a lessor duty during the test, and your cranking battery will like you.
    It only makes sense to remove all spark plug prior to this test!


    B.... He apparently doesn't mind if his starter motor and battery pull extra duty.
    Nor does he mind taking the extra and unnecessary time to R&R each spark plug individually.


    I would go with A, and I'd look at B as being rather silly.


  2. Not sure why one would care to do this, but if you've left this side's valve train fully in tact and operational, the answer is YES.

buick430 said:
it will make a oily mess on the side with no cylinder head because it feeds the oil to the rockers through the head.
Click to expand...
Will, not necessarily because the push rods will have been removed.


.
 
fastjeff said:
You can, but....why!

Jeff
Click to expand...

Because my friend found low compression on cyl 3 and removed the head to pursue it. He didn't think to check the bank on the other side until after he removed the one head and the intake manifold. He then argued that he couldn't check compression on the unremoved head. I disagreed and was right.
 
lottanerv said:
Because my friend found low compression on cyl 3 and removed the head to pursue it. He didn't think to check the bank on the other side until after he removed the one head and the intake manifold. He then argued that he couldn't check compression on the unremoved head. I disagreed and was right.
Click to expand...
Tell your friend that as long as the cylinder head is still in place, and as long as the valves are being operated by the camshaft, these 4 cylinders on that side will continue to pull air in, and will continue to compress this air, giving him the cylinder pressure readings that he's looking for.

However, by removing the one cylinder head, he's lost the opportunity to do a cylinder leak-down test that may have otherwise told him a bit more about #3.

You may also want to suggest that he eventually pulls both cylinder heads and have both looked at, and/or re-done.


.
 
When you perform compression test, do a dry then wet test:

With all spark plugs removed, test compression on cylinders,

Then, retest by squirting some oil onto top of pistons through each spark plug hole as you go down the line. This will help determine whether rings or cylinder head are suspect, unless of course there is a hole in piston.
 
That may work fairly well with an in-line engine, vertical cylinder walls and flat top pistons.
It does not work well with V engines, and not well with dished pistons.

In a V engine, gravity will cause the oil to pool at the low side of the rings landings.
Any oil volume within the dished area will increase cylinder pressure regardless.
 
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"it will make a oily mess on the side with no cylinder head because it feeds the oil to the rockers through the head."
If you look in the valley of your block you'll see an oil gallery ascending on either side from a main journal on the cam. These feed oil to the head which routes it through the rocker shaft stand. If the side with the head still has the valve cover on that side won't be an "oily mess". However, the other side is already open. Perfect for getting a stream of oil sprayed every time the cam turns to the correct position to feed it. Yes, "oily mess" is in your future. If you, by chance, have a full grooved bearing on that journal the oil will flow continually making the "oily mess" even bigger. Now remember, the oily mess is always contingent that the oil pump is pumping oil and there is oil to pump.
 
So I am correct in assuming the oily mess as the 318-340-360 series are NOT fed through the push rods like the SBC's (that we all know Rick is so familiar with:rolleyes:).........

will.
 
Rockers are fed from the shafts. This is why correct orientation of the valve train is a must. SAVE0015.jpg
 
Dave, I stand corrected...... and Will, you are correct, I am much more familiar with the SBC.

Dave, would there be a way to temporarily close off these ports?

With these oil ports open, the oil galley pressure may not be up to snuff.
Consider that cranking oil pressure may only be 5 to 10 psi to begin with...... if that.
Even if the cam followers are not being as hydraulically active as they would be otherwise, the cylinder side that is being checked has the intake ports wide open.
I'd think that he can still take decent pressure readings.

He needs to spin the engine over approximately 32 revolutions.
4 compression strokes per cylinder being tested..... = 8 revolutions.... X's 4 cylinders.... = approximately 32 total crankshaft revolutions.


.
 
My 440 (auto) cranks around 30 psi. Pulling a lifter or two out of the adjacent bores will drop the pressure to basically zero while cranking. That's not to say nothing will flow to and from the oil gallery. With no significant pressure one should be able to hold a piece of rubber or plastic over the oil gallery hole to stop any flow while cranking.
 
fastjeff said:
Face it: there's gonna be a mess!
Click to expand...
No argument there, Jeff.... but how much of a mess could approximately 32 slow revolutions cause?
Is this concern large enough to prevent him from taking these readings?

I'd think that a few rags would take care of this. I don't know.... I'm simply asking!
 
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I recall adjusting the rocker arms on a Chevy 348 that was in a 55 Chevy. (Memories!) The motor was hot and at high idle, with oil shooting all over the place---especially the hot exhaust manifolds. What a mess.


Jeff
 
As long as we're getting side tracked here....

Jeff, no kidding......... ah the memories!

I too owned a 55 2 dr hardtop back in 1966.
I built a 337 cu in SBC, w/ 12.5:1 C/R (yes, that's not a typo), Moon mechanical camshaft, Rochester F/I, Hooker Headers, Muncie transmission, Anson scatter shield, large '58 Pontiac rear end w/ 4.88:1 Perfection American gear set. The Pontiac axles and gears were huge.
I'm forgetting the name of the company who made the bronze inlay aluminum flywheels back then.
I remember that it cost me $55 to have the rotating assembly dynamically balanced. Try that today!
The car was almost perfect.
I wish that I still owned it. :( Seller's remorse!

I also owned a 58 w/ the 348 engine. The 58 was too heavy to make any fair assessment about the 348.
There was also a 409 and 427 version of this engine.
Remember the Beach Boy's song...... 409? :D


Should we get back to the 360 topic? :D


.
 
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