compression pressure
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compression pressure
what should the normal compression pressure be for a 74 truck 390? ford manual says "lowest reading must be within 75% of highest, to be within specifications." but lists no readings. i have 135 to 105. engine runs very sluggishly. what test should i run next to help determine problem? thanks.
FE "Freakin Expensive"
Joined: Dec 2003
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From: Smith Mountain Lake, VA
Well I dont know off hand...but a 25% difference seems to be to wide of a spread to me...I have always went within 10% in all the cylinders.. But with that said 105 to 135lbs seems to me a bit low.. JMO
~Russ
~Russ
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From: Long Island USA
My '74 360 had about that much compression. But that is a big range between 105 and 135.
I'd do the "wet vs. dry" test also - and see if the compression comes way up when you drop some oil in each cylinder. That'll tell you whether it's rings or valves.
Did you check the timing? What is it at? Also, is the mechanical (and vacuum) advance working? Take off the distributor cap, and rotate the rotor COUNTERclockwise and let it go. It should move somewhat and then come back easily and not have to be manually rotated back. That means the mechanical advance is working. Then, disconnect the vacuum hose to the vacuum advance can on the distributor, and suck on it with a hand vacuum pump or if the hose is clean, your lips
Make sure the distributor plate is moving and then comes back when you let the vacuum off.
I'd do the "wet vs. dry" test also - and see if the compression comes way up when you drop some oil in each cylinder. That'll tell you whether it's rings or valves.
Did you check the timing? What is it at? Also, is the mechanical (and vacuum) advance working? Take off the distributor cap, and rotate the rotor COUNTERclockwise and let it go. It should move somewhat and then come back easily and not have to be manually rotated back. That means the mechanical advance is working. Then, disconnect the vacuum hose to the vacuum advance can on the distributor, and suck on it with a hand vacuum pump or if the hose is clean, your lips
Make sure the distributor plate is moving and then comes back when you let the vacuum off.
Elder User
Joined: Mar 2004
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From: Klamath Falls OR
Cant never say whats an exceptable PSI is for an engine. All depends on a camshaft used (Hyd, Solid, Roller, Lift & Duration) and C/R. I have seen race engines @ 125 psi or so. My 428 ran 220psi. Some bled off more then others. The biggest concern is the 10% rule. From highest to lowest. Other then that it doesn't really matter.
Senior User
Joined: Sep 2004
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From: Douglasville,GA
what should the normal compression pressure be for a 74 truck 390? ford manual says "lowest reading must be within 75% of highest, to be within specifications." but lists no readings. i have 135 to 105. engine runs very sluggishly. what test should i run next to help determine problem? thanks.
FTE Leadership Emeritus
Joined: Jul 2002
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From: Long Island USA
Yup, and it's a pre-72, so the cam timing is allowing it to get that high. It'd probably be lower on the 72-up 360's.
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