i have 130 psi in every cylinder. is that normal? i've searched everywhere and can find compression ratios and all kinds of other mumbo jumbo but not actual psi specifications. my buddy told me i should have more like 180 but i think maybe he is thinking of a 390 and not my 360.

so either my motor is strong as an ox or its worn out but the wear is completely even?

 

The new rule of thumb is that the highest cylinder and the lowest cylinder should be no more than 15% apart. They use to give numbers but in todays PC world no one will commit. Then again I am always suspect of a compression test that is the same in all cylinders. I've never seen it happen except with a faulty gauge, even on a new motor.

 

bah. ya got me. yeah a few had 125-129 but i would rather say 130 for all of them.

 

The 352 in the olé heifer with 5k on her new soles is about 146 lbs on average so ... take that for what you will

 

Lucky dog!

My old Chilton's "Truck and Van" lists the '70 to '76 model year 360s and 390s @140 psi cranking pressure.

HTH

 

True, but a 1966 352 had a 9.3 compression ratio which would be well above the 1969 360 at 8.4 compression numbers. Hell, my 428 CJ had 220 psi when new with a 10.6 ratio and a month before it broke a piston and destroyed itself still had 210+/- 5 all around at about 50K miles.

 

As mentioned already, a compression test will do best to show the general condition of the engine. It's not all about getting "HIGH" numbers. It's about how the numbers relate to each other.

Just an example using a 4 cylinder engine:

#1: 140 lbs
#2: 85 lbs
#3: 75 lbs
#4: 170 lbs

Now clearly this engine has issues. #1 in all likelyhood is in good shape. #2 and #3? Looks like a blown head gasket or cracked block/head. #4? The un-informed would look at that number and think "Holy Crap! Great Compression!!"...ummm..not so fast! There might be a load of carbon build up in #4 creating a smaller combustion chamber.

Now an engine with all low numbers? Probably has a lot of miles or was not correctly cared for. A 'fresh' rebuilt engine? Yeah I'd expect higher numbers across the board. If it's been broken in correctly? Should show at the upper level of what the shop manual states.

What this means to the average joe mechanic? Quite a bit if one understands the relationship of the numbers, and can also be combined with a "Leak Down" test for cylinders that are running lower then expected. 15% is a good rule of thumb, as I have yet to compression test an engine with the same number on all cylinders. There will always be some a whisker higher..and lower.. That's just the nature of the beast.

For what it's worth? 120-130 lbs of compression? That's decent for an engine that has a few miles on it, and is still in good shape.

For new people researching how to do a compression test? Four items that can really hose the test:

1) Make sure the ignition is DISABLED!!! (you'd be surprised how many people forget this)

2) Remove ALL spark plugs.

3) Block the throttle to WIDE open.

4) Ensure the battery has a full charge, and in some cases have it on a charger as well to ensure max. starter current. You'd hate to get 1/2 way into the test..only to have the starter get weak. *sidebar note* always allow the starter a fair amount of time to 'cool' between each test run.

It's also a good time to either swap out the plugs for new...or at the very least examine the plugs for signs of problems, or normal wear.

Hope this helps

S-

 

i did everything except that. wont opening the throttle cause fuel to go into the cylinders and seal them up giving me an innacurate reading? i did it with the throttle just in the idle position. and then afterwards i wanted to squirt some oil in the cylinders to see if the readings would end up being any different but i didnt get around to it.

maybe i am just confused and "wot" is how its supposed to be done because in order to compress something you have something to suck in through the intake to compress? maybe it would be like trying to breathe in outer space?

 

With the throttle at WOT the air flow is so low that the carb will not supply any fuel into the air stream. With the throttle closed you can pull fuel though thru the idle circuit. It takes vacuum to do that and you shouldn't have enough vacuum at WOT and cranking speed to pull any fuel. What the WOT does is eliminate any air restrictions on the intake side so you get the real numbers of what each cylinder will pull.

 

Is it possible to do a compression test with the engine (390) on an engine stand by turning it with an air impact wrench? The torque converter and fly wheel are off the engine so I can’t turn it with the starter (I believe the engine stand would prohibit the fly wheel from being installed).

 

The engine stand shouldn't prevent the flywheel install, but the starter mounts to the bellhousing on the tranny so it does really matter. Don't use an impact wrench, as hit hits the system all the time with a bunch of hammer strikes and doesn't rotate things smoothly. Use a drill motor which acts like the starter motor when it comes to stresses on the rotating parts of the engine.