Cylinder #1 = 175 psi

Cylinder #2 = 174 psi

Cylinder #3 = 180 psi

Cylinder #4 = 180 psi

Cylinder #5 = 176 psi

Cylinder #6 = 182 psi

Cylinder #7 = 175 psi

Cylinder #8 = 179 psi

Now, assuming the rings were gapped correctly, would the unported CC of my Vortec heads create enough difference between the cylinders?

that looks pretty good

that looks pretty good

Would it be worth (on my next motor) actually doing this test and taking the heads off and taking some material off the CC to get those psi numbers closer or would that be too much effort for little to no gain?

You can rest assured when you lay your head down on the pillow tonight that your readings are darn near about as perfect as you will ever get them. Tomorrow do the test again, only in reverse order from what you did the first time, and at a different temp. Then try and figure out what from where.

They will never be perfect.

Thats a good question cam Those #'s look good but there is something to be said for perfection.

With a spread of 8 psi or less than 5% don't worry about it.

The spread looks great to me. IMO you never want to see more than 10% diff between any two cyl. and 5% is better for high performance engines.

You spread is real close. It's pretty safe to say you'll need get all 8 cyl to read the same.

Like Rick said, do the test a few more times.. Heck just pick one cyl out and re test it 5 times.. With diff engine temps, weather cond., etc and you probbly won't get the same results twice

Well then I guess I will let it go. One less thing to worry about (as long as it turns out similar on the next go 'round!). Thanks for the responses. :D

IMHO, the numbers you posted are well with spec.

Was the engine up to operating temperature? Were all the spark plugs removed before testing began? Was the throttle held wide open during the test? And were the same number of "hits" done on all the cylinders? If any of these items weren't done, it could affect the end numbers.

Cylinder #1 = 175 psi

Cylinder #2 = 174 psi

Cylinder #3 = 180 psi

Cylinder #4 = 180 psi

Cylinder #5 = 176 psi

Cylinder #6 = 182 psi

Cylinder #7 = 175 psi

Cylinder #8 = 179 psi

Now, assuming the rings were gapped correctly, would the unported CC of my Vortec heads create enough difference between the cylinders?

Thanks for bringing up this topic. I was thinking of doing the compression test on each cylinder as well. Unfortunately I don't know how to do it.

I really appreciate it if you could write down the steps in order to do the test.

Thanks

You can buy a compression tester gauge and hose at just about any auto parts store for $20-35

To use the gauge you just pull the spark plugs out, thread the hose for gauge into a spark plug hole, snap gauge onto hose.. Hold the throttle wide open and spin engine over with the key/starter for about 5 full turns.

Look at the gauge and write down the # the needle is sitting on and what cyl that you just checked.

Now do the other 7 cyl one at a time just like that, and when done look at your readings. If you have one or two cyl. that's more than 10% lower than the highest reading cyl, trying squirting alittle straight 30 weight engine oil into those weaker cyl through the spark plug hole, and re test.

If the reading comes back a good bit higher now, then that means that cyl has worn rings. If still low then its a valve prob. (valve not fully closeing)

72Z/28,
Do the test on all cyls dry and then again wet (a squirt of oil in the hole) if the numbers come up signifigantly you are leaking past the rings. a BIG difference, and its time for a ring and hone or a bore job.

Mark

what is a good compression reading for the street, mine was about 203-210 before? i am know putting a roller cam with sam duration buy more lift.

thanks

what is a good compression reading for the street, mine was about 203-210 before? i am know putting a roller cam with sam duration buy more lift.

thanks

the engine being seal'd up is the most important. and that they're all close to equal as mentioned earlier above. 200lbs is definately a pump premium number at the minimum. the older chilton full size motor manuals show 200lbs for the old LT-1 and LS-6 for back when 100+ octane was common at the pumps in the very early 1970's. we're doing it today on less octane. i personally think they were overkilling it for warranty purposes from g.m.

i've ran up to 150-160lbs on 87 octane,, and 160-165 on 89 octane,, and 170-185 on 91 octane with no problems. folks here have done more pressure with less octane. i think those are safe ranges to be in. i don't see any reason to push the envelope for a street engine.

but this can open up a can of worms for sure. i can think of 3 on here running like 210-215+ on 93 octane. i think that's pushing it a bit for the average guy just getting into the sport....... no room for error on pump gas there.

dynamic compression (meaning while running) comes into play with the volumetric efficiency at the torque peak. as well as many many many other factors. you might end up with more than your actual cranking compression at the torque peak with a well set up combo that achieves more than 100% v/e. hope it helps. jmo.

IMHO, the numbers you posted are well with spec.

Was the engine up to operating temperature? Were all the spark plugs removed before testing began? Was the throttle held wide open during the test? And were the same number of "hits" done on all the cylinders? If any of these items weren't done, it could affect the end numbers.

Actually my test was done on an engine stand! I blocked off my oil pressure fitting and then used a priming tool and drill to prime the motor up since it has been sitting for 6 months (I shoot WD40 into the spark plug holes once a month and cover it back up). Once primed, I stuck an old distributor in, wheeled the engine stand over to my daily driver and hooked the wires up to my truck battery while it was running, using a switch to turn the starter on and off.

I started with the right bank, front to back and moved to the left side and repeated. I made sure to turn the motor over about 6-7 times to get peak pressure each time. Apperantly I have pretty good oil pressure too because at one point my home-made cork (in the oil pressure fitting) shot out and sent a stream of synthetic 10 ft in the air! lol

you also mite try an losen,or tighten the vavles of the cylinder that you are testing,and see the number jump,or fall off.
I have done this to find that i had some valves to tight and I got more compression when I backed them off a bit.
hydralic cam btw.

sent a stream of synthetic 10 ft in the air! lol

If this is a fresh engine (new rings/hone) Do not run the engine with synthetic oil for the break in. The rings will not seat and you'll have a smoker for the rest of the time you own the engine.

what is a good compression reading for the street, mine was about 203-210 before? i am know putting a roller cam with sam duration buy more lift.

thanks

That's real good compression. Most high performance street engines are in the 180 to 195 psi range. More is better but once you get over 200 psi you have to get real picky about your timing, A/F ratio, inlet temp, etc on pump 93 octane. Detonation can be a prob at that psi.

With that said I'm running 212 to 218 psi cyl pressure on my iron headed 355 but I had to dail it in just right to keep it from pinging it's self to death. So far close to 3 years on this engine w/o any probs on 93 pump fuel.

If this is a fresh engine (new rings/hone) Do not run the engine with synthetic oil for the break in. The rings will not seat and you'll have a smoker for the rest of the time you own the engine.

It's actually got about 3,500 miles on it. I broke it in with regular oil and then after two oil changes switched over to synthetic.

Cam -- as has already been posted, those #s look good cylinder to cylinder. I don't think you have anything there to worry about.

Cam... Oh ok, cool deal. Just wanted to point that out about synthetic oil in case you hadn't broke in the engine yet