ULTM8Z

Is this too low for a 350 Chevrolet with 10:1 compression and a Comp Cams 262 roller?

All my cylinders exhibit this cranking pressure during compression testing and leak down tests all pass with flying colors. If 120 psi is too low, what could be the cause?

fun Pain

cause what? Pressure seems ok, they're all about the same...

ULTM8Z

In the DIY forum I've been posting for a very long time about a rough idle with very lousy emissions numbers. (and yes, I've gone through every possible scenario on ignition, fuel, EFI system... everything!). I've verified beyond a shadow of a doubt that I have fuel and spark, so the last piece of the puzzle is compression.

After I opened this latest thread today, I've been reading around the internet and other people with similar cams and compression are getting on the order of 160-180 psi. Since all cylinders are the same it makes me wonder if my cam timing is off or something- intake valve staying open too late and bleeding off compression during the compression stroke. That much of a loss can't be having an insignificant impact on combustion... Anyway, I just wanted to run it by you guys here...

84z28350

To me that seems a tad low for a 10:1 motor, but then again cranking pressure is dependant on so many things its not even funny...

If it passes a leakdown test 100% i wouldnt worry one bit about it. 120psi should be still enough to keep a motor running healthy.

Guest

All plugs out, WOT. Are you doing that?

ULTM8Z

Actually no.


I guess I need to rerun it.... Should I expect the cranking pressure to go up?

fun Pain

yes redo, without the throttle open, only the min. air will be allowed to enter. And with the plugs out it will spin faster. (overlooking some of the cam). That will boost the numbers.

ULTM8Z

I'll certainly redo it. But, if you spin the engine over enough times, the cylinder will bring in as much air as it possibly can, I would think. My compression gauge works on a cummulative basis- that is, it will add each compression stroke until it reaches a maximum and the cylinder is not pumping in any additional pressure.

Wouldn't the WOT and all plugs out would only affect it if you were to do only one or two rotations?

mnorton

Pull the plugs, prop open the throttle blade, charge the battery, and spin each cylinder four times. My engine with 150K mi. was 160 psi.

Viprklr

iTrader: (5)

Yes, redo. It does make a difference. 120 seems too low. I was pulling 180 cold on all 8 cylinders on my 92 with 92K.

ULTM8Z

I just read that a factory ZZ4 pulls about 190 psi. It'd be curious if pulling the plugs yields me another 70 psi.

88 IROC CONVERT

cam overlap on a modified engine makes compression readings inaccurate. I think the best thing to do is
1) take readings on all 8 cyl and compare all about the same should be ok.
2) do a leakdown test, you will need a compressor, back off the rocker arms, hook air line and gage with shut off to the cyl , pressurize cyl and close valve. and wait. do all and compare. good luck

ULTM8Z

Don't know if you read the whole thread...

Leak test was done successfully. Cam is very mild- 212° intake, 112° LSA. Not much more radical than a ZZ4. Static compression is 10.1:1. I'd suspect the readings need to be pretty close to a ZZ4 if my engine is setup correctly. The more I'm reading, 120 psi is way too low for my engine combo (though I still need to rerun the test and verify).

5678TA

my machinist's camaro runs at 120psi cranking.. 355 with 10:1.. its a nitrous motor but still runs 12.7 off the bottle.. it is kinda low but facts are facts.. it can work great..

ULTM8Z

Well, I pulled all the plugs out and opened the throttle blades...

180-190 psi on all cylinders.

Damn... I was hoping I found the problem.

So I have fuel, spark, and compression on all cylinders, but a lousy idle. Go figure.

justangel

My camaro has low compression and at a shop they told me that the engine was done. Should I just get another engine or rebuild mine? Need some advice on this
New camaro owner

bop11

The reason you need to spin fast and be at WOT is that moving air has inertia. The compression of the engine is not just the Volume at the bottom of the stroke divided by the volume at the top of the stroke times 14.7 (normal air pressure) as you were taught in school. With the throttle closed you are creating a vacuum in the intake which will act to reduce the amount of air entering the cylinder reducing the compression ratio. With the engine spinning fast, the air momentum, (velocity times mass) will cause more air to flow into the cylinder during the down stroke than would if the down stroke was slow. These two factors combine to raise your compression by a factor of 1.5. This is the lowest the car sees when it is running, if the exhaust system is designed as a good extractor.
Those compression numbers look very good. Could there be a small vacuum leak or exhaust problem?

ploegi

What injectors you using? What is base timing set at? Done any tuning for the cam/compression? Whats manifold vacuum look like at idle?