low cranking psi ??? - Team Camaro Tech
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post #1 of 7 (permalink) Old Sep 4th, 02, 01:56 PM Thread Starter
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Getting an average cranking psi of 74psi on a fairly fresh 350cid, bored .060 over. Did a leakdown test and the minimum loss was 8%. Maximum loss was 10%. Engine has late 70's smog heads and flat top pistons. The cam IS too large for the setup (NOT my doing) but I can't imagine it would be the sole factor in the psi being so low. Power falls off sharply around 2500-2700rpm and does not come back.

Any ideas?


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post #2 of 7 (permalink) Old Sep 5th, 02, 08:17 AM
David Pozzi
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Be sure to block the throttle blades open when doing a compression test.
If the car has anti-pumpup lifters, make sure they are not too tight. Most require a half turn from zero lash, some only take a quarter turn from zero lash!

Make sure you haven't got a flat lobe or two...

Ignition adavance and fuel supply are critical to higher rpm's.

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post #3 of 7 (permalink) Old Sep 5th, 02, 06:20 PM
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cranking compression of 74 is extremely low as you already know. Do you know if you cam was installed retarded? Even if 7 degrees retarded which would have to mean it's using the crank retarded keyway and a -3 degree offset bushing on the cam that should only account for around a 30 psi loss in cranking pressure. Also with the late valve timing the car should have improved top end instead of no top end. You'd probably be having some clearance problems as well, but maybe not. So I don't think its a cam timing problem.

If I understand you right there is only 8-10% variances from the highest and lowest cylinders? If this is the case it probably isn't a single or couple of wiped out lobes, but it sounds like you've got a severe restriction on the intake of all the cylinders(restricted exhaust would result in higher than normal readings) or your compression height on your pistons wasn't checked during the build up. David's right about the throttle plate, make sure it isn't closed. You can try a dynamic compression check to check the individual breathing of each cylinder, the cranking or static check just checks the sealing ability of the engine, not the breathing ability. If you'd like to know how to do the dynamic check let me know, I've found bad cams that way, but again it doesn't sound like you have an individual cylinder problem, if they are all that low it has to be a restriction at the carb or intake system I'd think. Either that or someone didn't check the compression height of your pistons when they built the motor and you've got the wrong pistons,rods and crank combo resulting in too low of compression height and you really are only putting up numbers like 74 psi.

[This message has been edited by SY1 (edited 09-05-2002).]
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post #4 of 7 (permalink) Old Sep 6th, 02, 06:23 AM Thread Starter
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Thanks for the response guys.

The motor came with the car so I'm not clear what went on during the build process.

I did have the blades open during the compression test. Fuel delivery shouldn't be a problem. Running a 600cfm Edelbrock 4bbl that is freshly (and correctly) rebuilt. Intake is an Edlbrock Performer. The distributor is also a new HEI. I'm using an MSD 6AL in place of the module.

SY1, yes, 8-10% varience between all cylinders. It was only 10% on one cylinder (#6 if I remember right), the rest were ~8% or a little over that. I suppose it's possible that there is a bad cam lobe or two. It idles like it's got a too-large cam in it but maybe I'm mistaking the lope for a wiped out lobe??? Possible?

I do get a little puddle of oil on the intake near #6 when the car runs. Vacuum at idle is about 15Hg...shouldn't be that high with a big cam. Supposing I'm losing 30psi due to the cam; it still doesn't explain where the other 50+ psi of cranking pressure went. Must be more than just the cam.

Well, one step at a time. SY1, if you could tell how to do the dynamic check I'd appreciate it.

Thanks again, guys!

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post #5 of 7 (permalink) Old Sep 6th, 02, 06:46 AM
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Could you borrow another compression tester? I'm not sure it would run with that low of cranking compression. Possibly your gauge isn't reading correctly.


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post #6 of 7 (permalink) Old Sep 6th, 02, 06:18 PM
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I agree with Jody. Try another gage first, at least you'll know if low compression is really the problem that way and not another culprit. 74 is just too low, especially if you're pulling 15" of vacum at idle. That tells me it's breathing okay and you'd probably be wasting time doing the dynamic check. My 331 with a .539 lift solid cam and a huge plenum crossram pulled around 7" at idle, and idle was set very high on this car, it wasn't a street car. 15" leads me to believe you're breathing fine at idle. Make sure the engine was up to operating temperature before doing the test.

Here's the dynamic check anyways.

1.Write down all the cylinders static (cranking) compression figures.
2.Hook up all your plugs except one. Put the compression tester in that cylinder and remove the Shrader valve from the tester. You can leave it in and bump it every couple of revolutions to get the same effect. Ground out the spark plug lead for the cylinder being tested so you do not damage the ignition module.
3.Start the engine and record the reading while engine is running and write it down.
4.Next snap the throttle as quickly as possible using the carb throttle linkage, not the accellerator. You want to snap it all the way open and closed without allowing it to accelerate. This allows the engine to take a big breath of air, but closes back to idle before it can respond.
5.Write down the reading noted during step 4.
6.Do this for all cylinders.

Dynamic (running) compression should be half of what the Static (cranking) compression values are, say 70-90 versus 140-180 for example. The snap throttle value in step 4 should be 80% of Static (cranking) compression.

If Snap value is much lower than 80% of Static then you have a restricted intake or supply problem. anything from an intake lobe or rocker or lifter. If Snap value
is much higher than 80% you have an exhaust restriction, same thing here exhaust valve restricted, plugged convertor or exhaust, collapsed lifter. what you'll need to determine is how many of the cylinders are high or low and that will tell you if it's an individual cylinder problem or one bank of cylinders has a problem or if they're all high or low together. The results will give you a starting point on where to look.

Like I said normal Static compression chekcs only check the sealing ability of the pistons and valves, not the breathing. Vacum gage tests look at how the entire engine is breathing, not individual cylinders.

You say you're reading 15" vacum at idle. If atmospheric is 29.92 for a standard day, (I don't know what the exact baro was the day you checked it, but 29.92 is considered the standard for horsepower and thrust ratings at sea level) the difference 29.92-15=14.92. This is what the engine is drawing in. 14.92" of mercury = approximately 7.5 psia. If you compress it at I'm guessing 9:1 considering flat tops and 76cc chambers, 7.5 x 9 = 67.4psi and this is what you'd expect to see on your Dynamic, running, compression check.

The snap test causes the manifold vacum to drop further and the air pressure in the cylinder increases.

Since all your readings are fairly close 8-10% I don't think the dynamic test is going to teach you anything, because I don't think it is an individual cylinder problem you have. I'd try a different gage and maybe look at you compression height of your pistons. Possibly ALL your valves are lashed too tightly or your cam timing is way off? It's hard to say.

Good Luck,

[This message has been edited by SY1 (edited 09-06-2002).]
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post #7 of 7 (permalink) Old Sep 8th, 02, 04:40 AM Thread Starter
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Jody, good point on the tester. I will try and get my hands on another one.

Dave, thanks a ton for the info. At least I now have a reasonable idea of where to start looking.

I know there is no exhaust restriction...I've already replaced everything from the headers back. The old system had glasspacks (yuk) that were blown out and causing a restriction!

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