Compression Test - Impala Tech
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post #1 of 4 (permalink) Old 07-16-2010, 09:50 AM Thread Starter
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Compression Test

Emery asked me a very good question. One I though the rest of the board could benefit from. So I am reposting his PM to me to share with the board.

Quote:
Originally Posted by iceman
Can you tell me the cylinder psi on a stock 1992 350 Chevy. I started a compression check . on cylinders 1 and 3 I have 87 and 90 psi.
the problem is this. the motor runs and idles perfect, however sometimes but not all the time when I start the car blue smoke comes out of the exhaust then quits.the motor runs good but the smoke is annoying.
I was thinking on getting the heads rebuilt hoping it would stop the problem. im thinking its the valve seals. Do you think i need to also replace the rings.
also, all the plugs showed alot or some sign of oil.
...
Emery
The actual static pressure is 14.7 times the compression ratio. So a stock 8.5:1 1992 350 would have 126 pounds of pressure if the intake valve closed at bottom dead center on the compression stroke (which it doesn't as it takes time to get air to start to move once the valve opens). This is why racing cams sound lumppy. They hold the intake valve open longer than a stock cam to give the air a chance to flow into the cylinder at high engine speed (air is a fluid just like water so you can not get it to instantly fill a gallon jug; just as with water, just because you turned the tap on it takes some time to fill the jug). Because the valve is open and the piston is trying to compress air in the cylinder by moving up the bore; at idle, the air is being pumped back out the intake instead of coming in, because the valve is left open. It is that lack of a full charge in the cylinder that looses you power to pull away from the stop light unless you rev the snot out of the engine (a good way to draw the attention of a cop). It is that air pushing up past the carb interrupting the vacuum signal that makes the car sound like a race car with a rough rumpity rump.

I told you all of this to point out that dynamic compression (what you measure with a gauge) is always less than the static pressure; because the actual compression of the charge doesn't start until the intake valve actually closes. So a reading of 87 to 90 pounds is about what I would expect. What you are looking for with a compression test is that every cylinder has the same measured compression number (or with-in 10% of it). If you are saying they are all in that range you have a very good motor and the rings are not the problem.

Chevy got away from the original rubber umbrella design that squeezed excess oil off the valve stem in the late sixties to save a fraction of a cent per part. They went to rubber oil ring oil slinger that usually gets brittle and breaks off from heat cycling. I will wager all of your oil is coming into the chamber by way of the valve guides.

Big Dave
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post #2 of 4 (permalink) Old 07-20-2010, 01:38 PM
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Great breakdown Big Dave. This is all dealing with what is called volumetric efficiency. The goal of choosing a cam for a N/A engine involves a number of factors, but the main goal is to fill the cylinder with as much mixture as possible for the given use of the vehicle. For the street you need some low end torque and a good power band throughout your RPM range, so you want to shy away from big overlap cams with tight LCA's. ("lobe center angles").

The issue of some of the intake charge backing up past BDC is called 'intake reversion'. The charge doesn't actually go past the carburetor though, it simply mixes back into the plenum and goes to other cylinders on the intake stroke. This reversion is very minimal but when running cams like this it will play games with power brake boosters due to lower idle vacuum. When you get into tight lobe center angles on a race cam you can run into exhaust reversion that can be problematic, causing intake charge contamination. Most average street guys don't have to worry about this.

Another thing that affects advertised compression ratios of a piston is the size of the chambers in your heads.
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post #3 of 4 (permalink) Old 09-03-2010, 09:09 PM
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Quote:
Originally Posted by Big Dave View Post
Emery asked me a very good question. One I though the rest of the board could benefit from. So I am reposting his PM to me to share with the board.



The actual static pressure is 14.7 times the compression ratio. So a stock 8.5:1 1992 350 would have 126 pounds of pressure if the intake valve closed at bottom dead center on the compression stroke (which it doesn't as it takes time to get air to start to move once the valve opens). This is why racing cams sound lumppy. They hold the intake valve open longer than a stock cam to give the air a chance to flow into the cylinder at high engine speed (air is a fluid just like water so you can not get it to instantly fill a gallon jug; just as with water, just because you turned the tap on it takes some time to fill the jug). Because the valve is open and the piston is trying to compress air in the cylinder by moving up the bore; at idle, the air is being pumped back out the intake instead of coming in, because the valve is left open. It is that lack of a full charge in the cylinder that looses you power to pull away from the stop light unless you rev the snot out of the engine (a good way to draw the attention of a cop). It is that air pushing up past the carb interrupting the vacuum signal that makes the car sound like a race car with a rough rumpity rump.

I told you all of this to point out that dynamic compression (what you measure with a gauge) is always less than the static pressure; because the actual compression of the charge doesn't start until the intake valve actually closes. So a reading of 87 to 90 pounds is about what I would expect. What you are looking for with a compression test is that every cylinder has the same measured compression number (or with-in 10% of it). If you are saying they are all in that range you have a very good motor and the rings are not the problem.

Chevy got away from the original rubber umbrella design that squeezed excess oil off the valve stem in the late sixties to save a fraction of a cent per part. They went to rubber oil ring oil slinger that usually gets brittle and breaks off from heat cycling. I will wager all of your oil is coming into the chamber by way of the valve guides.

Big Dave
This is a little confusing in that "static" is what you measure with a gauge and "dynamic" is calculated.
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post #4 of 4 (permalink) Old 09-03-2010, 10:55 PM Thread Starter
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You cam measure the dynamic compression as well. You just have to machine the head to accept a pressure transducer that is either epoxied into the combustion chamber wall, or is inserted in on the end of a special spark plug addapter. CompCams, Dart, and Edelbrock are doing research research on improving their CNC cylinder head programs using this technology along with the use of a Spintron that takes the actual combustion process out of the equation so there isn't any thermodynamic fudge factors used if all you want to study is chamber flow and wave harmonics.

Big Dave

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