Re: What compression at idle..?

Posted by Hobo,NC on July 05, 2011 at 14:28:43 from (71.30.82.155):

In Reply to: What compression at idle..? posted by JBMac on July 05, 2011 at 07:52:04:

Quoting Removed, click Modern View to see

Ran across this years ago and it has served me well

The volumetric efficiency test or running compression test

is a very good test to use any time you have a engine miss

that can't be traced to ignition or other basic systems or

when there is a lack of power.

Step 1. Perform a cranking compression test with all plugs

out and the throttle open.

Step 2. Record your cranking compression readings. Make

sure the charging system was up to snuff so all cylinders

were tested under the same conditions.

Step 3. Install all plugs but one. Install a standard

compression gauge in the remaining hole with the shrader

valve in place.

Step 4. Start the engine. Yes, it will run but will be

missing on the cylinder with the compression gauge. Burp

the gauge and allow the reading to stabilize. It will be

bouncing around at about 50 to 60 psi. (the throttle is

causing a restriction and the piston is moving fast

compared to cranking speed causing only a partial fill of

the combustion chamber).

Step 5. Snap the throttle wide open and return to idle. The

gauge will hold at the peak reading. This reading is higher

than idle because the peak comes at the instant the

throttle restriction is removed and the piston speed is

relatively slow.

Step 6. Record your readings for running snap compression

for all cylinders. The running compression reading should

be approx. 80% of cranking compression. (example: cranking

compression = 150psi running compression should = about

120psi)

If your running reading is below 80% of cranking

compression, the intake system is causing a restriction. If

the running reading is above 80% of cranking compression,

the exhaust system is causing a restriction. If the problem

is in one cylinder you either have a problem with a worn

camshaft, broken spring/rocker, carbon build up, etc. If

the problem is on all cylinders look for a restricted

intake system or clogged cat/muffler. The 80% spec is a

ballpark spec and should be used as such. Remember that

cranking compression is a measure of cylinder seal while

running compression is a measure of volumetric efficiency

or the cylinders ability to draw the air/fuel mixture into

the combustion chamber.

This is the readings from a Flathead ford V8

cranking

Test Numbers, first hit/ fifth hit on gauge

1) 80/120

2) 50/125

3) 75/120

4) 75/110

5) 75/115

6) 65/100

7) 85/125

8) 75/120

Run'n

1st number running compression 2nd # blip throttle

1) 50/115

2) 50/115

2) 50/100

4) 50/100

5) 50/100

6) 50/100

7) 50-115

8) 50/115

No need to go farther with any cylinder efficient test,,, problem was (dual carb set up) one barrel was stopped up.. I would have bet money it was a cam are valve issue before the test...

Replies:

• Re: What compression at idle..? pete 23 14:53:51 07/05/11 (0)

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