Compression Testing and Results

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May 24, 2006 | 11:09 AM

tractoman

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From: Sonoma, CA

Compression Testing and Results

After a dry compression test, I added a capful of oil to the lowest cylinder and tested it wet. It came up 5 psi. to 130 psi. I was expecting a larger increase if it has tired rings or is this a valve issue. I know a leakdown test will tell but I do not have the equipment. I tested cold, plugs out WOT. It's a 76 F250 w 460 C6
Thanks

May 24, 2006 | 03:41 PM

Bear 45/70

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From: Union, Washington

First off what are the rest of the numbers? There is a 15 percent (percent not psi) gap between high and low numbers that are acceptable. And a 5psi increase can or can not be normal. A truely proper compression test is done with a warm engine.

May 24, 2006 | 04:22 PM

tractoman

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I had 4 at 140, 2 at 135 1 at 130 and 1 at 125, so I am within the 15% margin. It doesn't smoke ir miss but I may be thinking about doing a valve job if it will even things out more. I have read and been told that a cold test is the truest way to check the condition of the engine since things have not heated up yet. I also get about 18 Hg on my vacuum gauge from the manifold port.

May 24, 2006 | 10:24 PM

Bear 45/70

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First off the engine spends very little time running cold, so checking compression cold is the fast, lazy way. With your numbers it will be expensive for the minor improvement you would see. If the manifold vacuum is steady when idling I would doubt you have a real valve problem. If the port is reading 18" of vacuum at idle it is not the ported vacuum the dist. needs to be connected to.

May 25, 2006 | 10:44 AM

tractoman

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Yes it is 18Hg at idle at the manifold port on the intake manifold not the ported vac port on the carb for vacuum advance and yes the vacuum advance is connected to ported vacuum port on the carb. In checking the engine cold I believe I would be getting the lowest compression results which is what I want to get a handle on the engine condition. I have not had it long and want to get it in good reliable condition.

May 25, 2006 | 01:20 PM

monsterbaby

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From: iowa

out of curiousity could you list what cylinders had what compression. raising it 5psi could have just benn the oil taking up room in the cylinder so thats not telling you anything, and if your vacumn is steady you at a min don't have an intake valve problem, and with the cyl within 15 psi you most likely don't have an exhaust problem either (you would be getting a popping sound out the exhaust when it was running if you had much of one anyway) but a block that has warped (not uncommon don't get concerened) can change cylinder cold preasure test the amount you listed. To give you an example, when I did the testing on my deck hieght with my stroker if I hadn't squared the deck the compression ratio on number 1 cyl would have been 13.23:1 the compression on #4 would have been 12.52:1 almost 3/4 of a point of compression which would make a real difference in a compression test, and the amount that the block was out of square was not excessive nor even close to the worst one I have ever seen that still ran decently. It's also the reason I always tell guys building engines to square deck the block as it will smooth the engine out and give you better power.

May 25, 2006 | 01:31 PM

tractoman

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My test was as follows:
Cylinders 1,2,4 and 8 at 140, cylinder 3 at 125, cylinders 5 and 6 at 135 and cylinder 7 at 130. Again, done cold with all of the plugs out, throttle wide open and about 5-6 revolutions for each cylinder. After the initial test I added a capful of oil to #3 and tested it again and it came up to 130. Are these numbers OK for a stock 460 with who knows how many miles?