396 Compression Test. - Chevelle Tech
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post #1 of 12 (permalink) Old Sep 30th, 13, 6:16 PM Thread Starter
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396 Compression Test.

My recently purchased 1968 396 Chevelle is using a bit of oil. I figured I'd have to get the engine rebuilt. I did a compression test yesterday (engine hot, all plugs removed, throttle wide open). 2 cyclinders were between 165-170 PSI. All others were around 180. I was suprised at the readings as I expected much worse. Am I correct in my assumption? Next check would be the heads for the oil consumption. Any comments or advice would be appreciated.

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post #2 of 12 (permalink) Old Sep 30th, 13, 8:29 PM
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Smile Re: 396 Compression Test.

Originally Posted by billbonn80 View Post
My recently purchased 1968 396 Chevelle is using a bit of oil...
How much is "a bit"? BBCs are known to use "a bit" of oil.

Any leaks?


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post #3 of 12 (permalink) Old Sep 30th, 13, 8:31 PM
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Re: 396 Compression Test.

Probably from the valve guides/seals...

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post #4 of 12 (permalink) Old Sep 30th, 13, 9:15 PM Thread Starter
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Re: 396 Compression Test.

Roughly a quart every 200 miles.
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post #5 of 12 (permalink) Old Sep 30th, 13, 9:43 PM
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Re: 396 Compression Test.

Originally Posted by billbonn80 View Post
Roughly a quart every 200 miles.
Do a couple of checks :

1) When you start it after sitting overnight, does it smoke and then clear up ?
That would indicate worn valve seals.

2) Get someone to follow you down the interstate. At 60-70 mph, let off the throttle totally. If it smokes, that indicates bad rings.

I'm thinking you need to rebuild your heads or better yet, buy new aluminum ones! It will take 100 pounds off the front end.

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post #6 of 12 (permalink) Old Sep 30th, 13, 9:58 PM
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Re: 396 Compression Test.

1 quart/200 miles is a lot.Get it hot and repeat the test but do a wet compression test,if the numbers increase by 10% or more,its got worn out rings.
Wet test add a couple of squirts of oil from an oil squirt can and then check compression again,compare the results to the prior test.

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post #7 of 12 (permalink) Old Sep 30th, 13, 10:45 PM
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Re: 396 Compression Test.

Do a leak down test near TDC and at TDC see what happens.
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post #8 of 12 (permalink) Old Oct 1st, 13, 8:25 AM
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Re: 396 Compression Test.

leakdown x2

back in the 80's a friend asked me to go 50 miles to help out with his moms car. we did a compression test and a 4 cylinders read good. after more diagnostics, we removed the head. to our surprise one of the pistons had quite a lare hole in it!

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post #9 of 12 (permalink) Old Oct 1st, 13, 12:03 PM
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Re: 396 Compression Test.

Originally Posted by billbonn80 View Post
Roughly a quart every 200 miles.
That has to be rings, never saw valves lose that much oil. Did it sit for a long time?

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post #10 of 12 (permalink) Old Oct 1st, 13, 3:11 PM
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Re: 396 Compression Test.

Using a compression tester to check the basic condition of an engine is better than nothing, but it is NOT the best choice. Compression test results can be inaccurate and inconsistent. This is because the condition of the battery, variations in ambient temperature, and the effects of the presence or lack of oil (if the engine has been sitting) around the rings which helps seal them, can all affect the results. This makes a compression test only a mediocre test at best.

Leakdown testing is by far the BEST METHOD for checking an engine’s basic condition. It is done by checking each cylinder at TDC of the compression stroke. And any leakage heard, helps to pinpoint where any problems are located. Air leaking out of the carb indicates a leaking intake valve. Air leaking out of the exhaust system indicates a leaking exhaust valve. Air leaking out of a breather indicates ring leakage. And air leaking out of the radiator cap opening indicates a leaking head gasket.

I’ve tested the 3 different types of leakdown testers.

One is a single gauge tester that reads leakdown percentage directly. This one is NOT recommended because its accuracy is typically not the best.

Another one is a dual gauge “low input pressure (typically around 35 psi or less, depending on the particular unit)” type that has one psi gauge and one gauge face that shows leakdown percentage directly. These are usually fairly inexpensive, and are also NOT recommended because of their typical inaccuracy.

And the last type is a matching dual psi gauge “high input pressure (usually can go up to 100 psi)” type. This type is convenient to use, and has good accuracy, making it clearly the best of the 3 leakdown tester types. So, if you decide to get a leakdown tester, do yourself a favor and get this type.

Note: Input pressure can be referred to in two ways, static and dynamic. Static means you set the tester’s regulator to the desired input pressure, say 80 psi (more on that below) with the tester NOT connected to the engine yet.

Then once you do connect the tester to the engine, the pressure will drop somewhat, becoming dynamic input pressure. You can then readjust the regulator to bring that dynamic input pressure back up to the original 80 psi, if you want. But I’ve found no difference at all in the final leakdown percentage results between doing that, or just letting the pressure drop somewhat and leaving it there. So, the most convenient method is to simply set the tester’s static input pressure to 80 psi and simply leave it.

The way to get to the final answer for a given test is:

For example, after you connect the 80 psi static pressurized tester to the engine, the left side regulator controlled gauge may say something like 70 psi after it drops, while the right side engine leakage gauge may say something like 65 psi.

You just plug a few numbers into your calculator, in the following manner:
You ask yourself, 65 psi on the leakage gauge is what % of the 70 psi on the dynamic input pressure gauge? And you punch into the calculator 65/.70 (don’t forget that its “point” 70 here) and the answer comes up 92.8, which means that the right side leakage gauge is showing or holding 92.8% as much as the left side input gauge. And because the original 70 psi dynamic input pressure was 100% of the dynamic input pressure, you simply punch into your calculator 100 – 92.8 = 7.2% leakage in that cylinder, which is your final accurate answer for that cylinder. That’s all there is to it.

For those who don’t use much math, that may seem like too much trouble. But if you read through what was done a couple of times, and then actually do it a couple of times, you’ll see that it’s no big deal at all. And you’ll be crunching the numbers freely after the first couple of cylinders.

There is no universally accepted input pressure for automotive leakdown testing. But the FAA (Federal Aviation Administration) has established 80 psi input pressure as their standard for leakdown testing on piston aircraft engines. And they allow up to 25% leakdown in those aircraft engines.

That 80 psi input pressure works perfectly fine for car engines too, so I use that as my tester’s input pressure as well.

And the reference chart I use for COLD leakdown testing on High Performance Engines is:

0-10% = good condition

10-15% = though not ideal, still acceptable

over 15% = tear down and repair recommended for optimum performance

(for non-performance daily driver/grocery getter type vehicles, over 30% = tear down and repair recommended)

As a point of reference, my 540ci BBC Street/Strip engine shows a COLD leakdown of about 3%, using conventional Speed Pro rings, with a top ring end gap of .021” and a second ring end gap of .027”. And keep in mind that anytime you do a leakdown test, at least with conventional rings, you will hear some air leakage. Even for the small amount of leakdown that my engine shows, I can still hear some air leakage hissing out of the breathers, from the ring end gaps.

Here’s the excellent leakdown tester that I use and like real well. It’s from “Goodson Tools and Supplies for Engine Builders”


On another note, if you’d like to see my entire 100+ motor oil “Wear Protection Ranking List”, along with additional motor oil tech info, here’s a link:


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post #11 of 12 (permalink) Old Oct 1st, 13, 3:17 PM
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Re: 396 Compression Test.

What do you do if you don't have a compressor?

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post #12 of 12 (permalink) Old Oct 1st, 13, 3:54 PM
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Re: 396 Compression Test.

Also listen through the carb (intake valve) and exhaust (exh valve) to see if you hear or feel any air. That would indicate a leaky valve/seat.

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