The Stovebolt Site Forums The Shop Area Engine Shop 261 compression

Before ripping it back out of the truck, I want to do a compression test on my engine. What's the range I'm looking for?

Thanks,
John

GM spec: 130 psi minimum (higher was OK).
http://chevy.oldcarmanualproject.com/shop/1954truck/54ctsm0617.html
http://chevy.oldcarmanualproject.com/shop/1961ctsms/61ctsms0701.html

I think another spec for all years was no more that 5-10 psi difference between/among the 6 cylinders "for satisfactory performance".

Since you're likely to get different "pressure" readings on the same cylinder from different gauges due to calibration variations, compression pressure is a variable thing. Anything over 100 is good on a stovebolt engine, and the thing I look for is consistency. If cylinder readings are within 10% it's OK, but 5% or less variations is excellent.

I test a compression gauge for accuracy by hooking it up to my air compressor and comparing readings to the line pressure gauge on the regulator. You can even dial up the pressure with the regulator to different values to check the compression tester over its entire range that way.
Jerry

It's going to be affected by air temperature, battery and starter condition, and oil viscosity, so write it all down for comparo.
As said, if they're all pretty close it's probably OK.

I will also add that the camshaft installed can affect cranking compression numbers; sometimes significantly. If you have a non-stock cam profile, your compression numbers will most likely go down, not up.

Got reason to believe that something ain't right?

When I rode in the truck for a few miles at the homecoming, I heard some potentially expensive noises developing and watched the oil pressure creeping downward as the ride progressed. I recommended at least a rod bearing change.
Jerry

JW51, during the teardown of my otherwise completely bone stock and crusty looking 261, it was discovered that someone had installed a "performance" cam. So, nothing is impossible.

Higher altitude will also result in less pressure, consistency is the key, like Jerry says.

Ed

I have always thought that a wide open throttle is required during compression testing. However I have found that it makes no difference. I don't understand why it doesn't.

Perhaps it's because almost nothing increases the performance of these engines?

We're dealing with a very basic engine design that didn't undergo any noticeable changes in engineering or construction methods from the 1920's to the 1960's. Look at a 194 or 207 engine from 1928-34, and a 235-261 from 1962. They will all be instantly recognizable as being a Chevy stovebolt type engine. There was a compression increase or two to take advantage of the improving quality and increasing octane rating of gasoline, but the basic layout of the ports, combustion chambers, piston design, etc. was virtually unchanged. The 3rd.-generation inline 6 from 1962 to the mid-1980's incorporated a lot of the small block V8 technology, but even those engines take a lot of major modifications to see a dramatic increase in performance.

You really can make a silk purse from a sow's ear, but it makes the pig look funny, and it doesn't hear well afterwards!
Jerry

Thanks for the input, Gang. If I was going to pull the engine and change the rod bearings anyway, I might as well check to see if there was anything else I needed to do while I was at it.

Ed -- you always make me chuckle. You do know that our elevation here on the Western Shore of the Chesapeake Bay is about 20 feet above sea level? This is not exactly "high elevation" ....

Your most exalted Serene Eminence, Mr. John Milliman Sir,
What is the problem with the 261

I had a long post written about doing a cylinder leakdown test in addition to checking compression, and the website ate it. I'll try again tomorrow- - - -it's past my bedtime.

Back when I was running the bars and chasing the ladies, if I didn't get to bed by midnight I'd give up and go home. Now I can't stay up that late!
Jerry

All the usual things increase the performance of these engines.

It just takes a magnifying glass, a micrometer, and a good imagination to measure the increase.
Jerry

Finally got the compression check done. Checked with two gauges on each cylinder (one was a shop built gauge made by Charlie, the original owner of the truck) and their differences were not discernible. With engine started and brought to a high idle for each test, all cylinders were 130 to 135 psi. Ambient shop conditions were: 144 ft AMSL(approx), 90 degrees (F), 30.09 inches of mercury and 80 percent humidity... welcome to the Western Shore of Maryland ...

So I will proceed with the original plan and just change the rod bearings. Once I pull the engine. Which may happen sometime between now and the Apocalypse ...



Thanks,
John

Jim -- Apparently the 261 has bad rod bearings.

Forensics report:

We pulled the rngine yesterday -- noticeable scoring on the rod journals and most failed a plastiguage test. Crank is coming out to be turned.

Thanks Jerry!!

I was afraid that was what you'd find. At least you didn't have to walk up and down the road with a big basket, picking up pieces! If the crank is really bad or already turned way undersize, any 235 crank is a bolt-in swap.
Jerry

Just think how pumped you'll be with all new guts in that baby. You might even be able to get "Charlie" out of his own way and scoot on down the road at dangerous, hair-raising speeds.

kl

Carl ... um, you should attend Homecoming sometime ... "Dangerous, hair-raising speeds ..." remember who you are talking too -- that covers a wide spectrum on the ole speed-o-meter ...

That territory begins about 35 mph in the '50 GMC 420 ...

[img]http://i160.photobucket.com/albums/t172/stovebltr/2016/Homecoming%202016/355_zpso5pcuedf.jpg[/img]
Ole Charlie has been clocked at speeds in excess of 70 MPH, you know

It is killing me that I have not been able to make it to homecoming!
Ole Charlie at 70? That does make the hair in my ears stand up, and well, the hair on that big mole on my back, but hair is hair...