The Stovebolt Site Forums The Shop Area Engine Shop compression test analysis help

I bought a '59 Apache in December. It has a mostly stock 283 V8 with about 60k miles on a rebuild. I gave it a tuneup and checked the compression, but I don't know what these results mean. Can one of you engine experts analyze this?

Cyl reading
1 155
3 145
5 145
7 120

2 155
4 110
6 110
8 145

Was the motor warm?

Did you have the carb open all the way?

Did you turn the motor over the same for each Cyl?

If no to any of the above please retest

Yes to all your questions.

LT

Did you adjust the valves?

Basically you are looking for the numbers closer than you have

If the truck runs OK to you.......Drive it till it doesn't

If you don't like the way it runs now It's time to take it apart

LT - Proffit told you right, you need to retest. Warm that engine good, say 15 min of running and rev it several times to blow carbon chips out and off ring lands and valve seat interface. Take at least 3 readings on each cylinder (dry), and throw out the high and low readings. Then shoot some oil ( a teaspoon or so) in each plug hole and take 3 more readings (wet) and throw out high and low. Take the average wet and dry and compare. This will tell you if rings are indeed worn as the wet readings will be considerably higher than the dry readings by 10 to 20 or more lbs. If you have a vacuum gauge connect to manifold (not ported vacuum on carb base) and watch readings. If smooth and steady, cool. If jerky, bouncing or irratic back and forth from high to low vacuum this points to valve and or guide wear. A cylinder leakdown tester is what you need to pinpoint which cylinder and or valves are the problem. Anyway something is puzzling in your comp readings that you stated and something does not seem right. 1. You stated that your 283 is "mostly stock" meaning what has been changed? Is it in fact a 283? Check block casting number on drivers side bellhousing flange, will be GM followed by 6 or more numbers. 2. Recently rebuilt by whom? And could they have possibly put the compression rings in upside down, or have excessive end gaps in the rings or have the rings out of position so the gaps line up? 3. Is the engine block leaking oil around the rear seal? Heavily? How about the front seal? The reason I ask is because the readings you stated follow the firing order and pulse up and down. As if a back pressure existed. Does the intake manifold have the stock oil fill tube? Is the crankcase draft tube in place behind the distributor? I think that was omitted in 1958 as part of the PCV mods. Stock 59 heads were the first year with the offset valve cover bolts. Additionally they have no accessory holes in the ends of the heads, and were not machined flat on the ends. Accessory mounting was from the exhaust manifolds and waterpump bolts. This was changed on the heads around 64 or 65 I think. Interesting that you stated that you have headers. So how are accessories mounted? Does your engine have breathers in the valve covers? Stock did not. If you are not venting the crankcase you cannot achieve correct compression readings because of back pressure from crankcase will not allow the rings to seat properly Its the same on the top side as compression rings actually need a little blowby into the ring land to properly expand the ring to the cylinder to seal correctly. Need more details? The readings you have do not suggest broken rings or blown headgasket but there is enough variance to be very concerned. Take more readings and get a leakdown test done to pinpoint trouble spots. Good Luck and Think Chevy!

Proffit and Chevyman,
Thanks a lot for the advice.
Chevyman, you had a lot of questions, which I'll try to answer.

I said the 283 is mostly stock. The engine was rebuilt by an engine rebuilding company in Los Angeles, Bonded Motor and Parts Co. It was installed by a mechanic in 1987. I have the receipt. The previous owner paid $1100 for the engine in 1987. Based on this, I think it is an off-the-shelf rebuild, nothing special, but was done by pros, so I would not expect incorrect assembly problems. The non-stock parts are the headers, the intake manifold, and the carb. The manifold and carb were Edelbrock. I replaced the carb with a new Edelbrock 1403, which is a 500 cfm carb.

I did verify that the block is a 283 by reading the casting numbers on the left side forward of the bell housing. According to a book I have, it is a 283 used in cars and trucks from 65-67. However, the heads have non-staggered valve covers. There is a tube that runs from near the distributor to the carb and has a PCV valve inline.

I will retest and I'll try the "wet" test you suggested also. I thought the compression readings were less than ideal, but I wanted to get an opinion from somebody who knows more than me. So far, I've tuned it up and replaced the carb, and these things made it run much better, but the bottom line is that it runs ok but not as good as I'd like.

Thanks again
LT