I'm doing a compression test on my 1971 LS5 and would like to know what the compression would be on a

factory stock LS5

factory stock 1974 LS4

and what should my compression be. The engine in my car is a 1978 truck block with 72 open chamber oval port heads.

Thanks guys.

 

LS5 160 psi according to http://www.gmheritagecenter.com/docs...t-Corvette.pdf
LS4 had .25 less compression so guess would be 150 to 160 psi

 

Okay that's what I was looking for.

cylinder # 1 has 135
# 3 has 125
all the rest are 150

 

isnt the rule of thumb all within 15% of highest

 

The LS5 should have 160 psi when compression test is performed as specified in the Chevrolet Chassis Service Manual. The LS6 should have 150 psi. Compression readings DO NOT correlate with engine compression ratio, because of the differences in camshaft designs. Higher HP engines usually have more 'aggressive' camshafts with more overlap; that results in lower compression test readings.

Your cyl #3 number is somewhat lower than it should be. You might have some valve wear, a valve seat wear problem, or some piston ring damage. If you engine runs, I would recommend that you run some SeaFoam through it (via a manifold vacuum line, per can instructions). You may dislodge some built-up carbon deposits that prevent the valves from completely closing. For less than $10, it's worth a try, anyway. Then do the compression test again to see if there is any improvement.

That amount of loss on cyl #3 will result in less performance from that cylinder...but that is not unusual for an engine with significant miles. If the engine build is recent, it would be cause for concern.

 

I know it's an old low compression truck block so I'm okay with the 150 compression. The main seals leak oil so I'm sure it's a high mileage bottom end. I agree #1 and #3 probably have valve or seat problems.
When I bought the car it ran terrible that's why I started with comp test. In addition to #1 and #3 problems it had one cracked spark plug that was not firing on #8 and a bad plug wire on #2. So now it actually runs better after fixing a few problems. Never heard of sea-foam thing so I will give it a try. Even if I get it running better it's not the performance I'm looking for. I'd be happy with 500hp.

Thanks for the advice.

 

Although lots of folks don't believe that SeaFoam actually works, lots of fleet operations....including many Police cruiser fleets...use the stuff because it keeps engines running well longer. The basic function of SeaFoam is to help 'steam-clean' the engine from the inside, while it is operating. So, if you use it, make sure the exhaust from the engine is not aimed at anything you want to have crud/carbon blasted on. (i.e., laundry on the line... )

It is good for putting into the fuel tank with a full tank of gas. Over the use of that tankful, it will clean carb or fuel injectors, as well as help clean some carbon deposits out. But, I recommend that you put 1/2 of the can into a full tank of fuel. In the can with the other half of the SeaFoam, install another 18-24" vacuum line with a small hand clamp at the middle of the line. Now, disconnect the main vacuum line from the intake manifold and connect the line from the SeaFoam can to the intake fitting. Start the engine and rev it to about 2500 rpm and keep it there. Now, release the clamp on that line and watch the cr@p roll out of your exhaust. All that black junk is carbon build-up that was in your combustion chamber or on your valves.

Once the SeaFoam is gone, shut the engine down and reconnect the vacuum line to the intake manifold. All done...for this year! If you don't drive that many miles, once a year is about right. If you drive it a lot, then maybe twice a year. It works for me and a lot of other folks. If others poo-poo using it, ask them if they have used it also. You likely won't hear back from them. Good luck.