428 Cobra jet Cylinder Compression

mark peterson · #1 (**permalink**) 10-12-2004, 03:03 PM

Does anyone know what the compression should be in PSI for a 428 CJ. I did a test last week and most cylinders were about 90-100 psi. This is by spinning the engine with a long handled torque wrench from the front pulley. Someone said 150-180 was normal.

If this is normal do I need rings. The engine has about 91,000 miles on it. Does not seem to burn oil. All plugs looked tan and clean. thanks, mark

Hotfingrs · #2 (**permalink**) 10-12-2004, 03:42 PM

First of all you can't do a cylinder pressure test that way.....You have to block the carb fully open, disconnect the coil wire, then spin the engine over at least 3 revolutions, until the gauge doesn't move. Then let us know what the readings are.

bomelia · #3 (**permalink**) 10-12-2004, 05:36 PM

Hi Mark, I am interested in how this is done too. BTW, you will get more response if you post this again in the FE Forum.

Hotfingrs, excuse my ignorance of such things, but what doe "block the carb open" mean? Is the reason for removing the coil wire to avoid accidental ignition? I used to do this test on my old MGB, and do not remember doing these things.

Thanks!

Mike

DAVID GAGNARD · #4 (**permalink**) 10-12-2004, 05:41 PM

A quick easy way to "check out" an engine by using a compression tester is to test all 8 cylinders,write down the readings,then one by one,give each cylinder a couple of squirts of oil and re-test,if the reading goes up,your rings are not sealing all they can,if the reading stays the same (low),then your valves are not sealing all they can.........

David

Hotfingrs · #5 (**permalink**) 10-12-2004, 06:58 PM

Mike, block the carb means that the throttle blades are all the way open, and you do whatever you have to do, to keep them open during the compression test. Yes you don't want an accidental spark to ignite anything. Also all plugs have to be out of the engine to get a true reading. The only reason to squirt oil into the cylinder is if you have a low reading...

bomelia · #6 (**permalink**) 10-12-2004, 07:21 PM

Thanks Jack...I do remember the oil trick now that David mentions it. Is the reason for blocking the carb open so that the cylinder gets plenty of air?

Mike

Excaliber · #7 (**permalink**) 10-12-2004, 07:45 PM

Cylinders GOT to get plenty air during the STARTER CRANKING phase in order to get a decent cylinder pressure reading.

Watch out for that coil wire shorting out or starting a fire near that leaking carb fuel.

Cylinder pressure is a tricky thing which can be affected by the cam shaft profile. The "suggested" numbers are only a "guide line". What your REALLY looking for is all cylinders within 10% of each other.

Hotfingrs · #8 (**permalink**) 10-13-2004, 02:59 AM

Ernie is right about the reasons for doing the above mentioned. Cylinder pressure doesn't tell you compression ratio, it only tells you the condition of the engine. As an example my engine is 10.5-1 compression. Last year I had 285 lbs of cranking pressure, this year I changed the cam and my cranking pressure is now 185 lbs, with the same CR. The cam was the only change made.

DAVID GAGNARD · #9 (**permalink**) 10-13-2004, 04:48 AM

As Hotfingrs has stated,you only need to do the "oil trick" if you think you readings are low.......As long as all cylinders are close in thier readings you know the engine is fine,if you have one or more that are a lot lower than the rest,then try the oil trick to help determine the problem......

Back in 1996,I built two 351-Ws,both basically the same,only diffrernce was the cam,one had a cam with lift around .500 and a fair amount of overlap,the other had a cam of around .425 lift with no overlap,the cylinder pressure was tested after both enignes had about 1000 miles on them and the difference between the two engine was around 75 psi,all cylinders were within 5% of each other on each engine..... Cam profile has a lot to do with cylinder pressure.......

I helped a buddy find a "miss" in his 351-W some years ago,we tried everything and could not find it,finally I suggested a compression test,7 cylinders were very good,one had only 20 psi,tried the oil trick,it stayed at 20 psi,when we pulled that cylinder head we found a burnt valve on that particular cylinder......the test saved us from tearing down the whole engine and/or pulling the wrong head......it is a useful tool in checking out an engine......

David