Compression test on my new lightning motor...help??

20Screw01 · #1 06-22-2009, 11:39 PM

So I bought a used 2000 lightning motor and with the throttle wide open and oil pressure cold the test is coming back at 120-125 on all cylinders. What is stock cold and what the heck do I do to see if its just the rings?

Thanks Dave

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05RedFX4 · #2 06-23-2009, 09:05 AM

I don't know what the should have when they are new, but if your within 5 lbs on all cylinders and you have a low of 120, I would think your rings are in good shape.

I remember the old smog motors from the 70's only putting out 75-80 psi when they were new, so if you got 120 on a used L motor, get it put in already and go enjoy it.

If you were to have an engine buider build you two identical motors and have one blueprinted to run N/A and the other blueprinted for forced induction, the N/A motor will pull higher numbers on a compression check then the blown motor will.

20Screw01 · #3 06-23-2009, 01:13 PM

Thanks Red, I hope your right! I just want to make sure before I run this thing. Anyone know what these put out on a compression test?

CobraKit · #4 06-23-2009, 04:00 PM

I will check Ford shop manual, but I would agree that 120 and variation you are seeing is not bad. When I rebuilt my engine, the high boost (14lbs on non L block) waisted the rings and the exhaust valve guides. Valves and cyclinder walls were fine.

20Screw01 · #5 06-23-2009, 04:10 PM

That would be great if you could check for me Cobra! Just want to put my mind at ease.

150xlt · #6 06-23-2009, 08:36 PM

Those numbers sound about right to me

CobraKit · #7 06-24-2009, 09:10 AM

Forgot last night. Promise I will do tonight and post. Ford shop manual (actually Heim) is crap. Its hard as h... to find things.

CobraKit · #8 06-24-2009, 08:18 PM

Manual did not call out. Said 75 percent diffrence allowed between high and low. Add oil on low to see if it makes a diffrence. If it does rings, if not valves. Chart showed max pressure from 134 to 250 and low to 101.

20Screw01 · #9 06-24-2009, 10:31 PM

Thanks Cobra! I dont know why I didnt think of testing my cobra but I went ahead and tested it (03 low compression motor) and it came back at 125-130 with 10k on the factory motor so i am going to run it!