I just bought a compression tester and did some tests. i did cylinders 5 through 8 right now and before i go on i have to know something.

when i cranked the engine over a couple times the reading on say, cylinder 5, reads 85 psi. then if i crank it again it goes up to 125 psi. which is the correct reading?

 

i believe you crank it about 3 times to get the actual reading. its been so long sense ive done one

 

sign on AIM jeremy lol...

 

Yes you should crank the motor for 3 revolutions, you might not have had a good seal on the tester the 1st time. I usually do each cylinder about 3 times for repeatability.

 

thanks for the info dude, alls i know is that if i keep cranking it my compression will sit at about 140 psi. i was worried this wasnt right but i guess it is, and 140 is great right??!!

 

Stock heads? Should be around there I think 160psi equates to like 10:1 compression if memory serves. What you are really after is uniformity between cylinders, no more that 2 to 5% difference between them.

 

yep, 3 cranks, and you want uniformity between the cylinders, 2-5% is a good call :P

 

Make sure you crank the same amount for all 8 cylinders.If you don't it will offset the readings.

 

You want all the nubmer within 10% of each number, thats what the ford shop manual says.

 

mustangfan is right, its 10%. of course if they are within 5% thats even better but 10% is ok