Low compression test numbers
01-19-2012, 04:53 PM
#1
Low compression test numbers ran a cold compression test on my LS1. All cylinders were 120psi across the board. All plugs were removed, coil packs disconnected and i put the gas pedal to WOT when cranking. A little history. The top half of this motor is rebuilt less than 50 miles ago. New head gaskets, valve seals the whole 9 yards. no coolant in oil or oil in coolant and no coolant or smoke in exhaust. car runs great. the reason for the low compression numbers I believe is my **** up while lapping in the valves. i only used 220 grit lap to lap in the valves. you are supposed to use 220 grit then 600 grit to finish. i assume this is the cause of my lower cold compression numbers. can i run my motor like, this will all be good? It runs strong and idles great
car has a 234/238 cam with 11* of overlap so i expect lower numbers than what most post up. motor has stock 10.1:1 compression ratio maybe even less than that because i used the felpro head gaskets
I know these numbers are low but the numbers are all the same for every cylinder and it was on a cold motor. i've heard you can add as much as 15-20 psi going from a cold compression test to a hot compression test. so with that logic it would put me around 135-140 psi hot. not great but runnable seeing as i know the cause of the low numbers.
car has a 234/238 cam with 11* of overlap so i expect lower numbers than what most post up. motor has stock 10.1:1 compression ratio maybe even less than that because i used the felpro head gaskets
I know these numbers are low but the numbers are all the same for every cylinder and it was on a cold motor. i've heard you can add as much as 15-20 psi going from a cold compression test to a hot compression test. so with that logic it would put me around 135-140 psi hot. not great but runnable seeing as i know the cause of the low numbers.
Last edited by 3rdCoastPowerSports; 01-20-2012 at 12:32 AM.
01-19-2012, 06:04 PM
#2
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Big cam on mild compression will do this. It's normal. It's also the reason that engine will be less responsive in the lower RPM range. It will most likely be doggish below 3500 rpm. I hope your putting this in something very light.
This is what happens when the "recipe" has too much or too little spice...
It's probably not your lapping although 220 is horrid. I do not use the coarse stuff at all. If it will not lap up nicely with a tiny amount of the fine compound it needs a proper valve job...
This is what happens when the "recipe" has too much or too little spice...
It's probably not your lapping although 220 is horrid. I do not use the coarse stuff at all. If it will not lap up nicely with a tiny amount of the fine compound it needs a proper valve job...
01-19-2012, 06:13 PM
#3
Big cam on mild compression will do this. It's normal. It's also the reason that engine will be less responsive in the lower RPM range. It will most likely be doggish below 3500 rpm. I hope your putting this in something very light.
This is what happens when the "recipe" has too much or too little spice...
It's probably not your lapping although 220 is horrid. I do not use the coarse stuff at all. If it will not lap up nicely with a tiny amount of the fine compound it needs a proper valve job...
This is what happens when the "recipe" has too much or too little spice...
It's probably not your lapping although 220 is horrid. I do not use the coarse stuff at all. If it will not lap up nicely with a tiny amount of the fine compound it needs a proper valve job...
thanks for your response i feel better
