Compression test question

[Mad Hatter]

Had to ask! I did a compression test on my 2012 Boss the other day. I got values in between 143 and 146psi, but we did the test with the engine stone cold. Should I do the test again hot, or is that fine since the variation was ok?? The day after the compression test, took the car to the dyno and got 403Hp and 380 torque on a Dynojet. Its sticking in my head that I may still have some engine problem I need to fix. Car sounds and runs fine though (Does go through a half liter of mobil 1 every two track events).

 

[wwilde001]

As far as I know cold is fine. Make sure you open the throttle body though when cranking (or equivalent). This can affect the values you get on certain engines because it can restricts the airflow when closed. I had this happen on a race motor once. I was expecting a certain value and got much lower than anticipated. Good Luck.

 

[Jabooh]

I have always done mine cold.

 

[blacksheep-1]

Yeah, you're good, the thing to look for is a percentage of difference between the cylinders more than anything else.

 

[302 Hi Pro]

Had to ask! I did a compression test on my 2012 Boss the other day. I got values in between 143 and 146psi, but we did the test with the engine stone cold. Should I do the test again hot, or is that fine since the variation was ok?? The day after the compression test, took the car to the dyno and got 403Hp and 380 torque on a Dynojet. Its sticking in my head that I may still have some engine problem I need to fix. Car sounds and runs fine though (Does go through a half liter of mobil 1 every two track events).

You are fine with your process. Cold compression is a bit lower psi than a hot engine due to expansion, but not as much as one might think as piston rings have the close running tolerances hot or cold. When hot, I would expect compression readings in the 150 to 160 psi range.

Also, You and Black Sheep are correct as it is the cylinder to cylinder variance that would be a concern. Variances of 10% or greater would be a real concern, but your running around 2.5 -3.0%.

As far as oil consumption during race conditions? Just know your engine is going to use oil, as all engines consume some oil and all engines will use more oil under race conditions. You'll know there is an excessive oil consumption if your showing white smoke thru the exhaust and your loosing spark plugs due to excessive carbon build up. Worn piston rings also produces excessive crankcase pressures from
blow-by and this introduces excessive oil thru the PCV system, (Peterson Can or in the intake manifold). In the latter case, this results in excessive carbon deposits on intake valve faces.

Oil consumption is resultant of a nice cross hatch hone pattern on cylinder walls. This maintains an oil film on the cylinder walls that prevents metal to metal contact between the wall and rings. The top half burns off on each power stroke of the cycle. Some engines burn more oil than others but the main variable here is the driver and how the engine run and its break-in process.

IMHO, your engine is fine as I am assuming you do not show symptoms described in paragraph 3?

Good luck and keep us posted on your concerns that lead you to perform the compression test in the first place.

Dave
302 Hi Pro.

 

[Mad Hatter]

Thanks for the info Dave!! Do not have any white smoke and recently installed a jlt catch can. All my problems come from some pretty bad logs that show all kinds of problems. My usb camera arrived to day, so will try to see the the pistons and such next week. My long sob story is on https://trackmustangsonline.com/index.php?topic=9375.0

Will change passenger O2 sensor as well since things got better and my CEL's are gone since I changed the driver side O2 sensor.

Happy and relieved to see the compression test was legit and my Dyno run was what I expected for my mod.

I do a lot of engine braking from a off road backround, but my instructors keep telling me that's what the brakes are for! Seeing the other comments on the forum maybe I should try harder to change my habits.