compression tests

[theitalian3]

anyone done them on the 1.8TQ or S3

what psi did you get?

just wondering what they were meant to be, before engine flush etc i got

168
168
162
150psi

and after they all levelled out at

170
170
167
170psi

but was just wondering if 170 was the expected figure?

 

[Jamesh]

Min is 7 bar which is round 100psi, the max difference between bores is 3 bar(43psi). These specs were taken from a Haynes manual.

 

[theitalian3]

which means?

how do i know what bar im running at? or if 170 is expected? afraid im not too clued up on this kind of shazam

 

[Jamesh]

A compression test is a way to find out if any of the bores are leaking.The reading you obtain is how much pressure a bore can hold. The lowest reading you should get is 7 bar (100psi). Anything lower indicates a problem, such as headgasket failure, worn piston ring etc... There should not be a difference of more then 3 bar (43psi) between any 2 bores (indicating headgasket failure between bores). Your reading roughly 11-12bar are well above the lowest reading.

Please note this is only how I understand it, if I'm wrong somebody please correct me.

 

[CLS3]

a difference of 3bar is a lot!! i've always thought that a difference of 1 bar between cylinders indicated that there was a problem.
Also if a pressure is too high then that can indicate trouble too.

 

[theitalian3]

so im running at around 12bar

sound about right? i have had someone explain what compression tests are and all that but wasn't sure what the stock figure should be for the 1.8TQ?

 

[jcb]

they should all be the same.
how are you doing it? cold engine? hot engine?
did you open the throttle body and disable spark and fuel?

You are not "running" any particular Bar, the compression reading is a consequence of the engine internals (i.e what piston/rod/combustion chamber you have) and the health of the engine.
If you have a valve leak you will get a lower figure
If you have slack piston rings you will get a lower figure

both these will produce sliding figures that will gradually drop away when you leave it at TDC. If you want to figure whether it is valve or piston then do a "dry" compression test then a "wet" one by repeating but pouring in a couple of ml of engine oil.
It seals the rings/bore better and you get a higher reading, if it stays the same it's your valves/head.

if you are "3 bar" different between cylinders it is f@cked
they should be within 10% or as close to zero as possible.

If you are running a 10:1 compression engine (Standard N/A) you can expect a 145psi or 10 Bar reading (14.5psi/1 Bar is standard atmospheric pressure, 10x Atmos is 145 or 10)
except your engine runs hot and air heats up when squeezed (put your finger over a bike pump nozzle)

you should see between 170-200psi on a 10:1 c/r engine

none of your engine should be running 10:1 as turbos they will all be between 7 and 9.5 depending on whether they are standard or rebuilt.

the number means nothing unless you have done them the same way.

http://www.pumaracing.co.uk/comp.htm

this is a good write up

 

[theitalian3]

they should all be the same.
how are you doing it? cold engine? hot engine?
did you open the throttle body and disable spark and fuel?

You are not "running" any particular Bar, the compression reading is a consequence of the engine internals (i.e what piston/rod/combustion chamber you have) and the health of the engine.
If you have a valve leak you will get a lower figure
If you have slack piston rings you will get a lower figure

both these will produce sliding figures that will gradually drop away when you leave it at TDC. If you want to figure whether it is valve or piston then do a "dry" compression test then a "wet" one by repeating but pouring in a couple of ml of engine oil.
It seals the rings/bore better and you get a higher reading, if it stays the same it's your valves/head.

if you are "3 bar" different between cylinders it is f@cked
they should be within 10% or as close to zero as possible.

If you are running a 10:1 compression engine (Standard N/A) you can expect a 145psi or 10 Bar reading (14.5psi/1 Bar is standard atmospheric pressure, 10x Atmos is 145 or 10)
except your engine runs hot and air heats up when squeezed (put your finger over a bike pump nozzle)

you should see between 170-200psi on a 10:1 c/r engine

none of your engine should be running 10:1 as turbos they will all be between 7 and 9.5 depending on whether they are standard or rebuilt.

the number means nothing unless you have done them the same way.

http://www.pumaracing.co.uk/comp.htm

this is a good write up

basically coil pack out, sparkies out, compression test hooked up 1 at a time and turn over the engine, do for all 4, then put back coilpacks and sparkies, do a full engine flush and then repeat first stage

did notice a big change in figures, well not massive but you can see for yourself above