SoNiCa

MY98 Impreza Turbo
cycl#1 = 121 psi
cycl#2 = 121 psi
cycl#3 = 120 psi
cycl#4 = 121 psi

I have searched the forum and a bit confused about the compession test results. According to other posts it should be around 150 psi but my results nowhere around it. There is no oil consumption (even very little) but I'm not happy with the performance of the car. I'm trying to find a bottleneck which restricts the engine performance. How can I check if there is any boost leaks at somewhere? Visually all the pipes seems okay but this is not satisfying me. Are there anyway or any tools to check it out?

harvey

iTrader: (48)

The fact that all four cylinders are the same indicates that this area is not a problem area. FWIW these are the same figures I got from a WRX recently. I do not put a lot of store in the figures themselves just the fact that all four cylinders are similar.
The guage is not necessarily accurate and it depends if all plugs or just one was out, the throttle was open or closed etc.

[Davey]

I agree with the above, I would work on the idea that the gauge isnt accurate, as long as the readings are similar all should be fine.

SoNiCa

Yes the throttle was open and injector's connectors were off. Any inputs for the boost leaks? How do you test your cars? It pushes 22 psi without any issue but I'm not happy with the performance. To me, if there would be a boost leakage Defi boost gauge wont show 22 psi(1.51bar) of boost, isn't it?(machanical;tapped into manifold) Contrary to this PFC commander&logs shows 1.35 bar which makes me sick.

R19KET

Defi's are notoriously inaccurate, and over read, particularly the boost gauges.

Personally, I would expect the 1.35bar to be more accurate.


Mark.

MazingerZ

Right here are the compression ratios of each cylinder, now you have to find the compression that the manufacturer states.

They are not that bad, at least the readings are very very close to each other.

if you were to be running an engine at 150psi that would mean that you compression ratio would be 10.2:1 which is relatevely high.

bear in mind that milage does affect the seal that piston rings provide, hence the lower compression with higher mileage.


Koji Kabuto

R19KET

Kojo,

I'm sorry, but that's not correct.

Normal compression is 8:1, and Subaru tolerance for this in PSI is 142-171psi.


Mark.

MazingerZ

How did you come up with that result? I thought that compression test were runned on atmospheric pressure,

hence the equation (compression ratio x atmospheric pressure) = PSi

in this case 8 x 14.69595 = 117.6 PSi

Am I missing something?


Koji

Bob Rawle

As Mark says .. in addition the lower limit is 138 psi and cylinder to cylinder tolerance is 7 psi total.

bob

MazingerZ

Mark and Bob

How about trying to prove it mathematically?

I just want to know if I am wrong so I can rectify.... if someone says something that does not mean they are right.... or does it?

Koji

sparkplug

Hot engine or Cold engine?????????????
I had a twenty PSI difference when mine was tested
Only problem is tring to get four sparkplugs out of a very hot engine
plus all the other bits you have to remove can take upto 30mins
but engine cools down quite a lot in this time.

R19KET

Spark plug,

I agree, but as others have said, it's not just the actual "values", but the differences between them that indicate if there are any problems.

Older engines can have relatively low figures, but run without problems.

Koji,

I'm afraid I'll have to let someone else prove it to you mathematically, but I can assure you, that you can't just divide the psi by 14.69595 to give a CR.

Personally, when appropriate, I use 14.5 for atmospheric, but that's a whole other discussion, and something else I'll leave to someone else to prove

Absolutely not................


Mark.

MazingerZ

His compression ratio is 8.0:1 which supports my theory as follows:

Cylinder Compression = (Compression Ratio) x (Atmospheric pressure)


CC = (8.0) x (14.69595 PSi)

CC = 117.5676 PSi


Allow for Gauge calibration error and you see how he is able to get such readings....

All you have to remember is that all the plugs would be out, the engine would be warm and the throtle fully opened. So what the piston compresses is the air in the atmosphere (14.69595 PSi at sea level) 1 unit when the piston is at the bottom of the Bore and 8 times that once the piston reaches the top of the bore

So where do you guys get your 150PSi from?

If you were to have 150PSi or 10.2:1 compression ratio in a turbo charged engine, there would be no cylinder head left once the full boost is reached.

Suggestions?

Koji

[Davey]

Have to agree with the above!

The c/r represents the maximum compression at the end of the stroke, so the atmospheric pressure has been compressed 8 times..

The results do sound good for a low c/r turbo block..

As long as it holds the pressure and all cylinders are roughly equal there are no problems.

hypoluxa

You cannot simply divide gauge readings by atmospheric pressure to get C/R. Your calculations do not factor in the universal gas law and the fact that the inlet valves close after BDC

As has already been mentioned, the fact that the readings are so close together indicates a pretty sound engine.

MazingerZ

So how do you do it properly Mr Universal Gas Law?


Koji?

R19KET

Koji,

It would appear that if YOU are the someone saying something, then you MUST be right !

The Subaru service manual states that 142-171psi is what to expect on a CR test, based on the 8:1 engine.

As hypoluxa has pointed out, you're not taking into account the valves opening, & closing.


Mark.

MazingerZ

where abouts in the manual? can you post it here?

ZEN Performance

The compression actually acheived will depend on the speed of compression. There is work being done to compress the air, and this will heat the air up, increasing the pressure further.

hypoluxa

Kojo,

Because the piston is ascending the bore before the intake valve closes, it actually pushes the air back out the inlet. At such low rpm cranking rpm it is impossible to capture a ‘complete’ volume (This is why long duration cams have a lumpy idle). Now when you compress a gas it generates heat, this will increase pressure way above what is expected, but remember you are compressing less volume.

Subaru have calculated in these factors, hence the 140-170psi range at cranking speed for 8:1 C/R.

Bob Rawle

Compression ratio is defined as the ratio of maximum to minimum cylinder volume and has nothing too much to do with cylinder pressures measured under cranking conditions at a defined speed and defined throttle opening. The valve geometry and opening and closing angles and lift when related to engine speed have a far greater influence.

Its not therefore possible to be as simplified as the above calculation might suggest.

The figures Mark and I have quoted are Subaru's own design information and limits, I suggest that their engineers have done the math for this engine.

It is stated that the throttle was open and the injector connectors off but were the plugs out ? that will affect engine speed and hence the actual pressure figs achieved. I always crank with throttle fully open, plugs out and crank sensor connector off, also engine warm, oil at 70 degrees minimum and take at least 5 full engine cycles to stabilise the reading.

cheers

bob