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[Webby the Bear]

Hi gays, I mean guys 

I checked on Autodata and for my 2.5 it says the compression should be 12-15 bar. (doing it this weeked)

That means that ''Good'' compression should be between 176.4psi and 220.5psi. just seems a bit high to me.

is this correct? i only ask cos i have found some ''oddities'' with Autodata

 

[dbug]

Compressions on my old Elite were around the 210-220 psi from memory.  Checked when I had a bad misfire - cured by replacing DIS pack.

[Webby the Bear]

Compressions on my old Elite were around the 210-220 psi from memory.  Checked when I had a bad misfire - cured by replacing DIS pack.

ok thanks dbug. at least that confirms that sounds correct 

[dbug]

Remember I said from memory, but pretty sure thats right - HTH

[Webby the Bear]

Remember I said from memory, but pretty sure thats right - HTH

it does help. cheers 

[Entwood]

If the info I have is correct .. compression ratio of 10.8:1 petrol,  22.5:1 TD

with atmospheric pressure being 14.7 psi .. then 10.8 x 14.7 = 158 psi petrol ...  22.5 x 14.7 = 330 psi diesel

so quite where the figures you quote come from I'm not sure ...

[Webby the Bear]

If the info I have is correct .. compression ratio of 10.8:1 petrol,  22.5:1 TD

with atmospheric pressure being 14.7 psi .. then 10.8 x 14.7 = 158 psi petrol ...  22.5 x 14.7 = 330 psi diesel

so quite where the figures you quote come from I'm not sure ...

thats exactly the calculation i was doing mate (except i did 10.8+1 X 14.7)

[aaronjb]

Some info about the calculations you should be doing here: http://en.wikipedia.org/wiki/Compression_ratio#Dynamic_compression_ratio

In short, it's not as simple as just 'CR * 14.7'..

[Entwood]

Some info about the calculations you should be doing here: http://en.wikipedia.org/wiki/Compression_ratio#Dynamic_compression_ratio

In short, it's not as simple as just 'CR * 14.7'..

correct .. when running .. but when cranking on a starter motor it is near enough ... and the article you quote even says so .. down the bottom !!

Additionally, the cylinder pressure developed when an engine is running will be higher than that shown in a compression test for several reasons.
The much higher velocity of a piston when an engine is running versus cranking allows less time for pressure to bleed past the piston rings into the crankcase.
a running engine is coating the cylinder walls with much more oil than an engine that is being cranked at low RPM, which helps the seal.
the higher temperature of the cylinder will create higher pressures when running vs. a static test, even a test performed with the engine near operating temperature.
A running engine does not stop taking air & fuel into the cylinder when the piston reaches BDC; The mixture that is rushing into the cylinder during the downstroke develops momentum and continues briefly after the vacuum ceases (in the same respect that rapidly opening a door will create a draft that continues after movement of the door ceases). This is called scavenging. Intake tuning, cylinder head design, valve timing and exhaust tuning determine how effectively an engine scavenges

The most important thing about a compression test is NOT the actual figure for each cylinder anyway .. it is the DIFFERENCE between cylinders that tells you what (and sometimes where) the problem is ... anything more than 15 psi difference should be investigated IMHO

[dbug]

Have a look Here 

And here

And Here

All results well over 200psi (so think 158 psi incorrect!), but as said its consistency across the cylinders as well as actual figure.  Having said that I would expect the V6 to have in excess of 200 psi

[aaronjb]

You missed the important bit in that link though, Entwood:

For example, if the static compression ratio is 10:1, and the dynamic compression ratio is 7.5:1, a useful value for cylinder pressure would be (7.5)^1.3 × atmospheric pressure, or 13.7 bar. (× 14.7 psi at sea level = 201.8 psi. The pressure shown on a gauge would be the absolute pressure less atmospheric pressure, or 187.1 psi.)

Dynamic compression ratio is going to be lower than the stated static compression ratio - I've no idea how much on the Omega but let's assume it's similar to the example above and our static 10.8:1 drops to 7.8:1..

In that case: (7.8^1.3)*14.7 = 212.3psi

As your quote says the number measured by spinning it on the starter motor will be lower than the number we'd see with the engine running, so perhaps the dynamic compression ratio is actually lower than 7.8:1, but I know I'm not capable of calculating what it actually is..

Suffice to say, the exponential factor (1.3 in the example above, further explanation is in the link) is the reason why the measured pressure is higher than multiplying static CR by 14.7 would imply.

[tidla]

 
I think my head just melted.

Throttle open and sparks and fuel turned off.Easier said than done?

[Webby the Bear]

i shall do the tests and see what results i get. ill post them up and we can debate the results 

just thinkin' out loud... if i remove scuttle and undo and lift up (as much as poss.) the cable tray will that give me enough access to the SP wells to get a.) the plugs out and b.) get the gauge in?

also can i disconnect the ICV for this test? that would help with access to plugs 2 and 4

[r1]

you dont need to know what they should be as long as they are all the same or pretty close.

[dbug]

i shall do the tests and see what results i get. ill post them up and we can debate the results 

just thinkin' out loud... if i remove scuttle and undo and lift up (as much as poss.) the cable tray will that give me enough access to the SP wells to get a.) the plugs out and b.) get the gauge in?

also can i disconnect the ICV for this test? that would help with access to plugs 2 and 4

Yep won't affect results