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Engine compression
http://international2cvfriends.com/forum/viewtopic.php?f=7&t=2789		 Page 1 of 4
Author:  Willami [ March 16th, 2011, 1:02 pm ]
Post subject:  Engine compression
Hi

I've had a look back through my posts and searched 'compression' - can't find ti anywhere!!!

Some may remember that I did a compression test about a year ago and it was really low on one side and generally crap.

After setting the tappets when I had the engine out after Xmas, i finally got round to doing a compression test at the weekend......okay I'll get to the point

engine is un-modded M28 - compression on both sides is 115 (can't remember unit of measure)

any good?

cheers
Author:  Rhythm Thief [ March 16th, 2011, 1:09 pm ]
Post subject:  Re: Engine compression
Doesn't sound too bad. I think an 8.5:1 engine should be around 120, 130 psi or so, but the classic sign of trouble is when one side is much lower than the other.
Author:  Willami [ March 16th, 2011, 1:16 pm ]
Post subject:  Re: Engine compression
Rhythm Thief wrote:
Doesn't sound too bad. I think an 8.5:1 engine should be around 120, 130 psi or so, but the classic sign of trouble is when one side is much lower than the other.


Cheers Rhythm - yeah, when we were setting the tappets, one side the exhaust was shut and the input was too wide, other side - the exact opposite!!!! so it all makes sense

The plug on one side is still a bit oily though - seals aren't great, but it pulls okay...and doesn't smoke too much

cheers again
Author:  Sean [ March 16th, 2011, 2:15 pm ]
Post subject:  Re: Engine compression
Not the best!

But many things can effect the reading

tappets should be spot on engine should be warm battery good and a good starter

crank it over Wide open throttle (and clutch down) for a set amount of turns...10?

9:1 you want to be seeing over 160psi and very little difference between sides.

over 150 psi for 8.5:1

readings vary, gauges arent the most accurate of things and a tired battery sluggish starter cold engine all vary the readings.

if your seeing lots of oil in the filter box, dirty internals to the points box, leaks and smoky starts then its a sign of worn bores.

Sean
Author:  2CViking [ March 16th, 2011, 2:19 pm ]
Post subject:  Re: Engine compression
Anything below 100 psi is worn (out) any variation of 10% is too much.

Also do the compression test without wide open throttle. This will indicate what compression available at cold start, since most cars are started cold maybe with use of choke.
Author:  Willami [ March 16th, 2011, 4:43 pm ]
Post subject:  Re: Engine compression
ah, you see - didn't touch the throttle, starter is just satisfactory, battery goes flat after leaving lights on for 3 hours (shite) and didn't touch the clutch....but all that said

i've got worn bores I'd say.......f**k it, it uses about a litre of oil every month, so could be worse. :roll:
Author:  ken [ March 16th, 2011, 6:42 pm ]
Post subject:  Re: Engine compression
Will et al,
the conditions required for a standard compression test include 'engine hot' and 'throttle wide open'.

If you don't observe those requirements, the results obtained cannot be compared to the expected readings of 150 psi for an 8.5:1 engine and 175 psi for a 9.0:1 engine in good condition.

Testing with the engine cold or the throttle closed can reduce the readings obtained by as much as 20 psi., so doesn't make much sense...

ken.
Author:  2CViking [ March 17th, 2011, 12:54 am ]
Post subject:  Re: Engine compression
ken wrote:
Will et al,
the conditions required for a standard compression test include 'engine hot' and 'throttle wide open'.

If you don't observe those requirements, the results obtained cannot be compared to the expected readings of 150 psi for an 8.5:1 engine and 175 psi for a 9.0:1 engine in good condition.

Testing with the engine cold or the throttle closed can reduce the readings obtained by as much as 20 psi., so doesn't make much sense...

ken.


So you are not interested in compression available starting a cold engine with the choke on but only pay attention to a warm engine with the pedal on the floor :roll:
Author:  ken [ March 17th, 2011, 2:26 am ]
Post subject:  Re: Engine compression
Viking,
your interpretation is incorrect.
I have no interest in wasting my time trying to guess what difference will be made to the results of a compression test by failing to follow the correct test procedure.

The same principle applies when carrying out an oil pressure test.
If Citroen specified that the oil should be at 80*C and the engine running at 6,000 rpm for that test, why bother to try checking it at 50*C or 3,000 rpm? :?

ken


2CViking wrote:
So you are not interested in compression available starting a cold engine with the choke on but only pay attention to a warm engine with the pedal on the floor :roll:
Author:  Sean [ March 17th, 2011, 9:27 am ]
Post subject:  Re: Engine compression
2CViking wrote:
So you are not interested in compression available starting a cold engine with the choke on but only pay attention to a warm engine with the pedal on the floor :roll:


:roll: right back@U :lol:
Its not that you are "looking to see what's available to a cold engine" you are doing a test to asses the amount of wear in the engine, and the standard method for doing that is hot and WOT.
By doing it cold you are also recording the drag of the cold oil and the restriction of the closed throttle.

Once you have a proper reading is it safe to assume that an engine that is down 65% when hot is still f**ked when its cold?

its not like the condition of the barrels and rings change whether they are hot or cold.

Just like my car if I test the compression I get 150psi - its really 165-170 but i have a duff starter should i continue to test it with the dead starter because "that's what is available to it"? so thats what the compression is even when its up and running??

Sean
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