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 Post subject: compression
Posted: Thu Feb 03, 2005 10:21 am 
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what should the compression of a stock 302 win be
also what is a wet test in relation to compression and what does it do

 

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Posted: Thu Feb 03, 2005 7:11 pm 
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8.9-9.0:1 ,wet test checking the volume of the combustion chambers with fluid (CCing the heads) to be done with plugs & valves in obviously
to work out the comp ratio work out the cc' of 1 cylinder divide that by the volume of combustion chamber +volume of head gasket +valve reliefs in pistons. hope that makes sense to you.

 

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Posted: Thu Feb 03, 2005 7:19 pm 
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i think the wet test you are refering to is when you check the compression with a compression tester guage, thru the sparkplug hole, by turning the motor over (with the coil lead disconnected) to get a reading of the pressure in the cylinder, then you squirt a bit of oil in thru the hole and check again, if the reading is higher then this generally indicates your rings are worn, the oil provides a better seal around the rings and helps them seal better temporarily to give you the higher reading. but remember even if your rings are good it will still give a bit higher reading, but you will notice it dramatically if the rings are shot.

when you first check (dry) i think from memory it should be around 120-150 psi, anyone correct me if im wrong.....

 

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Posted: Thu Feb 03, 2005 7:46 pm 
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yobboford wrote:
i think the wet test you are refering to is when you check the compression with a compression tester guage, thru the sparkplug hole, by turning the motor over (with the coil lead disconnected) to get a reading of the pressure in the cylinder, then you squirt a bit of oil in thru the hole and check again, if the reading is higher then this generally indicates your rings are worn, the oil provides a better seal around the rings and helps them seal better temporarily to give you the higher reading. but remember even if your rings are good it will still give a bit higher reading, but you will notice it dramatically if the rings are shot.

when you first check (dry) i think from memory it should be around 120-150 psi, anyone correct me if im wrong.....


You guys are talking about 2 differant things. Kenny is refering to measuring the volume of the combustion chamber with a liquid (usually water) to calculate the compresion ratio. This is done with the heads of the engine.

 

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Posted: Fri Feb 04, 2005 12:54 am 
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madmax wrote:
yobboford wrote:
i think the wet test you are refering to is when you check the compression with a compression tester guage, thru the sparkplug hole, by turning the motor over (with the coil lead disconnected) to get a reading of the pressure in the cylinder, then you squirt a bit of oil in thru the hole and check again, if the reading is higher then this generally indicates your rings are worn, the oil provides a better seal around the rings and helps them seal better temporarily to give you the higher reading. but remember even if your rings are good it will still give a bit higher reading, but you will notice it dramatically if the rings are shot.

when you first check (dry) i think from memory it should be around 120-150 psi, anyone correct me if im wrong.....


You guys are talking about 2 differant things. Kenny is refering to measuring the volume of the combustion chamber with a liquid (usually water) to calculate the compresion ratio. This is done with the heads of the engine.


yeah but "xwel" asked what a wet compression test was, "yoboford" had the right answer, what "kenny" said was for working out static compression ratio, that isn't classed as a wet compression test, I wouldn't even class it as a test, just a measurement.

 

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Posted: Fri Feb 04, 2005 1:00 am 
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std windsors are 9 to 1..
I have 7.8 compression and my comp test came to 150 / 155 on all of them...Good ole C.P pistons....

 

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Posted: Fri Feb 04, 2005 1:50 am 
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EBXR8380 wrote:
std windsors are 9 to 1..
I have 7.8 compression and my comp test came to 150 / 155 on all of them...Good ole C.P pistons....


The actual compression pressure can change relating to camshaft overlap etc as well, less duration more pressure.

 

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Posted: Mon Feb 07, 2005 12:20 pm 
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yobboford ford had the answer i was looking for but thanks for the info guys i also think the compression for all 8 cyclinders has to be roughly the same in all

 

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Posted: Mon Feb 07, 2005 12:34 pm 
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EEC can do a cylinder balance check as part of the diagnostic suite.... all documented in the Fordmods EEC-V doco i believe.

 

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Posted: Sat Jul 30, 2005 10:04 pm 
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I too have a 9 . 1 compression V8 and I'm wondering what compression ratio is 950Kpa as my compression tester only reads in Kpa
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Posted: Sun Jul 31, 2005 6:27 pm 
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To answer my own question here. The 9.0:1 compression in kilopascals should read 1,100KPa. So 950KPa is 8.0:1. So new rings in order.
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Posted: Sun Jul 31, 2005 11:02 pm 
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Should be around 160 psi...

 

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Posted: Sun Jul 31, 2005 11:08 pm 
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Yeah I worked that out. 160 PSI, 11 Bar or 1,100 KPa.
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Posted: Mon Aug 01, 2005 10:31 pm 
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Faircon wrote:
To answer my own question here. The 9.0:1 compression in kilopascals should read 1,100KPa. So 950KPa is 8.0:1. So new rings in order.


This is NOT CORRECT, it is imposible to work out compression by multiplying the CLAIMED static ratio by some value which I assume you are using is atmophere. Just think about it, if you fully understand how an engine works its obvious.

2 reasons are:
-Manufacturers do not blueprint a motor from the factory, rarely is the claimed compression ratio equal to the actual.
-The Cam opening and closing events are not at TDC and BDC so the engine cannot "statically" compress to the actual compression ratio.

Just closing the LSA by 2 degrees on a cam profile will noticeably increase the cranking compression. Most people assume its the other way around.

The proper way to check the engine without opening it up is to perform a leakdown test.
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Posted: Mon Aug 01, 2005 11:23 pm 
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I needed to know if it was time for new rings and my method was correct because the ring gap was 0.40mm which is towards the max ring gap of 0.50mm which explains to me why the compression tester indicated 950KPa when it should be 1,100KPa.
So I'm in the process of installing new rings and bearings which should be good for a couple hundred thousand or more K's.
I'm happy with my method.
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