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compression conversion? how does it work???? i.e. 10.1vs220 

 

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 Post subject: compression conversion? how does it work???? i.e. 10.1vs220
Posted: Mon Dec 29, 2008 8:19 am 
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hey all ...

umm ... dunno if im sounding blonde here, but i was wonder how to work out what ratio compression is ...

okie, i tested my motor's compression, and im getting around the 200 mark ... how would that work out ie. in a 9.1 compression? is there a math way of working it out???

thanks guys ..

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Posted: Mon Dec 29, 2008 11:16 am 
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You need to use your bore size at TDC and BDC and your fuel bowl size to work out your compresion ratio. I don't think there is a way to work it out off the compression tester. If there is it must be a carzy maths equasion. Standard compression is 9.3-1

 

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Posted: Mon Dec 29, 2008 2:49 pm 
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here is a rough idea..
well atleast a ball park figure..
my au engine had 200 psi std compression and std they are rated at 9.6-1..
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Posted: Mon Dec 29, 2008 3:55 pm 
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i was under the impression testing your cylinders compression (ie how much well it holds the air) isn't related to the static compression ratio of your engine (ie how much the air in the cylinder is compressed from BDC to TDC)...

 

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Posted: Mon Dec 29, 2008 4:08 pm 
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I wouldent mind knowing this formula either.

 

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Posted: Mon Dec 29, 2008 5:55 pm 
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theres no direct relationship between comp ratio and the pressure reading from a gauge, a std au will have a compression ratio of 9.6:1 from the bore and stroke divided into combustion chamber volume measurments which wont change , but if you take the rings out you'll have no pressure at all,

strange way to explain but hope this helps

 

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Posted: Mon Dec 29, 2008 8:11 pm 
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it all goes out the window when you change cams.

the intake valve closes After Bottom Dead Center. If the intake valve closes 30deg ABDC the piston is already on its way back up and loose that part of the stroke. At windover speeds air is pushed back out of the intake valve.

An engine fitted with a cam that closes the valve at 30ABDC will show more pressure than the same engine with a cam that closes the valve at 40ABDC

At (some) running speeds air is still entering the cylinder ABDC.

 

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Posted: Tue Dec 30, 2008 1:15 pm 
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tickford_6 wrote:
it all goes out the window when you change cams.

the intake valve closes After Bottom Dead Center. If the intake valve closes 30deg ABDC the piston is already on its way back up and loose that part of the stroke. At windover speeds air is pushed back out of the intake valve.

An engine fitted with a cam that closes the valve at 30ABDC will show more pressure than the same engine with a cam that closes the valve at 40ABDC

At (some) running speeds air is still entering the cylinder ABDC.


Bingo.....when you start playing with cams, you end up anwhere.
Some cams come with a warning/statement that might say stuff like " Minimum comp ratio required 10:1......11:1......12:1....etc..etc...
You could have an engine with a comp ratio of 12:1 & a dirty great big cam ,and doing a comp test and only get 160 to 180 psi.

 

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Posted: Tue Dec 30, 2008 2:28 pm 
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I had 250 psi per cylinder in my 250 xflow and it had acl 10-1 flat tops and an unknown amount shaved off the head.Should be almost the same as a 4.0
Sounds like your comp is standard and in good nick.

 

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Posted: Tue Dec 30, 2008 2:31 pm 
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Ive got one cylinder as low as 60psi, and then they work their way up to 105psi. Id stop complaining if i were you!

 

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Posted: Tue Dec 30, 2008 2:49 pm 
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Andrew J wrote:
Ive got one cylinder as low as 60psi, and then they work their way up to 105psi. Id stop complaining if i were you!



hows the search for a new engine going?

 

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Posted: Wed Dec 31, 2008 6:58 am 
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is there a website with a comp ratio calculator?? Providing you have all your specs, bore, stroke, cam ect....

 

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Posted: Wed Dec 31, 2008 9:13 pm 
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wholohan123 wrote:
is there a website with a comp ratio calculator?? Providing you have all your specs, bore, stroke, cam ect....


I think you still need to know the volume of your cylinder heads by cc'ing them with a burette
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