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How to work out a comp ratio 

 

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 Post subject: How to work out a comp ratio
Posted: Mon May 09, 2011 2:46 pm 
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Just wondering guys how would one work out how much to deck to block or shave the head to bump the compression ratio up to 11.1 or even higher i know you would need some hi comp pistons help along the way but is there a rule to doing this? :mrgreen:

 

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 Post subject: Re: How to work out a comp ratio
Posted: Mon May 09, 2011 3:11 pm 
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as a side note can you work out the comp ratio from a compresion test?

 

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 Post subject: Re: How to work out a comp ratio
Posted: Mon May 09, 2011 6:11 pm 
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ausedwagon wrote:
as a side note can you work out the comp ratio from a compresion test?


people will pretend you can. but you can't
the closing point ABDC will determin cranking (dynamic) compression pressure.

 

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 Post subject: Re: How to work out a comp ratio
Posted: Mon May 09, 2011 10:39 pm 
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Why would you want a 11:1 compression ratio?

 

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 Post subject: Re: How to work out a comp ratio
Posted: Mon May 09, 2011 10:43 pm 
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to make HP.
Big cams need big compression. Most people on here don't run enough compression for the cams they are using.

 

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 Post subject: Re: How to work out a comp ratio
Posted: Mon May 09, 2011 11:00 pm 
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tickford_6 wrote:
to make HP.
Big cams need big compression. Most people on here don't run enough compression for the cams they are using.


What kinda cam's that you can use for an AU would realy need that though?

 

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 Post subject: Re: How to work out a comp ratio
Posted: Tue May 10, 2011 1:37 am 
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shyun wrote:
tickford_6 wrote:
to make HP.
Big cams need big compression. Most people on here don't run enough compression for the cams they are using.


What kinda cam's that you can use for an AU would realy need that though?


809
Rough idle, strong upper mid
range and top end, modified engines only. Min 11:1 comp ratio.
RPM RANGE 2800-6200

MAN
OR
HI-STALL 290˚ 290˚ 247˚ 247˚ .541" .541" 108˚ HYD HYD

 

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 Post subject: Re: How to work out a comp ratio
Posted: Tue May 10, 2011 5:08 pm 
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shyun wrote:
tickford_6 wrote:
to make HP.
Big cams need big compression. Most people on here don't run enough compression for the cams they are using.


What kinda cam's that you can use for an AU would realy need that though?



Crowcams: 2232546 and 2232550, the 2232549 should have at least 10:1

 

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 Post subject: Re: How to work out a comp ratio
Posted: Tue May 10, 2011 5:39 pm 
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So on the subject how do you figure it out on average?
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 Post subject: Re: How to work out a comp ratio
Posted: Tue May 10, 2011 8:02 pm 
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A lesson in car math might help.

Displacement is half the bore x itself x 3.14159 x the stroke. The 4.0L Ford 6 has a bore of 92.26mm and stroke of 99.3mm, giving a displacement of 663.8cc (cubic centimeters) per cylinder, or 3983.1cc/3.983 litres total for all 6 cylinders.

If you know the compression ratio to start with, you can determine the combustion chamber volume and how much you need to shave to achieve the desired compression ratio. Assume a starting TRUE compression ratio of 9.0:1 (Jim Mock says an advertised EF ratio of 9.3:1 often measures out as low as 8.7:1 actual). That says the volume above the piston at the bottom of the stroke is 9.0 times the volume at the top of the stroke (the combustion chamber volume). Therefore, the cylinder displacement must be 8.0 (9.0 minus 1) times the combustion chamber volume. A displacement of 663.8cc gives a combustion chamber volume of 83.0cc.

So, to get a target compression ratio of 11.0:1, we need to reduce the combustion chamber volume to 66.4cc - 663.8 divided by 10 (11.0 minus 1) - a drop of 16.6cc. As you are already using the AU head gasket, you can't reduce chamber volume that way (the AU gasket is .030in/0.76mm thinner than the EA-EL gasket, reducing the chamber by 5.1cc). That leaves shaving the head and/or using high-compression pistons.

Each .010in/0.25mm slice off the head is worth about 1.4cc of chamber volume. If the chamber in the head was the same shape as the bore, this would be 1.7cc (using the displacement calculation) - but the chamber is oval, so you are getting less reduction in chamber volume. So, to get a 16.6cc reduction, you would have to shave about .115in/2.9mm off the head! I'm guessing that would get you clean into the coolant passages. Then there are the issues with a slack timing chain from moving the sprockets 3mm closer together. So, you are right, you would need high compression pistons to achieve a 2 point increase in compression.

The AU3 engine is advertised as 9.6:1 compression. If this is correct, you could achieve 10.6:1 compression - a full point - by shaving .060"/1.5mm off the head. Does anyone out there know for sure how much can safely be taken off an AU head?

 

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 Post subject: Re: How to work out a comp ratio
Posted: Tue May 10, 2011 10:18 pm 
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ive been told its safe to shave about 25 thou of the head but people tell me its better the deck the block so you can shave ur head if warped or for what ever reason.

 

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 Post subject: Re: How to work out a comp ratio
Posted: Tue May 10, 2011 11:29 pm 
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the limiting factor is going to be timing chain slack.

You can deck only deck the block so far, the AU gasket is only about 18-20 though so you'll need the pistons at least 15 thou down the bore, that only gives about 15 thou to remove from the deck of the block. Depending on actual deck hight of the block you have and how accurate the throw of each big end journal is, how accurate the rod lengths and piston compression heights are.

ACL high comp pistons with drop the volume by 5cc
The rest is going to have to come from the head.

Don't forget, for every 1mm you take off the deck or head, it gives you 2mm of timing chain slack that needs to be dealt with.

 

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 Post subject: Re: How to work out a comp ratio
Posted: Tue May 10, 2011 11:32 pm 
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Longer rods could do it

 

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 Post subject: Re: How to work out a comp ratio
Posted: Tue May 10, 2011 11:59 pm 
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I might give an engine builder a call and ask for his advice the engine im building at the moment is au s3 i see what advice they can give me on this subject :mrgreen:

 

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 Post subject: Re: How to work out a comp ratio
Posted: Wed May 11, 2011 12:13 am 
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lol ive got no idea what my comp ratio is, mls headgasket shaved head, BUT using ed pistons with a deeper dish

 

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