working out compression ratios
07-13-2019, 10:49 PM
Post: #1
working out compression ratios
Hi all,

Im trying to work out what CR pistons to go for when I rebuild my 16v. I've read online (now i might have got this wrong) that you are suppose to measure the deck clearance when the piston is at TDC and are able to work out, or at least help work out what CR you are able to run.

My question is, how can you do this or how can you check what CR you can go with if the engine is stripped to a naked block? I know that cams (if above stock) have a part to play as well.

Basically looking on different ways that I can investigate have CR I could possibly use.

Hope this makes sense!
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07-14-2019, 02:12 PM (This post was last modified: 07-14-2019 02:15 PM by totta Crolla.)
Post: #2
working out compression ratios
Apart from physically being able to achieve the actual ratio required, 4age's are not normally a problem, the limiting factor to compression ratio is generally the fuel octane and the ability of the ecu to control knock. Knock control is really just a failsafe and should only really be utilised when bad fuel is used.
As far as actually measuring, yes the piston should be installed in the block with the rings fitted and it should be connected to the crank.
Most piston manufacturers will tell you the specs. of the piston including any difference to the standard piston, raised piston crown cc etc.

An analogue brain in a digital World
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07-16-2019, 08:22 PM (This post was last modified: 07-16-2019 08:25 PM by oldeskewltoy.)
Post: #3
working out compression ratios
(07-13-2019 10:49 PM)kazi Wrote:  Hi all,

Im trying to work out what CR pistons to go for when I rebuild my 16v. I've read online (now i might have got this wrong) that you are suppose to measure the deck clearance when the piston is at TDC and are able to work out, or at least help work out what CR you are able to run.

My question is, how can you do this or how can you check what CR you can go with if the engine is stripped to a naked block? I know that cams (if above stock) have a part to play as well.

Basically looking on different ways that I can investigate have CR I could possibly use.

Hope this makes sense!


there are specifications listed "around". You do need to know your final deck height to properly calculate compression. I would strongly recommend using a compression calculator, this is the one I like to use - http://www.csgnetwork.com/compcalc.html

Specs you will need: bore, stroke, deck measurement, chamber measurement gasket thickness, AND bore, and piston dome volume.

It is common to need to machine the top of the block a small amount (minimum .003"/.075mm) to get a good seal for the head gasket. This is more frequently done when using a metal head gasket. This procedure changes the deck measurement.

While knowing the deck is important, what is more important is properly setting the squish gap (some refer to as quench gap). In a 4AGE a final squish gap of .032"/.8mm seems to work very well.

Squish gap is the distance the flat areas of the pistons are away from the flat areas on the head when the piston is at TDC.

How to "adjust" the squish/quench area - 1) different piston with the wrist pin height(aka pistons compression height) changed, 2) thicker/thinner head gaskets, 3) machine the deck. Often times you would use a combination of 2 of the 3, or in some cases all 3.

As to how much compression the engine can tolerate before detonation - a LOT has to do with the condition of the combustion chambers. Quality of fuel is also very important.

With a properly shaped chamber, I've run a stock cam on 10.8:1 utilizing 92 octane fuels (USA premium) with no detonation.

Dan -

You can |Sad .... OR you can ask for help!!!

OST Porting service - http://hachiroku.net/forums/showthread.php?t=19991
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