working out compression ratios...

**slide86** · 17-03-2006, 09:51 PM

howdy all.
Does anybody know of a website or does anyone have formulas for calculating the compression ratio of a cylinder??

I am using bigport ae92 4agze, colipacks, afm engine with a HKS 2.0mm head gasket. im trying to work out if it would be better to use this gasket or the genuine toyota one that came in the rebuild kit. the head is standard (its been faced) and the block has been decked.

cheers
huw

**oldcorollas** · 17-03-2006, 09:53 PM

easy

(volume of combustion chamber and head gasket and swept volume) / (volume of everything except swept volume)
= compression ratio.

for a motor where you know the stock compression ratio and stock swept volume, and thickness of crushed head gasket, you can work out all the other details, and then apply changes like you mention (ie thicker HG)

edit: for decking, it's the same as head gasket volume, but in reverse.. requires that you know how much has been decked.

**slide86** · 17-03-2006, 10:08 PM

man you are smart! hehe
what is swept volume? i havent heard of this before. and whats the best way to determine the volume of the CC?

Man, i could build an engine in a couple of days but i cant work this out! i should get the tradeschool books out i suppose!

**roadsailing** · 17-03-2006, 10:14 PM

oldcorolas is on the money but,

keep in mind that static compression ratio means nearly nothing.

if you want to get your compression to a certain level (for boost or whatever) just speak to whoever makes the headgaskets

**oldcorollas** · 17-03-2006, 10:16 PM

swept volume = volume "swept" by the piston as it moves.

as i said

(volume of combustion chamber and head gasket and swept volume) / (volume of everything except swept volume)
= compression ratio.

swept volume comes from bore and stroke.
head gasket volume comes from thickness of gasket and diameter of bore.
the compression ratio you KNOW for a stock engine

so.....

the only unknown in the above equation is the combustion chamber volume (which takes into accoutn and dish or bulge in the pistons)

start writing down numbers

oh yeah.. dynamic compression (taking into account valve timing) is more important than static compression.. but it's harder to work out

**RWDboy** · 17-03-2006, 10:19 PM

Originally Posted by oldcorollas

easy

(volume of combustion chamber and head gasket and swept volume) / (volume of everything except swept volume)
= compression ratio.

for a motor where you know the stock compression ratio and stock swept volume, and thickness of crushed head gasket, you can work out all the other details, and then apply changes like you mention (ie thicker HG)

edit: for decking, it's the same as head gasket volume, but in reverse.. requires that you know how much has been decked.

Yeah can be a total pain in the arse if the piston is crown/dish shaped and make sure that the spark plug is included as well

Could be as little as 1/2 a cc and it could still alter your static compression ratio by .1 or .2

**slide86** · 17-03-2006, 10:28 PM

ok. looks like a job for tomorrow. along with the 10000 other jobs i have to do.
wawawa

**oldcorollas** · 17-03-2006, 10:36 PM

Originally Posted by RWDboy

Yeah can be a total pain in the arse if the piston is crown/dish shaped and make sure that the spark plug is included as well

Could be as little as 1/2 a cc and it could still alter your static compression ratio by .1 or .2

actually, it makes little difference if you are starting from a known compression and modifying... since the stock compression already takes eveything into account

the shape or whatever of piston and chamber have no bearign if all you need to know is the volume (which you back-calculate from known CR)

**RWDboy** · 17-03-2006, 11:10 PM

Yeah but how accurate is the *known* CR

It's not like manufacturers provide that information.

Depends on how accurate you want your answer (new CR) to be. I admit it'd be a little pedantic to worry about every 0.01cc as you can always tune it later on anyway. Plus the dynamic compression, squish areas, materials used etc and other factors are probably more performance limiting than the last 0.1 of the static compression ratio.

**oldcorollas** · 17-03-2006, 11:26 PM

thats very true, but toyota quote their CR's.. such as 8.9 or 8.4 etc.. for a built engine i would have no hesitation in grabbing my burette and measuring, but for a bodgy measurement, for a rule of thumb choice between gaskets, it's prolyl ok, esp as SBS mentions, the dynamic compression is more important, and as yo mention, there are much more important design aspects...

however, here, since he is just deciding between gasket thicknesses based on CR, then the basic theory works .. however badly...

actually, that reminds me...

choose the gasket which gives you the correct quench height (i forget proper term) between piston and head... meant to be maybe 40 thou? too thick a gasket kills that, too thin risks head to piston contact...
perhaps this should be a more mechanical choice than CR..

fwiw on K motor (for quick reference.. since it is 1300cc), difference between 9 and 10CR is about 3mls, and headgasket (0.8mm) is about 3.5mls, so at a rough guess, 1mm difference in gasket should make at least 1CR difference.

but please calculate for 4AG adn let us know

(sorry, too busy atm)

**YelloRolla** · 19-03-2006, 02:47 PM

http://users.erols.com/srweiss/calccr.htm

here it is again.

**sillycar chick** · 22-03-2006, 10:50 AM

Hi,

Old Corollas has pretty much said it, but for some more info, read one of my threads from a few months back....

http://www.toymods.net/forums/showthread.php?t=2038

And there are so many helpful resources on the net, you just have to be patient and search for them.... heres one to start...

http://www.advanceautoparts.com/engl...0020701cr.html

I know youre probably thinking this is a lot of hard work to go to just to answer your simple question, but what is a better way to learn than to UNDERSTAND?? Good Luck, hope I was of some help.