i need to find out what compression ratio i have so that i will know what boost i can run

i have an a12 with a14 head surfaced 60 thou

get a compressiong tester take all the spark plugs out hold the compressiong tester firmily in the spark plug holes and get someone to turn the car over for 5secs or so then you will no the compression

Something about Pies, springs to mind

Area X Stroke + Chamber CC Divided by Chamber CC. But don't quote me on that

its pie x number of cylinders x stroke length x Cylinder Diameter divided by 4, some #### like that anyway but that wont work in this case coz its got a diff head on it and its been shaved

Try this....Volume at BDC/Volume at TDC= C.R.

Remember volume of combustion chamber has three components, head chamber volume usually expressed in CC (cubic centrimetres), volume below deck, and volume occupied by the compressed head gasket.
Happy computing....

your engine needs to be apart for a true CR ratio check.

okay -

measure the volume of your cylinder at BDC.

{(pi * r)* 2} * stroke

so {3.14159 * half your bore} squared * stroke length.

measure the volume of your head chamber with an eye dropper. don't let the water bulge either.

divide the cylinder volume by the head volume and voila - total CR.

don't forget to minus the head gasket thickness and any dish in the piston, or to add any raised piston crown volume.

You should also be able to work it out using physics....... boyle's law I think. I'll find out how difficult it is to calculate.

OK this is going to be a bit long winded, but not all that difficult.

Pro 240c is right in saying that you have to take it apart to do it properly, but you can get a reasonable approximation with fairly straight forward maths (or the exact answer with complex maths).

There are 2 main issues, one is the assumption of standard temperature & pressure (25 deg.C & 101.3kpa or 14.7 psi). and the other is the quality of your pressure gauge. I'll ignore these probs for the mo.

I'll use the A-15 (cos I know the numbers)
Bore 7.60cm, stroke 8.20 cm (work in cm rather than mm so you get cc's)

area= pi*r^2 (radius is 3.80cm)
so 3.1412* (3.8*3.8)
=45.3645 cm^2

* by stroke to get volume

45.3645 * 8.20 = 371.98 cc per cylinder (1488cc total..... is that what the book says )

Now the tricky bit. The cylinder is sorted, but you dont know the extra cc from the combustion chamber.

Boyle reckons that when you compress a gas,

pressure 1 * volume 1 = pressure 2 * volume 2.

if you rearrange it you can get:

V2 = P1 * V1 / P2


so, chamber cc = 14.7psi * cyl. volume (L) / comp. test result (psi)



e.g. 14.7 * 0.3798 / 170 = 0.032841 or 32.8cc

Sweet

But, the lunch bell hasn't rung yet. Now that you know the chamber size the original assumption about cylinder volume is incorrect cos you havn't accounted for the chamber. the new volume is:

371.98 + 32.84 = 404.82 cc

then you take 404.82, plug it back into the top of the equation to get a revised answer of 35.01cc

and again to get 35.19

and you can keep going till you get bored, or stop when you reckon inaccuracies in your pressure gauge will have a greater effect on the answer .

now that you know the cc's you can calculate compression using the equation in pro240C's post.

Man, I wish my year 9 teacher had put that 5hit into the context of hotting up cars, I might of done better than 23% and a note to the oldies!!!

Yeah compression ratio is just the ratio of the displacement+combustion chamber to the combustion chamber alone.

Example: If the displacement of one cylinder is .35 liters and the combustion chamber (the entire volume above the piston at TDC) is .05 liters then the compression ratio is (.35+.05)/.05 = .4/.05 = 8.0 or 8.0:1.

Now all you need to know is your displacement and your combustion chamber volume and you're done. Obviously measuring the combustion chamber volume is the tricky part. Don't forget the volume due to the thickness of the head gasket, the dish volume on the top of the piston, and also the space between the top of the piston and block deck at top dead center. These little bits can make a big difference. I know because I just went through all of that on my engine rebuild.

Michael