bojangz over at Honda-tech made this cool C/R calculator
http://www.bojangz.com/calculator/
this is really really cool!!!
but i just wonder how accurate it is. the main reason why I’m not sure if its accurate is that it conflicts some info on the FFSquad site.
here is the conflicting info:
according to ff squad:
b17a w/ usdm itr pistons: 10.6ish
b17a w/ jdm itr pistons: 10.8ish
according to this CR calculator:
b17a w/ usdm itr pistons: 10.04
b17a w/ jdm itr pistons: 10.5
Now, i really have no idea which is correct, but they are different by a significant amount.
In addition to that: I’ve been told a few times that the b16a p30 pistons are very similar to the usdm p73 pistons. According to this CR calculator the p30’s and jdm p73’s are similar, while the usdm p73’s are less than both.
so, again, i’m not sure who is correct here. Either this CR calculator, or the FF Squad and the few reputable tuner’s i’ve talked to.
anybody have any definite answers?
Hey, it seems pretty close to me. It got my setup right, my friends, and it got the stock cr’s right.
It’s on for near stock compression ratios. But off on others. To be sure, do your own calculations using the teg tips page. If you need any help, let me know. I know how to do this.
dan, i’m glad you responded. I figured you’d have some good answers. I wish that calculator was right on, cause it makes calculating different combo’s of things extremely easy.
I’ve read the teg tips CR math that you wrote up, and I consider myself a pretty math savy guy. But the part i still don’t get is how to calculate CR when you change the pistons. I understand how to find the cylinder volume, and i understand about the cylinder head volume and what not. But how do you factor in the differences in piston dome heights?
because i still want to figure out the exact compression ratios for various pistons in the b17a. Also, oem honda oversized pistons, are they .25mm over or .025" over? I’ve seen people write it both ways.
thanks dan!
Hey Dan,
How would I figure out my compression with stock b20z pistons, 0.43mm head gasket, and a b18b head? I’m thinking maybe 10 sumthing?
The stock b20z CR is 9.6 if that helps and 9.2 for the b18b
Here’s my list so far
www.geocities.com/rcnacura/b20list.doc
rcnacura: the b18b head is the same as the head on the b20 right? thus, using stock b20z pistons, and a b18b head you’ll have the stock compression.
as for the headgasket, i’m not sure what the stock head gasket is in thickness. But if .043mm is thinner than stock then you can easily figure out how much it will affect compression by reading dan’s teg tip on calculating compression. Just calculate it like you had the head milled the amount that the head gasket is thinner.
GOT IT!
My compression ratio would be 10.2421084662 or
10.24.
Not bad, but would liek to get it maybe a little higher.
Hey Colin, check this link out. That post will tell you how to calculate compression with different pistons. I’ll have to add that to the teg tips…or maybe I’ll just do a bunch of calculations on my own and post them up. Depends on what I have time for. hehehe. Let me know if you have any questions. Later.
Here is a question. How would I find my compression ratio considering my head was milled to an unknown amount? All other things being stock.
Not to long after I bought my car and before I even knew what a GS-R was, my radiator cracked and I melted my head. They flattened it back out, but I don’t know how much they took off the head.
-Steve
You need to know how much was taken off the head. A way to do this is to measure a stock head and then measure yours. The difference is how much yours was milled. There’s no other way to find your compression without knowing how much material was removed from your head.
Ok…where do I measure from? Obviously where the head meets the block, but that isn’t striaght line from there to the valve cover…its at an angle.
-Steve