The Temple of VTEC - Honda and Acura Enthusiasts Online - Articles - One Man's Experience With Building a Motor: Page 3

So what compression ratio do I get for pistons? Well, I didn't completely take every variable into account and let a very busy store owner try to answer this question. But now you don't have to rely on anyone else, I'll give you the equations necessary to calculate compression ratio and you can leave the sales people alone.

CR = compression ratio

CCV = combustion chamber volume

CV = cylinder volume

CR = ( CCV + CV ) / CCV

We'll have some fun and throw in the numbers for my car.

First you have to calculate the cylinder volume of just one cylinder. The equation to do this is:

CV = (stroke)(bore)^2(Pi/4)

CV = (8.9 cm)(8.1 cm)^2(Pi/4)

CV = 458.6 cc for a stock 1990 Integra.

My decision to bore the block is going to increase the CV thus increasing the compression ratio. CV for 20 over is 464.3 cc, not much but it still going to make a difference.

Now CCV is a tricky one, but we can rearrange the equation and use the stock compression ratio number the factor gives you (Note: Actually compression is LOWER then what the factory says. But we'll just use these numbers for the example. But this equation can be used to calculate actual compression ratio if you take the time to measure the CCV while your engine is out).

Rearranging the equation CR = (CCV + CV) / CCV we get

CCV = CV / (CR - 1)

Therefore: CCV = 458.6 cc / (9.2 - 1) = 55.93 cc

CCV with 20 over bore: 464.3 / 8.2 = 56.62 cc

Other factors include head milling and head gasket thickness. Either one directly affects the CCV. If you mill the head you are decreasing the combustion chamber volume. In my case, my JG head was milled 35 thousandths (0.0889 cm). To calculate how many cubic centimeters this changes the CCV, you use the equation that's used to calculate CV; milled head volume = (amount head was milled)(bore)^2(Pi/4).

mhv = (0.0889 cm)(8.15 cm)^2(Pi/4) = 4.64 cc.

Therefore, CCV = 56.62 cc - 4.64 cc = 51.98 cc

Most head gaskets fall between 0.015-0.020". Take into account the small amount they take off when they decked your block and you are down to 0.005-0.015". The 9.2:1 for a stock 2nd Gen LS already takes into account the head gasket, so I'm not going to even take it into account here. Besides, this will just end up being a rough estimate just to give you a good example.

Calculating compression ratio: (51.98 + 464.3 cc) / 51.98 = 9.93:1

So, before I even picked what compression ratio piston to buy, I'm already right around 10.0:1; knowledge I didn't have :( Since I was sticking to NA with NOS in the near future, I was recommended JE10.5:1 compression pistons. And yes, I did mention the bore and the milled head.

So we'll first recalculate the CCV using 10.5 instead of 9.2.

CCV = 464.3 / (10.5 - 1) = 48.87 cc

Subtract the milled head volume: 48.87 cc - 4.64 cc = 44.23 cc

Recalculating compression ratio: (464.3 cc + 44.23 cc)/44.23 cc = 11.5:1

*ooooops*

Well, around Pennsylvania we have 94 octane. The car even ran fine on 93 octane. But when I went on my little tour of the country, places like Iowa and Missouri only had 91 octane (it might be the whole mid-west :(). But the car ran pretty good even on 91 octane. Besides, detonation happens almost instantaneously. If 91 octane was going to ruin my engine, I would have never had made it to Texas. But next time I travel, I'm bringing a case of octane boost *grin*. But I probably wouldn't have lasted long on even 94 octane.