Can anyone provide me with an article or directions on calculating compression? I’ve searched, read a few articles, and am still confused. It seems like the common method is to use a liquid in the cylinders and head to measure how much volume there is. However my pistons extend up out of the block, so it’s not really possible to measure the volume with a liquid as there is negative volume…

My setup is:
B18A block
P30 pistons
OEM B18A rods
OEM B18A crank

The online compression calculators seem to put it anywhere from 11.5 to 13. I’m hoping it’s under 12.5 though. I’d like to find out exactly what it is to help determine if I’ll be able to run off premium occasionally, or E-85 only.

There is some variation there but they’re closer than the numbers you posted. What calculator(s) are you using? If you really want to nail down the correct ratio you can search to see if people know that one calculator is right or wrong (but chances are they are all mostly right but have small errors here and there. Or you can follow the actual formulas for CR. But for that you’ll need to search for, or calculate the variables needed for those formulas. The generic Zeal site above may help with that, or this G2IC Teg Tip or just google…

Also, if you’re running 11:1 you’re going to want to run premium (at least 91) all the time. It’s generally accepted that 11:1 is a good safe level for 91 octane as long as your tune is good. You can go higher on 91 but that will depend on your exact situation and setup.

If you’re planning on upping your horsepower and upping your compression then you should plan on upping your gas budget to match. You shouldn’t be trying to run regular gas and high compression. In order to do that your tune is going to be a “de-tune” to compromise for the lack of added octane level.

[QUOTE=Colin;2016427]
C-Speed: 11.01:1
Zeal: 11.61:1
FF-Squad: 10.99:1[/QUOTE]c-speed is the first cal that zeal put out. it was off, hence the numbers. the zeal you listed that is b-series specific is the best one i have found. didn’t know ff-squad put the cal back up. i would guess that they are off also. i would need to look at thier d-series specs to be sure. those are the ones i can spot off the bat with little verification.

My guess would be that when these calculators are off they aren’t off on a consistent basis. The formula is basic, I doubt anyone has that wrong. Where I would expect errors is in the variables for each. Such as cylinder head volume, deck height, piston dome volume… So all the calculations for one calculator may be correct unless, for example, when using PCT pistons if that value is incorrect.

[QUOTE=Colin;2016427]
There is some variation there but they’re closer than the numbers you posted. What calculator(s) are you using? If you really want to nail down the correct ratio you can search to see if people know that one calculator is right or wrong (but chances are they are all mostly right but have small errors here and there. [/QUOTE]

I forgot to mention my block and head have both been milled to deal with warpage…so I’m sitting a bit higher.

I can’t count the number of times I’ve read on here that you shouldn’t trust those calculators. Those all show a positive piston to deck height. It’s quite obvious that the piston tops sit higher than the deck, so i don’t see how those can be right. I think it’s got to be a negative value.

I read a thread on here, which i can’t find now, where someone (who i thought was credible) said the ratio with P30’s in a stock B18A setup was closer to 12.5:1…due to the negative deck height.

[QUOTE=Colin;2016427]
Or you can follow the actual formulas for CR. But for that you’ll need to search for, or calculate the variables needed for those formulas. The generic Zeal site above may help with that, or this G2IC Teg Tip or just google…[/QUOTE]

yea, that’s what i’ve been doing. The problem is that the articles, as well as the teg tips, all need the combustion chamber volume to calculate the ratio. And given that I have a negative volume, i don’t know how to measure it. The articles i’ve read say to measure the amount of liquid to fill the cylinders…but that doesn’t work when the pistons stick out.

thanks for the comments though…maybe i’m just overthinking it.

I know i need premium at least (89 octane up here at altitude, but works out to the same as 91 at sea level).

That’s my concern…that my setup is higher CR than acceptable for 91. I’m worried that premium will not be high enough octane. I’m planning to build it to run off of E85 (95+ octane), But I’m looking to find out if I’ll be able to run premium sometimes as well, or ONLY E85.

Actually i think i just found that thread I mentioned and I think i was confused…it’s the JDM B16B (PCT) pistons that were the higher 12.5, not the JDM B16A (P30) pistons I’m using.

according to the calculators, I’d be around 12(c-speed) - 12.7(zeal) with my decking figured in. I’d think 12 could be alright on premium, but maybe not 12.7.
As 90DB1T pointed out though that zeal one could be off.

regardless, can someone help me out with how to determine my volumes in order to calculate my actual CR?

This thread should help. It’s got some of the formulas explained, some diagrams and even some specs (hopefully they are correct). I haven’t crunched any of the numbers to see what they turn out like (I’m watching Southpark and not feeling like doing math at the moment)… but it’ll be a pretty simple task. Pencil and paper time!

[QUOTE=Colin;2016610]This thread should help. It’s got some of the formulas explained, some diagrams and even some specs (hopefully they are correct). I haven’t crunched any of the numbers to see what they turn out like (I’m watching Southpark and not feeling like doing math at the moment)… but it’ll be a pretty simple task. Pencil and paper time!

exactly. the only way to truly equate c.r. is thru precise measurement of the cc’s, p2d height, piston dome etc. i mean do any of those calculators list the 18a/b at a different deck height as the c1/c5? most people don’t even know that the 18a/b block is a few mm taller