Ive seen a few people who have said that they ordered one type of piston, yet they ended up with a different compression ratio. This is why I think that is happening.

By looking at their site, I figure they only have 3 different compression size pistons per bore size.
For example here are their B20Z Pistons:

FI 84.5 mm 9.26-9.66:1

NA (Low) 84.5 mm 11.41-12.07:1

NA (High) 84.5 mm 12.17-12.93:1


Now see how they actually list xx.xx - xx.xx per each bore (which originally led me to believe that you can pick anywhere in there and they send you THAT pistons)...but what I think it is, is that its a one size thing and then the customer has to deck the block to get a certain compression ratio.
It even says on thei site "...and piston-to-deck clearances of 0.020" and 0.000". "
So for example look at NA (Low) 84.5mm.
It states 11.41-12.07:1. I figure if you have a deck height of 0.020" you will get 11.41:1 and if you have a 0.00" deck height you will get 12.07:1...make sense?


Its a wonderful thing when you can actually understand what all these numbers are.

 

Another easy solution to this problem is the headgasket. Which is why I stock different headgaskets to go along with different setups. If a customer is looking for 11.7:1, I'll be able to bring them as close as possible to the desired number using X piston a Y gasket.

I have the gaskets in a variety of thicknesses and bores to compensate for almost any possible motor build-up.

 

Yep...and that kicks *** Mike!

The other nice thing about head gaskets, is that you can "reverse" the compression ratio. Say you go with 11.7:1 with a .030" head gasket and it detonates. You obviously can't add on to the deck height..but you can get a thicker gasket and drop the compression (Duh! )

 

Question: From Tuan's posted calculations an SI's combustion chamber is 42cc. If the head is ported and the valves are deshrouded, the cc volume changes correct? Will the porter be able to tell you the new cc?

 

This Belongs in Tech, Not ITR forum.

 

Yes, a machine shop can CC the head for you...or you can do it yourself.

Its a process of using a piece of glass/plexiglass with a small hole in it.
Put the piece of glass over the combustion chamber and inject a liquid from a measured instrument and see how mcuh liquid it takes to fill the chamber.

I will be getting my head deshrouded and Ill either try this or have the machine shop tell me.

 

HX_Guy: I would actually do this myself that way you know for sure that all of the chambers in the head are the same.