4 replies to this topic

[Gal]

Standard Pistons from what I can understand are 9.4 : 1, What would I need to run 10-12 PSi without any detonation. I am thinking 8.5 or lower.

Also there is a dish in the top of the piston as well, Although this is measured in cc? What does this mean?

And any recommendations on a decent supplier of pistons in standard 1275 size, No overbore.

[Turbo Phil]

9.4-1 is the compression ratio, this doesn't refer to the type of piston used.
The Metro Turbo pistons have a 12cc dish in them. This means if you put a flat object over the top of the piston, put a small hole in it and then proceeded to fill the dish area below with liquid, it would take a volume of 12cc to fill it.
The larger the volume above the piston, the lower the compression ratio. This is why people fit pistons with a larger dish or a head with bigger chambers when trying to lower the compression ratio.
8.5-1 should be fine to run 12psi providing the engine is correctly set up, at that kind of boost an intercooler would be a useful consideration.

Phil.

[Gal]

Alot of the places I have been looking though only do overbore sizes. But I don't want them to be honest.

I'd rather keep the original size but lower the compression. Hence the Piston question.

12cc is a large dish to start with though isn't it? or are there larger sizes?

[Sprocket]

Machine the top off the piston reducing compression hieght, increasing deck to piston clearance and there by incresing combustion chamber volume. As for how much to remove to achieve 8.5:1 CR can be difficult to calculate until the machining has been done. Standard turbo pistons are cheap because everyone seems to go for overbores so if you manage to get it wrong then it won't cost a fortune.

[Sprocket]

Alot of the places I have been looking though only do overbore sizes. But I don't want them to be honest.

I'd rather keep the original size but lower the compression. Hence the Piston question.

12cc is a large dish to start with though isn't it? or are there larger sizes?

MiniSpeed can sell you some 18cc JE Pistons at +20thou but will set you back 400+ notes. The best you can get suposedly.

http://www.minispeed...duct&id=je-18cc

Looks like there is a possibility to machine the Standard piston to the same crown profile that the JE piston has, without reducing the strength too much.