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drop the comp ratio further, which seems the wrong thing to do when one of the benefits of going 16v is the potential to run higher comp before the onset of knock.
I am shooting at lower CR because I want more boost/safety headroom.
Completely separated twin scroll EFR turbo should kick in sooner than stock turbo and give back some of what is lost with lower CR. (my theory, could be wrong)
Originally Posted by blade7
Interesting, I wonder if the original CR was calculated using a turbo head or the 2.7 head most people would probably use.
I have all versions of 8v heads, 968 head and most of piston types now. I must take some time and cc everything.
Left is my 8v Andial/Mahle piston, right is HMR 16V T piston. I want to get exact positions for those two cutouts. One bowl shape is as good as another.
sorry to go a little OT here, but with the 8v Andial piston bowl that shape...
which way should the piston be oriented in the bore?
should the flat, raised portion (not the dish) be on the exhaust or spark plug side of the cylinder?
i would think it should be on the exhaust side, so the flat spot matches up to the flat spot on the head as the dish matches the head chamber pretty well in shape. D shaped dish, D shaped chamber, shouldn't the "D"s line up?
I still think machining the 8v pistons to suit 16v will drop the CR a bit too far, and Wossner or Heritage Mahle 16v pistons would be more efficient...
What are the power goals preventing you from running the 16v NA pistons as-is, or maybe machining the bowl a little more?
For 6 cc it might be half a point of CR?
Running math again, how are you getting those CR numbers?
Using the SummitRacing CR calculator, all-stock 944S2 info (104x88, 42cc chamber, 0 deck height) but 1.5mm gasket, puts CR at about 9.5:1...and your 8v pistons are 10:1.
You sure you aren't plugging in 8v chamber by accident (56cc or so)...that would put the S2/8v pistons at 8.3-8.8:1 respectively.
I still think machining the 8v pistons to suit 16v will drop the CR a bit too far, and Wossner or Heritage Mahle 16v pistons would be more efficient...
Math says Ill probably be short at about 9.0:1 so Ill have to use at least 1.5mm HG to get near 8.0:1, but it will be difficult if bowl depth stays as is.
Originally Posted by V2Rocket_aka944
What are the power goals preventing you from running the 16v NA pistons as-is, or maybe machining the bowl a little more?
For 6 cc it might be half a point of CR?
Running math again, how are you getting those CR numbers?
Using the SummitRacing CR calculator, all-stock 944S2 info (104x88, 42cc chamber, 0 deck height) but 1.5mm gasket, puts CR at about 9.5:1...and your 8v pistons are 10:1.
You sure you aren't plugging in 8v chamber by accident (56cc or so)...that would put the S2/8v pistons at 8.3-8.8:1 respectively.
Id like to get at about 550whp +/- and not swap head gaskets every two months.
I did my calculation like this but deck height is just guess, I have no idea what will turn out since the block was decked. Ill probably compensate with HG thickness or if machinist wont complicate things complete bowl will be machined slightly deeper.
This is 10cc. I am bending plexi a bit with finger here, so water level is a bit higher than it should be.
Machined piston should get +/- 10 extra cc, making it ~43. For some reason I thought it was somewhere around 0.5 below deck, but just checked it and since my block was decked, it should even stick out a bit. Damn. It'll be hard to get much below 9:1.
Same calculation with deck clearance + 0.20mm is 8.819:1
maybe you can machine out the flat spot and maximize the dish.
or...drop it further down into the hole with a skimmed piston top or shorter rod, like your best friend raceboy
09-21-2015, 04:54 AM
