How well do the stock (and ported) cylinder heads flow?
The following table gives air flow data for stock 1997 GTS cylinder heads, as well as the flow numbers following a porting performed by Caldwell Development (the GTS-R folks). The stock head Intake port volume is 214cc, the Exhaust port volume is 70cc and the Combustion chamber is 73ccs, for reference.
|Valve Lift||Stock Heads||Ported Heads|
|(inches)||Intake CFM||Exhaust CFM||Intake CFM||Exhaust CFM|
|.50||229||185||247 (10% gain)||210 (13% gain)|
What does this mean? Well, after dynoing the GTS before and after, the modified heads (which also resulted in a compression ratio increase to 10:1 from the stock 9.6:1) were good for an additional 67 horsepower. When GTS-R headers (1 7/8" primaries, 42" long) were installed, the motor made 100 hp more than stock. Interestingly, there was little difference in power output between 1.6:1 and 1.7:1 ratio rocker arms.