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 |
| .05 | 30 | 26 | 32 | 28 |
| .10 | 61 | 54 | 61 | 59 |
| .15 | 91 | 88 | 93 | 92 |
| .20 | 118 | 115 | 125 | 116 |
| .25 | 144 | 136 | 155 | 145 |
| .30 | 167 | 155 | 178 | 170 |
| .35 | 186 | 171 | 197 | 193 |
| .40 | 203 | 180 | 214 | 204 |
| .45 | 217 | 183 | 231 | 209 |
| .50 | 229 | 185 | 247 (10% gain) | 210 (13% gain) |
| .55 | 237 | 187 | 261 | 212 |
| .60 | 242 | 187 | 270 | 214 |
| .65 | 240 | 186 | 260 | 213 |
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.