It is the stock exhaust manifold AND the cam.... and the lower stock compression.
The advantage of going with headers and a good exhaust is that it will help all across the RPM range. It will help with torque, power, and with mileage. IMHO you are on the right track with the cam type:
- Keep the duration low to maintain low/mid RPM torque
- Keep the lift high to help cylinder filling and lower end torque
- Look at LSA to either help mileage and lower end torque (wider LSA) or help top end power (narrower LSA)
- Cam advance helps low end torque; many cams have this ground in. (Example: If LSA is 112 and ICL is 108, then the cam has 4 degrees of ground-in advance.) More advacne helps lower end torque.
If you notice, I keep mentioning lower end torque. This is for all-around street manners. 318's are lower compression and suffer quickly from loss of low end torque if too wild a cam is used
Long term thinking, breathing in the head will only help high RPM power, and the low end torque can be managed with the cam choice.
