Unless you super or turbo charge, the path to more horsepower is to boost the air flow. Either make the holes bigger or move the air faster. You can always adjust the fuel ratio..............except in nitro motors where the fuel makes its own oxygen and it's possible to feed so much that the engine hydrolocks during compression goes boom.
Well there's just a bit more than that. You can also direct the air to the optimum place in the combustion chamber for a good burn.
1. work the air passages in the manifold and head to smooth them and direct the air better.
2. work the val