The second factor is combustion chamber volume in CC’s (cubic centimeters). This (along with the volume of the piston dome) affects the compression ratio of the engine. If fuel octane and kick starting weren’t a consideration, then generally "more compression ratio means more horsepower". Now back to the real world! With the limitations of the new fuel blends compression ratios in the 9.5:1 to 10.5:1 range should be considered maximum for a street engine. With higher compression ratios fuel octane becomes very critical, especially in an air cooled engine. Detonation (uncontrolled combustion) can do major damage in a short amount of time. It is important to know that by the time you hear pinging in your engine it has gone through a whole range of in-audible pinging, all doing damage. So if you have a choice it is better to go with a lower compression ratio. Another problem with the higher compression ratios is that the piston dome becomes a major factor in combustion chamber flame travel. I set combustion chamber volume to get the desired compression ratio. All heads are CC’d, and the combustion chamber volumes matched to give identical compression ratios in each cylinder.

    The third factor is combustion chamber finish. Especially on air cooled engines I prefer to polish the combustion chamber. This reduces heat transfer to the head and drastically reduces carbon build-up.

    Generally the more valve area you can get into a high performance combustion chamber the better. That being said there are several factors that effect valve area.

    The first factor is logistics. You can only fit so much valve into a given space. Also, since both valves will be open at the same time during the valve cycle, they need to miss each other. The larger valves do improve overall flow. For maximum performance these are worth the investment. . In the 1200 to 4500 RPM range (this is where a Harley spends most of it time) I prefer the stock valve sizes to the larger valves. If you decide to go with the larger valves I can install oversized valves and valve seats . While we are on the subject of valve seats, Shovelhead, Panhead, Knucklehead and Ironhead Sportster would definitely benefit from having new valves and hardened valve seats installed. They were all designed before the new no-lead gas. A high Nickel valve seat seems to work best in the aluminum heads.

    The second factor is valve seat angle. Multi-angle seats help turn the air/fuel mixture into the combustion chamber. Multi-angle seats flow much better than the conventional 45 degree valve seats. I grind multi-angle valve seats and then radius them into the valve pocket.

    The third factor is the backside radius on the intake valve. The shape and angle help direct the air/fuel mixture into the combustion chamber. This has a large impact on cylinder filling, especially at low valve lifts.     Next