I still wonder if Hitchcocks' are barking up the wrong tree with their 4 valve head. The probability of seeing bikes with both types of head going er... head to head on the same dyno must be on the very far side of remote, but I suspect at lot of us would want to see THAT video.
Bonneville salt flats next?
A.
Are Hitchcocks still continuing with the 4-valve head? I hadn't heard anything about it for a long time.
Anyway, as I have said before on this topic, there are some very good reasons for 4-valve design, but I don't really think that a street Bullet falls into the best application category for it.
The strongest argument for a 4-valve Bullet is the larger valve curtain area at lower lifts giving very good low lift flow. Also, there is the benefit of reaching 25% of valve lift/diameter ratio earlier with the smaller valves in the 4-valve layout. Other typically hyped benefits of lighter valves and overall flow increase from greater valve curtain area are much less critical on a low revving long stroke Bullet.
Not that I am against any of that, but it needs to be argued versus the increased cost of double the valve train parts and more porting labor, which is substantial.
In my production situation, which is basically one or two at a time, I cannot benefit from any mass production economies of scale. Our billet head is expensive to produce, but if Hitchcocks could use higher volume casting methods, then they could probably save enough on the castings to cover the added valve train parts. They seem to have enough cash to do production runs of 100 cast heads, but unfortunately I do not.
From my experience with street and racing Bullets, including BW's 500 racer, the engine can only utilize a certain amount of air flow, and providing more flow capacity just reduces midrange torque unnecessarily. Our billet head with a 2-valve layout and 34mm inlet port can flow as much as this engine can use up to the 6500rpm maximum rev limit of the Power Commander equipped GT535. You can see on Otto's latest dyno chart that horsepower was increasing all the way to the 6500rpm rev limiter. I see no crying need for more flow. Our design criteria includes attaching to the factory ancillaries such as throttle body and exhaust, etc, for installation without a massive amount of changes to the bike. If somebody wants to race, then we can make adjustments on a custom basis.