Well, I just happen to have the flow bench information on the AVL head right here, so we can discuss.
The AVL rockers do have about 10% lift multiplier ratio built in, and the cams have .300" lift on the intake, and .280" on the exhaust. So, we have max lifts in the vicinity of .330" on the intake, and .308" on the exhaust.
These flow figures that I have do NOT include the manifold stub or carb attached to the head. These are bare head numbers.
We are looking at a peak intake flow of about 157 cfm, and 123 cfm for the exhaust. This is very very similar to the stock intake flow of the Iron Barrel head at similar lifts, except the Iron Barrel exhaust port flows about 10 cfm more because it is a bigger exhaust port and valve. So, we find it no surprise that it also gives about the same power as an Iron Barrel engine which had its compression raised to a similar figure. Apples-to-apples, very similar indeed.
However, we find that if the lifts are increased, the stock AVL intake port does have some ability to flow beyond the stock Iron Barrel intake port, but the exhaust port has a problem at .400" lift and begins to "back up" and lose flow at that lift, which is an indication of a problem in the exhaust port. So, the exhaust port has some problems.
The AVL responds to a larger intake valve and to some porting of both ports. With a 1.8" intake valve and a 1.57" exhaust, things definitely seem to look up. Without any additional lift over stock we got up to 183 cfm after making these mods on the intake side, and got rid of the exhaust back-up issues at all lifts. So, a 26 cfm improvement on the intake side is getting us pretty close to the Iron Barrel Fireball flow rate. Not quite there, but getting close. Pretty good, especially for not having any change to the stock cams or rockers. With the Iron Barrel Fireball, we have higher lift cams to get our lift up to get the 195 cfm flow rate we have with that head.
But, if we add lift height, by cams or rockers, we can get a lot more out of this porting job. By adding Ace cams and the roller rocker package with 1.45:1 ratio, we can bring that lift at the intake valve up to .510" lift. And at that lift with this AVL ported big-valve head, it will flow 208 cfm at peak lift. That is a significant increase of 51 cfm(33% more) over stock, and in fact that is enough to support a peak hp rpm near 7000 rpm, with a carb of sufficient size, like 36mm.
Additionally, the combustion chamber improvement with the new compact shape is more compact and efficient, which allows faster and more complete burn, which in turn allows us to use a higher compression ratio without detonation. So, we get more out of it on the combustion efficiency end of things too.
This is what is going to give that elusive 40+hp figure that was alluded to when the bike was introduced. We pull more torque out of it with more flow, more compression, better combustion, increase the lift and duration to increase the rpm capacity, and push it to 7000 rpm, and you'll get your 40hp at the rear wheel, which will be around 47hp at the crank.
Essentially, right about the same specs as a Norton Manx 500.