The venturi mentioned above is part of a quote from the Roland Pike autobiography. The venturi is a 22 degree reduction in the port size down to 80% of the carburettor throat size, followed by a 7 degree increase in size until the port size is re-established. I do run the venturi on one of my bikes and it did clearly help and reduced spitback at low revs due to a large valve overlap. As for the top end, the seat of my pants could not tell, but roll on tests between 2 points did show a marginal improvement.
Just a bit more on ports. From memory Superflow gives us a formula that suggests that for every extra cubic foot of flow there is an extra half horsepower. But with regard to port size the idea of a large port being ideal is not the case. The real driver of power in an engine is the velocity of the flow, and that applies to the inlet and exhaust tracts. It is only with a high velocity flow that complete cylinder filling can be achieved, and with high enough velocities, figures well in excess of 100% filling are achieved by formula 1 cars. So the port must be shaped to flow without turbulence to maximise the flow.
Initially the main driver of the cylinder filling is the outward flow of the exhaust gases which sucks in the incoming charge during valve overlap and it is not until the piston is about half way down on the inlet stroke that the suction by the piston takes over. The piston then travels to the bottom of the stroke after which it starts to rise. When there is a high velocity flow through the inlet port, the momentum of the incoming charge continues to fill the cylinder as the piston travels up on the compression stroke and if you are running a 56 HS mk1 cam in the VH, the inlet valve does not fully close until about 80 degrees before tdc, (100 degrees abdc) ie the piston has travelled more than half way up the compression stroke and the cylinder is still filling? Well it can only still be filling if the velocity and momentum of the incoming charge is high enough. Otherwise there is a lot of spitback out the carburettor as the incoming charge is forced back out the carburettor due to the piston rising which greatly robs power.
That raises another point in that the most important event in the valve timing is the closing of the inlet valve. So to get the best out of the engine it is necessary to be able to alter the valve timing in steps of a couple of degrees at a time to get the highest possible cylinder filling.
Roland Pike in another part of his book mentions that Ariel complained to BSA about the poor power output of the huntmaster which on the dyno was down a few horses on the A10. BSA took a look at the huntmaster and fitted it with the smaller diameter exhaust pipes of the A10 and there was a corresponding immediate increase of a few horsepower. The smaller pipes operate at a higher velocity which aids extraction. Ariel however decided they preferred the looks of the over large exhaust pipes even though it robbed power.
While I am by no means an expert at porting I have spent a lot of time on a flow bench with various VH heads. I have used flow beads to check for dead areas and to reattach the flow and have filled in dead areas with plasticine, and have used cotton flags to look for turbulence and try to eliminate it, as well as to find out where the main flow areas in the port are. In that regard probably 90% of the flow is over the top of the valve, where much of it then rushes out the open exhaust valve during overlap (more power lost)
I have found that a good multi angle valve seat will do more to increase flow than many hours of shaping in the port. 90% of the gain to be made in the inlet port is in the bowl ie within 20mm of the valve seat. David Vizard has some excellent details in that regard. The reason why a 3 or multi angle valve seat and possibly a 30 degree back cut on the valve is important is that it improves flow at the most restricted part of the port. Think about it, the valve only opens to .430” with a HS cam and follower and only .318” with a standard VH cam, however for most of the valve opening duration it is open much less than that. So for the majority of the valve opening the most restricted part of the port is around the valve seat and not the size of the port that is feeding it. So that is the main area affecting flow and the main area for the flow to be improved.
To improve the remainder of the VH inlet port the floor needs to be raised considerably in the vicinity of the short side radius, and that also means the roof needs to be raised. That is not possible with the VH as there is inadequate depth of material around the guide and the spring seat is too close. It is also unfortunate that with the alloy heads the centre of the short side radius and the centre of the radius to the roof are not the same in the VH inlet port which makes it very difficult to improve flow and reduce turbulence in that area. That is where the late goldie is so good as it has a raised floor and a large short side radius, with the radius of the roof having the same centre as the floor.
We can all learn a lot from the late gold star ports but there is much more than a large port size to be considered in achieving more flow and power. Would enlarging the black Ariel ports increase power or is the slow valve timing such that there would be no benefit. But a 3 angle valve job would certainly help.
I have wrecked a couple of heads with hogged out ports so I would never recommend an amateur take on porting unless it is with the aid of a basic flow bench and a lot of time is spent first with flow balls and cotton flags in an attempt to understand how the port is flowing before any alteration is done.
This site where small high velocity ports are promoted is worth a read even if it is over the top. Do not consider filling the ports of an air cooled engine with epoxy.http://mototuneusa.com/thanx.htm