Just fixed this tiny little port so it's smooth. This is with std valve calibrating off the std port which is around [email protected]
". This head must be earlyish because these ports measure 29.5mm not 30mm. @ 28" the std port needs 3 vaccs the modified one needs 4.
So through a stock 30mm Concentric it's about 143cfm @ .385" and about the same @.350". The stock side is probably around 105cfm through that carb. Just the port is 146.66 and with an old bell-mouth 150. So again the carb takes a bit off. But this might be good with a pair of std GPs on a Spitfire. Though a good bell-mouth with no gauze on the 30mm Amal might do the same and be easier to live with.
My thought with this head was to make it a better breathing head that can use the stock GP carbs on a Spitfire, and make it perform noticeably better. I'm just thinking of selling one and advertise it on E-bay as a ported Spitfire head to suit GPs. Because it sounds a bit more special than Lightning head.
I just want to make sure it's a noticeable improvement if someone buys it so they are happy with it. I guess I should measure speed but I can guess its similar to the 34mm port.
The reason to test with a bell or radiused entry is to take the effect of the sharp entry away. Otherwise you can change flow quite a bit by accidently making that sharp edge a bit rounded. Having the carb on takes that edge out of the calculation and works better as well. On these figures it should give a good hp boost, but you don't know what people will do setting stuff up. If it gets near 70 engine hp it would be good.
Last time I tried this size port it was 137.5cfm just the port so 146 is pretty cool.