21-06-2022, 10:02 PM
Hugh is, based on my understanding and experience, essentially correct. The speed of air flow helps pickup fuel droplets and improve improve atomisation, if any or all of the carburettor, manifold and ports are oversized, you can get areas of slow moving air and subsequent fuel dropout. (highly polished ports cause the same, its far better to have a smooth but unpolished surface) If they are to small you restrict the volume of air that can be packed into a cylinder during an induction cycle. Hugh also alludes to inertia of the mix which can assist volumetric efficiency particularly during the early and late parts of a valve opening. The same is true of exhaust systems, getting them sized and designed correctly improves drag and can help scavenge cylinders particularly during the late overlap period of the exhaust cycle, smaller primary's 20 to 25mm are often far better than the 32mm+ that you regularly see with after market A7 exhaust manifolds. The hard part about precisely tuning an SU is knowing which measuring points are rich and which are weak, yes you can have both on the same needle. I have a large selection of needles and made up a calibrated rod that fits into the piston, once on a rolling road I can measure the fuel/air ratio at any given measuring point and adjust needle selection accordingly. Of course all of the above means needle selection dependant on numerous factors; alterations to valve size, Cam design, C/R etc and a reason why simply saying and AN or NO6 or whatever will rarely be the perfect solution. I agree that the advice often given here about a specific needle can provide a good starting point, but it seems many are still somewhat confused by the often touted AN advice and end up applying it to the wrong engine set up with disappointing results. Do as Hugh has done, find a Dyno operator with SU experience and good needle selection then the money will be well spent.
Black Art Enthusiast