|
Nice work, and should make a very well breathing B20 fly if you can get it right.
OK, let's start with the B20 motor & head itself, before you try to find out whcih carburretion you should go with.
Is the motor stock compression or built with a better bottom end with better psitons & higher compression?
Is the head stock or ported, and bigger valves?
Do you know if the motor can achieve higher rpm's than stock, and a cam that can flow more at high rpm's?
The answer to these questions are very important to which needles to choose. The stock needlse for a B20 (US spec) are SM, not KN, and hte SM are much leaner than the KN, especially at low flow, low rpm's. A well built B20 that can flow the max the stock twin SU HS6 carbs is 175 CFM X 2 = 350 CFM, and RR needles can accomodate a good A/F curve for such a motor, flowing 350 CFM and get 200hp at about 7000 rpm. If you take a B20 and add three SU HS6's, (525 CFM max), then you will need three carbs that cumulatively flow 350 CFM or there abouts. I've yet to see a race built B20 go much farther than 500 CFM, and that was done with EFI.
In answer to the big question, the needles will need to have a much leaner taper than even the SM, but that will be the best place to start. I wouldn't try any needle that is any richer.
When you get the carb's on, place toothpic's in the dashpot holes and go for a drive with the hood off and note the height the pistons advance under load, the higher the rise the better the needle is matched to the flow of the engine, depending on the tune and setting. Note this is straight out of the SU manual circa 1966, which I have a copy to scan for you if you like.
Like Webers, the needle matching in this case is going to be just as tricky as jet selection. You may need to go down one jet size (0.08 dia) and run much leaner needles than are available for the 0.100 jets.
I should also note that when Jaguar went from three SU HS8's on the XKE IL6, to two carbs, the HP went up by 40, and the engine flow improved by about 20%
|
