In reply to # 3889037 by Gerald O
Lots of misunderstandings in this thread.
First, the CFM flow rate of individual twin carbs on the MG cannot be added to compare with a larger carb or throttle body that is dumping into a plenum type manifold. The two twin carbs are not connected by a shared plenum so only one carb is flowing at any given moment.
Second, the flow rate required by an individual cylinder cannot be 'averaged', or taken as 1/4 that of the entire engine. Its peak flow requirement must be met, which is the same flow rate as the entire engine. So if the engine can pump 150 CFM, then each cylinder also pumps at that rate, but in an interrupted manner. Since only one cylinder is drawing through the carb at a time, a single 150 CFM carb cconnected to a plenum serving all 4 cylinders would, in principle, satisfy the flow needs. However, if each cylinder were only fed by its own separate carb, it would still need 4 carbs of 150 CFM each. In the case of the MGB, with two pairs of siamesed intakes, two 150 CFM carbs are needed. This is of course a simplification, as it does not take into account air velocity and momentum effects of the 'pulsed' or interrupted flow, which tends to raise the nominal flow spec required from the carb. In short, the twin HS4 or HIF4 carbs are the right size.
First, the CFM flow rate of individual twin carbs on the MG cannot be added to compare with a larger carb or throttle body that is dumping into a plenum type manifold. The two twin carbs are not connected by a shared plenum so only one carb is flowing at any given moment.
Second, the flow rate required by an individual cylinder cannot be 'averaged', or taken as 1/4 that of the entire engine. Its peak flow requirement must be met, which is the same flow rate as the entire engine. So if the engine can pump 150 CFM, then each cylinder also pumps at that rate, but in an interrupted manner. Since only one cylinder is drawing through the carb at a time, a single 150 CFM carb cconnected to a plenum serving all 4 cylinders would, in principle, satisfy the flow needs. However, if each cylinder were only fed by its own separate carb, it would still need 4 carbs of 150 CFM each. In the case of the MGB, with two pairs of siamesed intakes, two 150 CFM carbs are needed. This is of course a simplification, as it does not take into account air velocity and momentum effects of the 'pulsed' or interrupted flow, which tends to raise the nominal flow spec required from the carb. In short, the twin HS4 or HIF4 carbs are the right size.
Nice post Gerald! I know the B series can do 170hp on a single 48mm carb (mine makes 140hp on a single HIF6) so it does make you wonder how 500 cfm would be ok. How does that work?
Adrian
(who loves his aluminum flywheel)
Home built Eaton M62 Supercharger with 7.6psi boost, 8:1 compression, custom "supercharger" cam from Schneider Cams, Mikuni HSR48 Carburetor, cold air intake, ported head, matched manifolds, CB Performance computerized ignition, Fidanza 9 pound flywheel, Maxspeeding rods with Teflon wrist pin buttons.
