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SU's IMO would work better than any Downdrafts. Go to HS8's if necessary. Just finished crewing on a 544 built with an honest 175HP using 48's.Unless there choked down, they have nothing over the 45's. We figured on 5.5 mpg under race conditions using 110 Sunoco at $11.64 a gallon. And I'm a big fan of DCOE and DHLA Dell'Orto's having used both on Alfa's, BMW's and Cosworth Formula Junior and FB Engines. I just think SU's are better than any Downdraft Carb on a Pushrod non crossflow, low revving Engine like the B20's. I will be using HS8's on my P1800 to be used as a Track and Street Car.
 

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I'm a big fan of both SUs and DCOEs, but the Schneider 274F cam has 58 degrees of valve overlap, which means a decidedly lumpy idle with SUs. I understand exactly what you're saying about sidedrafts theoretically being better on I4 engines than downdrafts -- I gave a tech talk at a Volvo meet on that years ago -- but I've backed off that view quite a bit. I put a 38/38 on a 2174cc B20 with the same cam and a well-ported head as an interim measure (the customer ran out of funds before we could do the aftermarket injection that was originally planned), and I was surprised how well it worked.
 
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