this is more of just a display of dyno results, than an actual blog article. (this is not an engine i built, nor put on a dyno. these are results shared from another shop.) there's a common misconception that the generic CFM chart, found online which is designed around the small block chevies, is a one size fits all for all engines, in deciding whats the best carb size for a particular engine . it's actually pretty silly that carbs are even catagorized and labeled by cfm rating. there's a common argument that even though, late nailheads don't even flow 600 CFM, that putting larger than a 600 on them is over carburation, by those that havn't and arnt willing to try it. makes you wonder why the factory qjet on the 66 spread bore intake (715 cfm) performed better than the 580 cfm afb. the factory dual quad with two 580cfm carbs on it (around 800cfm) performed better than the 715cfm qjet. the 401 and 425 both share the 364 cylinder head, and therefore have the same breathing restrictions. the 364 really doesn't make more hp than the 401 and 425. the bigger ci nailheads make more torque with larger bore and longer stroke. the two biggest and most sensitive power robs on a nailhead are narrow camshaft LSA and under carburation.
the following results are for a 9 to 1 compression ratio 364 with TA 25 camshaft and very mild head porting. had this engine been 10 to 1, it would've been over 400hp. you can see that the engine made 47hp gain over the 650 carb and 45 pounds of torque with the 850, more than the 650!!!! that's with doing NOTHING else to the engine. the results for the dual quad intake, was one of our modified and ported offy intakes. you can see that two 600s performed slightly better than the 650 on a stock four barrel intake, but could not hold a candle to the 850 on a stock four barrel. 500s were drastically undercarburated. this is the limitations of the offy intake design and not simply because its a dual quad intake. on a 401, its likely that two 600s on a factory dual quad would put you in the same ballpark as an 800 avs2 on a factory four barrel. if seeking out a dual quad intake, the factory dual quad and eelco/edelbrock are the best ones. weiand in 3rd and modified offy in fourth. the offenhauser intakes are not usable unless modified as you cannot fit the tip of your pinky finger through the cross over runners. the gains and usability of the intake mod are greater with the smaller CI baby nailheads. many factory experimental intakes probably perform as well as the factory dual quad. all the ones we've seen are designed almost identical, aside from the factory released product being squashed for hood clearance.