Demon carb - VAC or MECH SECONDS??

Typically mechanical secondaries work best with a stick car with a fair amount of rear gear. An auto with not much gear will work better with vacuum secondaries. The issue is maintaining air flow through the carb, when you open the mechanical carb and engine can't rev quickly the velocity of the air entering the engine will slow and it will lay down. With vacuum the secondaries do not open untill the engine is ready so a slower reving car like an automatic with airplane gears will do better with this setup. At the strip the launch and 60ft will be better with the right secondary. Mike

Well I think the 650dp is a mechanical secondary so if it's clean inside you could at least use it to test with. Not sure the specs on your engine and drivetrain but I'd guess you'll do better with a 750 vacuum secondary. But the 650 is free so at least try it. I'm not sure but I think if you buy a 750 from Summit and you don't like it you can return it for a refund. I've tuned alot of carbs over the years, from 6 pack setups on big block Mopars to 2 barrel Holleys on 6cylinders. I've changed carbs at the strip from a 650 dp with mechanical secondaries to a 750 with vacuum and picked up 2 tenths with no other changes. It's just matching carbs to proper setups. The Demon is not a bad carb. I've been inside of a couple, they're pretty similar to the Holley. But I'd say I like the Holley best. Mike

Just don't make the mistake of getting too much carb. More cfm doesn't equal more hp.