Summit's CFM calculator uses volumetric efficiency of 85% for street engines, 110% for racing engines. For a 302 street engine it calls for 408 CFM for revving up to 5500 rpm. Only increases to 520 CFM if you wind it up to 7000 rpm. The racing engine calls for 528 @ 5500, 673 @7000 rpm. Planning on using an Edelbrock 500 cfm, myself.
That calculator is only theoretical.. not necessarily real world. The reality is usually 10-20% higher than that and why we see real world results favoring carbs in the 600-800 cfm range. What's already been touched on here.. and is more of an indicator of actual performance(torque spread and peak hp to peak torque width).. is the venturi size and its affect on booster efficiency and how the carb "comes up on the cam". This greatly affects the entire intake tract and aids.. or hurts.. response depending on the rest of the combo. In other words.. I would never recommend a small venturi carb in the 450 - 500 cfm range for anything around 300 CID with more than a stock or very mild rv cam that rarely see's anything north of around 5,200 rpm. From there.. a 600 will walk away from it every time while not giving up much if any on the low end. JMO based on many builds and friends.. and friends-friends combos.
I have a 72 with close to your set up and am running a 725 Demon carb. It is a beast, car was previously a strip only car, I am driving it on the street. It's a cold natured car, but does ok for the set up......just don't expect great mpg.......it ain't happening. Only time I had any miles to speak of, it was about 11mpg, but with 4.10s and C-4, I thought that was ok. I was running around 65-70 and turning around 4k rpm. The carb does hit hard when needed, I'm shifting around 6200rpm.
I have had trouble with the Demon race series carbs. They tend to wander around on A/F ratio when we ran them on a dyno. I like the 600 Holley 1850 carb for a street engine. Simple, not too big and not expensive. The Holley Street Avenger series offers a 570 and a 670cfm that would both work fine. The smaller carb will generally have better throttle response and part throttle cruise where the bigger carbs tent to work better at higher RPM's and are generally a little lazier on the bottom end and initial throttle response. Carbs are one place where bigger is not better. If you make an error, make it on the small side. It will have better manors and be more tunable but may not produce 100% of the top end power. Not a fan of the Edelbrock carbs. Never liked tuning them and resale is awful if you need to go bigger. A lot of guys are buying them (because they are shinny and pretty?) but I also see a lot of them at every swap meet bringing nothing. A carb guy I really respect said they work good out of the box but when it's time for a rebuild (or if you have to do a lot of tuning) they don't work as well as a Holley. I have no personal experience with them but he has a lot. I respect what he told me and from what I see on their resale value, I'll stick with my Holleys. JMO, SPark
we have had good luck with the edlebrocks, but then again we used to run carters, which is what they are. the current ones are being built by webber and can be tuned just fine. it really just depends what you are familar with in all reality. fwiw on my wifes car we have a 625 carter performer series carb. its pretty damn good. we removed a 650 holley to use this and havent looked back.
I have a Holley 600 cfm. Before I had an Eddlebrock 650 cfm. I had some serious HP gains right off the bat from switching before it was even adjusted. I'm sure theyre both good carbs, but I'll stick with the Holley. I have a 306 BTW. I doubt that makes much difference in cfm requirements, just for what it's worth...
Thanks I'm totally convinced about not going bigger than 625 CFM now, thanks to all BUT this info concerns me for several reasons 1. I like the Demon's Goggle valve as we are running a dual plane intake with the separator and it only seems logical that the Goggle valve and dual planes appear to be made for each other. 2. the 1 3/8" small triple stack primary bores should make it very street able then the Goggle valve with torsion spring allows for seamless transition from the small primaries to the Goggle valve which appears to act like a spread-bore while eliminating the inherent problems typical of spread-bores........now this is all based on my reading and limited understanding but can anyone verify or call on my thinking? p.s.I thought you were talking about the Demon Spark just realized you were talking Edelbrock...and yes they suck p.p.s. did notice you WERE talking about Demon Race series....not in my plans..The Street Demon is what I like
