I have asked you guys several questions about the build I am doing and have gotten great answers. I have still another one about my carb. The guy who is working on the engine for me, said I should put a 670 Holley on it. He is going to install bigger valves, polish and port match the heads to the hi po headers. We are installing a cam, not sure which yet, a dual plane intake and car has dual exhaust. I'm just not sure if the carb is too big. He says its not, and he has built some awesome engines I have ridden in so I am kind of taking his word for it. What do you guys think? Thanks

 

it depends on the cam and how hight you plan to rev it. that sounds big to me, but if you are going to put a very agressive cam in it, i've seen 289s with 740 cfm carbs that rev to 9k. something in the 500-600 range is fine for street cars with mild cams. 600-700 for race street/strip engines. 500 is fine if you plan to keep rpm below 6k, if you are worried about size a 600-650 cfm will work fine and be good to 8k. bigger carbs need bigger cams and better flowing heads and exhuast to match with them. Hope this helps.
Kip

 

if it's a vacuum regulated secondary carb then there's less overcarbing as the secondaries only open the amount you get pull in the primaries.

 

It would be a vacuum regulated secondary carb. I told him I dont want an overly agressive cam in the car. Any more info?

 

I'd say 670 or less (rather less).
let's see what the experts say, here are a few around who know a lot about carbs

 

I'm running the 570cfm on my 302, with AFR heads, Crane roller cam etc etc, and shift it at 6,300ish without any problems breathing up top. The throttle response is pretty wild. For me the tradeoff between 570 and 670 is the 670 might give better PEAK horsepower, but not necessarily better AVERAGE top end power(average power between 4,000-6,500rpm), but since in all reality, 95% of the driving I do is under 4,000rpm, the smaller carb = better throttle response, drivability and mileage. Fuel metering/atomisation is more important for power than rated CFM(within reason), so all things being equal, the smaller carb always has the metering advantage. Also consider that the cfm rating is the rated flow with a 1.5" Hg presure differential, so if you pull 2" of manifold vacuum from a slightly smaller carb it's gonna flow more than it's rated cfm, and atomisation will be better. The trick is to pick up extra power from better atomisation without losing more power from too small a carb.

 

67mustang302: which one's the one you have? is that a vac secondary? (what make/model)
I was thinking going down the route of ponycarbs new spreadbore 1.12 (autolite 4100).
If that is as good as a 4barrell as my autolite2100 is a 2barrell then it should be worth every penny

 

i have a 650 DP i just got from my dad im thinking about trying on mine. what i like is its a semi-spread bore carb (kinda an oddball) so it bolts up like a standard holley but the front two barrels are smaller (1.5" and 1.75" rears). it has ford AT kickdown linkage so im seriously considering trying it.

 

Mine is the 570cfm Street Avenger, which are all vacuum secondary. Autolites are good carbs, but I hear they can be difficult to tune.

 

I'd say he should be sorted with the same one so, maybe buy the one off my77stang