ORIGINAL: Norm Peterson

Valley - wouldn't it be simpler and cheaper to rebuild the 570? What exactly is wrong with it that you wouldn't expect a simple rebuild to fix? My point is that rejetting either of the bigger ones isn't going to be free in terms of either $ or time, and you'll likely end up rejetting it yet again once the 347 is up and running.


The basic carb sizing math is really simple.

[CFM req'd] = [CID] * [RPM] * [VE] / 3456

The toughest parts for way too many folks are (1)being honest about the numbers they dump into it, and (2) making too many assumptions about what a carburetor CFM rating really means.

Unless my memory is AWOL this morning, those 715 and 780 cfm carbs were installed for reasons of making them legal for Trans-Am racing. That much carburetion, in a motor that high-strung, was never really intended for street driving. Certainly not by any lowest-common-denominator sort of car buyer who might happen to wander into the showroom at the right (wrong?) time. But it had to be offered as such.

Norm

ORIGINAL: 67mustang302

You picked 3 engines that are all known for being way over carbureted and didn't drive that well on the street for 1. Second, carb CFM rating can be misleading and abritrary, carbs flow different CFM's based on the vaccum that's pulling the air through them. It's also common knowledge that engine technology especially in the induction systems/heads from cars in the 60's was very limited, they didn't know how and didn't have the technology to build engines with the efficiency they have today. And I think you misunderstood what I was talking about with the 2-2.5k rpm range. I was talking about the top end of the engine's operation under full power. 3,500-6,000, 6,500-9,000rpm. You need to make the best average power in the rpm range you operate in(unless it's a superspeedway engine, then you're looking at peak hp more than other applications). If you calculate carb CFM size based on max rpm, VE at that rpm, engine discplacement and manifold type, you get the required cfm FOR THAT RPM ONLY. Anything lower in the rpm range requires less carburetion. And as far as carburator sizes and you "being too old to be fooled" or whatever, you're stuck in the past. People like myself an others are running naturally aspirated 302's with 600cfm or SMALLER carbs and making the same hp at the WHEELS that the Boss 302's or the Shelby 289's made at the crank, and doing it with less rpm and MUCH better mileage. And as far as "not enough carb to feed a Boss 302," that's a load of garbage. I know guys running turbocharged blow through carb's with a single 600cfm 4 barrel and making over 1,200hp to the wheels, and guys naturally aspirated on MUCH larger engines with single 650 or 750 4 barrels that are running 10's all engine. And if you're 302 wanted more than 1,750cfm you had to have been turning at LEAST 14,000rpm, cuz that's the only way a 302cid V8 is going to be able to move THAT MUCH AIR with extremely high volumetric efficiency(greater than 130%)