I find it interesting that Barry Grant himself says to not go by any formula. He notes reduced throttle response of SOME carbs with larger cfm ratings, but it isn't always the case. Throttle response can be affected by different boosters, different pump cam profiles, and main air bleed changes. Holley developed '3-circuit' systems for Dominators, to increase throttle response, no matter what engine the carb was installed on. Smaller throttle blades and venturis are known to help with throttle response, but, the fact remains, not just any moron can work on a Holley. Edelbrocks make good street carburetors, but unless there is a space limitation, such as when running dual cars, run a Holley. As for a carburetor being 'too big' or 'too small', you cant really say that. The carburetor doesn't know what engine it's installed on, and, as they all say, it requires tuning to the specific application.

General guidelines:
Lighter vehicle can generally tolerate a larger carb
Dual plane intakes are better suited to so called 'over carbureting'
Taller(numerically higher) gearing can stand a larger carburetor

Mech. secondaries and Vac. secondaries
In some situations, a vac. sec. carb. can be faster at the strip, especially in a high powered, lightweight car on street tires. In some situations, a mech. sec. carb is better suited for a street car, such as when driver control is better than air valve control(i.e., slick roads, or an extremely heavy vehicle, where increased loads will open the secondaries prematurely). It also depends on driver preference.

 

SIGH......Ok, I have run a 650 double pumper on my 302 AND a 750 vacume secondary and also a 600 vacume secondary......The one that ran the best was my 750 vac secondary 3310-2 Holley. The engine was pretty mild..it only had the Edelbrock Performer RPM cam and intake set up with home ported heads and easy 9.5 to 1 compression. The exhaust was simple...a set of Headman headers into 2 and 1/2 pipes mandrel bent going through a set of Flowmaster muffs. Now, my transmission of choice was pretty odd in that I chose one that I would most likely never break and was a C-6. The rearend was changed from time to time with all three carb combo's depending on what was going on in my life at the time. But was either 3:00, 3:55, or 4:11. No matter which gears were in it, it always ran best with the 750. Now, what I havent mentioned, is that I spent the most time TUNING my 750 to get it to run "just right". I changed jets, secondary springs, powervalves, accelerater pump cams and accelerater pump sizes. I prefer Holley's simply because it's all I have ever used or known. I'm putting a 750 Holley 3310-5 on my 460 in my 3/4 ton 53 F-250. I know I'll have to TUNE the carb differently for the truck that I would in my little 67 mustang, but I'll bet I can get it to run almost perfectly. For those nay-sayers, I'm not a professional mechanic, or an engineer. Heck, I don't even work in a tire shop. I simply put on what I have seen other people use and it seems to work for them, so why not me? All you have to do is go to the track a few times and walk through the pits to see what works and what doesn't. I have had many people tell me that my carb is too big on my 351w, and all I do is show them my time slip with the 750 and then show them the time slip with the 650. If it aint broke, don't fix it! Now, in the previous posts, I have seen refrences to Ford, Chevrolet, and others using certain carbs because of their "extensive R&D". While they DO have a LOT of time in R&D, they use the best carburator for THEIR application. (which may not be the same as yours!) The smallest thing could be the difference. If you look, they seldom used different sized carbs on the same engine. Same engine size yes, same engine internals, no. One may have larger valves and can handle a bigger carb, while another may have smaller valves and needs a smaller carb. However both engines are the same size. Heck, if it is THAT important to you, put on what you have....TUNE it to work it's best on your set up, then go out and either buy or BORROW one of a different size, TUNE it to your needs, and see how it works. peace.

 

Wow, ya'll have been busy!

New problem. Why in the world did ford put a square bore carb on a spread bore? Or has someone swaped carbs on me with another Ford carb? I went to put the 750 on and noticed it. The only way I could get the secondaries to work would be to use a thick gasket. Even then though half of the back is covered up! I ended up not using the 750 due to the owner of the carb after seeing that didn't feel like letting me borrow it. (!Family!) It came off of his race car and didn't have a choke nor kickdown. So we just fiqured I might want to look at getting a intake. It's looking like I might as well just pull the engine and rebuild the darn thing! Don't want to right now but heck I don't want to buy a dual plane then need a single plane intake. This stinks. I'm going to go to some scrap yards and see if I can find a main body. Before I decide.

 

That manifold originally had the Ford/Motorcraft 4300D/4350-series carburetor on it.

The spread-bore layout on the square-bore bolt pattern is a dead giveaway.

That spread-bore pattern is why those carburetors were rated at 750CFM from the factory.

 

Ok, what was a 4300d/4350. The carb that I got off the truck was a 600cfm. From what Holley said. I alittle confused!

 

That was a spread-bore Motorcraft carburetor, designed and manufactured by Ford.

I don't have any pictures of what one looks like, but it has a spread-bore throttle body pattern, similar to the Quadrajet from GM; however, the bolt patterns are different between the Quadrajet and the Motorcraft 4300D/4350-series carburetor, and so are the primary throttle plate butterfly locations.

You might be able to do a search, and find a picture of one, but I know for sure you can find a picture in one of the books about High Performance Fords that are available.

 

So someone did switch out carbs! That interesting. I also got to looking and found a number of hoses pluged with bolts! I don't think my uncle knew that someone had done this. He always told me the truck got 8mpg no matter what you where doing. Although 10mpg would be great!

 

No biggie. Just surprising! He could have forgot.
So now I have 600cfm witha striped hole that wasn't right to begin with!

 

I think you can buy an adapter plate from auto-zone or advanced that will adapt the spread bore to standard bore and vise-versa. Have one in the garage sitting were I but an Eldebrock on a quadra-junk manifold. This adapter is about 1 inch thick just so ya know but doubt ya'd have hood clearance issues.


This may be a quick and easy fix compared to looking for a manifold right now.

 

True. Thanks