Who's up for name that Carb/application?

n/p

I had mine on a 306 with a set of ported 289 hipo's and a (x303) cam
was a runner for sure its a good street carb for a mild build

 

Nope...I've never seen a spreadbore carb on any smallblock Fords.

 

Most guys tend to over do it with CFM on stock/mild engines. They think it works because they havent tried other options. I used to run a 650 double pumper on a pretty serious 302 with fully opened up heads 1.94 1.6 valves,gears and a stall. Had a smart old racer talk me into running a 600 vacuum secondary. I never took it off after that day at the track. The car ran a quicker 60 foot time. That was my set up though. Lots of variables. You just have to experiment.

650 cfm is 650 cfm is true at wide open throttle but the transition from the primary to secondary is different on a spread bore vs square bore. Look at the numbers on the pump shooters and that says it all.

 

Since you have worked with double pumpers than you know that the secondary timing it adjustable. The blades just don't go from closed to wide open. only difference should be on the bottom end. Before the secondaries start to open 225 cfm versus 325 cfm, maybe better Tq or MPG. Honestly 50 CFM is going to make or brake your car? Don't want to start an argument I am just giving counter point.

BTW the boss 302 came with a 780 CFM

 

Agreed. I just found that some of the milder engines have trouble with that extra shot from the accelerator pump coupled with the quicker secondary action. Of course that can be minimized as well with tuning if you have the ability. Most guys dont have the experience to tune a carb to that degree. I find the 600 much more forgiving.
The Boss did have a big carb and huge heads. I believe the 780 is a vacumm secondary though. It also wasnt a very good street engine. That engine was designed to turn lots of RPM on a race track where it could use those huge heads. Ford didnt give two craps about the performance on the street. They only produced it so they could race it.
ANyhow in the end it all comes down to tuning I guess. Thats the fun with Holleys. Almost no limit to tuning. Too bad they dont stay in tune a bit better. I still prefer my 600 on my car over a 650 for the street. Thats my application though. Something a bit more aggressive may love that 650 double.

 

Ray, I just read where the "spreadbore" was used on the 351CJ (new one to me) and the 351 Boss in 1971 and 1972......but It was a MotorCraft Model 4300-D carb? Don't Know, I'm not that "UP" on carb ID's.

 

Wow, definitely a new one on me then.