how truth it is that if a change for a 4 bb carburator i'm going to save more $ on gas true or false? I have a 2 bb and I'm thinking to change.

 

in simple terms.

A 2 bbl has 2 intakes, if it is a 600cfm carb, each is capable of 300cfm

a 4 bbl has 4 intakes, if it is a square bore, all 4 are equal so a 600cfm 4 bbl is actually 4 150cfm bores.

if you don't open the secondary bbls the 4 bbl is actually a smaller carb than the 2 bbl, the problem comes when you put your foot into it and open the secondary bores.

personally I have a problem keeping my foot out of it, so I would probably get better mileage with the 2 bbl.

the other downside of switching manifolds and carbs is it is only part of the intake and exhast system, so your taking a TUNED 2 bbl system and making it an UNTUNED 4bbl system.

in order to do it properly you need the RIGHT intake, carb,heads,and exhaust that are all designed to go together.

 

Eric hit the nail on the head. Headers will help that much more, The same as the 4bl.

 

i would stick with the 2 bl if you offroad. the 2bl is less apt to flood or starve at angles such as driving on the side of hills

 

then i will sitck to the 2bb carb and i will look for some headers. thanks for the help

 

I had a 289 in my '67 Bronco. I swapped the 2bbl manifold and carb. for a 4bbl manifold (from a junk yard) and a new Carter 4bbl. The combo worked great for me, even with the stock exhaust and heads.

In my opinion, Edelbrock will be your best choice. They sell matched sets (600cfm Performer Carb. and #2171 EGR manifold) designed specifically for your engine ('77 351M, right?).

Also, 7fords said, "I would stick with the 2bbl if you are going off road. The 2bbl is less apt to flood or starve at angles such as driving on the side of hills".

Edelbrock makes an off-road Needle & Seat set (part #1465) that is spring loaded and designed specifically for hard 4-wheeling. This will avoid any starvation or flooding.

Hope this helps.

Jim

 

i agree with that statement i went from a 600 - 800 and i noticed that if i control myself even though i am not quite done tuning it that i save gas and the spring loaded needle&seat is a good idea even for everyday driving in my oppinion cause when people used to cut me off and i would have to slam on the brake i would stall occasionally and when i am in the desert its a much smoother ride

 

and actually if you want a 4bbl carb i got my old one still its a carter 625

 

I don't agree with the 2bbl in combo with the headers. I have that combo on my 351w, and it started giving me trouble after I put the headers on.

The other poster is correct that the whole stock system is tuned together. Change anything, even headers, and it gets out of wack. I had a pinging problem after I installed the headers, and had to change the jets in the carb up 3 sizes, and go to a 180 degree themostat to get it to quit. I found out later the reason is the egr on this engine works on exhaust backpressure, and when I lowered the back pressure with the headers, the egr wasn't working as it should. Theoretically the headers also leaned the engine out a little bit too, but I can't prove that. I did notice a lot more throttle response when I changed the jets though.

If you go with an aftermarket 4bbl carb, they are usually jetted rich enough that you will not have any problems with adding headers, and you can also get rid of the egr system with the aftermarket carb.

 

im in the process of changing a 77' 302 from a 2bbl to a 4bbl right now. the only thing im not real sure of right now is how the trotal linkage is going to hook up???? the 4bbl carb. and intake are the stock carb.(holley) and intake off of a mid 80's 302 bronco.

thanks for the help!!

thecowboy

 

i have trucks with 2 bls and with 4bls with the off road conversion kit in the carb power wise you cant beat a 4bl setup but with offcanter angles the stock 2 bl beats the 4bls hands down. my 2 bl trucks will sit on a hill almost straight up and idle all day