2brl vs 4brl

[WORNBRAKES]

I am in debate with a friend on which is more efficient. So I will raise the question here.
Is a 2brl more efficient than a 4brl and why?
Thanks in advance.

[jhow66]

No. Why is what you mean by efficent? (mileage or power).

[shelby987]

With gas at over $3/gallon on regular, I am gonna guess you mean fuel economy wise. All I know is that a 4bbl (non double pumper) will have a set of primaries and a set of secondaries, obviously the primaries are smaller than the secondaries, as the primaries main function is to keep your truck running when in situations where partial throttle is required (cruising on the highway for example). The secondaries should only come on at either a throttle position (mechanical) or a given engine load (vaccum actuated secondaries). This is not the case on a 2bbl carb, your two bbl's are at some stage of open all the time, so the amount of fuel being pulled into the engine is only determined by the amount of those two throttle blades opening, and not having a seperate set of throttle blades closed all together.

I would say if you were to compare 2 carbs of equal cfm ratings, like a 2bbl 500 cfm to a 4bbl 500 cfm, you would likely end up with better mpg with the 4bbl. That being said, a 4bbl will let you suck down a lot more fuel if you don't keep your foot out of it, as with all things, driving style will likely play a bigger role in economy than the carb selection.

I haven't had much experience with 2bbl carbs though, its either 1bbl or 4 thus far!

[JameslovesJammie]

My grandfather was shop forman at Chevy garage in the late 60's, early 70's and knows how to tune a carb. He has a 1972 K/10 with a 350/350 combo. He originally had a 4 barrel quadrajet on it, but took it off and put a 2 barrel on it for "increased fuel economy."

I don't know what 2 barrel he put on, but his gas mileage didn't get better. He actually lost about 2 miles per gallon and his truck doesn't tow as well.

A properly tuned Quadrajet is about the next best thing to fuel injection as far as efficiency. The small primaries are great for fuel economy, while the HUGE secondaries are awesome for towing and hard acceleration.

[Robert1970C20rstbukt]

A Quadrajet will get better mileage than a 2bbl, since the Qjet primaries on it are smaller than the 2bbl's main's as long as you don't stand on it a lot.

[jhow66]

Check, check and check.

[farside847]

has anyone ever gone from a 2brl to a 4brl and noticed a real increase?

[shelby987]

1bbl to 4bbl netted me a 2-3mpg increase, I would be a 1-2mpg increase wouldn't be too much of a stretch.

[Huck]

All of the above--my experience is running on the 4brl primaries is more efficient then on a 2brl. Also, running a stock intake is more efficient then any aftermarket intake. In addition, running stock exhaust manifolds has also been more fuel efficient then headers. I have tried several variations and it seems stock is more fuel efficient with the exception of ignition--HEI is better. Not only fuel efficient but my low end torque is best --stock.

[bryanw1968]

My .2 cents.....

I have done a LOT of carb tuning and installation on SBC's.
I agree with what most of the others said. A 4bbl will be a better choice.
Now, that being said, the biggest mistake that people make is over carburetoring. Chevy's escpecially don't like it. Consider the weight of the vehicle and how quick you need to move air through the motor. The heavier the vehicle, the faster you want to move the air to create torque. I'd either recommend a 650 Edelbrock or a Quadrajet at the same CFM or smaller coupled with a dual plane intake manifold. Some of the intakes are cut with a spread bore and some with a square bore. Go with the square bore if you don't have a high performance application.

Click here to see an example of the intake.

http://public.fotki.com/bryanw1968/73_spread_bore_corv/

[davidh]

I thought of doing this with a 283, 305 heads from 80's. mild cam rv or smaller then I have a 80's 305HO factory aluminum intake, topped off with a fresh quadrajet and keeping a manual tranny. I was going to run ram horns and maybe single exhaust through a 3" pipe too to keep some back pressure.

[special-K]

I built a 307 and put a Holley 450 CFM economaster on a stock 4bbl manifold and got a big increase in economy AND when I put the pedal down,more power.The spread-bore 4bbl is a very fuel efficient design for "best of both worlds" performance.The motor also had 1.94" heads and an RV-type cam.It outran stock 350s but got better mileage.(headers and HEI,too)

[68C15]

Originally Posted by farside847

has anyone ever gone from a 2brl to a 4brl and noticed a real increase?

not likely. the driver usually ends up using all 4 barrels.

[farside847]

Originally Posted by special-K

I built a 307 and put a Holley 450 CFM economaster on a stock 4bbl manifold and got a big increase in economy AND when I put the pedal down,more power.The spread-bore 4bbl is a very fuel efficient design for "best of both worlds" performance.The motor also had 1.94" heads and an RV-type cam.It outran stock 350s but got better mileage.(headers and HEI,too)

Cool, that sounds similar to my set up. Do you remember what MPG you got?

[Longhorn Man]

Originally Posted by davidh

I thought of doing this with a 283, 305 heads from 80's. mild cam rv or smaller then I have a 80's 305HO factory aluminum intake, topped off with a fresh quadrajet and keeping a manual tranny. I was going to run ram horns and maybe single exhaust through a 3" pipe too to keep some back pressure.

Dave, if the budget allows, go with daul pipes. you could go with a 2 or 2 1/4, since you aren't looking for real performance here.
There is no reason to WANT pack pressure onless you have a turbo.

[special-K]

Originally Posted by farside847

Cool, that sounds similar to my set up. Do you remember what MPG you got?

Ok,here`s the scoop...It was a K/20 with 4.57 rears,14.50/37 tires,and a 4spd manual and I got 12-15 mpg on the hiway.

[davidh]

Originally Posted by special-K

I built a 307 and put a Holley 450 CFM economaster on a stock 4bbl manifold and got a big increase in economy AND when I put the pedal down,more power.The spread-bore 4bbl is a very fuel efficient design for "best of both worlds" performance.The motor also had 1.94" heads and an RV-type cam.It outran stock 350s but got better mileage.(headers and HEI,too)

Isn't that 450 carb discontinued? I was looking at summit and jegs and didn't see it. Maybe it's just not listed. I did see a 390 4bbl holley that would probably be good on a 283, 305, 307 if it was a mild motor they had a chokeless 450 cfm but I don't know how I'd like that on cold Oklahoma morning or day.
A few years ago I had 83 C10, rebuilt 350, Oreily rebuilt Quadrajet. RV engle cam, duals, Older nonantispark HEI, mild curve kit, 3.08 gears, Going 60-65mph with an unloaded car trailer got 15mpg by checking the mileage by mile marker, not spedo. It was in FLAT Kansas too.

[WORNBRAKES]

Thanks. I won the debate and you all are having fun with the thread.