PICKING THE RIGHT CARBURETOR SIZE FOR YOUR MOTOR

[oldbee]

A little story about a ‘75 Monte Carlo. 400sbc w/2bbl. All stock; if u remember back then gas mileage was bad across the board. I bought a Holly 4-but that had total cfm the same as the 2-Bee. Picked up 2mpg & better throttle response. Wasn’t looking for more power, just better running. Air velocity thru those small venturri’s.

 

[BSB67]

A little story about a ‘75 Monte Carlo. 400sbc w/2bbl. All stock; if u remember back then gas mileage was bad across the board. I bought a Holly 4-but that had total cfm the same as the 2-Bee. Picked up 2mpg & better throttle response. Wasn’t looking for more power, just better running. Air velocity thru those small venturri’s.

Not an uncommon story. But you really cannot compare advertised 2 bbl numbers to 4 bbl numbers. They use a different dP. A 500 cfm 4 bbl would be a 700 cfm 2bbl. A 350 cfm 2 bbl would be a 250 cfm 4 bbl.

Those small Holley 450 cam 4bbl carbs on the S2P2 intakes made every factory 2 bbl car from the late 70s perform better in every meaningful way.

 

[oldbee]

Well, I’m still thinking air velocity thru the carb! But??

 

[BSB67]

Well, I’m still thinking air velocity thru the carb! But??

Yes. The primaries are still smaller in the small 4bbl verses the 2 bbl and can provide better signal and low speed fuel delivery.

 

[Officer Higgins]

A little story about a ‘75 Monte Carlo. 400sbc w/2bbl. All stock; if u remember back then gas mileage was bad across the board. I bought a Holly 4-but that had total cfm the same as the 2-Bee. Picked up 2mpg & better throttle response. Wasn’t looking for more power, just better running. Air velocity thru those small venturri’s.

Where you located oldbee im east of Cincinnati

 

[oldbee]

Where you located oldbee im east of Cincinnati

Way north, SE of Akron in Uniontown .

 

[Officer Higgins]

Many mopars up your way,?

 

[oldbee]

Many mopars up your way,?

Quite a few actually. Same allotment as mine is an original ‘66 Belv II auto/Hemi. 1of 700+

 

[LVMYRT]

check this video out about how to choose a carburetor for your motor from BAD *** Racing. I just posted it on FABO to

http://www.badasscars.com/index.cfm...ct_id=450/category_id=13/mode=prod/prd450.htm

one great informational video!!

 

[threewood]

I couldn't make it through the video. Did he ever get to talking about how to actually size a carb for a motor or did he just continue to ridicule people for poor decisions?

 

[65-440]

I couldn't make it through the video. Did he ever get to talking about how to actually size a carb for a motor or did he just continue to ridicule people for poor decisions?

Yes he did. What he says makes sense, whether there is real world truth to it who knows... however, after watching this I'm going down 100cfm on my 340 dart and am curious to see what the results will be.

 

[rumblefish360]

Yes he did. What he says makes sense, whether there is real world truth to it who knows... however, after watching this I'm going down 100cfm on my 340 dart and am curious to see what the results will be.

What size carb is on it now?

 

[451Mopar]

The sizing equation gives a picture of the amount of air flow at a specific RPM and VE, but the Carb size rating is with a 1.5" pressure drop so the carb is likely a restriction at high RPM. On the other hand, that mostly matters to race engines operation at WOT. On the street a smaller carb is easier to tune because it gets into the main metering circuit earlier. With a big carb on the street, you driv alot in the transition range, and most people don't know how to tune the carb there.

 

[65-440]

What size carb is on it now?

750 .... .488 comp cam, eddy rpm heads, stock bore

 

[rumblefish360]

750 .... .488 comp cam, eddy rpm heads, stock bore

Unless you have a big stall converter and high numerical gears, a 750 maybe a little large for the 340. I think that you’ll like a 650/700 better.

 

[65-440]

Unless you have a big stall converter and high numerical gears, a 750 maybe a little large for the 340. I think that you’ll like a 650/700 better.

It's s 4spd car ... so, when I first got the car out had a street avenger 770 on it. Ran great(at least I thought so). Drove it that way for 5yrs... fast forward to the 383 build in my coronet, was having trouble with an edelbrock 800avs. Put that 770 on that motor and there it stayed ! I built an AED 750dp and have had a stumble ever since. Accelerator pump circuit is good, very crisp off the line.. it's part throttle acceleration that's killing me... stumble stumble stumble go ! Drove with a vacuum gauge, played with power valves, up the primary jets a few times and got better but still there ..... it's in the primary side, all my testing done with secondaries disconnected... I've always felt the car was over carbed, even with the 770..... Guess I'll find out soon !

 

[BSB67]

It's s 4spd car ... so, when I first got the car out had a street avenger 770 on it. Ran great(at least I thought so). Drove it that way for 5yrs... fast forward to the 383 build in my coronet, was having trouble with an edelbrock 800avs. Put that 770 on that motor and there it stayed ! I built an AED 750dp and have had a stumble ever since. Accelerator pump circuit is good, very crisp off the line.. it's part throttle acceleration that's killing me... stumble stumble stumble go ! Drove with a vacuum gauge, played with power valves, up the primary jets a few times and got better but still there ..... it's in the primary side, all my testing done with secondaries disconnected... I've always felt the car was over carbed, even with the 770..... Guess I'll find out soon !

Of course the stumble is not because of the carb size. So if you put a smaller carb on and it runs better, you cannot conclude it is because of carb size.

 

[65-440]

Of course the stumble is not because of the carb size. So if you put a smaller carb on and it runs better, you cannot conclude it is because of carb size.

According to the dude in that video , it could very well be. A big gulp of air with little fuel as soon as the butterflies open up. It makes sense to me, not saying that's my problem but its worth a shot. although it never did that with the 770 !

 

[rumblefish360]

Of course the stumble is not because of the carb size. So if you put a smaller carb on and it runs better, you cannot conclude it is because of carb size.

Getting th bigger carb to give an adequate pumpshot coreectly timed is the problem. IMO.
Your statement has truth in it but not at 100%.

 

[BSB67]

Getting th bigger carb to give an adequate pumpshot coreectly timed is the problem. IMO.
Your statement has truth in it but not at 100%.

750 cfm on a 340, no problem at all. We've run the 4779 on several 340s. They run great.