2bbl vs 4 bbl

So I'm watching an episode of Big Muscle on YouTube and they're talking about swapping out a 500cfm 4bbl for a 500cfm 2bbl and throttle response and drivability being better. And this got me to thinking since I don't have a carb for my 307 yet, and a carb size calculator says I only need a 500cfm carb, why don't I get a 2bbl instead of a 4bbl?

Why not go with a 500 4-barrel with vacuum secondaries.

A 500 CFM two-barrel does not flow the same amount of air as a 500cfm 4-barrel. It's easier to quote than to explain: "The cubic free per minute, CFM, rating for 2 BBL and 4 BBL carburetors can not be directly compared. 2 BBL carburetors (and 1 BBLs) are tested with a 3 inch vacuum while 4 BBLS are tested with a 1.5 inch vacuum. In other words the CFM tests for 1 and 2 BBL carburetors are done with the testing equipment sucking twice as hard. If you want to compare the CFM rating of a 2 BBL to that of a 4 BBL multiply the 2 BBL rating by 0.707. ".

Also, the 4-barrel will have smaller throttle bores than the 2-barrel because the airflow is spread over four venturis, this mean that when you tip-in the throttle on the 2-barrel you will get more flow for the same degrees of throttle opening than the four-barrel so you might actually get less driveability since the 2-barrel will be more of an "all or nothing" sort of feel.

Last, smaller throttle bores have a better vacuum signal to the booster venturis because of higher air speed, you'll probably get better fuel atomization with a four-barrel than a two-barrel of equal size.

If it were me, I'd just put a Quadrajet on it and be happy. The tiny primaries give good off-idle and cruise performance and the huge secondaries give plenty of airflow for WOT.

Q-jets were used on everything from a 231 to a 455 because they only flow as much air as the engine demands, setup the secondary air door properly and you'll be fine and it won't bog or stumble.

Shawn

That knocks that idea down, and I thank you. It's exactly the kind of info I was looking for. I'll see if I can come across a decent deal on a Qjet this year at the swap meets.

Q-jets are awesome carbs when set up right. Almost no one sets them up right. Either take the time to learn it yourself* or find that one old guy in your town that can do it in his sleep.

• If you learn to do it yourself, you will eventually become "that old guy".

Too bad I tossed the quads I had years ago. The Q-jet, when properly jetted, is just about the ideal carb for a mid size V-8.

+1 for Q-Jets

I like Q jets but have to ask, why the 307 ?

I like the Holley economasters too, basically a 4-bbl Weber.

QJets: http://www.customcarbs.com/index.php/services http://www.cliffshighperformance.com/ http://www.smicarburetor.com/

2bbl Rochester Carbs are often built by circle track builders (or listed above)

A long time ago, I ran a modified Rochester 2BBL on an adapter plate on a Edelbrock Performer 4BBL intake. It ran better than the Holley 600 4BBL. This was on a 327.

Trans_Maro wrote: Why not go with a 500 4-barrel with vacuum secondaries. A 500 CFM two-barrel does not flow the same amount of air as a 500cfm 4-barrel. It's easier to quote than to explain: "The cubic free per minute, CFM, rating for 2 BBL and 4 BBL carburetors can not be directly compared. 2 BBL carburetors (and 1 BBLs) are tested with a 3 inch vacuum while 4 BBLS are tested with a 1.5 inch vacuum. In other words the CFM tests for 1 and 2 BBL carburetors are done with the testing equipment sucking twice as hard. If you want to compare the CFM rating of a 2 BBL to that of a 4 BBL multiply the 2 BBL rating by 0.707. ". Also, the 4-barrel will have smaller throttle bores than the 2-barrel because the airflow is spread over four venturis, this mean that when you tip-in the throttle on the 2-barrel you will get more flow for the same degrees of throttle opening than the four-barrel so you might actually get less driveability since the 2-barrel will be more of an "all or nothing" sort of feel. Last, smaller throttle bores have a better vacuum signal to the booster venturis because of higher air speed, you'll probably get better fuel atomization with a four-barrel than a two-barrel of equal size. If it were me, I'd just put a Quadrajet on it and be happy. The tiny primaries give good off-idle and cruise performance and the huge secondaries give plenty of airflow for WOT. Q-jets were used on everything from a 231 to a 455 because they only flow as much air as the engine demands, setup the secondary air door properly and you'll be fine and it won't bog or stumble. Shawn

Everything he said times infinity. Nice post, transmaro.

You can get thermoquads for pretty cheap if they are not a 1971-1972 mopar version...Those tend to be looked at as gold plated.

I have one I'm using on my 340. I just really like the idea of the dual personality and I know it's a match to the motor or at least Chrysler though so. You really can't go too far wrong.

In reply to curtis73:

Thanks man!