hi guys, i woke up and can't get back to sleep. astrodokk asked me a question about 3 two barrell carb intake manifollds for small block chevy's this afternoon. (he saw the hype on some car channel recorded on DVR). he noted the 3 2s could give him about 1000 CFMs of airflow. dokk-those generic carbs we discussed were 2GCs.
DOKK is trying to become another victom of overcarbing (as much as i've told him not to)
let's do the math:
a 383CI stroker will consume 383 cubic inches every two RPMs at 100% volumnmetric effencity (most street engines will achieve about 80%)
SO:
a 383 engine at 6000 will consume 383 X 3000= 1149000 CI.

to convert CI to CF you need to multiply 12X12X12 =1728.

now if you divide 1149000 into 1728 you will get:
664.93055CFM at 6000, thus a 650 carb would be perfect. in reality a little larger carb works better (your 750 is fine) but 900-1000CFM of airflow is a bad idea.
remember, i went from an 800 to a 650 on my pantera.

the math:
351CI at 3000 (6000 RPM) =1053000
12x12x12=1728
1053000 devided by 1728=609.375. so the 650 carb is just right...DOKK...

 

 

2bbl and 4bbl carbs are flow tested at different pressures. In order to make an accurate CFM comparison between the two, you have to divide the CFM of the 2bbl carbs by 1.414.

1000 CFM of 2bbls is about the same as a 700 CFM 4bbl.

 

Pontiac GTO 389 motor came with 3 2's. They were screamers too!

 

The L88 came with 1 (one) carburetor.

 

A 2V is rated at 3"WG, a 4V at 1.5"WG. A wide open throttle (WOT) CFM comparison has to be normalized (by the square root of 2, 1.414, factor).

Throttle response is also a function of the throttle plate diameter (in a simplified world), with the smaller plate offering a more defined off-idle response. Driver control is often defined by the off idle throttle response, so for most applications an under-sized carb provides the owner the best drive result.

Three 2V carbs have 6 throttle plates to share the WOT CFM, vs. a single 4V. Three 325 CFM 2V carbs flow about the same as one 700 CFM 4V carb, and with the six-pack each throttle plate flows ~115 CFM at WOT, vs. each of the 4V plates that flow ~175 CFM. Each 4V throttle plate flows about 50% more CFM at WOT, compared to the six-pack plates, making it (in theory) about 50% more difficult to achieve clean throttling off-idle with the similar sized 4V carb.

The same clean throttle response effect from the small throttle plates found with the six-pack and dual-four setups in the 50's was exploited by the radically different size primary and secondary throttle plates of the spread-bore carbs (the QuadraJet & ThermoQuad) in the late 60's. A well tuned Q-Jet offers the great off-idle throttle response of a smaller 100 CFM per venturi 4V carb (comparable to the throttle response of a 390 CFM Holley), with the WOT performance of a 800 CFM carb (comparable to the WOT airflow of the 780 Holley).

Multiple carbs are more than eye candy, but you have to know how to tune the set-up. We would often find the end carbs of six-pack setups disconnected, or with stiff vacuum pot springs to slow the secondary response to near decade lag times. Well tuned set-ups (the 340 AAR Cuda comes to mind) were a ball to drive.

When my friends and I were racing a 427 BBC one of the best street setups was twin 660 CFM 4V carbs on a TeamG tunnelram. The engine would idle clean at 850 cfm, and jet flames from the header flanges at WOT. We ran quicker ET's with a 1050 Dominator on a C454 manifold, but we lost some of the street-friendly throttle response of the dual-fours.

We were guessing a lot with our set-ups and I would have really enjoyed to listen & learn from some of the guys who grew up tuning the multiple carb set-ups, because when they are gone much of the practical knowledge will go with them.

 

People often make the mistake of comparing 4 barrel CFM to two barrel CFM by simply adding the trips up but the figures they come up with are way wrong.

Withn a 383 he should do well with the 3 2's

Doug

 

Uh, guys, ...guys! I saw the price for a new Edelbrock tri power intake manifold and also saw that it would be difficult to get those carbs anymore. Sooo, I'll stick with the 4BBL. Got an email about experience with tri power too. Since the car came with the 4bbl, I'll keep it that way. Stacy David on the Gearz TV show sure made it look cool and easy! I'm so gullible!

 

No doubt. Just like all the other shows. Nothing like showing days of work for some things in 1/2 hour.

larry