Carb Sizing

Tech Question


neil schroll, Mount Joy, PA, 1969 Dodge Charger 446

How do you know when to move up to the next larger cfm carburetor???

I am currently running a 750 cfm edelbrock, that is 12% richer than thestock edelbrock setting. However, This is the richest rod and jet comboallowed by edelbrock.

The car runs great through out the rpm range, Idle to 6,000 max. No bogsor flat spots.

Plugs show that I could richen things up just a bit, which is whatconcerns me some what. (Base ring is about 50% soot covered, and insulatoris just slightly off white.) No audible signs of detonation.

Carb math shows some where in the neighborhood of 770 cfm.

The car specs are as follows:
3.23 gears
440, 0.030″ over
9.75:1 comp (measured)
727 tranny
stock 906 heads, gasket matched
edelbrock performer intake
Headman headers, 2.5 exhaust
MSD ignition, 38 degrees total
MP 484 lift,241 dur@.050″,114 deg lobes
1/4 best: 12.97 @ 106mph w/ street tire
Max rpm = 6,000


Neil, CFM (airflow) and carb size are mostly independent of each other.Unless the mismacth is huge, A/F ratio will change very little if carb istoo small or too large.

Unleaded gas gives strange plug readings. UEGO is the only way (if you’reunfamiliar with this term, just Google it, or see Bold Beeper series a fewissues back.) Remember, too, there’s nothing to prevent you from drillingjets and/or filing metering rod power steps, although there will come apoint where internal passages are the restriction, not the jet.

Dual plane manifolds are much more sensetive than single plane to carb size.Personally, if the car were mine, I’d leave the carb and change themanifold! But you can try a bigger crab, only real way is trial and eror.Remember Chrysler put 800 CFMs on piggy 318s, and 580 CFMs on hot 440s!

By the way, your combo sounds like it’s ragged-edge for detonation. Becareful, and remember that any detonation you can hear is REALLY bad.


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