diff between big block and small block carbs?

[todd f]

What is the difference between big block carbs and small block carbs. Especially the original carter carbs

[Ausydad]

The carb itself shouldn't have any difference. BB's are built for torque while SB spin faster giving them horse. BB's usually like the carbs that are a little bigger in cfm's (750-850BB, 550-650 SB) to give them the proper amount for mixture and throttle response. The carbs themselves though (like the Thermoquad - 800cfm) are the same regardless. Hope it helps!

[dave571]

Well known engine builder and racer, Les Davenport once said to me,"..a carburetor isn't very smart. It only understands one thing, how much air is going through it. It doesn't know what engine it's bolted to."

That in mind, ausydad is right on the money.

The larger engine may require more air(cfm), but the operation is the same.

[todd f]

the main thing that i was wandering was if there was a difference in how they bolt on. Let's say I take a stock carb off of a 440, ans stick it on a 340. Will there be a problem? Is the bolt pattern different

[DEMON SIZZLER]

Small block Thermoquads for example, have a cfm rating of 800
and have smaller primary/secondary jets than the 850 cfm big block carb. Also the metering rods are different od on the 2 or 3 steps between the two carbs and the 800 cfm carbs have a larger id main/hi speed air bleeds which lean the circuits. The hp
AFB/AVS carbs also were calibrated/different cfm ratings like the
T.Q.'s depending on the application, DAVID.

[Ausydad]

You're wondering about whether or not some are squarebore (all four holes are same size) or spreadbore (secondaries in back are larger). Even if the intake is one, and the carb is another, go to your local parts house and get the Mr. Gasket adapter. This will covert the two to work together. My tip is to take a Dremel or something like it, and flow the adaptor before you put it on. You can still use whichever of the carbs.

[rb77413]

I haven't seen a differance in the bolt pattern between sb and bb. Only thing is of course to watch for spread bore, and spread flange. Like a Carter might not bolt on to a manifold originally used with a Thermo quad.


Bob

[5thAve]

Quadrajets and TQs have the same bolt pattern. Bigblock and small will be the same, until you get into different styles like rb says.

[vernvesco]

All Carter TQ's have the same bolt patterns. The Carter AVS and AFB's have eight or four holes drilled on the mounting base. Most after market units (Competion Series) had eight holes (when looking at a eight hole gasket) drilled out. Some of the Mopar Factory units only drilled out the four inner holes. You can always drill the outter holes.

But there was no different bolt patterns between SB-BB. All holleys (square-hole) if compared to a eight hole gasket will use the outer four holes.

All spread bores (holleys, TQ's,Quadrajets) same mounting.
Earily AVS,AFB (factory) used the four inner holes.
All After Market AFB and Holleys use the outer holes when compared to a 8-hole mounting gasket.