Carb ?

[1979chevystepside]

So I have the chance to buy a new carb for my k10 but I have some CFM questions. First what is the 4bbl quadrajet CFM? Im thinking about getting a 650 Edelbrok. But I also have the choices of a 600 and a 760 Holley. I have a stock 350 straight piped and will be getting headers shortly. Other than that, Im planning on keeping it stock. So experts whats the best?

and yes i searched

 

[dyeager535]

Stock Q-jets were 750 or 800CFM. GM had a reason they put which ones on which engines, but no idea what it was. The 80's 305 trucks I've taken carbs off of were 800's, the 350's are 750CFM.

With the wrong carb it's pretty easy to "overcarb" an engine. But just because the calculators say 500 or 600CFM is right, doesn't mean the calcs are right. David Vizard tested that, and made more power with a carb that would theoretically be WAY too big for the engine being tested.

GM put 800's on 305's and 307 (Olds) up until 1990, so at least with a Q-jet, bigger can be better for some reason. I somewhat suspect this doesn't hold true for aftermarket carbs, which seems somewhat odd since if vacuum secondary, it shouldn't matter. That explains why the calculators you see for fuel requirements seem to be so low. But you know GM was not wrong, so something isn't right in the calculations, or they are simply too generic.

 

[gmc4cw]

every truck is different. with that being said. my truck had a quadrajet on it when I bought it. never really had any issues with it. when I swapped in a new motor I ran it for a while. it was ok. I think it sat on the shelf too long. I swapped a truck avenger and I love it. I haven't stalled it yet. It has stumbled but that was when I had the truck standing on it's nose.

I had heard that the quadrajet works with a high cfm because the primarys are small. basically if you doubled the primarys the cfm rating would be very low. kind of a good idea though. just enough fuel to tool around and dumps it when the secondarys open.

 

[Hogback Fabrication]

David Vizard tested that, and made more power with a carb that would theoretically be WAY too big for the engine being tested.

(I'm not trying to call you out here Dorian, I just want to make sure people are not fooled by the misconception)


Bigger carbs make more power on any engine, period. There is less restriction to flow, which means more power. 4 barrel carburetors are rated at 1.5" of mercury which means at that flow (750 or 800 CFM for a Q-jet) the difference in pressure between the incoming air and the outgoing air is 1.5" of mercury. If, by the calculator, your engine doesn't "need" all that air, the pressure drop will be smaller than 1.5" and more air will make it through the carb and into the motor.

The reason that you hear people talking about carbs being too big for a motor is how they run at part throttle. Big carbs on engines that don't need them tend to run poorly on the street and suck gas.

The Q-jet is an exception to this. It will run well on anything from a straight six to a big block. That is my vote for which carb to run as well.

 

[1979chevystepside]

ive rebuilt this thing 100 times, shops have rebuilt it, and im plan sick of it. its design is stupid, i hate the whole metering rod idea. anyway it runs great for about a month then starts acting up and wont quit until it is yet again rebuilt. so my truck is street driven, but i dont need something with monster power, just enough to cruise.

 

[dyeager535]

Yeah, I wasn't inferring bigger is always better, and at some point a larger carb simply won't work I'm sure. (and we're talking over 1000CFM for sure)

As you mention, the primaries on the Q-jet are pretty small comparatively, and for the poster, the difference between the two CFM's is IN the primaries on a Q-jet. 250CFM on the primary side for the 750CFM variant, 300CFM for the 800.