Thread: Carb Sizes

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I have a question for you guys that someone asked me but I am not sure what the answer is truly. Does switching carbs (bore size I guess) from something like a 500 or 600 to a 750 hurt fuel milage? I told him that I think it does because for me it seemed like when I switched from a 750 to a 600 it got better milage... Could have been that it was tuned wrong, etc but who knows. So if they are both tuned correctly and you swap from a 600 to a 750, will it hurt milage at all?
Thanks!

I'm certainly not an expert, but it is a balanced formula between the carb, intake, cam and timing. Thats why I'm going with a matched set from Edelbrock, it takes the guess work out, for beginners like me! Their performer packages improve both economy and performance. I have also read threads about too small a carb actually causing worse milage, but don't know from experience if this is true.

I am not a huge carb guy either, but I think if you increase the air flow, you have to increase the fuel flow to have the proper mixture.

So will changing from a 600 to a 750 change the fuel milage? I see what you are saying I think.

I think it will because you will have to jet it to accomidate the increase in air flow. What mods are done to the motor, because if its a stock motor you are best to stick with the 600. and also is it a holley single fuel feed or double pumper, or an edelbrock carb?,

Holley with vaccume secondaries. It's not mine. A guy on another forum asked this question. I told him that it will hurt milage and then he says that it wont and I am wrong. Not sure why he asks a question and then answers it but whatever. I told him that when I switched carbs on my old Chevy truck and another car it helped milage. Sometimes people put much too big of a carb on a motor than what it really needs IMO.

"I told him that I think it does because for me it seemed like when I switched from a 750 to a 600 it got better milage... Could have been that it was tuned wrong, etc but who knows."


The carb can be tuned for mileage but driveablity may suffer with to big of a carb. The motor is going to determine the mileage and your driving habits.

Yes you are right that could have well been the issue. Thanks for the information. As for the driving habits that is true as well. I baby all my stuff and rarely get on it hard because I have no money rarely so the last thing I need is to get on something hard and have it break apart on me.

An X cubic inch motor will pull in X cubic feet per minute of air at X rpm's. It doesn't matter the size of the venturis or throttle bore, except to say that assuming the carb is large enough to provide X cfm, the smallest venturi will more properly meter the mixture because it is getting a stronger signal than a larger venturi. (same AMOUNT of air going through both, but the smaller venturi requires more VELOCITY to pass the same X amount of air).

When I encounter mind problems such as this, I take it to the extreme each way. Let's say the motor had a small 2-bbl on it that provided decent power, good mileage and good throttle response. You find a carb off a B-29 bomber that's about the size of a TV set and decide to adapt it to your world beater. It would easily meet the requirement for cfm of your X motor, but the venturis would be so big and the velocity of the slug of air going through them would be so slow that a proper air signal couldn't be established and I doubt you could get the motor started, much less have any kind of throttle response.

On the flip side, you decide to adapt the carb off your Briggs Rototiller onto your world beater. I suspect you could get the motor started with it, but little more than that. A 5-horse Briggs passes 14 cfm @3,600 rpm's. A 2-litre Pinto motor passes 17 cfm while idling at 500 rpm's. The tiny little venturi would be inadequate to provide enough air flow to feed a motor much larger than the Briggs. It would be like you running a marathon while breathing through a straw. Ain't gonna happen.

In my opinion, the smaller of 2 different carbs would provide a little better mileage simply due to the fact that the velocity would be higher through the venturis and the mixing would therefore be more complete and more precise.

For a real world example, look no further than the Rochester Quadrajet. When the GM engineers designed this carb (in my humble opinion, the very finest automotive carburetor ever built), they incorporated very tiny little primary venturis, just barely large enough to feed the motor through slow speed driving and small enough to provide excellent velocity, signal and mixing. They were much smaller than any of the 2-bbl venturis being produced at the time. To provide power, the Q-Jet used sewer-pipe sized secondary venturis. The best of both worlds. Ask anyone who has properly set up a Q-Jet and run it. Best gas mileage of any....ANY carb out there as long as you could keep your foot out of the secondaries.

Tech your the man, thanks for the info!

I think the carb from the B29 would use less gas because it won't start

Haha yeah that's actually true. Techinspector, the guy that asked me this question in the first place pretty much doesn't agree... He is like talking to a wall. Asks a question, I answer it, tell him what you said and he said Chevy GM stuff is junk and Ford makes the best fuel economy carbs. :rolls eyes:

Originally Posted by FMXhellraiser

Haha yeah that's actually true. Techinspector, the guy that asked me this question in the first place pretty much doesn't agree... He is like talking to a wall. Asks a question, I answer it, tell him what you said and he said Chevy GM stuff is junk and Ford makes the best fuel economy carbs. :rolls eyes:

.

I agree with Tech, but other things enter into it.

I swapped in a Holley vac secondary 600 cfm smog carb - jetted lean at the factory - for a 750 cfm Carter jetted/rodded for the altitude and found the mileage to be the same.
On a 462" Buick fwiw.

Pretty sure what happened here was the lean jetted Holley required more of a throttle opening than did the bigger Carter and pulled a little more fuel.


On the other side - and not sweating the mpg - little brother runs a total of 2200 cfm on his 500 cid engine....

The emissions holley doesnt surprise me. Probably running off the power valve once you open it up wide enough to get it moving. Any aftermarket carb tuned properly will get decent mileage and not require lean jetting. Also, I think if your little brother is running 2200 CFM on his motor, he's got to be running WOT a Lot, otherwise there is just no reason to have that much carb. And dare I ask, whats a 462 Buick? A 455 bored or stroked?