Decreasing carb size to increase gas mileage?

[nightmare]

Hey guys, I've searched to see if I could find the info that I'm looking for, but can't seem to find it. Anyhow, what I want to know is if I change my carb from 750cfm to 600cfm will I gain any mpg's? I have a '69 Longbed with a 350, 700r4, 31" tall tire, and 3.73 gears. I know that switching to EFI would increase mpg, but that just isn't an option at the moment. Any additional knowledge would be appreciated.

[CC69Rat]

What type of 750 is it? Holley? I'd just consider re-jetting the one I had if I were you. 750 is a little big for a 350 IMO. Is the engine built at all? (cam, headers, etc.? ) I had a built LT1 in a 69 Camaro with a 3310 jetted 70/76 and it was plenty. An out of the box 750 is 72/76 if I am not mistaken. ..

A 600 might make the engine work harder to produce the same power (?) thus eliminating your gas mi savings. Vs. a re-jet and you still get the air flow of a 750.. assuming it's a Holley.

Just my .02.
Chad

[69Beauty]

Is the Current Carb new or old? if it is then a rebuil and rejeting as cc69rat said would definetly. but overcarbing an engine is kinda a thing of the past. David Vizard put an 1150 cfm dominator on a mild 350 and it only lost 7ftlbs and gained 20 hp up top so it depnds on priorities i guess.

[crazy longhorn]

Originally Posted by CC69Rat

What type of 750 is it? Holley? I'd just consider re-jetting the one I had if I were you. 750 is a little big for a 350 IMO. Is the engine built at all? (cam, headers, etc.? ) I had a built LT1 in a 69 Camaro with a 3310 jetted 70/76 and it was plenty. An out of the box 750 is 72/76 if I am not mistaken. ..

A 600 might make the engine work harder to produce the same power (?) thus eliminating your gas mi savings. Vs. a re-jet and you still get the air flow of a 750.. assuming it's a Holley.

Just my .02.
Chad

My 383,as well as the last 355 that I ran ,loved that #70 jet in the 3310(primary side). I ran a purple spring on the secondary diapragm On both motors. I did not touch the secondary jetting......let them ponies run My neighbor is running the 3310, that I set up on my 383...on a 454 in his camaro, & hasnt touched it But to the ?....mpg....dunno! Theory is that a mild built sb WILL get better mileage with a smaller carb....dunno, I have a lead foot!. If you really want mileage, go spreadbore....small primaries(cruise)....kick it open, & it will run. Now I am a big fan of the Holley carbs,but.....bucks down, I have played with the Rottenchester......when that carb is set up right on, it WILL run with the best crazyL

[nightmare]

Originally Posted by CC69Rat

What type of 750 is it? Holley? I'd just consider re-jetting the one I had if I were you. 750 is a little big for a 350 IMO. Is the engine built at all? (cam, headers, etc.? ) I had a built LT1 in a 69 Camaro with a 3310 jetted 70/76 and it was plenty. An out of the box 750 is 72/76 if I am not mistaken. ..

A 600 might make the engine work harder to produce the same power (?) thus eliminating your gas mi savings. Vs. a re-jet and you still get the air flow of a 750.. assuming it's a Holley.

Just my .02.
Chad

Nope, it's an Edelbrock. But it's the 1901, which is the quadrajet replacement. I think that rejetting an Edelbrock would be similar to a Holley rejet as well. It's a crate engine with an Edelbrock intake and carb, and headers with an H-pipe. Other than that, there's nothing special added to it.

I've thought about the 600 causing the engine to work harder, but thought that it might be more efficient with a smaller carb.

Originally Posted by 69Beauty

Is the Current Carb new or old? if it is then a rebuil and rejeting as cc69rat said would definetly. but overcarbing an engine is kinda a thing of the past. David Vizard put an 1150 cfm dominator on a mild 350 and it only lost 7ftlbs and gained 20 hp up top so it depnds on priorities i guess.

The current carb is an older 1901 that came on the engine. I've rebuilt it hoping that would help, but only with the parts that came in the kit. I think that some of the brass bushings need to be replaced, but don't have the proper tools to do that part. Not only that, I can't seem to find the proper parts for it anyhow.

[nightmare]

Originally Posted by crazy longhorn

My 383,as well as the last 355 that I ran ,loved that #70 jet in the 3310(primary side). I ran a purple spring on the secondary diapragm On both motors. I did not touch the secondary jetting......let them ponies run My neighbor is running the 3310, that I set up on my 383...on a 454 in his camaro, & hasnt touched it But to the ?....mpg....dunno! Theory is that a mild built sb WILL get better mileage with a smaller carb....dunno, I have a lead foot!. If you really want mileage, go spreadbore....small primaries(cruise)....kick it open, & it will run. Now I am a big fan of the Holley carbs,but.....bucks down, I have played with the Rottenchester......when that carb is set up right on, it WILL run with the best crazyL

I haven't had too much luck with the Holley's, but that's primarily because I've been using the carbs that came with the vehicles. Rochesters or Edelbrocks, either have been the quadrajets, which seemed to work well. I've been able to tune them quite easily.

[Hubscrub]

i use quadrajets when i can they do good for mileage and power, and if not i have always run a 600 edelbrock on my 350's seems about perfect for the smaller V-8s

[crazy longhorn]

If you want a carb that "you dont hafta mess with", go 600 Edelbrock....on a mild build, it will give what you want out of the box. I love tunin.....run a 750 Edel on my 383...not too much to tune. if you really want to tune, that old GM Q-Jet will show you a bunch....not an easy carb to tune(bring you lunch!)crazl

[todd99]

what gas mileage are you getting now? with 3.73 and a 700r4 I think it would be pretty good.

[nightmare]

Originally Posted by Hubscrub

i use quadrajets when i can they do good for mileage and power, and if not i have always run a 600 edelbrock on my 350's seems about perfect for the smaller V-8s

The 1901 is supposed to be a "direct replacement" carb. I don't know what was on there originally, since this one came on the engine. I'm guessing that a 750 was original, but don't know for sure. So what type of mileage have you received with the 600?

Originally Posted by crazy longhorn

If you want a carb that "you dont hafta mess with", go 600 Edelbrock....on a mild build, it will give what you want out of the box. I love tunin.....run a 750 Edel on my 383...not too much to tune. if you really want to tune, that old GM Q-Jet will show you a bunch....not an easy carb to tune(bring you lunch!)crazl

I don't want to have to tune it all the time, I want to be able to "set it and forget it." However, I don't mind tuning it so that it works correctly. I just don't know if having a 750 on there is hindering my gas mileage.

Originally Posted by todd99

what gas mileage are you getting now? with 3.73 and a 700r4 I think it would be pretty good.

That's the thing, I'm not sure. I haven't been able to run a complete tank through yet as I just had the 3.73's installed. Anyhow, before them, I had the 3.08s and was getting about 11.5. I'm thinking that with the new gears that I should be getting about 17. We'll see though.

[DANTIP]

A Quadrajet is essentially a variable-size carb. It responds and mixes well at all throttle positions. If you want better mileage, teach your right foot to go as easy as possible. Quadrajet economy is very close to EFI. I can get 15+ in my 402 with a Quadrajet when I am easy on the pedal.

[Longhorn Man]

if you end up getting 17 MPG... weld the hood shut and never mess with it again.
I would expect more like 13 to 15

[69CST4x4SWB]

How can I figure out what model the Edelbrock is on my small block 400 and which model would run the best on it? The only mods that I know of is the Headmen tube headers and what looks to be Flowmasters or something designed similar and 2-1/2 or 3 inch dual exhaust. Have not measured it, so I am not sure. Thanks!!!

[ole dollor]

nightemare: i would thing your tires and rear end would be the place to start, it takes a lot of gas to pull 31" tires, and try at least some where around 340 gears, or 307, i have not seen your truck and how tall it is. so just with what you listed this is what i see....

[jocko]

Originally Posted by nightmare

I haven't been able to run a complete tank through yet as I just had the 3.73's installed. Anyhow, before them, I had the 3.08s and was getting about 11.5. I'm thinking that with the new gears that I should be getting about 17.

Huh? While a 3.73 is a good rear gear for a vehicle equipped with an od trans (like your 700R4), going from 3.08 to 3.73 will absolutely NOT increase your gas mileage. Did you simultaneously swap in the 700R4? If you were getting 11.5 with 3.08s and a 700R4, you might expect your gas mileage to even decrease (to 9 or 10 mpg) with the 3.73s in there.

IF all things are held equal, including your driving habits, you're going to be turning about 20% more rpm in every gear with a 3.73 as you go from point a to point b, so mpg will get worse. That is why ole dollar is recommending a lower (numerical) ratio than the 3.73. It caught my attention that you JUST installed the 3.73s in hopes of increasing economy.

[nightmare]

WOW, old thread. Still applicable though.

Originally Posted by ole dollor

nightemare: i would thing your tires and rear end would be the place to start, it takes a lot of gas to pull 31" tires, and try at least some where around 340 gears, or 307, i have not seen your truck and how tall it is. so just with what you listed this is what i see....

Originally Posted by jocko

Huh? While a 3.73 is a good rear gear for a vehicle equipped with an od trans (like your 700R4), going from 3.08 to 3.73 will absolutely NOT increase your gas mileage. Did you simultaneously swap in the 700R4? If you were getting 11.5 with 3.08s and a 700R4, you might expect your gas mileage to even decrease (to 9 or 10 mpg) with the 3.73s in there.

IF all things are held equal, including your driving habits, you're going to be turning about 20% more rpm in every gear with a 3.73 as you go from point a to point b, so mpg will get worse. That is why ole dollar is recommending a lower (numerical) ratio than the 3.73. It caught my attention that you JUST installed the 3.73s in hopes of increasing economy.

I originally had a 3.08 gear with a TH400. I was getting ~11.5. I wanted to increase that, and the consensus was to go with an OD trans. When I did that swap, my gas mileage went down to about 9.5. It was recommended that with an OD trans that the 3.08's were a bad gear for that combination, and that the 3.73's were the "optimal" gear ratio. This was because the trans wasn't allowing the engine to be in the proper RPM range that it should be in for driving, especially at highway speeds. I then switched to the 3.73's, and my mpg's went back up to about 11.

This was all on a 31" tire. I switched to a 29" tire, and that brought the rpms up. My mileage did suffer a little with this, but only a small amount. I would think that with this combination (stock 350, 700R4, and 3.73 gears), and a 600 cfm carb that the mpg's would increase.

[CC69Rat]

That 1901 is a 750cfm carb if I'm not mistaken. I happen to have a 650 (I think, I'll have to check) Edelbrock I am considering selling if you're interested.

It ran great when I took it off my GMC. Pics in the build thread. It was on the truck when I bought it and hasn't been on it long. It is a high dollar carb from Summit.

[PanelDeland]

I would think MPG would be better with the 600 or a 625.Theory(yeh right) is that driveability is better with a vac secondary of smaller CFM due to better vac signal to the carb.If you keep your foot out of it and drive mostly city the secondaries shouldn't open very often as long as the engine is getting the fuel/air it needs.Unfortunately with the specifics of any given vehicle/engine etc.combo you can only go by past experience to determine what to try.That can get more expensive than the fuel to continue with the MPG you currently have.