My 350 V8 mileage carb thread.

[Kane]

In my quest to have a strong running Chevy 350 that gets the same gas mileage as a AMC 258 i6 / 4.2l with a 4 speed I am trying out another carb to see if it helps. So far I have used the Holley 670 truck avenger, the Edelbrock 1405 600cfm carb and now I am trying a Quadrajet 4M from a 1978 350 car. I have every fill up documented since I built the motor and what carb and tune it had during that mileage.

Engine spec in a nutshell:

I have a mildly built Chevy 350 with Edelbrock Performer heads and flattop forged pistons making about 10-1 compression on 89 octane gas. It has dual exhaust with headers and Flowmaster series 40 mufflers. The only downfall to my mileage quest is the headers have large primary tubes and I have some with smaller primary tubes that I plan to swap at some point. This is important because the smaller primaries will give me better scavenging effect down in the lower rpm and cruising range to increase torque and gas mileage. CT loves to use ethanol based gas so my comparison is with that from the same gas station. I run a full MSD ignition system. My timing is the same for all three carbs at 12btdc idle and 36btdc for full mechanical advance plus another 6 or so for the vacuum advance. It all comes in at 2300rpm. I drive mainly short trips and 50% highway/city driving so the mileage is generally a little lower. So there is the background.

Edelbrock:

This carb is great for the ease of tuning and quick jet and needle changes. I have a full tuning kit that comes with all the jets, needles and spring you could need. I used the Edelbrock site chart to help dial the carb in very nicely. To try and get the best mileage I leaned out the primaries as much as I could to the point of low RPM surging and then went the other direction. The best I could get out of this carb was 13.5 MPG with poor drivability. When I fattened up the primary it drove great but mileage was in the 12mpg range.

Holley:

This carb is a monster for power but terrible on gas. When I am very nice I get 11mpg and I have seen less than 10 mpg a few times. The only tune I have done was to change the factory jets down 2 sizes for the primary and 1 size for the secondary. The carb runs rich still and smells of gas when the top is down. Also the idle jetting is rich and it struggles to keep an idle less than 1000 rpm. I really dislike tearing these apart so I just left it as is. The throttle response is awesome with the low end torque I can snap my head back into the seat when I step on the gas.

Quadrajet:

I bought this off of Ebay for $35 and bought a $20 rebuild kit. I could tell this carb has never been apart and hasn’t seen fuel in at least 15 years. I tore it down, cleaned it up and put it back together, checked all my adjustments and dropped it on the Jeep. The Jeep fired right up and settled into a nice idle at 850 rpms. I made some minor adjustment to the idle circuit and return spring tension, double checked my timing then took her out for a quick spin. Initial thoughts, this carb drives fantastic. It is very smooth and there is no hesitation from the primaries to the 4 barrels which is amazing. From what I can tell it makes good power but I haven’t laid into it yet. I plan to do a 30 minute drive after work if the weather holds out. I won’t be able to really drive it for a full tank until next week after I get back from a trip. I will run a full tank and then report on how it did for mileage.

 

[JR74CJ5]

~ Jr

 

[CJim7]

I was entirely pleased with the Q-jet on my 401 for the reasons you pointed out.
A very smooth carb and the small primaries should show you an increase in mileage..
The only problem that I've seen with many of these is the throttle shaft wear causing leaks. But a properly maintained Q-jet is worth it's weight in gold.

 

[Old Dog]

I've had a few Q-jets on small blocks, they are a very good carb that does well off road also.
The trucks they were in though were lucky to get 10 mpg.

 

[CJim7]

I've had a few Q-jets on small blocks, they are a very good carb that does well off road also.

That's what I was most impressed with....a huge surprise actually, having dealt with many different carbs. None held a candle to what my Qjet would handle.

...Hmm....maybe I should quote that that last sentence for poetic-ness. lol

 

[Petescj]

That's what I was most impressed with....a huge surprise actually, having dealt with many different carbs. None held a candle to what my Qjet would handle.

...Hmm....maybe I should quote that that last sentence for poetic-ness. lol

Your on a roll today my friend. On a serious note though just what kind of mileage is a AMC 258 i6 / 4.2l with a 4 speed supposed to get ? 15 mpg? 18mpg? Ive seen some outrageous claims before. Whats the magic number?

Ive got 35 in tires and an auto. I havent run a tank through yet but Ill know what I have by next week. Id be happy with 15mpg cause I know theres room for improvement.

 

[Kane]

My history with mine was 16 mpg with 33 inch tires and a lift on the jeep i have now with the same gearing. It is a good apples to apples comparison. The AMC 258 i6 / 4.2l was stock everything.

 

[Petescj]

My history with mine was 16 mpg with 33 inch tires and a lift on the jeep i have now with the same gearing. It is a good apples to apples comparison. The AMC 258 i6 / 4.2l was stock everything.

Ok so I assume we are going with the CJ5 in your profile. I know the T-18 is a 4 spd but its driven more like a 3 spd for this comparison. It doesnt make much of a difference for your test, I was trying to glean a bit of info for the AMC 258 i6 / 4.2l . What gear ratio is it by the way?

 

[CJim7]

the best I got was 18mpg....that was on a totally de-smogged AMC 258 i6 / 4.2l with a TFI ignition running 33's, 4.10's, and a T-5 tranny.

From what I've seen, that's about the best you can get. I've heard people boast about 20mpg.....but I'm skeptical on that claim.

 

[Kane]

It is the CJ5 . I had the T-176 in with the AMC 258 i6 / 4.2l so but of course the final drive is the same and i have 3.55 gears. I had a CJ7 with a T-5 / AMC 258 i6 / 4.2l combo on 31's and a hardtop and got about 18 mpg at best with a weber carb. Anything is better than 11 mpg at this point. I would love to change the headers and drop in a 700r4 to see over 16 mpg but i doubt that can happen with this motor. My old 327 got 14 - 15 in this very jeep with the edelbrock carb and the same driveline as the 350.

 

[Scooter402]

:lpopcorn:


I'll be watching this one. When I de-tune my 402, I'm looking to swap something in place of my Holley 850 HP. I kicked around the idea of a quadrajet, but never have really pulled the trigger on anything. The only other thing I have thought of is some sort of EFI setup, but I (at least going by what my wife tells me... ) don't have the spare cash for it.

Drive that tank empty already!

 

[CJim7]

:lpopcorn:


I'll be watching this one. When I de-tune my 402, I'm looking to swap something in place of my Holley 850 HP. I kicked around the idea of a quadrajet, but never have really pulled the trigger on anything. The only other thing I have thought of is some sort of EFI setup, but I (at least going by what my wife tells me... ) don't have the spare cash for it.

Drive that tank empty already!

How's your compression ratio? Thought about going propane?

 

[Petescj]

Thanks. I was just trying to get some "clean" information.

It is the CJ5 . I had the T-176 in with the AMC 258 i6 / 4.2l so but of course the final drive is the same and i have 3.55 gears. I had a CJ7 with a T-5 / AMC 258 i6 / 4.2l combo on 31's and a hardtop and got about 18 mpg at best with a weber carb. Anything is better than 11 mpg at this point. I would love to change the headers and drop in a 700r4 to see over 16 mpg but i doubt that can happen with this motor. My old 327 got 14 - 15 in this very jeep with the edelbrock carb and the same driveline as the 350.

 

[Scooter402]

How's your compression ratio? Thought about going propane?

It's between 11 and 11.5:1. Not really that high. I could go propane, but I'm thinking I'd have problems trying to make it street legal again, as that's my goal. I was planning on either doing some combustion chamber work to bring down the compression, or tuning the carb to run primarily on E85. I do have some service stations near me with it. Fuel economy (I say that jokingly... ) already isn't great since it is a race motor, so I'm not expecting any huge gains there.

 

[csuengr]

I can get about 13-15 around town and 16+ cruising to the mountains. I could get an honest 18 if I put in an Overdrive Transmission .

 

[Kane]

So I took the Jeep out for a spin last night and I drove it to work today. So far I really like this carb. It is very drivable, more so than the other two carbs. The engine still has good torque down in the lower rpms and pulls off the line really nicely. Last night I hammered the pedal a couple of times and I could tell when the secondary barrels kicked in. You can feel when the linkage starts to move through the pedal. I like that because it allows me to stay out of the secondary barrels when I want to. It gave a pretty good kick in the seat when I hammered it but I think the Holley 670 made more power overall. The engine did pull strong up to 6500 rpms when I let off. I plan to do some tuning over the next few weeks after I run a tank of gas down and then see how good I can make this carb. I have high hopes for some better MPG gains at this point.