My 350 V8 mileage carb thread.
Kane
Full Time Jeeper
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- Berlin, CT
- Vehicle(s)
- 1984 CJ-8 Restomod Scout axles 4 wheel disks, 258, ax15, D300.
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.
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.