Big Block MPG

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Jesus Chrysler

Jesus Chrysler

Forgiving Sins Against Mopar Since 1983
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Ontario, California
Just for fun i tracked my MPG's for the limited amount of driving I've been able to do.

1967 Barracuda, stockish (pertronix ignition) 383 4 speed 3:23.

Hot doggin' around town, one short trip with 3 burn outs.

14 MPG.

Not bad, i say! Once it's roadworthy enough for some highway time I won't be surprised if i hit 18MPG.
 
383 with 3:55 and get that! Most of the time my foot is in it. No tire burning though, seems a waste not to stick to asphalt and go!
 
My son's 440/4sp/3.23 Dart convertible has pulled 15 on the highway without spending much time fine tuning. Next time the car hits the road the 440 will likely be fresh which should help as the short block hasn't been apart since Chrysler put it together in 1970.
 
My extremely lo-po-POS, ‘78/400/727/3.55 w/800cfm TQ in a ‘79 Magnum riding on 246/60/15’s would get 18mpg’s on the best days.
 
Mileage is about tuning, overall car setup, and driving style. IMO 15 should be the low end of a mild 383 package. Make sure your alignment is correct, and tire pressures are up closer to the max pressure noted on the tires, and the vacuum advance is on and functioning properly.
 
Jesus Chrysler said:
Just for fun i tracked my MPG's for the limited amount of driving I've been able to do.

1967 Barracuda, stockish (pertronix ignition) 383 4 speed 3:23.

Hot doggin' around town, one short trip with 3 burn outs.

14 MPG.

Not bad, i say! Once it's roadworthy enough for some highway time I won't be surprised if i hit 18MPG.
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Obviously, your foot is too light.
 
Ottmundr said:
Obviously, your foot is too light.
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I took the car for my longest cruise since I’ve had it this past weekend. 70 something miles partially around town a little bit of twisty back country roads Some decent open highway miles in about 30 minutes of crappy traffic. Mileage drop to about 8.5. Has me wondering if gas is evaporating while the car sits for a week and doesn’t move.
 
440, 800cfm AVS, 727 auto, Stealth heads, Street Dominator intake, 1 7/8" headers, 238/242 solid lifter cam, 8.75 rear with 3.55 gears.

I averaged 10.8 mpg on the highway with an average speed around 64 mph. I shudder at what it gets around town lol.
 
Jesus Chrysler said:
I took the car for my longest cruise since I’ve had it this past weekend. 70 something miles partially around town a little bit of twisty back country roads Some decent open highway miles in about 30 minutes of crappy traffic. Mileage drop to about 8.5. Has me wondering if gas is evaporating while the car sits for a week and doesn’t move.
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More likely it's evaporating while sitting in that crappy traffic. Might want to look into a vapor return system and an insulating carb spacer if you don't already have one. Also how is the carb jetting? You might be able to lean out the low-load/cruise circuit without any bad effects. I picked up 3 MPG on my old 360 after installing an O2 sensor and finding out I was crusing at 13.5-14:1 AFR so I leaned it out to about 15.5:1. I also noticed after leaning it out the oil took a lot longer to turn black and no longer had any gasoline smell to it. And the exhaust wasn't so smelly.
 
BTW a gas station that recently opened near me sells ethanol-free gas, it's only 87 octane (mid-grade at this higher altitude) but my 5.9 Mag is only 9:1 compression, i'm gonna give it a shot a buddy of mine with a '70s Ford truck (360 FT engine) says it runs much happier without the ethanol.
 
MopaR&D said:
More likely it's evaporating while sitting in that crappy traffic. Might want to look into a vapor return system and an insulating carb spacer if you don't already have one. Also how is the carb jetting? You might be able to lean out the low-load/cruise circuit without any bad effects. I picked up 3 MPG on my old 360 after installing an O2 sensor and finding out I was crusing at 13.5-14:1 AFR so I leaned it out to about 15.5:1. I also noticed after leaning it out the oil took a lot longer to turn black and no longer had any gasoline smell to it. And the exhaust wasn't so smelly.
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I honestly have no idea what jetting it is. I want to say it’s whatever the factory put in a 383 67 barracuda Carter but I don’t know if someone played with it over the years. It does smell rich and I got the car running after years of dormancy and did nothing to the carb other than hose it down with spray cleaner. I’m still somewhat amazed that it doesn’t leak. The choke doesn’t work and one day I found the linkage screw for the accelerator pump sitting on the manifold after I’d been driving it for a bit. I do intend to pull it out and have it gone through by a shop but I haven’t gotten there yet.
 
mygasser said:
big block mpg is why i'm looking into lpg for the 383 in my 68 valiant.
neil.
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When I build my new motor I intend to go with a Holley super sniper efi. I am curious how it would effect mileage on my current 383 and I may end up installing the FI setup before I get the new motor built.
 
Jesus Chrysler said:
When I build my new motor I intend to go with a Holley super sniper efi. I am curious how it would effect mileage on my current 383 and I may end up installing the FI setup before I get the new motor built.
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it'll definately improve the mileage for sure. but with lpg i can get bottled gas for forklifts at less than one quarter the price of petrol here in the uk. so 'pound for pound' (or dollar for dollar) a 10 mpg big block will do the equivalent of 40+ mpg. plus i won't use petrol at all so i can use more timing advance as lpg is about 110 octane so i won't lose any power either.
 
mygasser said:
it'll definately improve the mileage for sure. but with lpg i can get bottled gas for forklifts at less than one quarter the price of petrol here in the uk. so 'pound for pound' (or dollar for dollar) a 10 mpg big block will do the equivalent of 40+ mpg. plus i won't use petrol at all so i can use more timing advance as lpg is about 110 octane so i won't lose any power either.
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Oh gotcha, I didn’t realize you were over there in jolly ol England.

I think LPG typically gets less MPG but if the price point justifies it I can understand your thought process. How many pounds of Her Majesty’s finest are you paying for a liter of petrol these days?
 
about £1.20 per litre for petrol and bottled forklift gas (lpg) is about £0.25 per litre. as for the loss of mpg on lpg as i say it's less than one quarter the price of petrol so i guess pound for pound economy will still be about four times cheaper than petrol. even close to that and it'll mean i can use my big block valiant as a daily and it'll be as cheap to run as my diesel mercedes and i know which i'd rather drive :thumbsup:
 
Jesus Chrysler said:
I honestly have no idea what jetting it is. I want to say it’s whatever the factory put in a 383 67 barracuda Carter but I don’t know if someone played with it over the years. It does smell rich and I got the car running after years of dormancy and did nothing to the carb other than hose it down with spray cleaner. I’m still somewhat amazed that it doesn’t leak. The choke doesn’t work and one day I found the linkage screw for the accelerator pump sitting on the manifold after I’d been driving it for a bit. I do intend to pull it out and have it gone through by a shop but I haven’t gotten there yet.
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You'll pick up a lot with a full carb rebuild and tune, if your carb is in good shape. Do you know if it's an AFB or AVS Carter? Either one is pretty straightforward to tune, the AFB more so since the secondary opening rate is essentially non-adjustable.
 
Jesus Chrysler said:
When I build my new motor I intend to go with a Holley super sniper efi. I am curious how it would effect mileage on my current 383 and I may end up installing the FI setup before I get the new motor built.
Click to expand...

Depends on how good your current tune is. I gained about 1 MPG going from a carb to EFI (FiTech), however, the drivability and throttle response improvements were worth it alone.
 
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