gas prices crunchin cash_time for TBI?

73_Mopar_Muscle · 11-26-2007, 03:23 PM

I have a 73 roadrunner 318, 4 speed but the gas mileage in this car is killin me!!

As a mopar enthusiast I refuse to give up this car or to stop driving it. What would it take to swap in the TBI from a mid-90's 318 and what could I expect as far as gas mileage? I already found an adaptor to put that throttle body on a 4bbl manifold but what else is needed to make the computer function properly. Thanks

CAT-TECH · 01-28-2008, 08:36 PM

I owned a 1990 Dodge Ram with a TBI 360, the truck still sucked on gas mileage(7-9mpg). But then again it has a extra-cab longbed with dana 60's front and rear and I never had my foot out of it. But if I tried I could pull 14mpg out of this thing on the freeway and staying at about 65-75mph. the TBI imrpoved the driveablilty and cold starts alot more than MPG. I always new the 318 as a really good Gas mileage engine, what size carb are you running? how about your rearend gears? What kind of cam lift have you got going on? How about miles on this engine?

440roadrunner · 01-29-2008, 06:19 PM

You say, "the mileage is killin me" but you don't say what it IS.

Back when I swapped a stock '71 340 into my '70 body, it could get (on the highway) nearly 19 part of the time, it was

'70 Plymouth RR body, 3.54, Dana rear, G-70 x 15 tires

Stock '71 340, with headers.

Ran either stock Themojunk Carter, or GM quadrajet carbs, and for awhile, ran Holley "spread bore." Best mileage was with a Quadrajet!!

Just about any good running, properly tuned 318 should be able to get more than 17, if you drive responsibly, and there is nothing wrong with the engine, and have something for "street" gears.

CAT-TECH · 01-29-2008, 06:23 PM

Normally lower CFM carbs help with mileage, My buddy ran a 390 CFM(I think it was an edelbrock) on his 318 in his 70 Dodge W-200 truck for awhile and he said it helped out his mileage.

440roadrunner · 01-30-2008, 09:49 AM

That's true, and trying to adapte a TBI is NOT the "magic bullet" here, and might just cause more problems.

There are MANY things besides the carb and intake which affect mileage.

Run a good hot 190 stat

Make sure the engine is running "up to temp" but not overheating.

Make sure your tires are not underinflated.

Check out your gearing / tire combo. Usually, higher ratios (lower engine RPM) helps with mild street engines, but in some hi perf. cases, if you get "too low" the engine can actually get worse mileage

Check your ignition advance. Is the mechanical advance working properly, and advancing all the way?

Ditto the vacuum advance. Is it working, and is it hooked to the proper vacuum port?

And, of course, there's your driving habits!!!

CAT-TECH · 01-30-2008, 10:57 AM

Anyway what mileage are you getting? also are you running a carb with mechanical or vaccum secondarys?

11-26-2007, 03:23 PM	#1 (permalink)
73_Mopar_Muscle Junior Member Join Date: Nov 2007 Posts: 1	gas prices crunchin cash_time for TBI? I have a 73 roadrunner 318, 4 speed but the gas mileage in this car is killin me!! As a mopar enthusiast I refuse to give up this car or to stop driving it. What would it take to swap in the TBI from a mid-90's 318 and what could I expect as far as gas mileage? I already found an adaptor to put that throttle body on a 4bbl manifold but what else is needed to make the computer function properly. Thanks

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01-28-2008, 08:36 PM	#2 (permalink)
CAT-TECH Member Join Date: Jan 2008 Location: Cali Posts: 42	I owned a 1990 Dodge Ram with a TBI 360, the truck still sucked on gas mileage(7-9mpg). But then again it has a extra-cab longbed with dana 60's front and rear and I never had my foot out of it. But if I tried I could pull 14mpg out of this thing on the freeway and staying at about 65-75mph. the TBI imrpoved the driveablilty and cold starts alot more than MPG. I always new the 318 as a really good Gas mileage engine, what size carb are you running? how about your rearend gears? What kind of cam lift have you got going on? How about miles on this engine?

01-29-2008, 06:19 PM	#3 (permalink)
440roadrunner Senior Member Join Date: Nov 2007 Posts: 272	You say, "the mileage is killin me" but you don't say what it IS. Back when I swapped a stock '71 340 into my '70 body, it could get (on the highway) nearly 19 part of the time, it was '70 Plymouth RR body, 3.54, Dana rear, G-70 x 15 tires Stock '71 340, with headers. Ran either stock Themojunk Carter, or GM quadrajet carbs, and for awhile, ran Holley "spread bore." Best mileage was with a Quadrajet!! Just about any good running, properly tuned 318 should be able to get more than 17, if you drive responsibly, and there is nothing wrong with the engine, and have something for "street" gears.

01-29-2008, 06:23 PM	#4 (permalink)
CAT-TECH Member Join Date: Jan 2008 Location: Cali Posts: 42	Normally lower CFM carbs help with mileage, My buddy ran a 390 CFM(I think it was an edelbrock) on his 318 in his 70 Dodge W-200 truck for awhile and he said it helped out his mileage.

01-30-2008, 09:49 AM	#5 (permalink)
440roadrunner Senior Member Join Date: Nov 2007 Posts: 272	That's true, and trying to adapte a TBI is NOT the "magic bullet" here, and might just cause more problems. There are MANY things besides the carb and intake which affect mileage. Run a good hot 190 stat Make sure the engine is running "up to temp" but not overheating. Make sure your tires are not underinflated. Check out your gearing / tire combo. Usually, higher ratios (lower engine RPM) helps with mild street engines, but in some hi perf. cases, if you get "too low" the engine can actually get worse mileage Check your ignition advance. Is the mechanical advance working properly, and advancing all the way? Ditto the vacuum advance. Is it working, and is it hooked to the proper vacuum port? And, of course, there's your driving habits!!!

01-30-2008, 10:57 AM	#6 (permalink)
CAT-TECH Member Join Date: Jan 2008 Location: Cali Posts: 42	Anyway what mileage are you getting? also are you running a carb with mechanical or vaccum secondarys?