Inline 6, gas guzzler??

[chris1htx]

Hello all,

I have an 84 c10 with an inline 6, 4.1 engine and it guzzles gas! There hasn't been anything added to the engine just the basic of what came off the line. The only reason why I think it guzzles gas is because of my heavy foot. Do you guys have any ideas of why it guzzles so much gas?
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[Rufton]

My brother's I6 is pretty thirsty also. You can put smaller jets in carb to gain mpg in some cases. Easy way to tune AFR is a wideband O2 gauge.

Next step is to find a torque curve for engine similar to yours. Then choose gearing that allows cruise at lowish rpm but make sure still in meaty portion of torque curve. Some went to trouble of OD swap and lost hwy mpg because cruise rpm to low for their engine setup.

[canadian trucker]

My old I6 was great, drove it to and from high school for 2 years, 40 km each way.
But you have to remember you are pushing around an 3500lb vehicle with shoe box aerodynamics. All with an engine that produced 155 hp on its best day.

[Rufton]

I used to get 19-21 mpg hwy w/ 1967 200 I6 3 spd manual 2800 pound car. Sometimes I wonder if they went out of their way for trucks to get bad mpg. I guess they were tuned and geared for payload.

[meatwagon83]

my buddy has an 83 3/4 ton with a 292/sm465 and i dont know what gears but it gets about the same mileage as my 454 dually i figure its because the engine has to work that much harder to keep the truck moving

[Rufton]

Low power and low mpg?
Don't think that's win win.

[donut]

What's your definition of "guzzles gas"? Some folks think less than 25mpg is thirsty.
15 is normal for me in my '03 Trailblazer. (I-6, 4wd, long wheelbase)

[BenfromTruro]

Hey folks! My first post here on 67-72, I gotta say this place is an amazing wealth of knowledge.
Anyway, My 74 I6 is also pretty thirsty, 10-12 mpg, but with 4.10's and a 3 on the tree, it's to be expected

[Rufton]

Originally Posted by BenfromTruro

Hey folks! My first post here on 67-72, I gotta say this place is an amazing wealth of knowledge.
Anyway, My 74 I6 is also pretty thirsty, 10-12 mpg, but with 4.10's and a 3 on the tree, it's to be expected

Welcome!
Is that around town mpg?

[kalbert]

Rear gear, tire size and transmission will make a significant difference in mpg in different driving conditions so it kind of matters what that I6 is mated to. In general though, worn ignition parts and a poorly tuned carb will make a 16 mpg truck get 12 easily.

[beamn7]

The family 78 C10, 250ci, 3 on the tree with 3:42 gears averaged 16mpg since it was new. Doesn't use much gas at all these days........maybe in another year or two....
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[lilpoindexter]

My '78 1/2t stepside went up to like 17mpg when I swapped in an '89 Camaro T5. It has 3.73's in the back. Honestly, I didn't buy it for the great MPG...I wanted a truck with something else than a 350. I usually just drive around alone in it, or at worst the bed full of yard waste, it fits my needs ok.

[undr8ed]

My 74 long bed has only had the 1 barrel rebuilt (just changed platic pieces to brass, so I guess it's a rebuild) since I've had it (about 2 years) and gets about 16 around town... I row gears, so I'm never in a hurry. My gas/shifting pattern is really close to my (a lot) earlier vehicles, so she goes through the rpm band. I'm hoping to adjust the valves, fix an exhaust leak, and I'm pretty sure vacuum tuning should get me closer to 20.......


I haven't checked the rear gears, but she sounds like a sweatshop sewing machine at a top speed of 65, haha

[Rufton]

Originally Posted by undr8ed

My 74 long bed has only had the 1 barrel rebuilt (just changed platic pieces to brass, so I guess it's a rebuild) since I've had it (about 2 years) and gets about 16 around town... I row gears, so I'm never in a hurry. My gas/shifting pattern is really close to my (a lot) earlier vehicles, so she goes through the rpm band. I'm hoping to adjust the valves, fix an exhaust leak, and I'm pretty sure vacuum tuning should get me closer to 20.......


I haven't checked the rear gears, but she sounds like a sweatshop sewing machine at a top speed of 65, haha

Could you explain your vacuum tuning plans?
I've only used vacuum for idle tuning.

[vegadesign]

i have a 250 l6 and i concider mine a gas sipper at 27mpg
for an old carb engine to get that is sweet

[jennerik07]

Originally Posted by vegadesign

i have a 250 l6 and i concider mine a gas sipper at 27mpg
for an old carb engine to get that is sweet

Stock carb and intake?

[Rufton]

Originally Posted by vegadesign

i have a 250 l6 and i concider mine a gas sipper at 27mpg
for an old carb engine to get that is sweet

27 mpg would be sweet.
Did you mean 17 mpg?

[vegadesign]

Originally Posted by jennerik07

Stock carb and intake?

single barrel rochester and stock intake.
so ya everything is stock except internals.
its all about breathing, moving air within the engine

i know it sounds crazy and that im doing something wrong but its all legit

[tlawre87]

We have a 79 nova with the 250. It has a single barrel Rochester. All stock with the TH250. Its a 4 door as well. on a 400 mile trip a few years ago it was averaging 26. And the car had been sitting for 2 years. Later on too found it had a blown intake gasket. Dad said back in the day the car would easily get 30mpg. Mind you this car only has 55K original miles on it.

[vegadesign]

honestly i think these engines are badass, from an engine builders standpoint and drivers
they are bulletproof and easy to work on.
they knew what they were doing when they designed it.
everyone thinks they are not that great on gas because its just like a v8 inline but its so much more

[rollin thunder]

Originally Posted by vegadesign

honestly i think these engines are badass, from an engine builders standpoint and drivers
they are bulletproof and easy to work on.
they knew what they were doing when they designed it.
everyone thinks they are not that great on gas because its just like a v8 inline but its so much more

right on! My 84 had an inline 6 with rochester staged 2 barrel. I absolutely loved it and that is what im going to put in when ready for the new engine. IMO cant beat a straight six. really torquey as well

[vegadesign]

amen!

[77chevyinline6]

my 77 had a l6 in it when i got it and it got around 12mpg i have a built 350 in it now and the gas milage isnt much differnt if any

[andrewmp6]

A over drive trans will help but your still pushing a heavy brick on wheels.

[undr8ed]

Originally Posted by vegadesign

i have a 250 l6 and i concider mine a gas sipper at 27mpg
for an old carb engine to get that is sweet

Internal porting/polishing? Mine has as pretty good leak at the exhaust, so I know my tuning isn't (can't be) 100%.

Waiting on mail order mufflers and after a quick tune, I'll see where she is for mileage and post back.