does reduced exhuast back pressure = more fuel usage?

[454FOTF]

I have a '79 C30 with 454 running headers and turbo mufflers.  I plan to put the truck to work pulling loads of hot freight.  I need maximum low end torque and the best fuel economy possible.  I am considering replacing the turbo mufflers with regular mufflers.  I am thinking this may improve throttle response, off idle torque, and fuel economy.  I can tolerate less horsepower if it brings better fuel economy.  I'm currently only getting about 7 mpg running around town empty.  My goal is 8 mpg on the highway with a load of approx. 10,000 lbs. including trailer.  Truck has 4 speed muncie and 3.73 rearend gear.

Any thoughts on the muffler change?

Any other ideas for mpg increase other than the typical tune up, alignment, and driving easy?  

[123 pugsy]

Lower rear end gears would help, but not sure about C-30's.

[454FOTF]

for heavy towing purposes, I think I need to keep the higher gear ratio.  I'm lucky, a lot of these trucks came with 4.10's

[roundedline]

I honestly don't think you are going to gain anything noticeable by swapping out mufflers to warrant the cost.

From my experience with 454's and 73-87 big trucks, mileage won't vary much from town vs highway.  Last one I had access to was a C20 with 454/Th400 and 4.10's.  It got 8mpg empty or pulling a 20ft pontoon boat.  

Chris Lucas
www.73-87chevytrucks.com
www.captkaoscustoms.com
Project Su
Jimmy 2WD Project

[Lt.Del]

If you will be able to get 8 mpg pulling 10k, well, sooner or later you will have to go UP hill.

I have an ole 79 w/ new 383 stroker new qjet and th350 and pulling around my 30' camper (about 7k lbs loaded), i am lucky to get about 7 mpg.  I have a 3.08 rear.    

SgtDel
aka "Andy"

www.delbridge.net

1979 Big 10 383 stroker
1991 Blu 'Burb 3/4 ton 4x4

[454FOTF]

I just remember when I was a kid I put headers and glasspacks on my olds.  It hurt my fuel economy and also the throttle response was not as tight.  I gained quite a bit of top end power though.  

Seems to me, you put more air through and engine it will need more fuel to go along with it.  Less air = less fuel.  Standard mufflers = less air.  

With the amount of fuel these trucks burn and the current price of fuel, gaining an extra mile per gallon is significant.  The math might surprise you.  On just a 1000 mile trip, the difference between 7 and 8 mpg is over 15 gallons of fuel.  

Anyway, if anybody has any ideas or experience, I would appreciate it.

I don't really understand that thing about going up hill.  My experience is that you get it back when you go downhill.

[roundedline]

Headers are a proven ( versus cast manifolds) to increase gas mileage, you probably got less in the Old because of wanting to HEAR the exhaust.

You analogy is correct, but your are on the wrong end of the engine.  The mufflers that move less air create more backpressure which yeilds less fuel economy.  If you didn't change anything on the motor from the pistons up, you are getting the same air in and capping it going out by putting restrictive mufflers.

You could always switch over to TBI it will get better mileage than a Carb'd motor, but it is a large initial investment.

Chris Lucas
www.73-87chevytrucks.com
www.captkaoscustoms.com
Project Su
Jimmy 2WD Project

[Lt.Del]

Quote:I don't really understand that thing about going up hill.

To be able to get 8 mpg pulling 10k lbs, seems to me that is possible only if going down hill.  

SgtDel
aka "Andy"

www.delbridge.net

1979 Big 10 383 stroker
1991 Blu 'Burb 3/4 ton 4x4

[VileZambonie]

Quote:I have an ole 79 w/ new 383 stroker new qjet and th350 and pulling around my 30' camper (about 7k lbs loaded), i am lucky to get about 7 mpg. I have a 3.08 rear.

Believe it or not you will get slightly better fuel economy from a numerically higher ratio like a 3.55:1 over your 3.08:1 for the simple reason that you will not have the throttle open as much.  

[1976Scottsdale]

Yup, my 76 with 3.42 gears outdid my 78 with 3.08's.  I even had more power coming out of the 76.  

[Lt.Del]

Is that just when pulling/towing or always????  
She does real well with mpg on the interstate, obviously, w/ lower rpms.