Can you tune a gas engine to get better MPG?

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Chris142

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Jun 5, 2003
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Location
Apple Valley, California
My old truck is killing me!

The stats are.
1973 International Harvester 3/4 ton 4x4 pickup.
Rebuilt I.H. 392 with a Comp cams #268 cam (268 duration,490lift, 214 duration @ .050)
BB Chevy valve springs.
4bbl Holley 600cfm carb with an electric choke.
2.5 inch duel exhaust with 3 inch flowmasters.
Warner t-19 wide ratio trans....No overdrive.
Dana 60 rear and a Dana 44 front axle with 4:30 gears.

Has 33x12.5x15 tires and they are properly aired up.

The truck weighs #5200 lbs empty.

Truck gets 5.90-6.20 mpg @ 60 mph empty!

Loaded with say a large boat on a 2axle trailer and it gets 3mpg! Although there's not a hill it wont climb.

I had it tuned for torque on a chassie dyno. We played with the jets in the carb and the advance curve in the distributer. It has a very fast advance now. It's all in by 2200 rpm.

FWIW 302 ftlbs of TQ to the rear wheels @1200rpm in 4th gear!!

I realize a big boxy truck with a 400 cu inch engine will not be great on gas but is there anything that can be done with say the ignition timing curve to bring the MPG's up a little? Would a slower advance curve increase the MPG? I don't think I can quicken it up any more.

The carb jetting is 14.3-1 under cruise and it dips to 12.5-1 under power. The next smaller main jet caused a lean stumble and the jetting was 15.5-1.

At this point I will give up some hp and tq to get a more reasonable MPG since regular gas is now $2.40 a gallon.

Any ideas?
 
Well, I can't comment on your carb settings. It's sized right for the engine (the CFM).....

..but I gotta ask you ..

Are you sure your speedo/odo is correct? I mean you're down there with my friend's 196? Ford dump body with a (I think) a 477 cube engine (dual clutch disks and all). He got 5 mpg plowing snow and slinging salt. Have you got the right speedo gear in there??

That type of consumption, if true, would have to be dumping some serious fuel out the pipes. Do you foul plugs often??
 
quote:
Originally posted by Chris142:
I had it tuned for torque on a chassie dyno. We played with the jets in the carb and the advance curve in the distributer.
Click to expand...
Just to be sure, you did set the mechanical advance curve with the vacuum advance disconnected, correct?
 
Short of having a 4000 rpm stall converter, I would go with the speedo theory also, that cam is not too large, and even with a 4.30 gear, it should get better than that.
 
Lotsa weight, lotsa gear ratio. Big tires.

All things that cut gas mileage.

What type of ignition system do you have? A msd, crane, or other multi-strike ignition will help mileage.

Total advance (vaccum and centrifugal) at cruise should be about 50-55 degrees. About 34-36 degrees centrifugal without vaccum advance plugged in, the rest coming from the vaccum advance.

If you get a lean stumble with the leaner jets, I would leave that alone.

Was the cam degreed in? I have a comp cam in my 340 and it was 6 degrees retarded at 0 setting on timing chain. If the cam is retarded, it will lower gas mileage and power.

Do you have synthetic lubes in all gear cases, axles?

You might have to back down from the tall tires and go to shorter tires but that would mean you have to go to different gear ratio in both axles (3.00?) Big tires in diameter and width cause a lot of friction.

Alignment? if the tires are aligned correctly they are fighting each other down the road, eating up power and gas mileage.

Headers might help. Also the 3 inch diameter core on the mufflers may be too much. May need to step down to diameter of the exhaust pipe to keep flow up. Also crossover between passenger and driver exhaust pipe will help.

Do you know the compression of the engine from measurement? If not, anything under 9 to 1 compression can lose gas mileage. Thermal efficiency increases greatly between 7 to 1 and 9 to 1 compression. Engines may be speced at 9 to 1 but maufacturing tolerances may come out to be much less. For example my 340 was rebuilt with 10.5 to 1 forged pistons. But the heads measured 75 ccs, not the blueprint spec of 65cc. Compression measured out to 9.25 to 1.

Dan
 
I'm somewhat familiar with holley's,
I'm guessing you have a 4150 or 4160. You should be able to change jets a couple sizes leaner and should not get a lean stumble. There are many other factors that can contribute to that. It's hard for me to offer any advice or suggestions without riding in the truck or seeing the setup, but fwiw make sure the throat of the carb is clean and polished and it can get a good amount of unimpeded air. Hood scoop?
grin.gif

If you don't have any carb books, I recommend getting Carburetors, how to tune & modify. It covers holleys well and goes over many basic & practical things.
Depending on how much effort & money you want to put into tinkering with the carb, Jeg's has a complete jet holley jet kit for what you're looking to do. You would also need a set of power valves- which would also cause the stumble. Jets and power valves go hand in hand on a holley, along with secondary diaphram springs. You should be able to lean it out some, safely, and get better mpg. How do the plugs look?
 
quote:
Originally posted by Dan4510:
Lotsa weight, lotsa gear ratio. Big tires.

All things that cut gas mileage.

What type of ignition system do you have? A msd, crane, or other multi-strike ignition will help mileage.

Total advance (vaccum and centrifugal) at cruise should be about 50-55 degrees. About 34-36 degrees centrifugal without vaccum advance plugged in, the rest coming from the vaccum advance.

If you get a lean stumble with the leaner jets, I would leave that alone.

Was the cam degreed in? I have a comp cam in my 340 and it was 6 degrees retarded at 0 setting on timing chain. If the cam is retarded, it will lower gas mileage and power.

Do you have synthetic lubes in all gear cases, axles?

You might have to back down from the tall tires and go to shorter tires but that would mean you have to go to different gear ratio in both axles (3.00?) Big tires in diameter and width cause a lot of friction.

Alignment? if the tires are aligned correctly they are fighting each other down the road, eating up power and gas mileage.

Headers might help. Also the 3 inch diameter core on the mufflers may be too much. May need to step down to diameter of the exhaust pipe to keep flow up. Also crossover between passenger and driver exhaust pipe will help.

Do you know the compression of the engine from measurement? If not, anything under 9 to 1 compression can lose gas mileage. Thermal efficiency increases greatly between 7 to 1 and 9 to 1 compression. Engines may be speced at 9 to 1 but maufacturing tolerances may come out to be much less. For example my 340 was rebuilt with 10.5 to 1 forged pistons. But the heads measured 75 ccs, not the blueprint spec of 65cc. Compression measured out to 9.25 to 1.

Dan
Click to expand...
I know the speedo is way off. My measurments are by GPS or highway markers.

Yes I adjust the base timing with the vacuum disconnected and plugged. It runs best with the base timing @ 17btdc. Any less and it won't idle.

I was running a Pertronix electronic ignition system. It caught on fire one night and left me walking!

I'm back to points with 28 degrees dwell.

The cam was not degreed in. This engine has no timing chain, only 2 steel gears. There is no easy way to adjust cam timing, no aftermarket timing components available.

No synthetics at this time....It's an I.H. and they leak! I'm running Sae30 in the trans and X-fer case although they call for Sae50.

It has not worn the front tires in 35,000 miles..I assume the toe-in is ok?

Nobody makes headers for this engine.

The heads were not milled, they were only surfaced and it has thick headgaskets. Stock compression is 8.0-1. I'm prolly 7.9-1 or less.

I found the specs for the dist. It has 22 degrees timing all in by 2200 rpm. So w/o vacuum advance my timing is 39 deg btdc.

The vacuum advance adds another 15 degrees and is hooked to ported vacuum.

The carb is a holley # 80457s with #64 jets(stock is #66) and a 4.5 power valve. It has the Black spring in the secondary diapharam(it came with this)
 
I would say your cam choice is wrong. Most older style comp cams have a lobe seperation of 110 degrees, which makes for lousy mileage right off. check your cam specs to see if that's the case. If it has a 110 degree lobe seperation cam in it, switch to cam with a higher (I.E. 114 degrees) lobe seperation and with lower lift and less duration. This will help out low end torque but you will lose a few horsepower. Dont go overkill on valve spring stiffness. If you dont have one, dual plane intakes are better for gas mileage than a single plane intake. The bad thing is that this is not exactly a beer budget chevy small block.
 
No aftermarket intakes available! It has a cast iron factory 2plane hi-rise with a 1 inch 4 hole spacer I put on today to see if it changed anything.

The pickups and Travelalls normally had a low rise intake, the dumptrucks and busses got the hi-rise.
I got the truck from the original owner and they said the intake was original.
dunno.gif



The cam has 214 duration.
 
First thing to do is to go to an electronic ignition. Crane makes a good one xr-700. I bet a lot of the problem is marginal ignition.

Also, a good coil, spiral core wires and brass inserts in the distributor cap.

Dan
 
Drop in a Cummins.
grin.gif


In addition to what has gone before...

Are you running a reasonably high temperature thermostat ?
 
I can't imagine getting that much gas through an engine without it coming out the tail pipe.
dunno.gif


A friend of mine had a 197x Chevy tandom wheel pickup. It had a 160 gallon fuel capacity (120 gallon tank in the bed) and a 440 CI engine. Pulling an 18' boat we would get 9 MPG running 65 MPH.

We changed the engine to a 427 CI and it did not do much better.

But to get 6 MPG is really hard to understand.
dunno.gif
 
Actually, I was just thinking about it and my friends truck had a 454 CID engine in it. The 440 CID was a Dodge engine.
 
Something is off with fuel/spark.

That carb is handy -- reasonably sized -- but crude compared to a Rochester QuadraJet or Carter Thermoquad [which would be my choice]. These two carbs are more than a generation advanced over the Holley, and are far better at part-throttle operation (which can be adjusted!). After that, I'd choose a BG carb (Road Demon), sized and fitted by an authorized dealer, such as FBO Systems http://www.4secondsflat.com/. (See tech articles).

Agree on 114 cams.

Hot thermostat (195F)(experiment), exhaust crossover, best TOTAL timing, synthetic fluids/greases, etc.

7.90 cr is not ever going to be a mileage champ in any vehicle. 8.8 cr is best, IMO, but a work truck or tow vehicle has to be built carefully to take advantage.

If there is an HEI ignition retrofit, go for it. Otherwise, an MSD with adjustable timing control. That distributor of yours should be checked for proper operation. A cracked shaft, etc, can cause timing to retard, vary, increase. Weirdly.

Mount vacuum gauge on dash to learn to best use accelerator, transmission. It can reveal problems at cruise otherwise hard to detect.

What do the guys on http://www.binderbulletin.org have to say?
 
quote:
Originally posted by TheTanSedan:


What do the guys on http://www.binderbulletin.org have to say?
Click to expand...
Most guys are at least getting double digit mpg. Maybe 9 pulling a trailer.

Tonight I put a Carter AFB I had laying around on it. It is really doggy with it(yes it's gettin full throttle
rolleyes.gif
)

The carb spacer seemed to make the Holley lose power across the board and I'm wondering if it did the same with the AFB.

I may pop it off tomorrow and see how it runs.

The shop 2 doors down is a hotrod shop and they have a chassie dyno. They are pretty busy but they said they will put it on the thing and see if they can find anything wrong with the mixture or ignition one of these days.

I'll let ya'll's know what they find.
 
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