Seeking fuel efficiency @ 1500-1750 RPMs
 

My quest is for the best gas mileage in my 1986 F150 with 4.9L engine and Duraspark conversion. Several years of "tweaks" has it in the 18-25 MPG range. It has a re-curved distributor and Carter YF carburetor from a 1970 F350. The transmission is 4 speed manual O.D. and the differential is 3.08 LS with stock rims and 215/75 X 15 tires.

Usually, when discussion in this forum centers around increasing performance, some of the most often discussed items are: F.I. exhaust manifolds with Walker Y-pipe and 2 1/2" piping, the H.D. exhaust manifold with 2 1/2" piping or headers [all with more freely flowing mufflers than stock]; Clifford or Offenhauser intake manifold with 2 or 4 barrel carburetor [390-600 CFM]; re-curved distributor; advance cam timing either with stock cam or aftermarket cam kit; zero deck the block and or shave the head; clean-up valve pockets, back-cut the valves, polish combustion chambers and piston tops, etc.

The above listed changes reportedly "wake-up" the engine and some post of easily turning 4500-5500 RPMs with a smile. :)

Okay, when going for the best gas mileage [55-62 mph], the engine in my truck turns ~1500-1750 RPMs. Rarely does the engine ever see beyond 2500-3000 RPMs.

Given the highway cruising engine speeds listed above for my truck, is anything to be gained by changing from the stock set-up regarding fuel efficiency @ 1500-1750 RPMs?

Thanks!

I think where you're at, you're only going to be seeing mpg gains by allowing the engine to keep doing exactly what it's doing, but having an easier time doing it.

A larger, freer flowing exhaust is not going to make the engine work as hard to push it out of the tailpipe.
A hotter spark / higher compression is going to make a bigger, more powerful explosion with the same amount of fuel.
Roller rockers are going to reduce friction every time a rocker arm rises and lowers.
Etc.

Kinda the same idea as airing up your tires to reduce rolling resistance. Using thinner oils in the transmission/differentials. Etc.

For a cam, who knows, you may find one that increases torque at that RPM, which may raise efficiency.

I am hoping the cam I selected will help in my current build. The stock 300 cam is a 268 gross duration cam with a paltry 192 degrees at .050 valve lift (where things actually begin to work). Lift is .425". It seems that the stock cam takes way too much time to open and close the valves all the while squandering fuel and vacuum in the process. I had Comp grind me a a cam based on their Extreme lobe profile. My cam is 249 gross and 206 @.050 with .443 lift. A note on the gross duration: Comp measures this at .006" lift so pure gross duration is probably around 252. At any rate the idea is to fill and empty the cylinders fast and not waste much on seepage out the exhaust. I am working with 26 degrees of overlap and a 112 degree LSA. Both are good for vacuum which helps response and MPG.
This is going in a truck with 3.50 gears, 35" tires, and a NV4500 5 speed. Our cruise RPM levels are similar. I have the EFI ex. manifolds, DS II dizzy, Offy DP intake and currently an Edelbrock 500 carb. I intend to play with ignition curves and an Autolite 4100 carb as well.

Thanks, AB!

Right now, I work within the constraints of nothing internal to the engine... that will come whenever a rebuild is mandated. Right now, at 189+K miles, the engine has never been opened and shows no signs of tiring.

The tires on the truck [Michelin LTX M/S] indicate [sidewall] a maximum pressure of 35 PSI. Normally, I run 35 in front and 32 in rear. With 50+K miles, the wear pattern is good and flat. On the recent drive to the OK GTG, where the truck returned 25.7 mpg on 2 tanks, I cheated with the air pressure and ran 41 PSI in front and 38 PSI in the rear.

Synthetic is being used in the engine [10W30], transmission and differential [75W90]. Future plan is to repack the front bearings using synthetic.

Have you ever read the output of the O2 sensor at cruise speeds, to see if you are currently lean or rich?
Jim

Jim,

At the OK GTG in September, 2014, Gary Lewis used his AFR instrumentation to check this. The O2 sensor was mounted in the factory location [exhaust manifold].

Warmed up and @ idle = 12.5:1

Cruise @ 62 mph on level ground = mid 15's

Power = a low of 11.5:1, but normally 12.0 - 12.5:1

Climbing a long hill = 17.0:1

An O2 sensor is one thing I would have loved to have had back in the day that I had my 1bbl.

The output of the unused O2 sensor, after a DS2 conversion, is a very valuable and unused diagnostic tool.

You can compare it to having a vacuum gauge in the dash. You will find yourself constantly looking at it and using it to adjust your cruise speed / RPMs to achieve the best fuel air mixture.

Here is a good web site with info on how a O2 sensor works.
WHAT THE HOME MECHANIC NEEDS TO KNOW ABOUT O2 SENSORS

You will need a cheap DVM with a low voltage range read out, like 2 VDC (2000m) full scale, like this one at $6 at harbor freight.
Digital Multimeter - Save on this 7 Function Digital Multimeter

First off, play with the idle mixture to see how it works. Connect the negative of the meter to the battery negative terminal and the positive lead of the DVM to the O2 sensor. Set the DVM on the 2 volt (2000m) scale. Start the truck up (cold) and wait / watch the meter to see the O2 sensor come online. Note the voltage, then play with the idle mixture to see the numbers float around. .7-8 volts is what I use.

For those of us with an auto trans get someone in the cab to put it in drive (brake on) and park to compare the read outs.

If the above idle testing works out, the next step is monitoring while driving your normal weekly patterns.

The voltage/current signal from the O2 sensor is weak so use a larger gauge wire to run in the cab and a good ground in the cab so you don't add an resistance which might lower the voltage reading.

Bottom line is if you are currently running at optimum fuel air mixture, little if anything can be cost effectively gained changing EXH INT manifolds. Changing EXH / INT manifolds also adds in a new problem set to solve for winter driving.

Jim

Thank you. What about replacing the two original catalytic convertors with a more modern single 3-way one and using a turbo type muffler in an attempt to reduce the back pressure?

At the rpm level you are targeting, a "freer flowing" exhaust may not gain you much. I have went that route with a carbed 81 and an EFI 91 with little gain on the 81 and no gain on the 91. I probably went too big with the 91. Something folks often forget with exhaust is that charge velocity helps create a vacuum for successive exhaust pulses. Too small of an exhaust diameter equals restriction, but too large can impede scavenging.