Gas monster
holley 600 vac carb 68 main 4.5 power valve new proform ign set at 18 in 42 total no vac no pinging stock pistons low comp. iv made a 100km run onley hiway 55mph average 2.2l/10 km
2.2l/10km converts to 4.54km per liter, converts to 10.26 mpgUSg.
Cruising on the hiway, at an rpm after which the cam is no longer allowing reversion in the intake; is all about;
1) vehicle weight and shape,
2) cam overlap,
3) rpm and tune.
>At 65=3100, your combo has the reversion thing beat;
see note-3
> Your truck is a brick, so the only way to slow the consumption of fuel is to slow down.
> you can't do anything about cam overlap, except to replace the cam with one of less overlap.
see not-6
> that leaves the tune;
Without getting into the engine, you need to concentrate on getting the Cruise-Timing right. This starts with modifying the distributor;
see Note-2.
Here's the thing;
there is an optimum time when the piston is at the optimum position to transfer the energy of the expanding gas pressure above it, to the crank. Generally, this occurs at or near 25 to 28 degrees AFTER TDC. So all you gotta do is find that point, at your chosen cruise rpm, and then try your best to achieve it.
Finding it is easy,
see note-1, but achieving it, and maintaining it can be difficult. If you can achieve it, your throttle-valves will be at a minimum opening for that rpm/speed, and then you can begin decreasing the fueling until the engine complains about it. That is how to get fuel-economy with the current set-up.
Note-1
to find your Cruise-timing, first decide if you are willing to drive your brick slower than 65mph.
Second; verify that your Speed-O meter is accurate.
Third; determine your cruise rpm for whatever minimum roadspeed you have chosen.
now comes the hard part. Rev your engine up to that rpm, and simultaneously give it timing advance while trying to keep the rpm at the chosen mark. Once you get close, you will need to be more accurate, so use your fast idle cam speed adjusting screw to fix the rpm. Then add or subtract timing until the rpm has risen to it's highest. Then return the speed to the mark, and repeat one last time. Now, read your timing with a dial-back light. Write it down. Then return the rpm to idle. This will require changing the timing back to whatever it was before beginning the test. Let the engine run for a bit, so it cools itself.
Now; what number did you get. This number signals the optimum, no-load timing, at your chosen Cruise-rpm. Don't be surprised to see a number in the 50's
Note-2
Achieving a number higher than your Power-Timing, requires a working, and adjustable, vacuum advance system. And you need to know your manifold vacuum at your chosen Cruising Roadspeed. This may be different than the rpm at that chosen roadspeed.
Here's an example;
Lets say your Power-Timing at 3100 is 30 degrees. And lets say your no-load Cruise-Timing was determined to be 54* .. First, lets subtract about 3degrees for the load at 65mph, so our target is 51*. Subtracting the 30 from the 51, means that we need 21 from the vacuum advance. This is how far retarded your no-vacuum advance engine currently is. If the optimum crank position for the 400 cubic-inch engine is 28*ATDC, now you can see that 3100rpm, the timing is 53 less 28 equals 25* retarded. and 28 less 25 equals a pressure transfer angle of just 3* ATDC. With the piston this close to TDC, that pressure is mostly just driving the rod straight into the crankshaft, with only a small amount of the energy actually getting to the flywheel. The proof of this is the 2.2l/10km fuel consumption figure.
Ok so, in the above example, the VA needs to bring in 21 additional degrees over the Power-Timing at 3100 rpm.
Of course, this is just an example, your engine's requirement may be different. But now you have an idea of what to do.
Note-3
To find your minimum Cruise-Rpm, put a vacuum gauge on the intake, then just road test it in Second gear. Watch the gauge. You will see the vacuum rising with rising rpm. Then the Vacuum will peak, and if you continue to speed up, still in Second gear, eventually the vacuum will begin to fall again.
The LOWEST rpm at which the vacuum was the highest, signals the first time that all the air in the intake was moving in the same direction, namely towards the intake valves. In your combo, this might be in the range of 2000 to I'm guessing 2400rpm. So your cruise speed could come down as low as that point, whatever it is, but 3100 is obviously high enough.
You can run at a lower than optimum rpm, but suffer the consequence of a lower vacuum reading, which also signals a greater throttle opening, and so, more fuel consumption. Depending on how far into reversion you get, you may be able to run economically down there by carefully and meticulously fudging the low-speed circuit.
Note-4
Just FWI; my alloy-headed 367HO with a 270/276/110 cam liked 60 degrees of Cruise-Timing at 2240rpm=65mph in overdrive.
Same engine, now with a 276/286/110 cam, now likes 54* at same 2240 rpm, at same ~177+psi cylinder pressure.
One combo I had, ran at 65=1600 in double overdrive. Yeah that was about 400 rpm into reversion but with careful carb tuning, that combo was able to pull 32mpgUSg on one certain day-trip.
In metric, I think that converts to ~11km/liter; or 24km per 2.2liters, just over double what you are currently getting. Now I don't expect your combo can achieve that, but you can see that for cruising, the tune can make/break your fun-ratio.
Note-5
there are plenty of good ideas coming at you, but IMO, you can do a lot with tuning, and especially by slowing down.
Note-6
Overlap is the time spent in degrees, that both the intake and the exhaust valves are open with the piston near TDC. The bigger the cam, usually, the more overlap it will have.
With headers, overlap is good thru the midrange and is a large contributor to making power. But at low-rpm, it is terribly disruptive in the intake, and makes tuning the idle quality quite difficult. Hence the idle-lope.
By Cruise-rpm, things are usually looking up, so in steady-state, you can tune for it. But getting to cruise rpm can really suck the fuel.
