Probably not, if your unit accelerates rapidly enough to satisfy your needs in it's current configuration.
And you are cruising at such a low rpm.
And it has enough reserve power to climb hills, in Second, and still operate satisfactorily at the higher elevations ........... then I doubt you can make your current comb any better.For sure, splitting gears will not be advantageous.
But if you are saying '
as a future event, then that sorta means that you are now cruising at 70= 2400/.78 = 3100. If you are happy at 3100, and the engine still has power left in her, such that you are still on the Primaries, I would leave it alone... I think. lol.
The point is this, it is highly unlikely that you could ever recover the buy-in costs in fuel savings . It just cannot happen. So the buy-in cost would have to be justified by other means. Like gear spitting for acceleration, or NVH; which is noise, vibration, and harshness. With your tractor-like torque and looks to be perfect gearing, you won't justify gear-splitting either. That just leaves NVH.
But if your rig cannot maintain 70 mph, at 2400, then the whole thing goes up in smoke; and in this case forget the overdrive.
Furthermore
It may be impossible, with mechanical timing controls, to give that bad boy engine of yours, the cruise timing it will want at 2400; and if that happens, you could actually LOSE fuel-economy! which is sorta the point of running an overdrive in the first place. (see note 2)
So what I did was ; I got me a stand-alone, dash-mounted, dial-back, timing retard box, with a range of 15*. Now I could, for cruising only, add up to 15 degrees to that 43, for a total of 58*; BadaBoom! And now I was saving money at the pump on every tankful. That was 2003 or so, so Ima thinking by now, it has paid for itself, cuz she has over 100,000 miles on by now.
I hung a GV od behind a Mopar A833 overdrive box, and got me a double overdrive, for 62=1530rpm. At 1530 it did not want 58* anymore, but same combo was 85mph=2100, so 56* was back in the ballpark. I could tell you that on a certain daytrip in 2004, from point to point, with a tuned for economy carb, that she got me 32 mpgUS, but nobody believes that, so, you didn't hear it from me.
The point is this; there comes a hinge-point in the rpm band, different for every combo, when fuel economy will be as good as it gets, and less rpm, will just make it worse. And this hinge-point depends very heavily on satisfying the engine's need for advanced ignition timing.
My 367 combo has closed-chamber alloy heads and has always had the Scr adjusted to maintain a cranking cylinder pressure of from 177 to 185 psi, This has yielded a tight-Q on every combo from .040 to .032. So as for fuel-economy, this combo has a lotta potential, probably more than most HotRod engines. That is what I designed it to do, namely; Performance with Economy.
How it applies to you;
IMO, for fuel-economy;
High pressure is of utmost importance, so that your engine is inherently powerful at small throttle openings. And
IMO, a tight-Q does not get the recognition it deserves.
IMO, after that, ignition timing is everything.
Once the engine is built, all you have left is rpm and ignition timing.
______________________________
Note 1
To help you find the hinge point of your combo; try this;
Make sure your Vcan is working, and that your TDC mark is accurate. Bring your dial-back timing light, unless you balancer is marked out to 60*, then a standard is fine. You will need to get both rear wheels off the ground, no fooling. Ok , in neutral, with the brakes locked, rev the engine up to whatever rpm she currently cruises at. For the rest of this test you must strive to keep the engine at that rpm, whatever it is.
Ok now read the timing, and write it down. Then begin to add timing, while reducing throttle-opening to keep the rpm relatively fixed. Add some more timing and reduce the throttle opening. Repeat as often as needs be until the rpm no longer increases with an increase in timing. You have reached a plateau where no more timing is tolerated. From here on, more timing will just slow her down, as she fights excessive timing. So back up to the last time the rpm increased, and read the balancer. Write it down.
Now put the timing and rpm back to where you found it.
What did you find?
You found the timing that the engine likes to cruise with at no load. She probably won't want that much pushing wind, but you found the theoretical best timing for cruising at that rpm.
That's your ceiling.
The timing she will in real-life want under load is gonna depend on how hard the engine has to work, to maintain your cruise rpm.
I'll take a guess that you found say 50/55 degrees at 2800. Whatever it was, take 6 to 10 out, for your next roadtest. This needs to be done with the Vcan, so as not to upset your Power-Timing. If your engine Vacuum is so low at your cruise rpm, (see note 4) that the Vcan has already cut out, then yur pooched. You will have to figure out a different way to engage the Vcan for cruise only.
If your Vcan cannot achieve the additional timing, you will have to modify the stops on it. Take the can off and you will see a number scratched on the arm; like 9L or 13L or something. The number indicates the amount of advance the unit is designed to supply... in distributor degrees, which you have to double for crank degrees, on a 4-stroke engine. The thing that controls this are the external stops on the arm, that crash into the back of the can at full vacuum. You cut part of those off on the can-side, to get more Vcan timing. I cut mine for 22*. Trailbeast cut his for 24* IIRC, so lets say 24 is the most you can get. But only cut off enough stop to get what you need, to get to your target. 4* will already be noticeable on the fuel gauge.
Inside the can if it has flats on the side of it, is a lil adjuster which delays the operation of the can. So if you need it to come in slower,or drop out faster, you have a lil adjustment window. That screw works backwards, that is to say, all the way CCW will allow the fastest operation; But do not force it; when it stops it stops. If you crank it right out, I don't think you will ever get it back in. I think the size is 5/32 allen socket head.
If you cannot get to your target with the Vcan; your next option is the stand-alone timing-retard box. This is a great tuning device. Best $170 bucks I ever spent. You put it in the center of it's adjustable range, then reset your timing to whatever you need. Then on the roadtest, on a flat level straight section of hiway, you can advance or retard up to 7.5 degrees either way from where you set the timing, and watch what happens to your speed. I always retard first, about 3 or 4 degrees, I'm guessing because my unit is not calibrated.
If the speed decreases that is what I'm expecting. So then I hammer it over about 7 degrees advance from where I am, expecting the roadspeed to increase past the initial speed. You need a very steady foot. Or do what I did (see note 3). I made a throttle stop that always stops the throttle in the exact same place. It's spring loaded so I can still drive thru it whenever I need to.
If the speed increases past the initial, I again take timing out to prove it ain't the grade or the wind. If the speed drops, then I crank it full advance and wait. If the speed again advances, that is what I want to see, but now I am outta adjustment. So I leave it there and rock on. Next time out, I reset the dial-back to it's center point, reset the base timing plus 7 degrees and start all over. In this way, the speed is ever increasing; so I have to readjust my throttle stop each time, so that I don't fudge up my parameters.
Every time the speed increases,with additional timing, your engine is making more cruise power. So you just keep on going this way until one day more advance causes the speed too slow down . Then back up the timing to the last good setting and Leave it that way for a trip or two, to prove that you reached the limit.
Read the cruise timing and write it down.
Now during all this testing, you have been increasing the Base Idle-Timing... which simultaneously increases the Power-Timing. If at any time you are driving deep enough into the throttle to wake up the ugly detonation rattle, you will have to fix that before continuing. Cut off some stop material on the Vcan arm, 5 to7 degrees atta time, and reset your base timing back to where you started.
And when you finally find the perfect cruise-timing, you will have to put your Distributor back to where it started, in terms of Power-timing at least, and then modify the Vcan stops one last time to bring in the rest of the Cruise-timing to whatever you wrote down.
Then reclock your dash device to 3 or 4 degrees advanced and set your Power-timing one last time to whatever you started with, that is known not to have created detonation. Now you have a hill-climbing retard device, and/or you can dial in some advance when the engine is cold. Just don't forget to reclock it when you put it into drive.
And finally, with 3 or 4 degrees advance still on the control, you can keep on searching for the magic Cruise-advance number, or even test the PowerTiming from time to time.
When the day comes that you believe she is the best that she can be; Compare the current cruise timing to the starting cruise timing, then come back here and post up your results in terms of fuel-mileage increase/decrease, and the amount of work you had to go thru to get it.
Note 2
I'll give you an example. This is in no way applicable to your motorhome tho. It's just an example.
My 367 was very happy running 62=2770 with my 4-speed. It liked 50* or more of cruise timing. Which was fairly easy to achieve. 14* idle plus 14 mechanical , plus 22 in the Vcan= 50*
After I installed the GVod, the rpm came down to 2160, but now it sorta wanted 60* of cruise timing, 56 at the minimum. I still had the 14 idle plus 22 in the V-can, but the mechanical was down to 7*, for a total of 43* So there was absolutely no way to get 56, much less 60. So, in this case, the GVod did nothing for fuel-economy....... because my combo was previously right on target.
Note 3
My throttlestop is just a coat hanger wire bent in a J-shape. The J fits around the throttle pin where the KD rod usually lives, and the straight end travels forward to the anchor for the throttle return spring. I twisted that bracket 90* and then drilled a hole in it, then shoved my coat hanger thru it. But before I did, I slipped a little cable clamp onto it. Back at the J end I bent a lil u on the very end just big enough to accept a weakazz throttle spring. How it works: I slide the J-hook into a position that I thing will allow my combo to cruise with the pin just touching the closed end of the J. Then I lock the cable clamp on the front end, right where it touches the anchor. Finally I install my weakazz spring one end on the hook I made on the J end and the other end to wherever is convenient. So now, when I press the gas pedal far enough, the KD pin will eventually stop when it hits the end of the loop, and the spring holds the loop there so I can feel the increased tension as I continue pressing. So I still have full throttle available. But it is easy to find the throttle stop as I approach my desired cruising speed. It took me a couple of tries to find a suitable spring to make results repeatable; and it took several tries for me to hit the right length loop, to always stop at the chosen roadspeed. but I doubt I got $5 in it. and 5 minutes to make it. The cable clamp is off a bicycle I think. The spring was surplus. The coathanger was courtesy of the Mrs. And the whole thing comes off in a couple of seconds
Note 4
And finally, lets visit the situation where your throttle opening is too large to activate the Vcan. This will happen when the primary throttle blades go too far past the vacuum port, and the signal starts to decrease, ultimately falling to zero. So Tee in to that line and put the vacuum gauge on the windshield where you can see it, or just bring it into the cab whatever is easiest.
Ok bring a clipboard with a sheet of paper on it, and an ecrit (writing implement in French, probably one of the four utterable words I still remember), and a helper. Find a flat level hard abandoned road and get your tank up to about 50 mph in Drive. Have your helper record the vacuum readings at about every 5mph
IN STEADYSTATE CRUISING. This is very important. Your speed must not be increasing nor decreasing. Keep on going to 70/75 mph or to whenever the vacuum falls to below say 6 inches; whichever occurs first.
Now plot a graph with speed across the bottom,increasing from left to right, and vacuum up the left side from zero to whatever maximum you saw . If the max vacuum does not occur at your selected cruise speed that may be a problem. But if it is plummeting to zero by 70 mph then you are pooched. Whatever vacuum you have left at 70 is all the Vcan has to work with, and if it cannot bring in it's entire load, with the internal screw on the minimum preload, then you will need to find an alternative way to get that advance. Again, if the spark-port vacuum has fallen to zero you are pooched. At that point you have to figure out how to use manifold vacuum to get what you need. But manifold vacuum is waaaaaay less sensitive, I mean the range will be very small. and there is no zero point, even when the throttles are at WOT. Ok so I have not built a system for those conditions.
But if your spark port has a minimum vacuum of say 6 inches at 70 mph, you can still use it to trigger a vacuum amplifier and RELAY manifold vacuum to the Vcan with some kind of proportional control. You can get an amp like that off a 70s/80s slanty. It's the little round ball planted usually on the valve cover. Ok so I have not built a system for these conditions either.
So far I have always been able to use the sparkport as is.
Well not quite true, I did have to lower it on one combo. I suppose you could raise yours? depending on how far it needs to go.
Anyway this post is getting really long, and I'm getting really hungry.
