Spitballing;
I have heard that it is possible to engage the A518 overdrive between gears. But the .69 ratio makes it kindof mickey-mouse IMO. The A518/618 ratios are
2.45-1.45-1.00-.69 for splits of .59-.69-.69 Splitting gets;
2.45-1.69-1.45-1.00-1.00-.69od Split gears shown in red. the splits are;
.69-.86-.69-0.0-.69 See what I mean. Spitting first and second looks cool on paper, but when you drive it, IDK.... Outshifting at 4000, First to second is 4000 x .59=2360, a drop of 1640, whereas into overdrive is 4000 x.69=2760, a drop of 1240, a betterment of 400 rpm, and shifting from 1od to second is 4000 x.86=3440, a 560 rpm drop. That's cool.
Ok but now, as the going gets tougher,and you start to buck wind, yur back to outshifting second at 4000 to; 4000 x .69=2760 into Drive, OR INTO OVERDRIVE SECOND_, same ratio, and yur doing 59 mph@ zero slip, say65 at 10%. Good thing you got all that torque and only 5 more mph to pull.
But I just gotta say one thing; 400ftlbs at 2400 at WOT, is 183 horsepower. whereas 400 at 3480 (in drive) is 265 hp.
Let's say your rig requires 100 hp to cruise at 70 on flat level hard ground; I mean just for arguments sake.
How much throttle, will it take at 2400,in overdrive, to generate the 100hp, to maintain 70 mph; how far is your throttle open? I mean IDK.
And how will that compare to in Drive at 2400/.69= 3480rpm? I mean at 3480 this engine has the potential to make 45% more power at WOT, so it seems to me it will require less throttle-opening to make the 100hp. Just asking cuz I don't know.
For best fuel economy, you might want to run the primaries WOT/ near WOT, so she's up on the mainjets which you can tune, and the boosters do what they do best.
But, to do that, I'd be willing to bet a dollar, the 2400 rpm cruiser will like a different carb than the 3480rpm cruiser.
Ok but now I'll tell you this; scietifically speaking;
To generate 100 hp, requires the same amount of fuel irrespective of rpm. The formula dictates .5 pound of fuel per horsepower per hour. You can fudge this around a lil but it will be darn close.
It don't matter if you have 360 cubes or 440 cubes, or 2400 rpm or 3480; the 100 hp requires a given amount of fuel to generate. From a good design to a poor design after optimized, there's not gonna be a whole lot of variation in the fuel used to make the required 100 hp.
Likewise as to heat. 100hp is 100hp. whether you create it at 2400 or at 3480, approximately the same amount of waste heats will be generated; the difference being in the ring tension and valve springing. Those you can control. And then
there's ignition timing;
Cruise timing at 2400 will be hard to optimize. At 3480 it's easy. Ima guessing that you will lose, IMO, more cruise economy to insufficient timing than you will gain in the rpm difference, IMO. Like said earlier, IDK what your engine will want for cruise timing, but in all likelihood the most you can get with the factory type Distributor, at 2400rpm, might be 40/42* .
Will it be enough?
IDK
But Ima guessing it could be 8/10 degrees or more short.
But at 3480, you can easily supply up to 56*, and maybe a lil more, And she probably will be happy with less than 50*;yes, I am guessing.
What I am thinking is 2800.
At 2800 you might be right on the max available cruise timing of 50/52. IDK if you will need it all or if she will want it, but at least there it is. But gearing the back to get 2800@70mph, in .69 overdrive with 33" tires is not gonna happen, lol, unless you use the loc-up. My math says 5.56s will get you 3000@10% slip/ 2740 in loc-up. The question is, will 5.56s survive? and for how long?
The point is this; IDK how hard I would try to get the rpm down, in striving for fuel economy, in an application like this, which already has quite a bit of aero going for it.
Just spitballing.
