Beefing up A500 overdrive
As far as I'm aware most of the loss in auto transmissions is in the torque converter. Lock up pretty much fixes that.
Overdrive gets the engine working at the most efficient part of the power band but is subjected to high torque at lower revs. This is why I'm looking to strengthen the overdrive, not because I have crazy horsepower.
maybe so but torque output is directly related to throttle input, ignition timing, and the resistance of the powertrain/vehicle to move and accelerate.
If your goal is fuel-economy;
with 3.23s and .69od, your cruise rpm works out to about 65=1800 which is a little on the low side..... because you just cannot, without a computer, give the engine enough ignition timing.
If you are really interested in fuel economy and are really gonna cruise with 3.23s at 65=1800 in overdrive/LU, then you will need a stand-alone timing computer to optimize the cruise timing, AND you will need a spreadbore carb with tiny primaries, (triple-booster is better) and a matching hi-velocity intake, AND a sharp tune. To make the 250BHP, with these restrictions, you may need a solid-lifter cam, and a free-flowing dual exhaust. A decent cylinder pressure close to 155/160 psi would be a bonus.
On my dash I have a variable timing retard switch with a range of 15 degrees. My Idle-timing is usually set to 12 to 14 degrees, with the device set in such a manner that I can advance up to 6, and retard up to 9. With it fully retarded the Idle-timing drops to about 4/5 degrees. This is just enough to keep the engine running, while in first gear (manual trans with an over-all starter gear of 10.97), at about 550 rpm, with the throttle closed, idling thru the parking lot. Any less rpm and the engine is dangerously close to stalling. But if I nail the gaspedal, all hell breaks loose, lol.
The point is between the minimum and the maximum torque that your engine can produce, you control it with throttle opening, and to some degree, with ignition timing.
With the factory-type un-modified distributor, at the factory timing spec, you'd be lucky to get 24* of cruise timing@1800. If you modify it and kick in a bunch of Idle-timing, maybe you get 46 degrees@1800, which is not likely gonna be enough. But it's a pretty good start. In the end, your engine at cruise, is likely to want 54 to 60 degrees, can't say exactly, cuz I've never had a Poly and never tuned one...
Having said the above; 2200 to 2400rpm is a good cruise target, for optimizing the timing.. With 27" tires and .69overdrive, this maths to,
3.91s for 65=2180 in overdrive.
This will get you a starter gear of 10.71, a very nice tire-spinning number.
and the bonus is 55= 2700 in Drive/locked up, and perhaps 3000 unlocked at WOT, perfect for passing slowpokes.
I have driven this combo for several years in a second generation Dodge Dakota and I liked it very much.
For your application, I would not consider the GearVendor, even tho I have one in my HotRod and like it too.
Ima liking post #20
