ETA:Working on building this trans now, and found a possible third option, please check my math! LATER ETA: This doesnt work. I recently picked up a Borg Warner overdrive for my '48 Plymouth. It is crusty and obviously leaks, so I will freshen it up before installing it. When I do, I can swap in a different ratio second gear, but looking at the numbers, I am not sure how to interpret them to know which ratio will give the best spacing between 1st, 2nd, 2nd OD, and 3rd Is this math right for calculating the ratio of a gear combination: Input shaft has 19 Teeth. It is in constant mesh with cluster gear with 32T= 1.684 Cluster gear has 25T, meshed with speed gear with 25T= 1 1x1.684= 2nd gear ratio of 1.684. is this the right way to figure this? Current ratios in the trans are 2.57, 1.83, 1.281, 1.00 '1940' gear set is 2.57, 1.55, 1.085, 1.00 3rd option that DOESN'T exist: 1940 Cluster gear with '41-56 second speed gear creating: 2.57, 1.684, 1.179, 1.00, .07 Car is around 3200lb, with 95HP, a 3.91 rear, with a 28" tire. It is used as a family cruiser, but gets enthusiasticly driven from time to time. I have the gear clusters for either choice. Thanks guys!
If I’m right in guessing the chart really reads “1st, 2nd, 2nd over, 3rd, 3rd over” I don’t think it makes much difference. Knowing the engine would help. But if it’s original I don’t think you’ll notice a difference. Certain long hills and low on power, 2nd over can be useful. But I’ve only used that when towing. Regular car, normal driving, it was mostly 1-2-3 3over. Others are in situations where it might be different. I’ve been using those old R10 and R11’s for over 50 years. Use the best looking set.
Your present gear-set is better "Current ratios in the trans are 2.57, 1.83, 1.281, 1.00" The "1940 gear-set" renders 2nd O/D totally useless. Both gear-sets will have a 3rd O/D ratio of 0.7:1 So you end up with 5 forward ratios 1st 2.57, 2nd 1.83, 2nd O/D 1.281, 3rd 1.00, 3rd O/D 0.7 It is nice having a workable ratio between 2nd and 3rd
Yeah, I could only tailor the chart so much. And in my write up I left out the 3rd O/D, .7, since it is the same with either gear cluster. Engine is mostly stock 95HP flathead six, with dual exhaust. It will likely be getting a dual carb intake and milled head before this trans goes in. This is what confused me looking at the numbers and corresponding speeds. The supposed Mopar flathead guru swears by the 1940 "fast second" in an overdrive as being better, but the 1.085 2nd O/D seems useless with only around a 5 mph change in speed, vs having a 10-15 mph spread between every gear.
You want the ratio spread [rpm drop] between gears to be widest at the lower gears where there is the most torque multiplication Then progressively get closer together as you shift up the gears 1st 2.57 x 68 % = 1.83 2nd 1.83 x 70 % = 1.281 2nd O/D 1.281 x 78 % = 1.00 3rd 1.00 x 70 % = 0.70 3rd O/D 0.7 the closest ratio spread [or RPM drop] is between 2nd O/D and 3rd at 78% So if you're revving to 4000 rpm in 2nd O/D it will drop to 3120 rpm in 3rd [4000 x .78 or 78%] The ratios ^^^^ are quite linear for a transmission 0.7 O/D in 3rd would be a "low load" cruising gear. With a 26" tall tyre and 3.91 rear end ratio, it would rev at 2475 rpm cruising at 70mph [in 3rd O/D]
Oh man, 700 r4 and a 3.73 gear is a wonderful combo. Just need to run the rpm up high enough to cover the 1-2 drop. The low first gear DOES get it moving, the OD works on highway. 700 r4 with a 3.08 gear kinda sucks. Trans Gearing - It's all a tradeoff - based on whole driveline (engine torque band, rear gear). Same with a wide ratio 4 speed. If it's not matched up well, it can be disaster. @Kerrynzl has it right. Figure you're ratios. Then find engine rpm at gear changes considering rear gear ratio. Match to your engine powerband best you can, with either trans choice or rear gear change to keep it in powerband and have good final drive rpm on highway.
That "fast second" works great in non O/D 3 speeds, wish I had it in my 49. I would not use it in an O/D transmission.
Unless I am doing my math wrong or missing something, It looks like "medium" second gear is also an option. Trying to duplicate the math @Kerrynzl did previously, it would look like this: 1st 2.57 x 65.5 % = 1.684 2nd 1.684 x 70 % = 1.179 2nd O/D 1.179 x 84.8 % = 1.00 3rd 1.00 x 70 % = 0.70 3rd O/D 0.7 I added a new chart in the OG post, but it seems to be a happy medium between standard "fast first" second and 1940 "fast second". Second overdrive would give about a 10 mph difference from 3rd, vs the 15 of the standard 2nd, but in reading about the BW overdrive it sounds like in normal driving it most likely won't kick out of overdrive between 2-3 shift, so really shifting would be 1-2-3-3od or in town 1-2-2OD-3OD. I dont have a tach, but it seems that most power is from 1800-3800 With the standard ratio, unless I wind 2nd all the way out, getting onto the expressway the 2-3 shift leaves me lugging along.
Hey, HW; Pretty good info from Kerry. Fwiw; when I was playing w/3spOD's, I found out I couldn't manipulate gear-ratios like I wanted, & that what seemed to be the most preferable(for me) came only in very weak trannies. Given my druthers, I'd look at the trans-final-drive(in OD), figure out how much power I had to use at what rpm & what rpm I was willing(or had to) to run it at, then go down from 3->1 in the most even steps I could get. Read: even steps 'twixt 1->2, 2->3, so's that the OD split the difference as even as possible, for the most useful rpm drop 'twixt gears. Another way to look at it, would be nice even rpm drops from 1, (*maybe* 1-OD), ->2, ->2-OD, ->3, ->3-OD. Old-school-5(or 6) speed(equivalent) trans. Usually, you can't get anything close to a decent gear-spread. Adding an OD to a 4sp is only really good for the last OD gear. You're not going to get a literal even gear-ratio-spread 'twixt all the gears, but as long as it's even 'twixt the 1/2 & 2/3(on each side of the OD "split"), you'll be happier. Best little 3sp spread was a T-90 in a 4cyl Scout, w/an aftermarket Warn OD on(or in) the transfer case. But that box was a seriously weak-sister... ;( . The Stude v8 3spOD wasn't bad, but I still didn't like the lousy gear-spread, as 2OD was short spread to 3. Almost pointless, except for around town & some slow(er) 2-lane roads(depending on the speed & terrain). Still, better than nothing, or even just the std 3sp(& maybe even better than a 4sp - depends on what you're after & its' use. . Marcus...
FWIW, the best ratio spread, and I've calculated them all, and in the strongest transmission, is the Borg-Warner T89 found in Studebaker trucks, among others. Low first gear and just perfect spread between second and third gear to make second overdrive a really usable gear. 3.17, 1.75, 1.26, 1.00, .722. If I'm feeling frisky, I will use first overdrive, 2.28 and make it a six speed. jack vines
Thanks guys. Unfortunately, reality doesnt work as well as my math- two big gears= not enough space center to center. Duh. I will have to stick with the stock, standard '41-56 ratios. All good. Jack, Do you run your Overdrives with stock wiring and kickdown? If so, how do you find best to kick it out of OD in between gears? Thanks!
I wire the kickdown circuit to a pushbutton on top of the floor shifter knob. This allows for part-throttle downshifts. AMC used it first on early '60s Twin Stick Ramblers. jack vines
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