Was reading an '86 HR mag and a guy put a 3-speed manual behind his 3-speed automatic "as an under/overdrive." This raises many questions...

Application: this was in a '30s Ford hot rod. I know space is of the essence, but let's eliminate the obvious and think about this where it could realistically fit, like under a truck or van.

The big thing to me seems to be that if the 3rd in the auto is 1:1 and the 3rd in most 3-speeds is 1:1, where's the overdrive in that? The numbers otherwise involved (rear end, auto gears) are giving me a brain cramp...

Can someone make sense of this? It seems like a great idea in theory with the right gears in everything (whatever that would be, don't ask me after a week of work )but that sure doesn't mean it is.

Just the fabrication involved is befuddling. Of course, this guy's driveshaft was probably a foot long and he'd also "engineered" fake Hemi heads & valve cover covers for his "stock 302." There's some kind of genius here, but I can't quite get a read on it.... Thoughts? Experience? Theories? Let 'em fly...

~Scott in TX
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62 Plymouth Valiant "Alvin" 170
74 Dodge D100 "Joe" soon to be 225
74 Roadrunner "Sarge" 360

In the owner's brain. There is none. But it would permit the selection of a taller-than-ordinary rear axle ratio for a very tall final-drive ratio while still maintaining off-the-line acceleration. In practise, one would probably never use the manual transmission's 1st gear, only 2nd (for an underdrive) and 3rd (for direct).

Let's assume he was using an A230 3-speed. Those were available with two ratio sets:

3.08 or 2.55 (first)
1.70 or 1.48 (second)
1.00 (third)

The ratios in an ordinary Torqueflite are 2.45, 1.45, 1.00

So let's guess he put a 2.26 rear axle in this vehicle. Ordinarily, that would give effective drive ratios (trans ratio times rear axle ratio) of:

5.54 (first)
3.28 (second)
2.26 (third)

For comparison, here are the effective drive ratios of an ordinary Torqueflite in front of a 3.23 rear axle:

7.91 (first)
4.68 (second)
3.23 (third)

And the effective drive ratio of an A833 4-speed in 4th gear (0.73) in front of a 3.23 rear axle is 2.35.

So, if we put that A230 between an ordinary Torqueflite and a 2.26 rear axle, we get the following effective ratios:

A230 trans with 2.55 2nd gear, in 2nd gear:

14.11 (first)
8.35 (second)
5.76 (third)

These numbers are equivalent to having a 5.76 rear axle behind an ordinary Torqueflite.

Then if we shift the A230 into 3rd, we get our
5.54 (first)
3.28 (second)
2.26 (third).

So third-under and first-direct are almost identical. It would be really cumbersome to drive a vehicle equipped this way. Your upshift pattern would look like this:

A230 in 2, Torqueflite in D until it reaches 3rd gear, then A230 into 3 and Torqueflite downshifted to 2, then shifted back into D. And let's not even start talking about downshifting in sequence!

This is a neat idea, but it falls down in practice. All it gains you is five or six unusably low gears. What are we going to do with this 14.1 first gear...scale the reichstag???
I agree with you, the guy's a genius. He figured out a smart solution to "Honey, I've told you to get that damn transmission out of the middle of the garage floor! This is your last warning, and I mean it!".

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

The only place I've heard of this type of modification is in off-road/4WD applications.

Putting an extra trans between the primary drive trans and the (divorced) transfer case would give one some hellacious crawl ratios and torque multiplication out the wahzoo.
Hmmmm....maybe the guy did do that in order to build what is termed a "doodlebug". (A car that has been modified into a general-duty farm workhorse. A not uncommon practice in the 30's to war years.)

Roger

That would be a good read--must be one funky collection of stuff in that guys rig. Certainly no special O/D in that setup, if it wasn't for the cost a GearVendors Overdrive/Gear Splitter is the cats meow if you need that sort of thing. I wouldn't mind one in my truck so I could run a numerically higher rear end, and have some better shift points, but I won't be strapping a 3 speed manual to it any time soon !

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Slanted D150

Might this serve some sick purpose for a strip/ street setup? You know 30 mpg to the track then 10 sec 1/4 et then 30 mpg home?????
Frank

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Scrapple: Because a mind is a terrible thing to waste.
73 Duster - Race Car
66 Dart Wagon - DD
178" FED
82 D150
All Slant powered

I built a setup like that for my DeSoto about 25 years ago. I called it an underdrive. I used a Ford 3 speed(all synchro from a Ford van) with 1st and reverse removed.

Its not much different from Brens setup.......

You need a drive coupler and a case coupler. Both are an easy fab.

My DeSoto had the room under the floor....and a long enough wheelbase.

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Yeah....Im the one who destroyed this rare, vintage automobile.....

Uh...good idea! Imagine the carnage and fun sounds if one were to shift the aux trans into Reverse at speed...

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I think there was guy down the road from me in Florida (1973) that had a swamp buggy running a 283 with a powerglide and 2 manual 3 speeds.

It was 2 wheel drive and had large tractor tires on it with snow chains.

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'72 Duster 198 stock cam, 3:23's Hookers on jack stands for 8 years in the driveway
'79 Maxivan 360 Offy Qjet Comp RV cam/rusting in the driveway.
93 D350 160HP Cummins Auto Dually Clubcab needs a injector pump
2005 Golden Couch Buick

The article didn't go into detail at all, but I do have a copy of the "cutaway artist's rendition" profile of the car. I just don't know how to post it up here... and if I can't figure that out, I should stick to 1 tranny at a time!

It was pure hot rod, a slammed Ford coupe. I mean, they had Gear Vendors stuff in '86...

Ok, someone hook up an 833OD with reverse & first removed (now that's genius) behind a 518 overdrive with high three gears. You could get... and uh... ogawd I need an aspirin sandwich.

I can't imagine using it except for "Ok, road trip, this lever goes in 3 and stays there" and "Ok, punk in a 5.0, eigth-mile, this lever goes in 1 and stays there." Otherwise it'd be way too much work.
Awesome, man. And thanks for uncramping my brain on those numbers. I ran out of toes really fast...

Sandy in BC - just for info purposes (and to have it posted for searchers), I'd love to hear more about what you did & how it worked out.

~Scott in TX

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62 Plymouth Valiant "Alvin" 170
74 Dodge D100 "Joe" soon to be 225
74 Roadrunner "Sarge" 360

Naw, naw, naw, that's different. That's emergency backup transmissions for when the Powerglide grenades!

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Somewhat off topic... When selling class 8 trucks for a living in the 80's I sold a new truck with a 10spd direct main box and a 4spd double overdrive aux.

I drove a 72 Osh Kosh 6x6 with a 1673 Cat. It had a Fuller Road Ranger 10spd OD a 3 speed box behind that with low, direct, and OD.

Top speed? 67mph

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'72 Duster 198 stock cam, 3:23's Hookers on jack stands for 8 years in the driveway
'79 Maxivan 360 Offy Qjet Comp RV cam/rusting in the driveway.
93 D350 160HP Cummins Auto Dually Clubcab needs a injector pump
2005 Golden Couch Buick

Yup but that Osh Kosh would pull the mountians in two.
Frank

_________________
Scrapple: Because a mind is a terrible thing to waste.
73 Duster - Race Car
66 Dart Wagon - DD
178" FED
82 D150
All Slant powered

Never got it stuck. It would hop in the Florida "sugar sand" almost could jump out and throw stuff under the tires as it jumped!

It had a Jiffy Lift Eagle block crane on it and haul 900 8x16 blocks at a time.

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'72 Duster 198 stock cam, 3:23's Hookers on jack stands for 8 years in the driveway
'79 Maxivan 360 Offy Qjet Comp RV cam/rusting in the driveway.
93 D350 160HP Cummins Auto Dually Clubcab needs a injector pump
2005 Golden Couch Buick