Tallest gearing for FWD transaxle?

[SteveM]

I am doing some noodle-scratching, trying to figure out what type of transaxle (either a "factory" or aftermarket solution) offers a REALLY tall final drive ratio.

For my application, which is a front-wheel-drive diesel truck, I'm already using what I believe to be one of the tallest factory offerings, in the form of a VW 02A/J, code CTN transaxle.  This is a 3.16 R&P, combined with a 0.76 overdrive in top gear, for an overall effective top gear of 2.40:1.

As we all know, diesel engines are not known for being high-revvers.  I'd like to find some taller geared options, even in the form of aftermarket ($$$$) transaxles.

Of course, the other option is to use bigger tires, but there is certainly a physical limit with respect to how large of a tire will fit under the nose of a 1984 Dodge Rampage.

Any help would be greatly appreciated.  Innovative solutions are encouraged.

Steve.

[SPARKY]

how large is that ring gear and pinion compared to a 7.5 gm for example

Also there are truck rear ends that have what we call bull gears very similar to planetary gears  but perpendicular to the axle shafts

[Milwaukee Midget]

Hate to say it, Steve, but I'm thinking you may well be into custom gearset territory with this one - BUT -

Have you considered any automatic transaxles?

I'm trying to think of large engined, FWD cars from the late 1980's, before computer controls were more effectively used to get decent gas mileage.

The Taurus was available with a 3.16 and a .742, which gets you to a 2.34 - not a huge step, but a step in the right direction.

The 440T-4 GM, which I think was used behind the Buick V6 in the Toronado could be had with a 2.84 and a .705 OD - that's ~2:1.  Granted, it's a 4 speed, but you've got more torque than most, and not a lot of weight.

And if there is ANYBODY who could make that kind of a lash up work, I've little doubt it would be you.  

Sorry I missed you at Indy.


 

[gkabbt]

If you remember, I'm using a race only Neon 31th non-lockup automatic in my Rampage because of the gearing.
I don't have all the transfer gear / R&P specifics on this laptop so I'm going by memory (or lack of.....LOL!!!)
There are 4 factory transfer gear sets available and two factory R&P sets.
With the 2.85 R&P and a .91 transfer gear you get a 2.60.
With the 3.05 R&P and a .91 transfer gear you get a 2.78.
When I get home, I post up all the available gearing with the 4 transfer gear sets with the 2 R&P's. Unfortunately, the other 3 transfer gears sets go the wrong way!

My transmission guy had special transfer gears made for drag racing. I'm going to start out with the stock stuff and, if that doesn't work, I'll go to custom stuff like him (already talked to him about it). Got to be a lot cheaper than the $19,000 trans we saw at PRI.....that was a SWEET FWD trans though.

These are VERY strong transmissions and you "might" be able to adapt it to your application.....current or future!

Gregg

[gkabbt]

Here is the info for the R&P's and Transfer Gears available for my transmission:

Ring & Pinion gear ratios:
2.86----60 tooth ring and 21 tooth shaft
3.05----61 tooth ring and 20 tooth shaft

Transfer Gear ratios:
  .91----52 and 57 teeth
1.04----47 and 45 teeth
1.06----56 and 53 teeth
1.22----60 and 49 teeth 

Possible final drive ratios:
2.60----  .91 transfer and 2.86 ring
2.97----1.04 transfer and 2.86 ring
3.02----1.06 transfer and 2.86 ring
3.50----1.22 transfer and 2.86 ring

2.78----  .91 transfer and 3.05 ring
3.17----1.04 transfer and 3.05 ring
3.22----1.06 transfer and 3.05 ring
3.72----1.22 transfer and 3.05 ring

As I posted last week, my transmission guy had special transfer gears made for drag racing.
This is an option that I might use if the 2.60 is not tall enough. 

Gregg

[Rex Schimmer]

Steve.
I always get concerned when people start talking about large percentage overdrive set ups, such as 24 to 30%. Even with an engine that makes retaliatively  good torque for its size asking it to still pull strong when you reduce the engine rpm by 30% is asking an awful lot. Aero loads, in horse power, go up by the cube of the speed increase so when you get it screaming in 4th gear at 7000 rpm will there be enough horse power at 4900 rpm to maintain speed much less actually increase it. My thinking is that you do a gear chart and overlay that with your horse power and speeds and you should be able to see what the maximum over drive can be. Although with a turbo engine like yours maybe you go to really high boost when you drop it into overdrive, might work.

Rex

[geh458]

Gregg, can the gears position in the transfer case be switched like in a quick change rearend?  If so, swapping the position of the 60 & 49 tooth gears, would yield a ratio of .816, and give you a final drive ratio of 2.33 with the 2.86 ring, and 2.47 with the 3.05 gear.

Just a thought.


Gifford

Here is the info for the R&P's and Transfer Gears available for my transmission:

Ring & Pinion gear ratios:
2.86----60 tooth ring and 21 tooth shaft
3.05----61 tooth ring and 20 tooth shaft

Transfer Gear ratios:
  .91----52 and 57 teeth
1.04----47 and 45 teeth
1.06----56 and 53 teeth
1.22----60 and 49 teeth 

Possible final drive ratios:
2.60----  .91 transfer and 2.86 ring
2.97----1.04 transfer and 2.86 ring
3.02----1.06 transfer and 2.86 ring
3.50----1.22 transfer and 2.86 ring

2.78----  .91 transfer and 3.05 ring
3.17----1.04 transfer and 3.05 ring
3.22----1.06 transfer and 3.05 ring
3.72----1.22 transfer and 3.05 ring

As I posted last week, my transmission guy had special transfer gears made for drag racing.
This is an option that I might use if the 2.60 is not tall enough. 

Gregg

[Sumner]

Steve.
I always get concerned when people start talking about large percentage overdrive set ups, such as 24 to 30%. Even with an engine that makes relatively  good torque for its size asking it to still pull strong when you reduce the engine rpm by 30% is asking an awful lot.....Rex

I feel exactly the same and it is one reason running a transmission where you have control over an overdrive gear and what it is, such as the G-Force 101A and others like it, can make a big difference on that last gear change.

I have a spreadsheet on my site that can help with that to some degree.  Here is a screen-shot of part of the output for one scenario ...



You can enter tire dia., rear end ratio, and gear ratios and the last gear can be an overdrive gear if you want and not the 1 to 1 shown above for 4th (3rd could be 1:1).  You can also enter a figure for the HP needed to run 100 mph.

Above I entered 50 HP to run 100 mph.  If I go down to 200 mph I see that I need 400 HP.  I can also see if I shift from 3rd at (7595) to 4th that I'll loose 1661 rpm on the shift and will be at 5933 rpm in 4th.  Now if I had a dyno sheet for the engine I could look at the HP at 5933 and as Rex mentioned is it at least 400 at that rpm or not?  If under that we know the car won't have the HP to accelerate.  The larger the HP over 400 at 5900 the better the car will accelerate.

Note that I can also look above or below 200 mph and quickly see the HP needed for that speed and the rpm's before and after the shift to see if I might do better shifting sooner or later.

You can find the spreadsheet here...

http://purplesagetradingpost.com/sumner/bvillecar/bville-spreadsheet-index.html

It is down the menu and called...

RPM's for 4 Speed - Excel Spreadsheet Format Version: 01-16-14

If someone is running a 5 speed I could probably make one up for that also or maybe change the ones for the motorcycles to include it,

Sumner

[gkabbt]

Gregg, can the gears position in the transfer case be switched like in a quick change rearend?  If so, swapping the position of the 60 & 49 tooth gears, would yield a ratio of .816, and give you a final drive ratio of 2.33 with the 2.86 ring, and 2.47 with the 3.05 gear.

Just a thought.

Gifford

Gifford, The world would be a better place for me if Chrysler had made these transfer gears straight cut like quick change gears. Unfortunately, these gears are all helical cut (for quietness) and need to remain as a set.....so I've been told.
The gears my transmission guy had made were straight cut and if I were to go down this path, mine would be as well.

Gregg

[awelker]

Gregg,

Do you have an exploded view of this transmission?  If you were going down the custom gear road I would look at making a new final drive gear set. Depending on the design you may be able to do a replacement spur gear set without too much trouble.

Andy

[gkabbt]

Andy,  You do have a very valid point of doing a custom final drive gear set. That might be a better road to go down!
I need to get my junk running using stock parts and see what happens before I get into custom stuff.

Transfer Gears:  Stock, top  Custom billet, bottom



Custom billet R&P



Gregg

PS,
You coming back out to Wilmington in 2016?

[awelker]

Those are some nice looking gears. If you have somebody who has already made those you should be able to do anything you need.

Always good to have the next few steps in planning. These type of things always take longer than expected.

Don't know about Wilmington next year, depends on salt conditions. We had a good time last year and the three day event would be nicer, if I get my upgrades done.

[SteveM]

Thanks for the replies, guys. 

I definitely spend "quality time" with gear charts, engine HP charts, etc.

Since I'm messing with diesel stuff, the torque curves tend to be pretty flat, which is good.  Even with that benefit, it's always preferable to have a "close range" set of gears whenever possible, especially through the top 2 gears.  A bigger spread at lower speeds doesn't hurt as bad.

I'm still looking, and trying to keep all options open.  The fact that my truck needs to remain FWD is important, and there's not a whole lot of room to do a longitudinal engine installation, either.

Keep letting those good ideas flow.

Steve.

[JimL]

I worked on a fwd project where we ran up to 29" front tires by cutting away and moving the original fender aprons, notching and boxing the unibody to clear the axle lines, and then "clocking" the transaxle 28 degrees upward (on the engine) to get the axle lines to work.  That's mostly "black-smithing" work and not so expensive as custom gears.  You might look at your setup to see if there is a way to clock the transaxle.

[vwpsycho]

Hmm.
I suppose I should check in more often...

I expect you've already arrived at a specific transverse VW 5 speed 'box setup already, but..

FWIW, VW has been using nothing but 6, and 7-speed transaxles in anything but the most pathetically cheap (like 2.slow) applications, since 2003 or so.

Our (gasoline turbo-powered) GP/S Passat uses a stone-stock longitudinal FWD 6-speed found in USA-market '05-'08 2WD Audi A4s. Model: 01X. Code: GVD.

3.75 R&P, .658 6th. So with an effective top ratio of 2.4675 and 23"-26" tall front tires, we've made a zillion 200+mph Bonneville runs. We allow it to spin to 9000 rpm, though, so your Diesel, -even raggedly spun to 6500- will likely run out of gear, with only 350 or so HP, if your drag area is remotely like the Passat. The Euro TDI version of the 01X has a .575 6th, giving it a better (2.15625) chance of high speeds with a diesel.

All that said... None of that really matters, since you are not cutting your pickup apart any time soon and twisting the engine 90 degrees.

But my point is that -while spending $10,000-$20,000US on a dreamy custom gearbox totally sounds really fun. your wife and kids will not like life on the street, even in Herky-Jerky MO!

There are plenty of transverse 6-speed combinations.

Example:
USA   2009 Jetta  TDI   2.76 r&p, .81 6th= 2.2356 overall
     2010-2013 Jetta TDI 2.76 r&p, .72 6th= 1.9872 overall

The 6 speeds actually have less overall length than the 02A/02J 5 speeds, and have room for a Very Large flywheel and clutch. And until you start making over 600-700 ft.lb, you won't break one. Let me know if I can help.