Using a FWD Drivetrain for a mid engine AWD setup?

So I was looking at LS4 stuff thanks to Knurled and remembered an episode of one of the 4wd shows on power block doing a mid engine buggy with a Saturn drivetrain (auto) mounted sideways with one axle powering each end of the car. This got me thinking about running an LS4 with the 4T65 (FWD Auto that came attached to it) in something using a Miata rear end (I have one of these) or IRS setup and some sort of FWD setup for the front, or some VERY small IFS truck setup. For a bigger car, a t-bird IRS and explorer 4wd IFS might be the ticket. One issue I see is that the driveshafts would be VERY unequal length (Short rear, long front).

this is an excellent idea. I propose we toast, and then get started right away.

I guess one problem would be that many front axles have an offset front diff, because the engine is usually in the way, but maybe you can get away with that with CVs on both ends of the front driveshaft.

Do any serious rock crawling people make reverse cut rear diff gearsets? if so, getting another IRS rear diff on the front but using FWD CVs, Spindles, brakes, steering gear, etc might work.

Wouldn't the unequal length of the drive shafts actually work to your benefit here? I see it as you lose more torque to the longer drive shaft (its heavier, so accelerating it takes more energy), so you actually might be rear-biased a bit!

is it the miata or fc mazda rears that share the same gearset size as s2ks and kia SUVs?

Maybe the Kia SUV front diff has the reverse-cut innards you would need to bolt into the mazda IRS?

In reply to singleslammer:

Seems like you would have gearing issues since you wouldn't have 1:1 gears in the diffs. You essentially would be using two final drive ratios. A 9.x:1 gear ratio is fine in a rock crawler with big tires, but wouldn't be my first choice for a road going car.

Elsmere wrote: In reply to singleslammer: Seems like you would have gearing issues since you wouldn't have 1:1 gears in the diffs. You essentially would be using two final drive ratios. A 9.x:1 gear ratio is fine in a rock crawler with big tires, but wouldn't be my first choice for a road going car.

This. Only probably 12-16:1

Mazda 79-82 12A: 3.909 Mazda 83-84 12A: 3.933 Mazda 84-85 13B: 4.076 Mazda 85 12A: 3.909 Mazda 86-92 13B 6PI / Turbo 5 speed: 4.10 UK models 4.30 Mazda 1988 Convertible: 3.91 Mazda 1989+ Convertible: 4.10 Mazda 86-92 13B 6PI Automatic: 3.91 Mazda 1989 GTUs 5 speed: 4.30 Mazda 93-95 13B-REW Manual: 4.10 Mazda 93-95 13B-REW Automatic: 3.91 Mazda 09-current 13B Renesis Manual: 4.777 Mazda 09-current 13B Renesis Automatic: 4.3 Mazda 04-08 13B Renesis Manual: 4.44 Mazda Miata 5 speed: 4.10 or 4.30

Kia Sportage 98-02 4x4 (front diff): 4.78 Mazda B2600 87-88 4x4 5spd (front diff): 4.44 Mazda B2600 87-88 4x4 Auto (front diff): 4.10

^^all of the above will direct bolt into the Mazda rear end. All are large spline differentials.

(From: Rotary Club trans info)

But now I am just confused... if a front diff gearset works in the rear of an RX7, then is the REAR diff 'reverse cut' on the trucks and SUVs? or does the t-case output opposite rotation?

OR! am I just crazy thinking that the front diff would need opposite rotation?

4th gear is OD at .70, and the tallest final drive looks to be 2.86 from wikipedia.

So lock the thing in OD and never let it shift, it would end up being a gear between 1st and 2nd. That does pretty much ruin the fun.

According to Wiki though:

"Gear ratios are remarkable in the 4T60 family in that there are two points in which the transmission can have different gearing (the drive-chain sprockets and the differential) resulting in up to 12 different available gear ratios. The combined gearing of the two is the overall transaxle ratio and is called the "Final Drive Ratio", and the different ratios allow the use of the transmission in multiple applications based on the engine and vehicle."

Maybe you could fab your own gears for a custom final drive?

RPM * .7 * 2.86 * say 3.5 ish (depending on what you can find) = RPM * 7.01

In reply to rcutclif:

A low pinion front axle generally uses the same ring & pinon as a rear axle of the same type. The "Reverse Cut" ring & pinons are found in the high pinon axles.

I did bookoos of researching a similar FWD platform for the same reason, but couldn't come up with acceptable axles. The center section out of an H1 Hummer offered about the highest somewhat readily available source for a high ratio IFS/IRS housing with it's 2.56:1 (they use portal gear boxes to further lower the ratio at each hub/wheel) but the Hummer junk yards are proud of them. I've got a spreadsheet at home where I was tying to find what might and might not work.

Most of the easily sourced IRS axles that come to mind just don't have a high enough gear ratio. Unless you're okay running a 27-29" diameter tire at all four corners the highway RPMs are just too high.

Now if you've got a set of RS200 axles (1:1) just laying around, you might be on to something.

• Lee

What if instead of the auto get one of the 5-6 speeds that were in the Grand Am. Remove the differential from the case and replace it with a chain using the same size sprockets on the transmission's output shaft and the yokes to the front and rear axles?

(2)- FWD engine/ transmissions sounds easier.

Yeah, I didn't think about the finals multiplying. The tbird irs front and rear with the 2.93 (i think that is the lowest) and only use the top three gears. This would probably be limited to auto x with tall tires.

You have good company, as a surprising number of people go down this same path, not figuring out the total gear ratios from crank to ground.

Notes from the spreadsheet I mentioned earlier.

Highest gear (numerically lowest) ratios I came up with, for semi-ready available IRS differentials that some form of LSD is also available for were the following:

Ford 7.5" 2.73:1
Hummer H1 2.56:1
BMW E30 2.79:1

Given how this would be used, would it be possible to remove the diff and weld a spool to the sun shaft completely bypassing the final drive? This would force a 50/50 split 100% of the time but that is ok.

http://www.ganzeboom.net/uploads/images/4t60e-4t65e-440-t4-.jpg

In reply to singleslammer:

Couldn't you just weld the final drive gearset to lock the carrier to the "ring gear" to spin the diff 1:1?

In reply to Kenny_McCormic:

I am not versed in these things, would that work? Nice thing it appears that the diff is easily accessible from the outside so a complete tear down wouldn't be necessary.

In a different episode, they used 4wd truck drivetrain and turned the engine and transmission/transfercase around. Then using oposite rotating differentials got the layout to travel in the right direction for the egine in the rear/midship.

Given how this would be used, would it be possible to remove the diff and weld a spool to the sun shaft completely bypassing the final drive? This would force a 50/50 split 100% of the time but that is ok.

Yes, but that doesnt get rid of the final drive gear reduction issues. If you weld or otherwise lock up the planetaries that are in the diff carrier, you get rid of the final drive gear reduction (which you couldnt even do if it had an actual ring and pinion for final reduction instead of a planetary set). If you weld the side gears inside the diff carrier you get a 'lincoln locker' 50/50 split. Do both and you get both. Seems workable to me!

In reply to singleslammer:

Next time I see my dad I'll ask, he knows those GM integrated diff boxes pretty well. Though I'd think based on the diagram that welding the planet gears on the diff carrier to the carrier would work.

So I found my old post through a Google search. Go figure...

Kenny - did you ever ask your dad about this? I still love this idea!

The kind of setup you're talking about here is commonly used in small rock-crawling buggies - a FWD powertrain in the middle, mounted sideways, the original axles going to the front and rear diffs. They have ultra-low gears because of the double differential reduction but this is exactly what you want in a rock crawler.

Make me think back to high school auto mechanics. We built a tractor out of an old Plymouth

We coupled two 3spd transmissions to get the required low speed.

Theoretical- 3x3= 9 to 1. Don't know the actual ratio.

In reply to singleslammer:

No, but I remember it came up again on here again a while ago, it was agreed upon that it was definitely possible to weld it up for 1:1.

singleslammer wrote: I still love this idea!

me too!

Yes, the final gear reduction from those GM trannies is a planetary you can weld or lock up. So it is possible.