I've probably lost my mind

[convert2diesel]

This is most likely the strangest construct that I have ever envisioned. Have found a 1978, 26 ft. GMC Motorhome for sale with a blown Olds 403 engine. Been intrigued with these motorhomes since they were introduced some 40 years ago.

The better looking one and I have been planning on some long distance, 6 month per year traveling during our pending retirement and as such have been researching the appropriate RV for that purpose. We do like the GMC for its driving characteristics. Many describe it as a 26 foot hot rod with plumbing.

Having said that, it is after all a 40 year old motorhome with some inherent issues even when it was new. The beefed up Olds Toronado front subframe and drive system being top of the list.

My idea as to replace the entire front subframe with one from a GM 3/4 ton 4X4. Have done some initial measurements and it will work. That will allow me to install, hopefully a Duramax, 6.5 Turbo or maybe a Banks VM Moterie if the bank account allows. Coupled with either an Allison or a 6L80, it should make for a very tractable drive train.

My question is this. Would a good mechanical transfer case have a strong enough front drive to be used exclusively (the motorhome was front wheel drive) with the rear drive either covered over or a modified yoke installed to seal up the rear of the transfer case?

Will also be doing some major mods to the rest (stretch it 4 feet and install a living room slide) and a complete body off restoration, but the drive system is the biggy.

As always, any input, (other than I'm crazy...I already know that) would be appreciated.

Bill

[Rumpelstiltskin]

A gear driven Transfer case would take the abuse and would be strong enough, chain drive will have issues and oil problems.
I honestly would look into custom gear boxes, and redirect power units.
I wish I could offer more help, but I think you will be opening a can of worms trying to convert a transfer case.

[arveetek]

Very cool idea!

However, I'm afraid that the front diff from a 1 ton truck would not be strong enough for full time use lugging around a motorhome. The front diffs were never intended to be used for full power, only when split between the rear and front diffs for part time use. Think of it this way: in a 4WD 3500 truck application, the rear diff would usually have a 10.5" to 11.5" ring gear, while the front diff would only have a 9 to 9.5".

Having said that, I'm not sure what size the original Toronado "ring gear" is, but chances are, whichever engine you swap in will have more power output than the original motor had, so it would probably need a beefier drivetrain anyway....

What's so bad with the Toronado setup anyway? My dad is actually using that TH425 tranny/transaxle in a project he's currently building: a 1964 Corvair that will be converted to a mid-engine Chevy 350.

Casey

[convert2diesel]

A gear driven Transfer case would take the abuse and would be strong enough, chain drive will have issues and oil problems.
I honestly would look into custom gear boxes, and redirect power units.
I wish I could offer more help, but I think you will be opening a can of worms trying to convert a transfer case.

I agree. Atlas apparently makes a gear driven TC that is built to stand up to 700 ft/lbs or better. However, would have to see how they configure the output shaft bearings etc. Probably not intended for full time front drive only.

Very cool idea!

However, I'm afraid that the front diff from a 1 ton truck would not be strong enough for full time use lugging around a motorhome. The front diffs were never intended to be used for full power, only when split between the rear and front diffs for part time use. Think of it this way: in a 4WD 3500 truck application, the rear diff would usually have a 10.5" to 11.5" ring gear, while the front diff would only have a 9 to 9.5".

Having said that, I'm not sure what size the original Toronado "ring gear" is, but chances are, whichever engine you swap in will have more power output than the original motor had, so it would probably need a beefier drivetrain anyway....

What's so bad with the Toronado setup anyway? My dad is actually using that TH425 tranny/transaxle in a project he's currently building: a 1964 Corvair that will be converted to a mid-engine Chevy 350.

Casey

Not too concerned about the diff. My "B" and "D" body cars had 9.5 diffs and I have seen them built to significant specs. The 1/2 shafts on the 3500 are beefier and the output shafts also seem to be heavier.

My issue with the original setup is that the newest 425 available would still be over 35 years old, plus does not play well with the newer engines. Loved the older torgue monsters (the original Olds Toronado 455 was rated for over 500 ft/lbs.) but again they are also over 40 years old.

The reason I want to go with the swap over to the 3500 subframe is to allow me to go with newer powertrains and more substantial suspension bits. Just going with the newer unit front wheel bearings is reason enough. Better brakes, heavier torsion bars, etc. make for a better setup. The original GMC motorhome weighed in at 12,500 lbs. I assume I will be adding another 1,500 to 2,000 lbs with my modes. Well within the specs of a 3500.

I am going for reliability here and the ability to get it fixed when on the road. Finding a good used 425 in a junkyard while 2,000 miles from home is not my idea of a good time.

Thanks for the input. Finding the right transfer case is still my primary concern.

Bill

[trbankii]

Atlas is a standard in the off-road world. TWF is another: http://www.trailworthyfab.com/Hero-Transfer-Case/

I know they’ve built some transfer cases for custom applications such as 6x6 and such. They could likely build something for your requirements.

The other thing to consider is a high pinion front axle so that you’re operating on the drive side of the gears. A low pinion axle in the front is operating on the coast side of the gears - similar to a low pinion rear axle operating in reverse. The downside is that the high pinion axle will likely create some driveline clearance issues over a low pinion axle.

[convert2diesel]

Atlas is a standard in the off-road world. TWF is another: http://www.trailworthyfab.com/Hero-Transfer-Case/

I know they’ve built some transfer cases for custom applications such as 6x6 and such. They could likely build something for your requirements.

The other thing to consider is a high pinion front axle so that you’re operating on the drive side of the gears. A low pinion axle in the front is operating on the coast side of the gears - similar to a low pinion rear axle operating in reverse. The downside is that the high pinion axle will likely create some driveline clearance issues over a low pinion axle.

Good point about the high/low pinion. Never thought about it but you're right, it would be a consideration.

TWF is interesting. Wonder if they make a single speed TC. Either that or a 2 speed with OD? Would open up my trans selection to allow for a low speed/high torque engine. Actually considered an LS gas option but they seem to want to see reasonably high revs to achieve the peak torque numbers. As the engine essentially sits beside you in the GMC, noise is certainly a consideration.

Thanks. Obviously needs some more thinking before pulling the trigger on the project.

Bill