Hello everyone,
I need some recommendations on a build im doing on my 1973 gmc motorhome, my goal here is MPG and more power then stock.
The motorhome weight is roughly 10,000 lbs, its a 455 and th425 trans with 3.08 final drive. 29.5" tires.
I picked up a transmission/final drive core from a 71 toronado, it has 2.73 gears in it.
I also have a built 9:1 compression 455 with ported big valve C heads that just needs a cam to match the set up.
I would like to rebuild the toro set up and use a higher stall converter to make taking off with that 2.73 better.
I realize this will be hard on the transmission and differential, can anyone recommend some upgrade/rebuild parts to put into this? Also a torque converter? Roller cam to match?
any input is appreciated,
Thanks,

 

Check the GMC motorhome sites (gmcmhforum.com and gmcmotorhome.com are a couple). Members there often recommend Manny Trovao who lives in San Jose CA as he specializes in rebuilding the TH425 for the motorhome applications. His contact info is on those sites somewhere maybe under the resources tab. I spoke with him a few years back and he knows his stuff. You can talk to him about the drive ratio options and he may recommend the switch pitch conversion on the TH425 as it helps launch the heavy rig.

 

I’m pretty sure the chain and sprockets are all the same, which means the only thing affecting gear ratio is the final drive. If at all possible, keep the lower gear. Your moving ALOT of weight, every little bit helps. The 425 trans is very similar to the TH400, a lot of the components will interchange. The same durability upgrades for a 400 could be adapted to the 425. Anything you can do to improve the torque output of the engine will make it more drivable. Keep the stall speed low, for the same reason.

 

A good upgrade for the TH425 is the replacement of the gear train from a TH475, HD truck transmission like would be found in a motor home or box van. They have straight cut gears that are a strength upgrade.