Allison 6 Speed GMC 401

[tommr]

Next move on the motor will be the new GMC 8.0l engine Bill. will try to run it on propane but will do gasoline if I cant get components. Chevrolet was a sponsor this year and hopefully next year. the 8.0l now has a part # and a price attached in long block crate motor form. they introduced it last year but no units have been available til recently. will see what they can do going forward on price and if they can open up the parts bin. Tom

[turbobill]

Hmmmm, sounds like it's a big block Chevy. Nothing wrong with those. I have a 454 (468 actually) that's been wearing a turbo since 1986.

I might be interested in the 401 if you decide to let it go. I'm also in the northeast.

[turbobill]

Did you ever installall the 8.0?

[tommr]

Have not done anything Bill aside from the last body lighting harness and conversion to all LED lighting last year. Engine conversion is not possible until rear suspension and axle can be worked out. Rear axle is unserviceable as there are no replacement parts available. The very short driveshaft requires everything in the back be laid out as a unit to achieve acceptable driveshaft angles. More pressing is front suspension and axle. Needs complete replacement. All tall order stuff. Our last big trip was Myrtle Beach SC. Did 625 in a single day. The 401 with overdrive allowed speeds up to 65 mph @2200 rpm and delivered 8-9 mpg. Previously speed was pretty much limited to 55mph @ 2900 rpm. Motor has a lot of miles so keeping it under 3000 pm made sense. Leaning towards a Duramax. GMC is re entering the Medium Duty market in 2019 with the 4500, 5500 and 6500 Silverado. There will be an air brake option on the Class 6 6500 Series trucks with an engine driven air brake compressor. The 8.0l gas engine had air brake compressor mounting brackets available but Duramax did not until now. You kind of need that factory engineered mounting system on these serpentine belt drive systems for reliability. Not all that simple to devise your own mount system. In the old days of separate V belts stuff like that was fairly easy. In all bus needs tall order stuff to keep going. In the meantime its collecting dust in the warehouse. Tom

[turbobill]

What is worn out in the front end/axle? Is there a problem with the rear axle or are you worried about the lack of parts?

As much as I like my Chevy engines, one does not belong where a GMC big block V6 resides lol. Besides, Chevy used them too, so it could be argued they are a Chevy engine!

[tommr]

You are right Bill on the Chevy/GMC point. ive always considered the Chevy a GMC product in the larger sense.
The bus rear axle is like the original 4 speed Spicer. Replacement internal parts exist nowhere. The Allison modernized that aspect of the bus. And most of the Allison swap can be retained with an engine update. My guess is that the axle would not handle the torque of a Duramax engine. The 401 had to relocated 6 inches aft and 1.25 inches off center to get a good driveshaft angle. So you need a replacement axle assembly in place before a new engine cradle can be fabricated.
Front suspension has original air brake chambers and they are unique to the Timken steer axle. Bus was garage kept in CA and sat East in a barn for 24 years. We store it inside. Air system up front makes almost no water so those rubber diaphrams are still working. But they cant last forever. Doesn't make sense to be out on the road when they time out so bus is parked. Brake drums for front and rear are not available and it should have 4 new ones. Drums could be hogged out of a piece of iron for crazy $. Modern air chambers could probably be adapted as well with machine work but all of it is expensive and you still have obsolete components. Front needs leaf springs replaced as well as spring eyes. Front is also a little primitive with a buggy spring ride up front. Theres no sway bar system either. A modern steer axle with disc brakes, conventional shocks and a sway bar is very doable. We need a component mfg to help us with dimensional data and maybe some engineering oversight. The guys at SAF Holland have offered some limited assistance with their air beam components but support would not include and actual axle assembly. The aim is to use new components throughout. As I said its all tall order stuff so we need buy in from one of the big axle builders. We have something unique with this vehicle and a unique purpose for it. Tom

[turbobill]

Perhaps components (front and rear axles) along with springs etc. could be used from a more modern transit style pusher school bus. Most modern school buses (last 30 years or so) are diesel powered and thus the axle ratio's are more friendly towards lower speed engines. They often turn up cheap and are generally well cared for. Grabbing one just out of school bus service may be a very economical way to score some good usable later model parts.

As for the engine, the modern diesels (Duramax included) are complicated and very expensive to repair. The emission controls, electronics and electronic components can be an expensive nightmare.

Power wise, the turbocharger saved the modern diesel from extinction. A naturally aspirated diesel produces less HP and torque than a same displacement gasoline engine. My point here is that the turbocharger works wonders on a gasoline engine just as it did on the diesels. Mild boost (6 lbs or less)on the 401 would yield wonders in the power and feel of your bus without over stressing the engine. The 401 could be turbocharged with quality components for less than the cost of a set on injectors for the modern electronic diesels lol. With a lower numeric axle ratio modern axle, some boost might be necessary and still be less stressful than a diesel on the bus.

If you are worried about the age and mileage on the 401, over the years I have sourced rebuild parts for my GMC V6's pretty cheaply. I always felt that an already good running engine could always be freshened (rings/bearings/gaskets/seals) without going to the elaborate expense of boring/turning the crank, and having to find rare and expensive replacement oversize parts. I've had good success with just freshening suitable older engines over the years if that's all they needed.



Do you do all of the work on the bus or hire it out?

[tommr]

The bus builder made engine chassis mount kits available in the 60s when these V6 motors were starting to become available. An owner could swing an entire V6 with the GMC SAE 3 flywheel housing, air brake compressor, brackets and pulleys from a 6500, 7500 or 8500 Series truck. Its was a quick and cheap upgrade that was light years ahead of the Buick 8. While our motor is very good mechanically it is underpowered. It will climb any hill or grade but it is be slow. Ive thought of adding a turbo but just not inclined. Later Duramax is getting pretty complicated but the earlier pre 2012 is very dependable and components are readily available anywhere. Aftermarket support is huge as well. Torque and HP is very adequate to keep up with interstate traffic in hill country. With availability of engine driven air brake compressor bracketry the swap is much more attractive. Fuel mileage would also be better than a gas engine. Axles could come from a school bus but what we really need is dimensional data from an axle builder. You could climb under a lot of vehicles and maybe not find what you need. Ive pretty much done all the work on this. Ive had help with some aspects of rewiring. Subbed out some light mechanical issues when it came out of the barn and subbed out a clutch R&R. Tom

[turbobill]

The pre Jan 1, 2007 D-Maxes and others are the simplest (for electronic engines anyway). That's when DPF became mandatory and urea came along around M/Y 2011. There might be delete kits for that stuff if you get a later engine.

Is there room width wise for a 90degree V engine in the bus?

[tommr]

Yes. the BB Chevy engines fit fine. Not all that different than in size to the V6. The 8.0L has made its way into new production school buses and those bus builders have designed a accessory drive system that incorporates an air brake compressor. It makes the BB Chevy an attractive option and crate engine prices are good. That said a Duramax eliminates 85 gallons of gasoline from the bus. 06.5-07 LBZ is the preferred Duramax swap engine. Aftermarket wiring is available new and reliability and parts availability is excellent. The 2012 and later LML/LMM Duramax has very little swap support from the aftermarket. You cant run them without the emissions. An entire body harness is required to accomplish that swap. Not likely a harness and a computer is ever available. But who knows?