better fuel economy with an automatic

[gus]

In theory AT always have lower ratio rear ends so they should get better mileage in lock up.

In practice there is so much slippage getting to the lock up stage that the mileage is probably always lower. An extreme case in which the AT goes for thousands of miles in top gear lock up would give better mileage, but that never happens.

Slippage also generates a lot of heat which is fuel vanishing into thin air.

I think lock up is powered by hyd pressure so the hyd pump also uses a bunch of energy.

The only energy loss I can think of for a manual box is some of the gears turning around in oil and the trans oil pump circulating oil, but that would not be much.

[RJ]

Short GMC Powertrain History Lesson -  (There will be a test!!)   

GMC designed the powertrain around tires that turned 495 revs per mile, going all the way back to the pre- and post-war Silversides.  With the 1:1 bevel gears, this gave 60 mph at 2100 rpm.  This combination was perfect for the time period, when the Interstate system was still on the drawing boards, and most highways were still one lane in each direction.  A few freeways existed around major metropolitan areas, but that was the exception, rather than the rule.

Fast forward to the late '50's.  President Eisenhower authorized the Interstate system, and the freeway race was on.  Greyhound had their Scenicruiser on the roadways, but was unhappy with it's twin 4-71s powerplant, and was experimenting with other combinations.  Come late 1959, when the "New Look" or Fishbowl transits was introduced as 1960 models, along with them came the first V-block 71s - the 6V71.  This was followed in 1961 with the introduction of the 8V71 in the PD4106, and as an option with the transits.  Greyhound immediately cycled all remaining Scenics thru Marmon-Herrington to get the new engine and other modifications.  The 8V71 was a hit, and became the engine of choice for the bus industry over the next two decades.

One of the things GM discovered while doing development work on the 8V71 for the highway models was that the increased torque of the 8V, multiplied by the gears in the transmission, would blow out the bevel gears.  This is why the bevel gear sets on subsequent models, both transit and highway, are before the transmission, not after, like the 4104's.  By placing the bevel gears first, they were better able to withstand the greater torque output.  A side effect of this, however, was that the engine/transmission powerpack had to be shifted slightly, and the rear axle pumpkin moved to the opposite side of the coach centerline, in order to maintain proper u-joint angles during operation, or they, too, would be short-lived.

Now remember, GM was still using tires that turned 495 revs per mile for their development work.  Because of the additional torque available, the engineers discovered that they could overdrive the bevel gears somewhat, while using the same basic internal transmission ratios that had worked so well up to that point.  This lowered the engine rpm at any given road speed, thus giving the 4106 virtually equal fuel mileage at 60 mph compared to the 4104.  Turns out, this is translates to a 450 rpm drop, because the 4106 runs 60 mph at 1650 rpm with these tires, compared to the 4104 running 2100 rpm at the same speed.  Fleet operators liked that, a LOT, since they were paying the fuel bill!  (So did the drivers - because the 4106 would flat get up and MOVE next to the '04, thus negating the equal fuel mileage claims.  One reason so many '06's out of the factory were governed at 1700 or 1800 rpm initially!!)  Thus all subsequent GMC manual transmission buses, be they transit/suburban or highway models, were equipped with an 0.808 bevel gearset.  Coupled to the 4.125:1 rear axle, this gave an overall 3.333:1 final drive, compared to the 4.125:1 overall of the earlier models. 

(This "tall" ratio is also why reverse seems way too high a gear for our purposes, but you must put that, too, into perspective.  It wasn't designed for wiggling into a campsite at Oh Dark Thirty - it was designed for backing out of a flat & level bus depot parking stall.  It's that simple.  Perspective, folk, perspective!)

Note that when the 40-foot 4903s came out, because of their increased weight over the 35-foot models, the rear axle ratio was changed to 4.375:1, yet the bevel gear ratio remained as is.  About this same time, the 12R22.5 tires started becoming the standard tire on buses, yet they stuck pretty close to the original 495 rev/mile overall sizing.  You also have to remember that the V-730 was originally developed for transit use, where operators are more interested in 0 - 30 mph, but with occasional short runs on the freeway.  Thus the bevel set came with the 0.875:1 ratio, as a compromise for both types of operations, city & freeway.  There's an urban legend out there that says you could also get a 1.04:1 bevel set in the V-730, but in my 29+ years in the industry, I never saw one.  If it, in fact, did exist, my guess is that you'd have only found it in early production runs coupled to the lower-powered 6V71 engine, and only installed in transit buses for operators like San Francisco's Muni, where hill-climbing ability is important.

So much for the history lesson.  I think you can see, just from my short (?!?!) dissertation, that there's a lot more that goes into all this than simply slapping on a particular tire size or choosing an arbitrary transmission ratio.

John -

So how does this affect you?  Let me illustrate with a real-world example:

There was a fellow about 10 years ago here in Fresno that did exactly what you're wanting to do - install a V-730 in a 4104, behind the stock 6-71 (non-turbo in this case), and using a rear axle out of a 4106.  I visited with him often during the change-over, so was pretty familiar with what was going on.  He did a beautiful job, you'd have thought it came from the factory that way when it was finished.

I talked to him again, after he got back from a trip to Texas, to see how it all worked out.

His first comment, when asked about the trans swap, was basically "Worst damn mistake I've ever made in my life!"

Elaborating, he said around town it was fine and a lot easier to drive, but on the highway the bus couldn't keep up with traffic, would climb the 6% grades we've got around this valley at 15 - 20 mph, as opposed to 30 mph with the stick, and his fuel mileage dropped from 10-11 down to 7.5 - 8.

He kept it another two years, but never made any long trips again, until a stroke took away his ability to drive and he sold the coach.

Last I heard, it was in the Oxnard/Ventura CA area, where the owner pulled the 6-71 and installed a 350 hp 6V92TA.

A 6-71T puts out just about the same HP/torque as a stock coach (not truck) 8V71, so those folk who have given you numbers for their buses with the stock engine hooked to the V-730 are very real world.

Your numbers do pencil out that theoretically you should get better mileage, based on the gearing, but reality isn't the case.  Too much loss in the automatic.

OTOH, if you were to switch to 11R24.5 tires, and bought the ones that turn only 470 revs per mile, as opposed to the 502/504 your current tires are turning, you'd pick up some of that mileage loss attributed to the Allison.

But it still wouldn't equal the mileage you're getting with the manual box now.

If you really want an automatic, the best bang for the buck is a 6V92TA bolted to the V-730.  Pump the six up to 350 hp, and you'll have plenty of power for climbing Rocky Top, and still get around 7.5 to 8 mpg - especially if you also invest in the taller 11R24.5 wheels & tires.

FWIW & HTH. . .

[Barn Owl]

6V92TA bolted to the V-730.  Pump the six up to 350 hp, and you'll have plenty of power for climbing Rocky Top, and still get around 7.5 to 8 mpg - especially if you also invest in the taller 11R24.5 wheels & tires.

My father's 4106 has that setup and it is exactly what RJ says. Simply wonderful!

[johns4104]

RJ, Sounds like what I need to do is run a 4106 trany and rear axle with my 671t.

Thanks for all the info

[RJ]

Sounds like what I need to do is run a 4106 trany and rear axle with my 671t.

John -

Do you already have the 6-71T? 

If so, pull out the measuring tape, as you'll have to do a lot of number crunching to see if this would work.

If you don't have the 6-71T, then simply swapping in a complete 4106 powerpack would be a lot easier.

Don't forget to upgrade the cooling system - the '06 radiator's a whole lot bigger than the '04's.

FWIW & HTH. . .

[johns4104]

RJ,
Yes I already have the 671 it is a fresh rebuild setup for a turbo.
Thanks, for all the info it gives me lots to think about.
I guess I need to get my conversion (interior) down first before I tackle the drive train.
The 671 with a 4spd runs good.

John
Just planning my moves!

[RJ]

The 671 with a 4spd runs good.

If it ain't broke - Don't fix it!!

[johns4104]

Thats right RJ keep it simple!!!