Gear Ratio

[rooster]

I am re-powering my 1979 Newell 38 footer.  Original 555 Cummins went out.  Installing a 6V92T Silver with a 747HT Allison.  I need to change out the Pumpkin to conform to the new lower top RPM of the Two Stroke.  I am leaning toward a 3.42 but would appreciate any advise from anyone familiar with this situation.  Thanks   Pete

[JackConrad]

Our MC-8 has an 8V71, Allison 740 and a 3.73. I drove a 102A3 with an 8V92, Allison 740 and a 3.33. both seem to be well matched for the engiines. Looks like the 3.42 should would good.   Jack

[TomC]

What is the gear ratio you have now?  Have you considered using an Allison 6spd World transmission instead?  With the power that the 6V-92T puts out, you could use the 3000 series-which is smaller and lighter then the HT series.  The first four gears of the 3000 series are almost identical to the HT747, but you would have a .75 and .65 overdrive. 
The 3.42 gear ratio would work also-it will give you 1800rpm at 65mph.  Good Luck, TomC

[wg4t50]

For sure the 3.33 or the 3.43 or what ever will work in that rear will do fine, I know of a 3.08 with the 6V-92 @ 300-350HP,, It works fine too with the 4 speed auto box.  As uaual, are you after power or mpg ?
Seems like a simple question don't it.    

If your using the DDEC, consider the lower numbers, if the mechanical engine, go with the 3.33 / 3.43
Enjoy

[luvrbus]

A 3:36,3:33 or 3:38 is ideal for your set up gives the long 3rd gear works great with a 6v92 IMO

good luck

[rv_safetyman]

I am lost here.  How can we talk about gear ratios without knowing tire size?

I suspect that the Newell would have the same general size tire as most of our buses, but I think we need to verify. 

I doubt that the tire size would be off more than 10-20%, and if that is the case, the replies are correct.

Jim

[luvrbus]

Jim he has to change the gears those engines in the Newell were 270 hp at 3300 rpm and 235 hp at 2800 rpm and his probably has the 24.5 tires they came with you do see 22.5 on 40ft's not enough to make a difference in the gearing with either size

good luck

[edroelle]

How will you use the coach?   I would ask 2 questions.   Will  you be running in hills and mountains or flatlands?   And, can you be satisfied with a top speed of about 72 MPH (3.73 axle) ?

Generally, the higher horsepower 8V92s use the 3.42 axle where the 6V92s were fitted with the 3.73.

Driving in hills and mountains occasionally, and usually driving 60-65 MPH, I would probably choose the 3.73

Ed Roelle
Flint, MI

[TomC]

When I changed my truck conversion from the 13 spd single overdrive to the Allison HT740, I was concerned about startability and cruising speed.  My truck with car on board I figure will weigh around 36,000lb-then pulling a 10,000lb boat brings me to 46,000lb (which was my empty weight when it was used as a tractor/trailer combo).  With the Caterpillar 3406B at 400hp and 1375lb/ft torque, my startability will be around 34% (Grapevine is 6%).
In 4th direct my rpm vs speed will be- 1400 @ 50mph, 1500rpm @ 53mph, 1600rpm @ 56.8 (which will be very fuel efficient), 1700 @ 60mph, 1800 @ 64mph, 1900 @ 67mph, 2000 @ 71mph, 2100 @ 74.5mph, 2200 @ 78mph.
With the high startability of Allison transmissions, I would shoot for gearing high, or with a low numeric number ratio. Changing rear end will do very little for hill climbing (that's taken care of by the gears in the transmission), but will affect overall top speed rpm dramatically.  For instance, going from a 3.73 down to a 3.42 or 3.08 means at 65mph (with 11R24.5's) you'd be at 1923rpm with the 3.73 compared to 1763 with 3.42 or 1588 with 3.08.  Good Luck, TomC

[rooster]

Thank You for all the input.  I will post picture of bus when I figure out how.  The Allison is newly rebuilt so I will run with it even though a 6 spd World would solve my problems.  I need to try to relocate the turbo which is mounted on top of the engine so I can lower the bed a bit.  Sounds like a 3.42 Ratio would give me highway speed without overheating on New England grades.  The pumpkin has a 14 bolt pattern.  What information do I need to locate the gearset I need.  I tow a Jeep Commander so nothing too strenuous.  I plan to use same radiator hoping that the two stroke runs cooler and it will be adaquate??  Pete

[TomC]

Unfortunately, 2 strokes are the hardest to cool-and with the fact that the 6V-92TA will put out more power, you'll probably need to upgrade to a larger radiator.  Measure your present radiator to see what you have.  To cool a 6V-92TA, a 1200sq/in (length x width of the actual core [not including the tanks]) with 6 row core should be used.  On my truck with a 8V-92TA I had a 1350sq/in radiator with 6 rows that was just enough.  Good Luck, TomC

[wg4t50]

TomC,
You got that right about the 2 stroke engine requiring more radiator.  I had the 12V-71 in a MCI for many years, radiator was mounted across the rear, used a normal 54" pusher fan, radiator was 5' X 6', had recored with all the cooling capacity including the wrinkle fins. On a hard pull, it still heated, so installed a home made spray system, used window washer fluid, (it does not corrode the nozzles, water sure does), that solved all heating issues, only needed the spray on heavy pulls. Always enjoyed making fools out of cars and trucks on 3 lane interstate mountains.
Now that I am too old to play those games, I have a store bought job with the ISM500 & Allison 4000R.
Love reading these stories.
FWIW

[luvrbus]

LOL the 6v92 won't run any hotter than his triple nickel he is replacing those things ran hot in 30 degrees all 4 of those engines the 378(v6),504,555 and the 903 were a pain to keep cool

good luck

[rooster]

I forgot to mention that the Engine/Tranny is a mechanical series.  I am trying to make this coach as simple as possible.  I have an 06 450 Lxi Wanderlodge and every time you phart, something goes out. These units were built with the front end overloaded,  Multiplex wireing and altogether too complicated.  Every trip was a problem of some sort.  It is parked in the barn with 11000 miles on the clock.  Really Sad, but Life Goes On.   Pete

[luvrbus]

Rooster,that is one nice bus you have parked the LXI450 is nice piece if it wasn't for the problems BB left you guys hanging with, the same with some Prevost conversion people are trying to dump those also it's a shame to pay a huge amount for a BB or Prevost and nobody can figure out the electronics they are so complicated.

You will take a hit on that one I seen 3 slides 2006 LXI sell for under 200,000  

good luck