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Author Topic: HT740 in 4905  (Read 5599 times)
RJ
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« Reply #15 on: November 12, 2009, 08:35:05 AM »

Bryan -

Let me clear up a few things in this thread:

The HT-700 series transmissions are for T-drive configuration powertrains.  They will not fit in the V-drive GMC layout.

Until the newer "World" series of electronically controlled automatics, all the Allisons had a top gear ratio of 1:1, just like the manual gearbox.  The major difference is in the bevel gear ratio used for the V-drive 730.

The stock bevel gear ratio for the Spicer 4-speed is 0.808:1, starting with the 4106s.  Prior models used a 1:1 bevel gear ratio.

The stock bevel gear ratio for the V-730 Allison is 0.875:1.

The stock rear axle ratio for the 35-foot GMC highway bus models, starting with the 743, was 4.125:1.

The stock rear axle ratio for the 40-foot GMC highway bus was 4.375:1.

Coaches equipped with automatics do run slightly hotter than manual transmission models, but not to the point of overheating unless there are other issues with the cooling system, most of which are NOT transmission related.


All of the above is well documented in various places.  What isn't as well documented is the following:


The optional bevel gear ratio in the V-730 was 1.04:1  (This is EXTREMELY rare, like new rear glass for a Scenicruiser.)  Muni in San Francisco used these in their Fishbowls, but all of those coaches have long since been scrapped.

The optional automatic in the GMC was the VS series, up until 1977 or so, used with the stock rear axle.

The optional V-730 (using the stock bevel gear ratio), when installed in the 40-foot Buffalo, also included the 4.125:1 rear axle, in order to maintain performance relatively equal to the manual transmission.

The 4.375:1 rear axle was an option on the 35-foot Buffalos, but one that was rarely installed.  (Primarily for mountain operators.)

Installing a V-730 into a 4-spd 40-foot Buffalo will lower your top speed by about 5%, and increase your fuel consumption by about the same.  Without also installing the 4.125:1 axle, some of this can be regained by using 11R24.5 tires that turn 470 revs per mile.

All GM highway buses had their powertrains designed around tires that turn 495 revs per mile.  When replacing tires, keep this number in mind to maintain stock performance.

There is a good calculator on Daris's site that allows you to play with all kinds of combinations.  Click on the "MPH Calculator" in the LH window here:

http://www.thebouthilliers.com/4106/

Using Daris' calculator can give you a good idea as to why a stock 35-foot GM will "run away" from a stock 40-footer at any given rpm!

Finally, only about 235 4905s were built with the V-730 option during the last three years of production (1977 - 1980).  I have a delivery roster with the VIN's of these somewhere in my files (still unpacked after my recent move).

FWIW & HTH. . .
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RJ Long
PD4106-2784 No More
S13406 1978 MC-5C Converted
6V71/MT-644
S14947 1980 MC-5C Shell
6V92/HT-740
Cheney WA
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« Reply #16 on: November 12, 2009, 09:10:14 AM »

Thanks RJ, that helps lay it all out.  Roll Eyes

Bryan
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wildbob24
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« Reply #17 on: November 12, 2009, 09:10:53 AM »

I knew RJ would jump in and straighten us out. Smiley
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P8M4905A-1308, 8V71 w/V730
Custom Coach Conversion
Greenville, GA
bryanhes
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« Reply #18 on: November 12, 2009, 09:20:04 AM »

RJ,

I did go to Daris calculator and plugged in my tire diameter and gear ratio. Tires are 12x22.5 and rear gear 4.375:1. It appears to be considerably off from what my actual is. Unless My Final Vehicle Record has the wrong gear ratio on it. I can run 75 MPH and it does not feel like I am close to 2100 RPM and seem to have alot left. Is the gear ratio on the axle stamped to a tag or something that I can verify?
Just when I thought it was all figured out  Undecided LOL

Thanks,
Bryan
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bryanhes
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« Reply #19 on: November 12, 2009, 09:29:31 AM »

To get the MPH to show what I think the bus would top out at I think I must have an old Mopar 3.23 gear ratio! LOL!   Grin Cheesy Cheesy.

Bryan
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bryanhes
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« Reply #20 on: November 12, 2009, 09:35:48 AM »

RJ,

If the reduced speed is only by 5% I should still be able to run 75 MPH if I chose to. Not that I want to all the time anyway. I know the ole girl will do 80 MPH with no problem on GPS. I have hit that speed going up a grade on my way to JV's place in Texas. Not purposely, just did not realize I was running that fast, oop's  Roll Eyes.

Bryan
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« Reply #21 on: November 12, 2009, 09:57:46 AM »

See the assembly number found on the transmission nameplate. Look up the assembly number in the Allison parts manual, which will list it as an A or B group code. Group code A is the 0.875:1 ratio and group code B is the 1.04:1 ratio."

I looked at my nameplate just out of curosity the numbers are as follows      Serial # 0510015641
                                                                                                       Part# 6882760 80 D 26
                                                                                                       Model # V730 D

I don't know what the "D " is but it is after "A" and "B" so assuming there was a "C" 

Interesting
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68 5303 Fishbowl 40' x 102"6V92 V730  PS, Air shift  4:10
1996 MCI 102 D3 donor bus for parts, ( maybe Smiley )
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« Reply #22 on: November 12, 2009, 11:46:58 AM »

Paso One,

I was refering to the rear axle ratio. Figured there must be an ID Tag that will list the ratio. But the V730 info may come in handy as well.

Thanks,
Bryan
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