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Author Topic: Drop box and diff gears  (Read 4480 times)
Ed Hackenbruch
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« Reply #30 on: October 13, 2011, 01:54:29 PM »

Paul, even though she hasn't posted here for a while, i don't think that Nellie will appreciate being called "old".  Grin         Brian,  "Real" men don't feel the need to conform to what others define as "Manly". And they don't care what those people think of them either.  Grin
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1968 MCI 5A with 8V71 and Allison MT644 transmission.  Western USA
bevans6
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« Reply #31 on: October 13, 2011, 02:04:12 PM »

Ed, where were you when I was buying a bus with a 4 speed Spicer?  That was the time for advice on personal philosophy!   Grin 
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1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
Spicer 8844 4 speed Zen meditation device
Vintage race cars -
1978 Lola T440 Formula Ford
1972 NTM MK-4 B/SR
lostagain
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« Reply #32 on: October 13, 2011, 02:21:05 PM »

The 4.11 gears (now sitting on my shop floor) were fine. It is that I was spending all day at 65 mph top speed and 2200 rpm. So now with the 3.36 it will be easier on the 6V92 at 1800 rpm, and hopefully some fuel savings. I have the HT740 so startability is not an issue with torque converter 1st gear. We used to drive MC5s all over the Canadian Rockies up some steep hills with no problems. Some had slow gears, some had the fast rear ends. I don't remember how often they replaced clutches: we just drove 'em, lol.

JC
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JC
Invermere, BC
1977 MC5C, 6V92/HT740
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« Reply #33 on: October 13, 2011, 06:08:02 PM »

The 4:11 gears are useful in metropolitan work, where top speeds might be 50 mph most days.  The 4.11's would be perfect for a bus that never got on a freeway.  On the other hand, starting on that steep approach to a light would be a lot harder with the 3.4's.

Brian

  My very first car was a 1966 Pontiac GTO. It had a 421HO, six pack carbs, 4 speed, and a 6.13:1 posi. It got around metro Duluth just swell, ease the clutch out in third from a stop on just about any hill, and if it wanted to argue you just lit em, lol. POS car but was fun while it lasted.

  I always just thought the road coaches with sticks stayed out on the highway, inner city stuff was all 2 speed autos. But yeah, if you had low gears like that and stayed in town it would make more sense. Maybe lower gears than that yet in a town like Duluth. When I was young I used to like watching the semi trucks climb Thompson Hill at 20 mph. They got a lot more power these days lol.
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Ed Hackenbruch
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« Reply #34 on: October 13, 2011, 07:53:54 PM »

 JC, i ain't no expert but i think you will see a bit better mpg and a higher top end. Smiley I also think that you will find that a 55 mph speed limit sucks because at 55 you will either be in 3rd gear up around 21-2200 rpm or in 4th at about 1500 rpm. Sad  Therefore, to keep from lugging it in 4th you need to be running about 63 mph or more. Leastwise that is what i have found with my 8v71, Allison 644, and my 3:36 gears. Smiley
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1968 MCI 5A with 8V71 and Allison MT644 transmission.  Western USA
luvrbus
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« Reply #35 on: October 13, 2011, 10:16:34 PM »

1500 rpm at 55mph doesn't cause a problem on a 92 series that is in the peak torque curve fwiw starts at 1300 rpm

good luck
« Last Edit: October 13, 2011, 10:22:16 PM by luvrbus » Logged

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GMC5303Wonder
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« Reply #36 on: October 14, 2011, 03:14:48 AM »

Sooooo not to change the topic or be a thread theif... but does anyone have a recommendation for my 1965 tdm 5303 fishbowl 6v91 with the 4spd spicer for a similar set up for hwy travel? Her top speed is about 65 but the detroit is sccreamin pretty good at that speed....
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Big Noah
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lostagain
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« Reply #37 on: October 14, 2011, 06:53:28 AM »

Ed, is your 644 ratio 1 to 1 in 4th?

The only concern I can foresee with cruising at 1500 rpm in 4th is keeping it cool on a hot day, is that is what you mean by lugging it. So I guess I could down shift to up the revs and I also have rad misters I can use.

GMC5303: assuming you have a 6V71 (not a 6V91). The 71 series doesn't mind revving on the governor all day as much as the 92 series.

Another point is the economical justification of swapping diffs is questionable at best. The diff and drop box cost me $3500. If I sell my take out 4.11 gears for half that, I still have a spare drop box. I did it "just because". I might see a little savings in fuel. And some have said that it will be easier on the 6V92, extending the intervals between overhauls. But at 10 to 15000 miles a year... I didn't do it for the return on investment. I'm just looking forward to the improvement in drivability.

JC
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JC
Invermere, BC
1977 MC5C, 6V92/HT740
Ed Hackenbruch
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« Reply #38 on: October 14, 2011, 07:27:15 AM »

JC, yes it is 1:1 in fourth.  And for optimal cooling for a 5A with a 8v71 i read somewhere that the rpm need to be in the 1700 range.  1500 rpm in any gear, (lugging), and my temp gauge will start to rise and i can see more unburned in my exhaust. As Clifford pointed out, and i hadn't noticed that you had one, your 92 series will be a little different. Smiley
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« Reply #39 on: October 14, 2011, 09:05:20 AM »


Another point is the economical justification of swapping diffs is questionable at best. I might see a little savings in fuel.
JC

  What were you getting for fuel economy with the 4.11 gears?

  If you can run 10 to 15 miles an hour faster and not burn anymore fuel, the time factor alone could be worth it. I think you made a good choice, and if you ever need that drop box youll be smilin.

  Back in the 90's there was an around the world tour for antique cars (none newer than 1955). We up met up with them going through Duluth Mn. (after they had crossed Africa, China, Alaska and Canada) and one that stood out was a 1926 Bentley. What I found interesting were the various extra mechanical bits, driveshaft, etc., that were bolted under and around the car. You likely would never need the whole box, but a few choice pieces out it could be a lifesaver far from home. I still havnt given up a trip to Alaska someday, so thats how im thinking (the worst possible problem), lol. Like bolting a good cylinder head to the bottom of the floor somewhere.
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GMC5303Wonder
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« Reply #40 on: October 14, 2011, 10:23:54 AM »

JC

That is very true......it would take a lot of miles too offset the difference in cost of fuel. Being my fish will probably only get. 5k miles or so a year ill prolly just leave er stock
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Big Noah
1965 GMC 5303
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bevans6
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« Reply #41 on: October 14, 2011, 10:40:21 AM »

I think the concept of economic justification in this hobby is an oxymoron...  You should do the upgrade if doing the upgrade will make you happy.  Happier driving the bus, more fun, spending the money doesn't make you cry, useful maintenance anyway, you always wanted  to change a diff gear and you could cross if off your "bucket list"...   Wink

Brian
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1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
Spicer 8844 4 speed Zen meditation device
Vintage race cars -
1978 Lola T440 Formula Ford
1972 NTM MK-4 B/SR
lostagain
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« Reply #42 on: October 14, 2011, 11:00:08 AM »

What were you getting for fuel economy with the 4.11 gears?

I don't know. I just fill it up when it needs it. I think around 6 or 7 to the Cnd gallon = 5 or 6 m/USgal.

I am not going to carry the spare drop box on the bus with me, but if I ever need it, I could get the neighbour to ship it to me.

JC
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JC
Invermere, BC
1977 MC5C, 6V92/HT740
luvrbus
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« Reply #43 on: October 14, 2011, 05:07:16 PM »

 Smiley
« Last Edit: October 14, 2011, 05:09:06 PM by luvrbus » Logged

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lostagain
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« Reply #44 on: October 17, 2011, 05:25:30 PM »

Finally all buttoned up, wheels back on this afternoon. And test drive.

Running great, as expected, and FAST.

This is the GREATEST improvement I have done to this bus. It now drives like it should: 40 mph in 2ND, 60 in 3rd, and 80 in 4Th (yes I tried it,  Grin) at 2200 rpm. 1800 rpm at 65 mph. And the loss of power is hardly noticeable.

I am so happy about it Smiley. Well worth the effort.

JC
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JC
Invermere, BC
1977 MC5C, 6V92/HT740
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