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Old 08-26-2018, 12:27 AM   #1
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Opinions on ISC gearing.

I'm planning a gearing swap from my current 7.17 gearing ratio. (It is painfully slow) My bus is a 36 ft RE bluebird high ceiling. Current weight with all the seats is around 23k. I'm not sure what my final weight will be after conversion but I want to go as light as I can.

The drive train is an ISC 260 with an allison B300. The tires are 11r22.5 (loaded radius of 19.3 inches).

From what I have read ISCs like to cruise around 1800 rpm on flats and I would like to cruise about 65 mph. I will be driving both rolling interstate and mountain roads.

Using the gearing calculator on spicers website I narrowed it down to three gear ratios depending on wether or not I can get 6th gear unlocked.

4.78 if I get 6th (.65 overdrive): 66 mph @ 1800 rpm

4.44 for 5th (.75 overdrive): 62 mph @ 1800 rpm

4.10 for 5th 67 mph @ 1800 (might be too high)

Any opinions on these ratios or my assumption of 1800 rpm cruising?

Also does anyone have similar bus size, drive train, and axle ratio thats been converted and how does it work for you?

Thanks

Ted
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Old 08-26-2018, 12:47 AM   #2
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Go for the 4.10, no one has been able to get bluebird to send them a letter to unlock 6tn gear
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Old 08-26-2018, 08:07 AM   #3
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Quote:
Originally Posted by Kubla View Post
Go for the 4.10, no one has been able to get bluebird to send them a letter to unlock 6tn gear
Thanks Kubla,

I am thinking 6th gear isn't going to happen but I'm going to have my local shop look i to it. I may want an updated shift program from allison with the new rear end.

Ted
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Old 08-26-2018, 09:47 AM   #4
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I have a very similar drivetrane in my Bluebird except 5.38 gears.

I would sure like to have a little bit longer legs on the highway. It is geared just right in town.

I would be reluctant to go as tall as 4.10 for fear of losing in town driveability.

I did find an outfit in Florida that claim to be able to reprogram our TCM's without a letter from Bluebird.
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Old 08-26-2018, 04:53 PM   #5
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Quote:
Originally Posted by PNW_Steve View Post
I have a very similar drivetrane in my Bluebird except 5.38 gears.

I would sure like to have a little bit longer legs on the highway. It is geared just right in town.

I would be reluctant to go as tall as 4.10 for fear of losing in town driveability.

I did find an outfit in Florida that claim to be able to reprogram our TCM's without a letter from Bluebird.
Thanks Steve,

I am a little worried the 4.10 could hurt mountain road performance. I'm also not sure the ISC would have enough power to hold the trans in 5th when cruising on the flats. Thats where it would be nice to be set up with the 6th gear. Better to downshift .65 to .75 than .75 to 1.

If my shop can't come up with anything I'll have to get the info for the shop you found.

Ted
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Old 08-26-2018, 07:02 PM   #6
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wow!! a 7 something rear? that's the highest number i've heard in a differential.

i don't know why, but i have never seen a pusher with a ring gear in the 4's. that said, a 260 ISC should not have any problem pushing that.

ol trunt has mentioned horsepower/grade/ gearing charts, but i've never found anything to the sort.

i have a FE 190hp 5.9. - 4.78 rear and make about 67 @25oo rpm on the flats. a 4.10 swap would be nice.
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Old 08-26-2018, 07:17 PM   #7
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And I thought mine was insane at 5.38 and only good to 45 mph!
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Old 08-26-2018, 10:22 PM   #8
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They also made a 8.17 for that axle. That would be snail pace... but a snail that could pull anything.

Ted
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Old 08-26-2018, 10:31 PM   #9
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Quote:
Originally Posted by turf View Post

ol trunt has mentioned horsepower/grade/ gearing charts, but i've never found anything to the sort
I'm going to check with my local shop and see if thier version of Powerspec has the ISC. It has a gearing calculator that lets you put all the variables in like engine tuning, GVW, frontal area, transmission gearing, and intended use economy vs power. The free version I downloaded had newer and larger engines but no isc.

Ted
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Old 08-26-2018, 10:35 PM   #10
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Quote:
Originally Posted by TJones View Post
Thanks Steve,

I am a little worried the 4.10 could hurt mountain road performance. I'm also not sure the ISC would have enough power to hold the trans in 5th when cruising on the flats. Thats where it would be nice to be set up with the 6th gear. Better to downshift .65 to .75 than .75 to 1.

If my shop can't come up with anything I'll have to get the info for the shop you found.

Ted
When thinking of gear changes, think of how long you'll spend going over mountains compared to how much time you are actually travelling down the road saving fuel with the higher geared bus. There was 2mpg difference in 10 mph between 80-70. I went across the country and am sure glad my ratio was not lower. 43 hours driving, maybe 2 of it was in mountains, Just sayin
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