allison B500

[ruthi]

Is there a reason that someone would block out the 6th gear in a Allison B500 transmission? Would it not really be needed if the gear ratio on rear end is high? I think the gear ration is 4. 25.

[HB of CJ]

The first reason is that 6th gear would drop the engine rpm down too low for efficient operation at the envisioned practical maximum legal cruising speed.

Another reason is that the existing engine doesn't have enough power to pull 6th gear, which is almost the same as the first reason, but different also.

Another reason is that for legality, city fathers don't want high powered fire apparatus capable of going 110 miles per hour, which they might be able to.

The limiting top speed seems to be the tires which nowadays are limited to around 75 mph.  Tires blowing out from heat failure is a bad business practice.

Finally, some say 6th gear on this tranny is somewhat weak and the factory will not warrant the Allision if 6th gear is used in some applications.

The fix is a bigger, more $expensive$ Allision transmission.  I for one would want all 6 gears that are available, but this is not always possible.

Some time ago, my brother took his fire engine down to the Detroit Diesel dealer ship and asked the computed guy what he could do to increase power.

The computer guy said he could give the 8V92TA something over 600 hp AND 6th gear, but he would would have to kill himself first.  Smog laws.

[belfert]

If my 1995 and your 1998 are the same the gear ratio should be 4.10.  I was told the reason that 6th gear is locked out is because the RPM could drop too low and the B500 would just downshift to 5th.  6th gear should just be unlocked for those times when it can be used.  So what if it downshifts when it can't be used?

I talked to my local Detroit Allison dealer about unlocking 6th gear and they said it would take $600 plus a letter from an engineer at the original manufacturer.  Even if I could get the letter from MCI it didn't seem worth $600 for this.  If I gained anything on mileage it would porbably take a long time to pay back $600 for me at least.

[Hard Headed Ken]

Ruth,
I'm reasonably sure that when we checked the information in your ECM it had the differential ratio listed at 4.1. 

Ken

[TomC]

Figuring you have 12R-22.5 that are 485rpm tires (in general) and with the 4.1 ratio, with the B500, 4th gear direct your engine speed will be 1822 @ 55, 1988 @ 60, 2154 @ 65.  In 5th gear, which is a .74 overdrive, your engine speed will be 1349 @ 55, 1471 @ 60, 1594 @ 65, 1716 @ 70, 1839 @ 75, 1962 @ 80. In 6th gear, which is a .64 overdrive. your engine speed will be 1272 @ 60, 1378 @ 65, 1484 @ 70, 1590 @ 75, 1696 @ 80, 1802 @ 85, 1908 @ 90, 2015 @ 95, 2121 @ 100.  If you have an electronic 8V-92TA, you could probably use the 6th gear, but otherwise, I wouldn't worry about it.  At least your drive train speed will be OK.  At Freightliner, we figure a maximum driveline speed of 3,200rpm, which is at 96mph.  Using the 6th speed would be perfect for a 4 stroke engine.

It is interesting to note that in an effort to get more MPG out of the engines, Allison on the 4000 series truck transmissions used with the DD engines have them shifting at 1600rpm and into 6th gear dropping to 1100rpm on light throttle.  Allison is slowly getting more confident with their transmissions.  I remember when the HD4060 first came out-it was restricted to 430hp and 1450lb/ft torque.  Now the 4000HS has a 560hp rating and 1850lb/ft torque (in gears 3-6).  Good Luck, TomC

[ruthi]

Thanks Ya'll 

[blue_goose]

There is no reason that you can't change it to the 6 speed.  YOu will be fine with the 4.10 rear, if it needs 5th it will go there.  Give Ted Keating a call at 267-716-7332 or 800-997-4858.  He is the man to talk with about your world trans.  Ted will reprogram your transmission for a lot less than anyone with Detroit.
Jack

[buswarrior]

Minimum input RPM for 6th gear for that specific model year of B500?

They were often paired up with 4.56 gears.

Somebody knows....

happy coaching!
buswarrior

[TomC]

On the older B500 and HD4060, 1400rpm was typically the lowest rpm allowed in 6th gear-which with your 4.56 gear ratio and 485rpm 12R-22.5 tires works out to be 59mph.  Good Luck, TomC

[ruthi]

Actually, 4.1 is the ratio.

[TomC]

That works out to be 65.5mph at 1400rpm.  Good Luck, TomC

[Hard Headed Ken]

Tomc, do you think 6th gear unlocked will be of any value on Ruthi's Dina? I remember reading somewhere that the fuel efficiency peak for the 12.7 is at 1400 RPM. It looks like peak torque is at 1200 and remains reasonably flat to about 1550. I have another reason for asking. The transmission gear set I'm using should put's me about the same. .74 top gear ratio, 3.36 final drive and 315/80 22.5 tires. I come up with about 1400 RPM at 70 MPH. Is that close?

Thanks,
Ken

[belfert]

That works out to be 65.5mph at 1400rpm.  Good Luck, TomC

My tires are 11R24.5 480 RPM and I have a 4.10 rear end.  Would I still be around 1400 RPM at 65 MPH with a B500?  It might be worth giving Ted Keating a call if that is the case.

[TomC]

Personally-I would just leave the 5th gear top in place-I don't think there will be much of an improvement using 6th gear.

As to using 315/80R-22.5 tires that are 492rpm with 3.36 gearing.  In 4th direct you'll be at 1653 @ 60, 1790 @ 65, 1928 @ 70, 2066 @ 75. In 5th overdrive of .74, you'll be at 1427 @ 70, 1529 @ 75.  I don't think 6th would work.

11R24.5 tires at 479rpm and 4.1 ratio works out to be-1963 @ 60-which is very close to the prementioned speed figurings.  Good Luck, TomC

[NEO/Russ]

If anyone needs a new ECU for a B500 I have one.  I had it for a repower I was doing, but sold the chassis.  New in '03, never used.  Contact me direct through my email on my profile if you need one, or a new accumulator, used oil to oil cooler all for S60 and B500 and we can talk.

Russ