8v92 with a six speed Allison

[lhwood]

Has anyone tried it?  I'm toying with the idea of putting a modern 6-speed Allison behind my DDEC 2 8v92 powered 102-c3. 

I currently have a HT741.  The B500 is a bit shorter so I'll have room to make the switch.  My goal is to have more available gears for take-off and hill climbing.  It's likely an intermediate step along the way to an engine/tranny swap to a Cummins M11/B500.

Thanks in advance!

Durwin

[Van]

Many factors involved here. Hp/torque, gearing. Last time I did a swap to a 60 series w/6spdOD we installed a 453 ring gear. Why not go with a 754 Allison.

[lhwood]

I've thought of going to a 5-speed but the low and high gear ratios are the same as the 4-speed.  That's really not what I'm after as I'd like to drop the cruise rpm's a bit and also get a lower first gear.  The only way I see to do that is to go with an overdrive transmission and a gear change.  I've got the hp/torque to cruise at a lower rpm.

[buswarrior]

Which differential ratio do your have now?

You likely will only use 5th gear.

The first of the S60/B500 came with 4.56 differentials, geared for power, not economy, and turned somewhere 1700 on the fleet limited highway.

The trick is satisfying the transmission electronic inputs.

Keep digging!

Happy coaching!
Buswarrior

[TomC]

The four gears of the HT741 and the first four gears of the B500 are identical. So all you're going to get is two overdrives which are .74 and .64. What rear end ratio do you have now? If you're at 65mph now at 1800 when it shifts to 5th you'll be at 1,332-which is a bit low for the 8V-92. If you're at 70mph at 2100 and then shifts to 5th you'll be at 1554-which is good. 6th probably won't be usable till 80mph. That's why the rear end ratio for HT740 (3.55) is different than B500 (4.56). Unless you also change you're rear end ratio, changing to B500 won't do anything for the first 4 gears since they are the same as you're present HT741. Good Luck, TomC

[lhwood]

I realize that re-gearing the rear end will be necessary.  I currently have 3:21's and at 70mph the engine is spinning a tad over 1800rpms (according to the DDEC and GPS).  It seems that the engine has plenty of torque to move the load at a lower rpm and was shooting for 1550-1600rpms at 70.  With that, a drop from 6th to 5th would put me back at 1800rpms at 70 if needed.  The change in rear end would then give me a lower first as well as hopefully a bit better economy.

[chessie4905]

Don't count on spending all that money will gain enough fuel economy to be worth it. You are still moving the same amount of weight. And 2 cycle Detroits arent known for fuel economy. Cruising at 62 to 67 will make a difference.

[lhwood]

Understood.  Like I said earlier, it is an easy first step on my journey to an engine/tranny swap.  If I like the results fine, and we'll stop with the tranny, if not that part is taken care of and we'll go ahead and do the engine.  The cost will be the parts as I'll do the labor myself and we have the bus on the road a bit more than average so will recoup the $ quicker than some.  As for mileage, I can't find any real world numbers on how much gearing and rpms affect the 2-strokes.  4-strokes can give up significant efficiency by running outside of their "sweet" spot. 

Thanks for the input.

[Busted Knuckle]

I realize that re-gearing the rear end will be necessary.  I currently have 3:21's and at 70mph the engine is spinning a tad over 1800rpms (according to the DDEC and GPS).  It seems that the engine has plenty of torque to move the load at a lower rpm and was shooting for 1550-1600rpms at 70.  With that, a drop from 6th to 5th would put me back at 1800rpms at 70 if needed.  The change in rear end would then give me a lower first as well as hopefully a bit better economy.

3:21's ? I doubt it, but once I thought I was wrong and it turned out I was mistaken!
Most likely 3:33's or 3:71's.
  BK 

[lhwood]

You'll have to mark this one down then.  

3:21's ? I doubt it, but once I thought I was wrong and it turned out I was mistaken!
Most likely 3:33's or 3:71's.
  BK 

Seems like they did use 3:21's some.  My bus has them and another I was researching as a parts/donor bus also had 3.21's. 

Copied directly from MCI's build sheet and verified with the tag on the carrier housing: "REAR AXLE ! ROCKWELL !61143WX-74 !NKA91077131
DIFF. CARRIER (STATE RATIO) ! ROCKWELL !3.21:1 !NKC91071686

Regards,

[buswarrior]

If you are turning 1800 now, i don't know that 1600 is going to get you anything more in economy?

The old timers here had some harder to find charts for those engines. IIRC, you're in the sweet range now. And for the 2 stroke, the sweet range is wider than the 4 strokes.

Still pushing the same wind and carrying the same load. If you were spinning north of 1900, yes there'd be a bit better economy, but your 3.21 is already doing what you need, vs the old standby 3.7, and the old economy 3.33.

By all means, this is a fun exercise, change out stuff and have fun, but don't be expecting any fuel savings out of it, your 3.21 has already got you almost as far as you can go now.

A matched 4 stroke/transmission geared up right won't get you much better than what you burn now.

Happy coaching!
Buswarrior

[lhwood]

buswarrior, you may well be correct on the efficiency capabilities of a 2-stroke.  The 4 strokes are able to best 5.25mpg by 25-35% in RV and Semi applications. From what I can tell 6.5-7 isn't unheard of with Series 60 engines and the M11 or ISM will do better than that.  My usage and that mpg difference could pay off the parts and pieces fairly readily.  We intend to keep this bus for a while so don't mind putting some sweat in to a swap.  The conversion is nice and well made and the bus is in excellent condition with around 200K on it.  The only detraction is the engine/tranny and typical 2-stroke drinking habit.

As for "Happy coaching", we are loving the ride and quality of a bus conversion.   We've owned "Shirley" for a bit over a year and have added almost 20K miles to her clock. 

Regards,

[luvrbus]

I am looking for 3:21 ratio if you decide to sell one and the one I am replacing is a 4:56 ratio MCI called it a mountain gear ratio in the MCI 9 with a N/A 8v71,the 8v92 TA does not like 740  with 4:56 gears for sure.You will need to upgrade to a DDEC lll for the WT B500 or program it for a stand alone application     

[lhwood]

Luvrbus,  I'm a little unclear on the rear end gear sets.  Are they standard heavy truck parts or do they have specific models for "pusher" axles?  Also, what am I missing with ddecII that the III gives me?

regards,

[TomC]

If you want to aim for 1800 at 80mph, then we divide by .64 to get 2812 in direct at 80. Divide by 80 and multiply by 60 we get 2109 rpm at 60 in direct. Divide by 485 rpm for 12R-22.5 tires and we get a rear end ratio of 4.35. Good Luck, TomC