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Old 06-17-2017, 01:44 AM   #1
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Overdrive on a Crown Supercoach: How High is High Enough?

In reading the manual on my Fuller T-905 transmission I was sort of amazed to see that it can be rebuilt as an overdrive unit, which would change 5th gear from 1.00 to somewhere around 0.65.

I say somewhere around 0.65 because I have the M gear set, and while the M is listed, it is not listed in the table on overdrive ratios.

And amazed because that's what I love about nice, old, rebuildable, versatile machines.

Which brings me to the real question, what's the right transmission for my bus? Figure I'm optimizing for 1) fuel efficiency and 2) fun and easy to drive anywhere.

The truth is, the jump from 1st two second is about 1,000 RPM, which is too big in my opinion. Going up any kind of grade it's hard to get to second gear before the vehicle slows down too far.

The rest are 500 RPM apart, and she goes 62 mph at 2,000 rpm in fifth. If I rebuilt to an overdrive at 0.65 then I'd be down at 1250 RPM at 62 mph. But would she cruise at 62mph@1250RPM. Maybe because I'm nearly empty and not hauling a few thousand pounds of human flesh.

I'd been thinking a ten speed was the answer, but the real question is which ten speeds?

Another night I'll sit down and do the math. Here are the ratios for the deeply curious.

[IMG][/IMG]

Welcome your thoughts,

Adam
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Old 06-17-2017, 01:45 AM   #2
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And here's speed and torque curve.

[IMG][/IMG]
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Old 06-17-2017, 02:20 AM   #3
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Here's what the numbers say:

[IMG][/IMG]

Which is to say I should get 4.7 with current gearing.

With overdrive and cruise down at 1,300 RPM (if that's even possible), I would still be at 6.9 mpg. By percentage that's a big improvement, but I was hoping for more like 10 - 12 mpg.

There's always a chance this set of curves isn't actually what I'm running.
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Old 06-17-2017, 03:17 AM   #4
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You need an RTO-910.
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Old 06-17-2017, 09:53 AM   #5
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I see now that the images don't show on phone. They do show in browser in full site though. Will debug later.


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Old 06-17-2017, 09:57 AM   #6
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The numbers you ran were all at full load. 62 MPH will be the same load, determined by frictional and aerodynamic load, say 150 HP, not full load at the various RPM points you're looking at.
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Old 06-17-2017, 09:59 AM   #7
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Rameses - RT0-910 is what I was thinking. Looks like Overdrive ratio on that is 0.81. Fuller has the length on the 910 at 28.9" which is a little longer than the 27.6" for the T-905.


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Old 06-17-2017, 10:05 AM   #8
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Ennonne - yes, I pulled those off the web. Full load is hopefully higher fuel consumption than whatever load I have at 62.

Time to get my own numbers.


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Old 06-17-2017, 04:19 PM   #9
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I went through this very sme thought process with my Crown. Rebuild the T-905M (that's what I currently have) or get the rto-10. The manual only states that the t-905a and 905b can be rebuilt to have overdrive, I'm not sure about all of the other 905 models. But, just like you, I decided a 10 speed way to go.
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Old 06-17-2017, 09:25 PM   #10
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Quote:
Originally Posted by upbeat View Post
Here's what the numbers say:

[IMG][/IMG]

Which is to say I should get 4.7 with current gearing.

With overdrive and cruise down at 1,300 RPM (if that's even possible), I would still be at 6.9 mpg. By percentage that's a big improvement, but I was hoping for more like 10 - 12 mpg.

There's always a chance this set of curves isn't actually what I'm running.
I think you should be getting a lot more than 4.7 MPG. A friend of mine's Crown tandem with a Big-Cam Cummins and RTO is getting about twice that! I don't know what differential gearing he has, bus I suspect it's higher than Crown's usual 4.1 - he can easily cruise at 75 MPH, with plenty more to spare. His was a Needles CA bus, so it probably has much higher overall gearing than most for the long runs across the desert. (It also has factory A/C!) Here's a short video of Brian test-driving it, pretty well for not having driven a 10-speed for a while:

John
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Old 06-18-2017, 12:41 AM   #11
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I think that if you put an OD into a 5-speed you will be extremely disappointed with the result.

The shift points on the 5-speed are almost too far apart already. Moving to 4th direct and 5th over means the splits in the lower gears are even further apart. And as you have noted, 1st gear is a big jump already. Moving 2nd higher will make a bad situation worse.

If you want OD your best bet would be to put in a 10- or 13- speed dual range transmission with at least one OD gear. With the dual range the low gears are close enough that most of the time you will never use 1st gear in order to get started and with each gear having a 300-500 split you will be able to skip some gears on the way up.

Because the 2-cycle DD's run the best when they are run up against the stop I would not try to run the engine too slowly. It will tend to lug a lot, which is something DD's do NOT like to do. Ideal operating range is between 1,900 and 2,100 RPM's. I personally would not try to cruise down the highway with a DD at less than 1,500 RPM's.

Most Crowns with the 6-71N/T/TAC have HP ratings between 210 HP and 300 HP. Most of the Crowns I have known regularly got 7-9 MPG out on the highway. Speed made some difference in fuel consumption but not enough to want to camp out in the slow lane.
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Old 06-18-2017, 12:54 AM   #12
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Appreciate your thoughts on the matter. I've got a Cummins 250 in this one, but can't imagine they want to cruise at less than 1500. Think you're right about ratios. Just got off the road for the afternoon and 1000rpm jump is just too far. I think the trick is going to be 10 speed and also change the differential gear so I get 300-500 rpm jumps all the way, with one or two nice low gears to crawl around and one or two OD to cruise. Will spend a while and get it planned out. Plenty to do in the meantime to get everything reliable as is. Lost power twice on the highway today due to some intermittent electrical lines. Need to get that sorted, then have an appointment for two new steer tires on Monday. Father's Day trip to the reservoir tomorrow.


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Old 06-18-2017, 12:57 AM   #13
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Quote:
Originally Posted by Iceni John View Post
I think you should be getting a lot more than 4.7 MPG. A friend of mine's Crown tandem with a Big-Cam Cummins and RTO is getting about twice that! I don't know what differential gearing he has, bus I suspect it's higher than Crown's usual 4.1 - he can easily cruise at 75 MPH, with plenty more to spare. His was a Needles CA bus, so it probably has much higher overall gearing than most for the long runs across the desert. (It also has factory A/C!) Here's a short video of Brian test-driving it, pretty well for not having driven a 10-speed for a while:

John


Great video. Curious about the benefit of the big cam. I know they came out with it in response to the oil crises. Was going to look into switching over if there's something to it. I do energy work for a living, so just can't help myself when it comes to pushing the numbers up.


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Old 06-18-2017, 08:42 AM   #14
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from ym experience and talking to people with various cummins rigs.. most all of the cummins motors (except maybe the big rig stuff).. Like to cruise in that 1800-1900 RPM range. I dont know your specific engine, but just seems thats how cummins factory builds.. which would be about right if you were crusing at 55 MPH..

something with that .8 or so overdrive seems ideal if you are looking for that 65 MPH cruise or so..

Speed does make a difference.. at least in both of my busses it does.. perhaps because im also running up RPM;s, we will see once my trans swap is finished if its speed or RPM's that reall affect MPG.. I'll have a 6th gear thats likely not feasible to run until im up in that 70 MPH range. so I can test theories of speed vs RPM when it comes to economy.

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Old 06-19-2017, 01:26 AM   #15
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Overdrive on a Crown Supercoach: How High is High Enough?

Quote:
Originally Posted by upbeat View Post
In reading the manual on my Fuller T-905 transmission I was sort of amazed to see that it can be rebuilt as an overdrive unit, which would change 5th gear from 1.00 to somewhere around 0.65.



I say somewhere around 0.65 because I have the M gear set, and while the M is listed, it is not listed in the table on overdrive ratios.



And amazed because that's what I love about nice, old, rebuildable, versatile machines.



Which brings me to the real question, what's the right transmission for my bus? Figure I'm optimizing for 1) fuel efficiency and 2) fun and easy to drive anywhere.



The truth is, the jump from 1st two second is about 1,000 RPM, which is too big in my opinion. Going up any kind of grade it's hard to get to second gear before the vehicle slows down too far.



The rest are 500 RPM apart, and she goes 62 mph at 2,000 rpm in fifth. If I rebuilt to an overdrive at 0.65 then I'd be down at 1250 RPM at 62 mph. But would she cruise at 62mph@1250RPM. Maybe because I'm nearly empty and not hauling a few thousand pounds of human flesh.



I'd been thinking a ten speed was the answer, but the real question is which ten speeds?



Another night I'll sit down and do the math. Here are the ratios for the deeply curious.



[IMG][/IMG]



Welcome your thoughts,



Adam


Adam--
Normal shift points (with top of previous gear at 2000 rpm) are 1-2 1000, 2-3 1300, 3-4 1500 and 4-5 1500. Splits- 1000/700/500/500. Our RT 910 Fuller 10 speed can be converted from direct 10th gear to 9th gear direct and .81 OD IN 10th, which would make it an RTO910 model. On the T905, it has a very deep 1-2 split, a medium split 700 from 2-3, and fairly fast splits 3-4.& 4-5 500 split. A .65 OD would hardly reachable split between 4-5 unless 4th was significantly changed as well... Once you master the slow split 1-2 of the T905, things get much easier to drive it. Our RT910 usually has 400 splits top to bottom, and 600-800 splits if you skip shift (i. e. 2-4-6amd back to 400 from 6-10th gears.unless you had 4.63 or 5.13 rear gears, a 0.65 wouldn't even be attainable to reach shifting unless you lugged it in your 4-5 shift. Your rear end is too low.. if you change the differential to 4.11s, you would do much better topspeed wise and retain drivability. I drove a mountain geared bus just like your own... 60-61 mph at 2100. It was more recently changed to a numerically lower (and thus a higher geared) rear end and it now cruises in the 70mph+ range.
Cheers,
Deanna


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Old 07-05-2017, 01:06 PM   #16
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Hi Deanna,

Thanks for this helpful and detailed post. Can you tell me more about my options for rear end gears?

Bus is currently in the shop and it looks like we're going to clean-up and re-seal the differential, so now would be a great time to figure out what's actually in there, and maybe swap for higher gearing.

Thanks,

Adam
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Old 07-06-2017, 09:04 AM   #17
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Smile It's always a tradeoff

Quote:
Originally Posted by upbeat View Post
Hi Deanna,

Thanks for this helpful and detailed post. Can you tell me more about my options for rear end gears?

Bus is currently in the shop and it looks like we're going to clean-up and re-seal the differential, so now would be a great time to figure out what's actually in there, and maybe swap for higher gearing.

Thanks,

Adam

Adam-
You're not working with a BigCam engine and honestly an RT 910 series trans is a remarkable addition. With your 250-a small cam engine...and the T905, I'd first check to see what ratio you currently have aboard. If your cruise speed is in the low 60 mph range a bump up (lower numerically) one or two would assist in highway legs and likely in fuel mileage. With a small cam Cummins, if you plan to do any climbing (mountains, long grades-Altamont/Waldo/Grapevine/Sherwin-I'd suggest you not go too deep, because you will lose lower end grunt, and will create a second gear bus in the mountains. I drove many 218 Detroits and 220 Cummins Crowns which were flatland buses but very slow in the hills due to flatland gearing. Balance is essential. Figure out where you are likely to go, and balance your Crown's power with mileage, performance and drivability for what YOU WANT to do. 4.11s might be a little too tall for your drivetrain in the hills, but if you're currently up in the 5-1 range, a shift to the mid 4s would possibly give you the balance and drivability you're seeking. Comments based on real world experience in driving and spec-ing these buses for almost 50 years. There is no PERFECT answer. There are good and workable ones.. Ask what your mechanic thinks when you find out what numbers you currently have aboard in your pumpkin.
Hope that helps.
Deanna
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