I recently purchased a 1991 U300 with a 6V92TA that's supposed to have an Allison 4 speed in it. I've made several long road trips with it, including from Tennessee to Maine just now. On the trip I noticed that the Allison has a gear in between 3rd and top gear. Is it possible that I have a 5 speed instead of a 4 speed? Does the 4 speed have an optional overdrive or something? or perhaps a two speed torque converter? or perhaps a two speed differential? This Allison has a hydraulic retarder... I'm just trying to explain to myself how it can go from 1500 rpm at 60 mph to 1800 rpm when climbing a hill, until I press the third gear button, then it downshifts again to 2000 rpm... By the way, this thing is quite the hill climbing machine through the Appalachians. I set the cruise control to 60 mph and passed all the loaded semis going up the hills and all the other motor homes too. Going down the other side they all passed me because the speed limit was 70 mph in most places, but I only do 60 mph. The thing got 10 mpg and made it all the way from Tennessee to Maine on one tank of diesel! (laugh)... I was VERY pleasantly surprised. Thank you Foretravel, Detroit Diesel, and Allison for building such an awesome machine :)
Scott
You are feeling the lockup of the torque converter, sometimes it feels like a full gear, you have the 4 speed I bet.
Dave M
Like Dave says, you have a 4 speed that goes into torque converter lockup half way through second gear so it shifts 4 times like a 5 speed. I thought the same thing the day we bought ours.
Important to know the lockup speed to plan a steep road decent so you don't get a shock if it goes into lockup. Has more power and the transmission runs a lot cooler locked up.
Yes, 60 MPH gives a good 10 MPG, even a little more if it's all flat.
Pierce
Tell me more about lockup.
Of late I've been annoyed by the tranny downshifting when I don't want it to (for example, shifting from 5th to 4th at 50 mph) and then heating up in 4th.
best, paul
Paul,
Most all late automatics have torque converter lockup now. This allows a direct connection between the transmission input from the crankshaft and the part of the transmission with all the gears when locked up. Most of the heat generated in automatics comes rapid movement of the transmission fluid around the close fitting paddle/turbine looking parts inside the converter. Without lockup, the engine side of the torque converter turns more rapidly than the transmission side generating friction/heat. This heat is reduced by an exchanger that gets rid of part of the excess into the engine coolant so both temps run warmer in city driving where the lower gears and lots of starts from a dead stop occur.
VW Syncro Vanagons used a special fluid that instantly turned into a solid when an electrical current was applied. Why it worked was unknown at the time it was introduced.
Not sure when your 6 speed Allison goes into lockup but the downshifting from 6th to 5th or 4th is a function of engine loading and computer programming. Translation: it's a pain in the rear not to be able to keep the transmission in a higher gear at a lower speed when loafing along sightseeing. U300s shift into or out of 4th (high gear) at 48 MPH so we have to put up with more RPM and use more fuel where a manual transmission lets the driver control the shift points. Automatics do keep a low time driver from damaging the engine or with a Detroit 2 cycle, keeps the driver from accidentally causing the engine to run backwards if you stall it leaving stop signs.
So, summing it all up: Automatic transmission lockup lowers transmission and engine temperatures extending component life, gives more power to the rear wheels, lowers RPM and increases fuel mileage. Not a bad deal.
Pierce
Paul,
If the tranny is coming out of lock up, it means you're asking for more torque than the tranny computer wants to allow in lock up. Your only choices are to either ease up on the accelerator, or manually shift to a lower gear.
On Paul's 4000 series Allison 6 speed, the torque converter locks at higher RPM in 2nd gear-- it feels like a mini-shift. It is always locked in all higher gears. It will not unlock in higher gears if functioning as it should.
My concern is going down hill. No pressure on the accelerator. The downshift from 5th to 4th at 50mph, or 4th to 3rd seems to me to increase the tranny temp.
best, paul
On my Silverleaf going downhill with not fuel I do not see the gear shift till I hit the retarder or manually downshift it. Maybe I am doing it faster than the auto transmission will but I usually manually downshift into whatever gear I finished the climb in. That is usually 4th. I am hot from the pull and it takes a bit to start to cool down. The tranny is locked on the way up I think and I will annually downshift as needed to rep the RPMs up. The OP has a 4 speed and a lighter unit than mine. I will run just about 8 mpg on the same trip. A bit more if I am running 55-60 a bit less if I am running 65-70. So, it costs me a mpg but on a long trip I can cover over 100 extra miles in a day. Running cross country that saves me a day at the cst of extra fuel.
I was sorta shocked at the hill climbing performance and the fuel efficiency of the two stroke Detroit Diesel with the four speed Allison. I've heard people rave about the two stroke engines ever since I was a kid and my dad was a diesel mechanic. I've always wanted a two stroke but I thought people were exaggerating as usual. The thing literally passed every other motor home and every loaded semi going up the Appalachian mountains with the cruise control set at 60 mph... and it got 10 mpg... I dunno, I just wasn't expecting that. Sometimes other people with motor homes come over to ask me about the Foretravel and they always ask what I get for mileage. Most of them have Cummins diesels and they usually say they're getting about 8.5 mpg, but they're all towing a car and they probably drive 70 mph, and they all have flat fronted motor homes nowadays. I presume the angled windshield of the U300 must make it a bit more aerodynamic than the newer front entry motor homes... still, I always thought two stroke engines were less fuel efficient than four strokes so I was pleasantly surprised... Another nice lesson I learned on this trip is the value of getting off the interstate, even by just a couple of miles, to find a waaay better fuel price. I'm not towing a car, and my motor home is "only" 36' so I can get to the diesel pumps (with difficulty) in supermarket gas stations and fuel oil distributors and other places where you can save at least $50 on a hundred gallon fillup. With a 150 gallon tank I can easily cruise a thousand miles before I have to maneuver through another obstacle course to get inexpensive fuel again... Filling up at a truck stop on the interstate is a ripoff by comparison and I think I'll do the obstacle course from now on... as long as I don't hit anything... cuz that would cost a lot more than the diesel!
Paul,
Coasting should NOT raise transmission temperatures. Of course, use of the retarder going down a hill certainly WILL.
Brett
RRadio,
To be super accurate on your mileage figures, first level the coach and then stick the tank with a stick, welding rod, tape measure, etc. The tank on your U300 is 25 inches tall and each inch off the bottom is exactly 6.00 gallons. A steady hand and good reading glasses will get you within a gallon. Just measure how far the liquid is up the stick after it hits the bottom of the tank and multiply the inches by 6.
The biggest diesel engine in the world is a 2 cycle in a container ship and is the only diesel engine to exceed 50% efficiency.
The lower Foretravel Uxxx models with the B Cummins engines will get even better mileage. Lighter, less drag, etc. Don't expect 10 mpg in the Rockies. A trip up Highway 1 last week on the California coast saw 7.8 MPG for the trip but probably less than 5 MPG along that road. Lots of weight with constant ups and downs along with good horsepower burns a lot of diesel. I keep ours about 60 mph and keep 110 psi cold in all six.
Pierce
'93 U300/36
I did stick the tank before and after I filled it at the beginning and end of this trip, and I also looked in with a flashlight to see if I was filling the tank to the same level, which was to the bottom of the filler neck. I'm pretty sure I really did get 10 mpg. A few more long trips will confirm it for me. Do you have a 6V92TA also?... and you sometimes get over 10 mpg on level ground?... I've never heard of any class A motor home getting even 10 mpg, but everyone I know is towing a car and they probably drive 70 mph, and they all have newer front entry motor homes with flat noses, which probably makes them less aerodynamic at highway speed.
The placard by the driver's seat says to run all six tires at 90 psi, but the previous owner was running them quite a bit higher than that. He told me he reduced the pressure a little bit because it rode so rough. When I first got the coach it had over 95 psi in all six tires. Before I started this trip it had 92 psi in all six. I was gonna ask Foretravel if they have a reason for running the tires at 90 psi or if it's okay to increase the pressure... if it will be any advantage that is? I'm not even fully loaded now I'm sure, so the tires really don't "need" maximum pressure in them. Have you experimented with different air pressures and seen any difference?
My 1994 U300 6v92 4 speed has only broke 7 MPG one time running on flat land. Most often between 5.5 MPG to 6.5 MPG running 60 to 70 MPH. I do carry a full load and pull a 2006 Dakota Club Cab 4x4. ::)
Sure. But why does retarder use raise tranny temps so more than retarder temps?
best, paul
Weigh your coach. It's about $9 to weigh it at a truck stop. You may be able to weigh it at a grain elevator, metal recycling yard, etc. Get weights for the front and rear axles. If you can get weights per side, that would be helpful. The manufacturer of your tires will likely have a web site with a table showing appropriate pressures for the your tires and weights. You should know your tire size, model, loading, and axle weight to find the proper pressure. The listed pressure will be for a cold tire at the lowest ambient temperature you expect to encounter. Higher pressures are OK up to the maximum rating for the tires and wheels.
I think Brett's U240 has averaged over 11 mpg all the time he has owned it. Aero drag kills mileage on any vehicle and is the largest factor affecting mileage other than topo changes. At lower speeds, the air/fuel ratio a diesel runs increases with a lot more oxygen molecules available for the amount of diesel sprayed (injected) so they are much more efficient than a gasoline powered RV especially in part throttle driving like in cities.
RV, cars, tire manufactures all placard tire pressures for reasonable comfort not best performance, fuel mileage or wear. Have always run my cars, RVs, fire dept apparatus at the max. listed cold pressure on the side of the tire. All bus companies do the same. Imagine the same for trucking companies. The bad highways here in California do sometime require a tighter seat belt. Higher pressures do give a firmer ride. On the other hand, brands with the really flexible sidewall radials move around a lot less at the higher pressures and give a more secure feel in side drafts or turns.
My 4107 Buffalo bus (35 feet) had a fairly flat nose but got even better mileage than our U300. Same size engine (8V-71 Detroit) but over one hundred HP less, 225 vs 350 HP. It was a bit lighter with a high percentage aluminum and full monocoque construction.
Yes, I have the same engine as yours. I met another 6V-92TA owner that had averaged 7.9 mpg (w/toad) in the five years they owned it with a lot of time in the high country.
Not long ago, the Union Pacific Railroad made the largest order in the history of railroads with EMD (Caterpillar) 2 cycle engines. Most of their routes are here in the west. Amtrak on the other hand uses GE (General Electric) 4 cycle diesels in the east. On upgrades, the 4 cycles will make a odd "chuff-chuff" and are easy to tell from the 2 cycles for railroad fans. Both have their pluses and minuses.
To me, they all missed the boat in not adopting the twin crankshaft, opposing piston 2 cycle diesel with no cylinder head, no head gaskets, no exhaust or intake valves.
Pierce
Napier Deltic - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Napier_Deltic)
Pierce, I worked on the Deltic engines, in fact served my apprentiship in English Electric/Napier making the gearbox parts for them for 7yrs before leaving for Canada. They were a very respected unit and had tons of horsepower. Moved over to the Sundstrand Helicopter engines for a while too. Gosh that was 1960 to 1969, my how time flies
John H
I was just talking to a lady in the campground here in Maine who has a small class A with a Ford V10 gasoline engine towing a tiny Honda Fit and she said she just barely makes 10 mpg with it on level ground. I forgot to ask if she has a 6 speed transmission... she probably does... She said the performance of the gasoline V10 is terrible in the mountains. I was thinking 10 mpg was excellent fuel efficiency for a V10. She didn't seem too thrilled with her V10 and sounded like she wished she had a diesel. She asked me a lot of questions about mine. I have no buyer's remorse at this point. I keep finding new things I like about it all the time. It's nice to buy something and find out you got more than you bargained for. It doesn't happen very often in my experience! haha
I will investigate the tire pressures further. The thing that bothers me about running at the maximum air pressure is all the blowouts I used to get with my car when I ran tires at maximum pressure. I would get belt edge separations, especially on the front tires, then the tread would strip off and the tire would blow out a few seconds later. It happened several times until I lowered the air pressure, then it never happened again... Hopefully truck tires are better built than car tires. I do NOT want to change a tire on this motor home beside the highway with trucks roaring past me just inches away.