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Stall speed of torque converter in Allison bus transmission?

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I'm going to present just a very simplified technical discussion FIRST, and then ask my quesiton so readers can understand WHY I am asking the question.

Most (not all!) "automatic" transmissions use a torque converter along with the transmission itself. In our Eagle buses, those of us who have automatic transmissions all have one model or another of an Allison transmission, and all have a torwue converter. This serves at least 2 important needs:

1. The bus can be at a stop with the engine running and in gear, without stalling the engine

2. The torque converter is designed to "slip" at low engine rpm, allowing the engine to "wind up" a bit to a higher rpm despite the slow, or even "zero mph" current vehicle speed.

This second function is important because it enables two really good things:

1. The engine is allowed to spin faster than it otherwise would for the current vehicle speed. This increases the power available, since most diesel engines have either flat or INCREASING torque curves at low rpm, and power produced = Torque x RPM / 5250. So, the greater the rpm, the greater the available power

2. When the torque converter "slips", i.e. when its rpm does not match the engine's rpm despite their 1 to 1 nominal coupling, it multiples the engine's crankshaft torque by a lot - up to about 3 times - depending upon variables of torque converter design. This enables a bus to apply more torque, and thus power, to get moving on uphills, brake out of mud, etc. The only negative associated with this is that slipping produces a lot of HEAT, so if the torque converter is allowed to slip too long, the entire torque converter and transmission assembly overheats and then self-destructs. So, moderation is key.

Above a certain engine rpm, which again depends upon the design of the torque converter and the engine torque, there is no more "slip". The torque converter rpm = engine rpm for that engine rpm and all higher engine rpms. That specific rpm is called the "stall speed" of the torque converter. This is admittedly simplified a bit, as the more technical readers will know.

Now to get to the reason for my question:

I have a really good, proven performance computer model that I have used for many years when modeling vehicle drivetrain performance for cars, trucks, and motorcycles. It generally gives me pretty accurate results, and is very helpful when trying to optimize things like rear axle gearing, and engine dyno curves for given circumstances. However, all the accumulated historical data in it is for cars, trucks, and motorcycles, not buses.

I do have torque converter stall speeds for many different vehicles I have modeled over the past couple of decades in it, but the stall speeds for cars and trucks, especially for the PERFORMANCE cars and trucks I have modeled and optimized, are radically different than the ones for buses! To give you just one example, the stall speed for a factory stock Chevrolet SSR retro pickup was about 1800 rpm, which is pathetically low for a performance vehicle, especially a vehicle that is HEAVY (4800 lb or so in the case of the SSR) with only a 5.3 liter gasoline engine. The souped up SSR ended up with a stall speed that was much higher - too high in fact (because the engine's final power output exceeded early expectations by a lot): about 3700 rpm. Its pretty typical for performance car (gasoline) engines, whose useful power ranges often run from about 2000 rpm to about 6000 rpm, to have converter stall speeds in the 2200 to 3000 rpm range.

However, a bus spends most of its cruising time at 1500 to 1900 rpm (with the lower end of the range being many of the 4-stroke engines, and the higher end being the 2-stroke Detroit Diesel engines like in early Eagles). PEAK engine power is often restricted via the governor versus any obvious airflow or mechanical limitations) - the power is restricted simply by the governor mechanism limiting the engine rpm - and is often in the 1800 to 2100 rpm range. This is a MUCH lower rpm than for a typical gasoline engine car or truck, and so obviously the torque converter stall speed needs to be different as well.

Not knowing the stall speed is a problem for the modeling software. Without knowing it, it cannot accurately model the performance at low rpm.

Does anyone out there know what stall speeds are egnerally used on bus transmissions? Specifically, at least at first, what stall speeed would probably apply to the troque converter in my 1979 Eagle with 8V71N engine, which originally came from the factory with a manual transmission, but was retrofitted later with an Allison HT740? I say"probably" because I suspect that not all HT740 transmissions came with the exact same stall speed, but I am hoping that if someone put the HT740 in there, he or she had the good sense to select a BUS model HT740, versus one used in say a bulldozer. If he or she did not, obviously my little modeling experiment will have limited accuracy at lower rpm.

ANYONE out there have access to this kind of information?

If I can get the information, I can treat you all to a "virtual" (on the computer) 1/4 mile accceleration run by a 1979 Eagle conversion bus! THAT should be fun . . . :D
Jim Gnitecki
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1979 Eagle Model 05 reborn around 1997
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I forgot to mention:

TYPICALLY, if you floor the throttle on a vehicle, from a dead stop, the "flash" reading that you see on the tachometer is at least close to the stall speed, although it DOES vary due to variables like the weight of the vehicle. So, if any of you recall what the tachometer flashes up to when you fllor it froma dead stop, that would at least give me a number.

While it might or might not be accurate for MY bus, it's a better starting point than what I currently have, which is NO data.
Jim Gnitecki
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Jim,

I am doing a little research on the torque converter stall speeds. There are several different stall speeds depending on the horse power and torque of your engine. What is the hp & torque on your 8V71N engine?

You are right about bus model transmissions. They have a smoother shifting which is accomplished through modification of the valve bodies in the transmission. If anyone is installing an automatic transmission, make sure it came out of a bus and not a dump truck or make sure it is set up as a bus model.
Daniel Lenz
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Jim, the stall speed on a 740 is 1200 rpm foward and 1200 rpm reverse, the 740 is the same transmission in a fire truck,highway truck or bus just different shift points and trimmer adjustments an easy adjustment to make.

good luck

Clifford
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Thank-you both!

Smoothjazz, my 8V71 started out as a 280 hp. The governor has evidently been raised somewhere along the way, as even with the 3.73 gearing and 478 rev/mile 11R24.5 Toyo tires, it is more than happy to run at 80 to 85 mph when I get distracted from policing the speed. I calculated that it could not do that if it was still governed to 2100 rpm, as 85 mph requires 2526 engine rpm.

I spoke to the former owner today. He is not very technical. He had it modified by a Detroit Diesel expert he knew after he complained he wanted a bit more power, but did not ask what the man did to it. I have found invoices that show replacement injectors that are "N65" size, and he told me the service provider "did a lot of work with governor weights and the rack."

The dealer who sold me the bus described the engine as a "318". I suppsoe he means it was retrofitted to 318 specs?

The Detroit Diesel tables and graphs available to me are not conclusive as to what happens with the torque curve as engine rpm is raised past the original 2100 rpm governro setting. SOME of the data suggests that engines spun to 2500 versus 2100, without injector changes, make about 13% more power, which would get a 318 to almost 360 hp. My own more pessimistic projections of the falling torque curve suggest more like 338 hp or less.

My bus, with its roof raised 8 inches over factory, has a measured frontal cross sectional area of 93 square feet, taking into account the actual frontal area measured via counting squares in a properly setup and scaled photo, and adding for the roof projections, mirrors, and tire frontal areas exposed below the chassis. My bus is, per the dealer, lowered about 2" versus stock on its ride height. I'm puzzled a bit by its ability to hit 85 mph. That seems a bit TOO fast. Data from Caterilalr suggests that a coach of 90 sq ft frontal area and about the weight of my 34,180 pound bus should take 305 hp to cruise at 85 mph PLUS the power driverted to power the cooling fan. I estimated that power for the fan to be 43 hp based on the Cat data, but maybe it is less for the DD 8V71?

I did check the speedometer versus Intestate mileage markers, in segments that were either 5 miles or 10 miles in length, at 72 mph (for ease of calculation via mind versus caculator while driving), and the error was negligble (the time per mile drifted a bit due to driver error probbably, but averaged 50 seconds).

I'm trying to build a decent bus model, but am still missing some pieces and some verifications for other pieces.
Jim Gnitecki
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1979 Eagle Model 05 reborn around 1997
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Jim,

As you almost certainly know, the Allison MT 740 transmission was designed for use with Diesel engines up to 445 nhp (332 kW). Net input power (max) = 445 HP (332 kW); Input speed range = 1900-2400 RPM; Net input torque (max) = 1435 lb ft (1946 Nm).

Worth mentioning, though, is that the Allison MT 740 has a three-element torque converter (single-stage, 3-element, polyphase) which provides smooth, shock-free operation. Moreover, a choice of torque converters permits matching the HT 740 to a wide variety of engine and vehicle applications.

Available stall torque ratios are:

TC 487- 1.50
TC 488- 1.70
TC 497- 2.72
TC 470- 2.83
TC 495- 2.39
TC 496- 1.78
TC 498- 2.45
TC 499- 1.91

I would contact the local Allison distributor to obtain the data and information for a particular application. They can run a SCAAN (for System for Computerized Application ANalysis) quickly and accurately through their computer terminal. It helps taking the guesswork out of bus re-power decisions and this way you can be sure that you get the most efficient engine/transmission configuration for your coach.

Chris
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Heres one for you Jim I have a FS 740 behind my 500 HP 8v92 ever hear of a FS Allison not very many made, they were made for the 8V92 sliver fuel saver engines good luck

Clifford
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luvrbus
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chrisber wrote:Jim,

I would contact the local Allison distributor to obtain the data and information for a particular application. They can run a SCAAN (for System for Computerized Application ANalysis) quickly and accurately through their computer terminal. It helps taking the guesswork out of bus re-power decisions and this way you can be sure that you get the most efficient engine/transmission configuration for your coach.

Chris


Chris, I was aware of the SCAAN capability, BUT I believe it is only a computer program that the Allison dealer runs to determine what configuration of Allison transmission and torque converter to offer to you when selling you one, or when checking to see if a tranny setup from one vehicle could be successfully used in a different one, or what changes might be required.

I don't believe there is any associated capability to determine what is already within a vehicle, without checking physically, which could also involve some disassembly.

Also, I don't think an Allison dealer is going to be keen to do anything for free since there is no potential sale involved (I certainly would not blame him).

I'm hoping someone is going to be able to reply with this posting and say soemthibng like: "The stall speed for a bus applicaiton is almost always xxxx rpm (as an earlier response to this posting seems to say)" OR "There is a tag on every torque converter that, once read, gives you enough info for an Allison dealer to tell you ALL about it". :)

Jim G
Jim Gnitecki
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1979 Eagle Model 05 reborn around 1997
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luvrbus wrote:Heres one for you Jim I have a FS 740 behind my 500 HP 8v92 ever hear of a FS Allison not very many made, they were made for the 8V92 sliver fuel saver engines good luck

Clifford


Clifford: Combining your reply with Chrisber's, we can probably see why a variant model of the HT740 was needed for your engine: the 500 hp rating of your engine, which exceeds the recommended hp capability of the normal 740.

Building a variant was the RIGHT response. Not all transmission suppliers practice "right responses"!

In the case of the Chevrolet SSR retro pickup, a vehicle I am very familair with as I wrote a 320 page best seller e-book on it, Chevrolet dropped in the 400 hp Corvette LS2 engine in the 2005 model year in order to address critics' complaints that the heavy truck needed more power. Unfortunately, they offered it with either 6-speed manual Tremec transmission and heavy duty rear axle, OR the wimpy 4L65E automatic transmission and standard rear axle. This engine made more torque than the automatic transmission and standard rear axle could handle, and also generated shock loads on DOWNshifts with the 6-speed manual that caused problems with even the Tremec and heavy duty rear axle.

GM "addressed" both problems by using the powertrain computer to REDUCE engine torque, and applied that computer software patch quietly, without telling the owners of the vehicles, when the vehicles were being serviced! No kidding. The more astute owners who had bought the optional auxiliary gauge package that included a funky "torquemeter" could actually SEE the reduction in peak torque via the gauge once alerted to look for it!

And we wonder how GM could degenerate to the point of bankruptcy?
Jim Gnitecki
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1979 Eagle Model 05 reborn around 1997
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Chrisber: You meant "HT740", not "MT740", right?
Jim Gnitecki
("Jim G")
JimGnitecki@gmail.com
1979 Eagle Model 05 reborn around 1997
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