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Stall speed of torque converter in Allison bus transmission?

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Note also that the nominal maximum input engine rpm for the HT740 is 2400 rpm, so engines regoverned to 2500 are technically above the maximum recommended rpm for that transmission.

Since many firetrucks were regoverned to 2500, I wonder if they got a normal or modified HT740?
Jim Gnitecki
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Sorry for the blooper :oops: , I meant HT 740 -- thanks for setting the record straight. And BTW, Clifford in all probability meant HT 740FS instead of FS 740...

Anyway, to continue this thread, there is an additional tidbit I’d like to add (old guys like to tell old stories):

The HT 740FS is a transmission configuration Allison added to their line of products around 1975, I believe. The suffix FS stands for “Fuel Squeezer.”

This particular transmission was primarily designed and configured for the Detroit Diesel Corp (DDC) Fuel Squeezer engines in the TT series. The governed speed was limited to 1950 RPM and most likely saw use in trucks only.

At that time DDC also announced a 307 horsepower version of its popular 6V92 turbocharged and aftercooled truck engine for on-highway applications. Called the "Fuel Squeezer Plus 307," this engine model was designed and manufactured for trucks in the 60,000 to 90,000 lb. Gross Combined Weight (GCW) class. Hence, the 307 horsepower rating provided the trucking industry with a higher output engine for good performance while still retaining the fuel economy advantages and other improvements which characterized the Fuel Squeezer Plus line. The new model produced 307 HP at either 1,800 or 1,900 governed RPM. The user gained the advantages of good fuel economy and excellent over-the-road performance.

With a properly geared vehicle, this engine was more fuel efficient than previous non-aftercooled, lower horsepower models and had added performance. The Fuel Squeezer Plus 307 user also gained the benefits of added durability, reliability and performance built into the Fuel Squeezer Plus line. Redesigned cylinder liners, matched turbocharger, revised GM unit fuel injectors and an aftercooler were used to produce higher thermal efficiency of the engine. These improvements also led to lower exhaust emission levels and fuel economy increases of up to six percent over the original Detroit Diesel Fuel Squeezer engines.

The Fuel Squeezer engines were introduced by DDC in 1975 with an anticipated 10 to 20 percent improvement in fuel economy when compared to the turbocharged engines operating at 2,100 rpm. The Fuel Squeezer Plus engines were announced in 1978 and offered additional fuel economy benefits as well as a number of mechanical refinements.

As far as I can remember DDC also introduced an 8V92TTA Fuel Squeezer engine rated at 335 HP that could be modified to 422-435 HP.

Now, how a 500 HP 8V92 can properly perform on an Allison HT 740FS is therefore somewhat puzzling to me.

Chris
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Chris, it works good been in the bus since 1992 without problems and fwiw the tag says FS-740 good luck
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Chris: Following up on luvrbus's comments:

Manufacturers don't always take their own "ratings" as gospel. In General Motor's case for example, they put the 4L60E automatic transmission, the weaker predecessor to the slightly beefed up 4L65E, into a LOT of high power OR heavy vehicles, despite the fact that they were exceeding their own recommended maximum horsepower ratings or weight ratings for the transmission. They just figured that the occasional warranty claim by an owner who actually USES the power frequently was a net lower cost than beefing up the entire production run of transmissions.

In fact, I THINK it is the 4L60E that has been used in a bunch of 454 V8 powered motorhomes - vehicles with big torque ratings AND vehicle weights up to 22,000 pounds or more. I think some of them actually had GVWRs of 26,000 pounds! This is like pulling an Airstream trailer with your minivan. It works for a while . . .

I think in the case of these 454 motorhomes, it was purely a price point constraint. I know that when we went shopping for a new National 36 foot motorhome in 1999 or 2000, we could have bought one of those or one that was IDENTICAL except for having the diesel pusher Freightliner chassis. The price difference was, no kidding, about $20,000. Of course, once you drove both versions, you concluded that you'd better find the money for the diesel version! No comparison in performance.

Another common thing is that an original transmisisn, probably with the correct original identification info on a tag, proves inadequate for the owner's needs, and gets upgraded by an Allison dealer during a rebuild, without the reference information on it being updated. I know that when I did that hot rod Chevrolet SSR retro pickup truck project in support of the e-book I was writing about the SSR, I got the 4L60E transmission slipping very shortly after installation of the Magnuson supercharger kit and hot cam (575 crank hp!), and Greg Ducati's qizards at Phoenix Transmission rebuilt the transmission to handle the increased power. NO identifiers were put on the transmission when Phoenix did that.

Sure made a HUGE difference in performance though. Greg had it set up so tight that the tires on the 5100 pound vehicle chirped on NORMAL (not hard acceleration) shifts. At 50 mph, flooring the gas pedal broke the tires loose when the tranny hit 2nd. For safety reasons, and to limit premature when on OTHER drivleine components, I had Greg soften it a bit.

The upgraded tranny proved to be very durable. I put nearly another 20,000 miles on it before selling it to one of the e-book buyers, with nary a whimper.
Jim Gnitecki
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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".


As always, Jim, the devil is in the details and I don’t think you’ll be able to attain a clear-cut answer to that question here, since performance parameters have apparently been modified.

As I itemized via my previous post, a choice of eight (8) converters and their corresponding stall torque ratios permits matching the HT 740 to a wide variety of engines. Your transmission’s converter is in all probability set up to have one of them and is complimentary to the original performance characteristics of the DDC 8V71N and drive-train of your coach.

The correct match is usually made at the factory or in the shop of a qualified Allison distributor and tested on a dynamometer. Therefore, the part number and serial number of your transmission ought to reflect the transmission’s application and setup, including the stall torque ratio.

Honestly, I feel uncomfortable to say this again, but your Allison distributor has the necessary background information in their database to interpret the P/N and S/N of your transmission and give you the information you are looking for.

I went through exactly the same scenario when I acquired a low mileage Allison HD 4560 six-speed (from a PACCAR demo) for my DDC Series 60 engine. Luckily enough, I found an impressive shop and met exceptional people who not only identified and decoded the original configuration of my transmission, but also modified the torque converter to exactly mach the performance characteristics of my newly acquired DDC S-60 as well as the dynamic power requirements for my coach. Moreover, they thoroughly tested my transmission on their dyno. and reprogrammed the TCU to contain the most current firmware and appropriate parameter setup for the engine and drive-train.

Likewise, the friendly and down-to-earth people at Stewart and Stevenson Power in Albuquerque and El Paso answered all of my questions without hesitation and helped me understand not only Detroit Diesel DDEC III/IV idiosyncrasy, but also the pitfalls and follies associated with Allison transmission adaptation, setup, testing and installation.

Just in case you’re interested here is some contact info:

Industrial Automatic Transmission Servive, Inc.
Irving Maintenance Facility
2339 E Grauwyler Rd.
Irving, TX 75061

Telephone:
800-999-8726 (TRAN)
972-438-1406
Fax:
972-579-7466


Chris
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Chris, it works good been in the bus since 1992 without problems and fwiw the tag says FS-740 good luck


It is not my custom or habit to be other that considerate with my fellow bus enthusiasts. What’s more, I joined this board with the impression that I can voice my opinion on technical subjects and either proof or disprove a hypothesis without remorse. I also had the insane notion that some background information may actually be entertaining, but may also help others understand the many engine and transmission configurations that are available.

Perhaps I was wrong in believing that mastermind group thinking is appropriate here. That is, participants raise the bar by challenging each other to create and implement goals, brainstorm ideas, and support each other with total honesty, respect and compassion.

Whatever works for you, Clifford, or what the tag on your transmission depicts certainly isn’t questioned here. What may be of interest are the technical details on how your high-power setup was achieved or what exactly your modified transmission’s stall torque ratio is.

Chris
Christian Berlit
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Chris, please don't be offended by what Clifford said. I saw nothing bad in it. He simply said it has been working well for him in his bus and his usage.

About going to an Allison dealer: Yes, I suppsoe I could go to one, but honestly, I can't afford to spend any money right now on the transmission to improve or optimize it, and I'd feel guilty asking anyone to take time off from their income producing work just to help me gather data. So, I don't feel right going to a commercial shop because I'm not right now a paying customer, and I don't like to try to get freebies from people who need to make a living.

I suspect there is someone on the board who might be able to get me the converter stall speed if I can figure out where to look for the serial number and other identifying information on the transmission, and it is entirely possible that they might simply verify what an earlier poster has already said: he believes the stall speed was 1200 rpm for normal bus transmissions as provided by the factory.

Until I get different information, I'm just going to plug 1200 rpm into the variable field for torque converter stall speed in my software, and see what results I get. If later information shows that the stall speed is something different, I'll plug in the corrected value and re-run the simulations.

Note that the stall speed is not going to affect normal "cruising" performance, but would have a potentially notable impact on acceleration where the combination of road speed and gearing put the engine in a position to spin faster than a direct coupling would cause. THAT's where I have to be very careful in not drawing too many conlusions until the stall speed has been verified.

Note that if the power output of the engine has been increased since the torque converter was specified for it, the stall speed can be notably raised similar to what happened with my Chevrolet SSR enigne and torque converter when it produced more low and mid range power than the tranny shop had envisioned - a nominal 3000 rpm stall speed became more like 3700 or so. Since we KNOW my engine has been altered, we are working with approximaitons now to begin with.

I can't afford a dyno run right now, so I'll simply work with what I have. I may even be able to do the modeling for someone else who ahs a KNOWN configuraiton of engine and transmisison in their Eagle.

Chris, I think this discussion is really good. Give it time to develop and mature. :)
Jim Gnitecki
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Hi Chris,

I agree with Jim, Clifford just gave you his experience and nothing more. Sometimes it's hard to really grasp in what vain a response is made, as this is such a flat medium. We are all here to learn from each other, ask questions, seek answers and gain knowledge.

Please continue to ask away, don't be offended by the response, everybody here is only here to help. If I knew half as much as Clifford, I would be able to help more often. ;)

I enjoy reading and learning!

~Paul~
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The reason why I submitted a rebuttal is not because I felt offended. On the contrary, I am not easily intimidated or sensitive to off-the-cuff remarks. I just felt that one-liners do not help us understand complex commercial-grade automatic transmission issues. This includes appreciating what is involved when one begins to futz with it or when a re-power is under consideration.

How or on what levels of understanding you all pursue your hobby, is an unknown to me. So, I can only speak for myself. I have learned to be fond of Eagles and therefore love to dive into the innards of my bus only to discover that I am at another point where I know what I don’t know. It makes me a little less dangerous, I guess.

The reason why I am taking the time and making an effort to explain “Stall Torque Ratio,” is because it is one of the most misunderstood aspects of torque converter construction. Even the semantics differ as some transmission experts often call stall torque ratio “Torque Multiplier.”

Here is what I have learned:

The stall torque ratio is the amount of engine torque that the converter can multiply at a particular RPM. So (and by definition), stall torque ratio is when the torque converter’s turbine is at zero RPM. At this stage the converter is at maximum designed stall. This will produce a positive rotational push on the turbine and increases the torque to the input shaft of the transmission, multiplied by the designed stall torque ratio of the torque converter.

Now, let’s assume you selected torque converter configuration “TC 495” for your Allison HT 740. In this case the stall torque ratio of 2.39 would multiply 1000 lb-ft of engine torque to 2390 lb-ft of torque at the transmission input shaft.

Often, the misconception of stall torque ratio is that more must be better…

This is not always the case. Higher stall torque ratio applications typically are aimed at engines with limited low RPM engine torque (somewhat typical for a Detroit two-cycle). Yet, with high stall toque ratio converters, there are some trade-offs. What you take at one end you give up on the other. Typically, a torque converter with a high stall torque ratio, such as the TC495, will be much less efficient above its rated stall speed. There is a sacrifice in higher RPM efficiency to achieve high stall torque ratios. That lower efficiency translates into less horsepower transmitted to the tires over an RPM range.

The sticky situation with a high stall torque ratio converter is that it is only high while the vehicle is not moving. Maximum stall torque ratio occurs at wide open throttle (fuel rack) with no rotation of the transmission input shaft. As the input shaft starts to rotate with vehicle forward movement, the stall torque ratio will drop off much sooner than a converter with a lower stall torque ratio. So, in this example an Allison TC 495 converter for the HT 740 with a stall torque ratio of 2.39 would show that at the starting line, but torque multiplication would drop off much sooner than a converter with a lower stall torque ratio.

In conclusion, for my DDC S-60/HD 4560(P) setup of I chose the Allison TC541 converter configuration and stall torque ratio of 1.9 with a Kp-Factor at stall of 46.2 lb-ft [“Kp-Factor“ defines torque converter capacity. “Kp” is the ratio of converter pump speed (RPM) divided by the square of the pump torque (lb-ft)].

This particular converter with a lower stall torque ratio will multiply torque for a longer period of time than a converter with a higher stall torque ratio. It will provide an excellent match for my new engine that develops 430/470 HP @ 2100 RPM and 1650 lb-ft @ 1200 RPM as well as the drive-train (new .411 differential gear) of my coach. This configuration, I believe, will also endow me with excellent performance (for a change) and best possible fuel economy, to boot.

Here is another example: For a drag race, lower stall torque ratio is gentler on the slicks at the initial launch, but it will pull harder for the remaining 1,305 ft. of the 1/4 mile. Fewer races will be lost at the starting line from excessive wheel spin. Lower stall torque ratio will be more efficient and transmit more torque and horsepower to the tires. This translates into lower ETs and higher trap speeds!

Chris
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Great write-up, Chris! I can see that you too have visited the Yank Converters website in the past.

I would add one more dimension to the discussion: heat.

A higher stall speed torque converter generates more heat than a lower stall speed torque converter, because there is by definition more slip.

And, as you point out, the higher stall torque converter continues to have more slip with increasing rpm, unless a transmission is equipped with a "lock-up" torque converter. So, unless there is a lock-up clutch, the higher stall converter will BOTH reduce fuel mileage slightly and generate more heat.

That heat can become critical in situations like high ambient temperatures and long uphill grades.

On my Chevrolet SSR pickup project, I had a lot of auxiliary instrumentation onboard, due to the nature of the project. One of the things I noticed immediately after the installation of the higher stall speed converter was that the automatic transmission temperature being reported was notably higher than before. I'm talking 200 degrees versus 175! On really hot Texas days (105 degrees), after a burst of acceleration, the gauge would climb higher to around 220, and would require a looooong time to come back down.

On the other hand, the SSR was a very heavy vehicle, and the extra torque multiplication REALLY made a difference on getting a good launch, and improved the 0-60 time by a few tenths. That's a HUGE impact in the rarified world of hotrodding.

For a bus, I think I would favor yor approach of a more conservative stall ratio, even though a higher stall converter would theoretically enable starting easier from a dead stop on steep upgrades.

Jim G
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