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csimo
08-31-2002, 14:08
I found a long article about diesel engines and automatic transmissions on the International web site. Here are a few paragraphs which I find interesting. The bottom line points out that diesel engines tied to auto trannys usually don't use all the published torque available. I'd like the REAL numbers as described in this article for the GM, Ford, and Dodge products. That would be interesting!

From: http://www.internationaldelivers.com/school_bus/178.html

"It's easy to claim "I've got more torque!". It's more difficult to understand the process of engine-converter matching and why that claim is meaningless. Bear with me, please.

You do know that the common method of rating transmissions is with the maximum engine torque level that can be absorbed. In the case of manual transmissions the peak torque produced by the engine is balanced against the capability of the transmission. For instance, the Spicer 6+ (code 13MAG) is rated at 530 Lb.Ft. and we couple it to our engines that have that peak torque value. In actual operation the transmission is subjected to that torque level whenever the engine passes through the peak torque speed range while operating at full throttle, either on acceleration or lug down when climbing hills.

Allison also rates their automatic transmissions and approves their application based on engine torque output. But, in reality the AT-500 series transmissions are never subjected to the engine peak torque. The operating characteristic of the torque converter (which is a fluid, not a solid, coupling) is such that it reaches the maximum torque transmission level at zero output speed. This condition is referred to as "stall". At the stall point the gearbox of the transmission is subjected to the maximum torque level which is determined by the engine input torque and the torque multiplication ratio of the converter. It is this condition that Allison uses to establish the "real" rating of the engine-transmission combination.

You can experience the stall torque situation by operating a vehicle as follows: with the engine running and warmed to normal operating temperature, transmission in Drive, set the service brakes tightly with the foot pedal and press the accelerator pedal to the floor. The engine speed will rise from low idle to a speed above peak torque and below maximum horsepower and stay there. This is stall speed. Do not hold this condition for more than 10 seconds because all of the engine power is converted to heat in the transmission fluid and you can overheat the system.
For purposes of best acceleration performance the engine and converter combination (or "match") are selected to cause the stall point to occur at an engine speed higher than peak torque where the resulting torque level is less than the peak. Since this match point speed and torque level can vary from one combination to another the advertised rating of the transmission would be hard to define and describe in simple consistent terms. Every matchup is evaluated on an individual basis with care taken to not exceed the gearbox capability at the converter match point."

Colorado Kid
09-03-2002, 09:00
Since the D-max makes at or very very near peak torque from 1600 to 3000 RPM I would say...yes it does.

Since mine has a 6 speed there's no doubt. :D

LarryM
09-03-2002, 10:32
I believe only the 6 speed equipped trucks can experience full rated torque in actual operation. In order to achieve the full 520 ft/lbs at 1800 RPM, you must use full throttle. With the Allison, (at 1800 RPM) the computer causes a downshift long before you can get a full throttle condition in any gear except 1st (which is fuel limited anyway).

This has been my major complaint since day one. My next pickup WILL have a manual transmission even if I have to change brands to get it.

Colorado Kid
09-03-2002, 13:36
My point is that, for example at rated speed (3100 RPM for 2001 and 2003) if the D-max is making 300 HP it is also making 508 Lb-Ft of torque. That's close enough for me! The Duramax 6600 (in stock form) doesn't so much have a torque "peak" as a torque "mesa". 98% of peak torque at peak horspower RPM is more than a bit unusual, particularly in diesel engines. If your Allison will hold ANY gear at ANY RPM between 1800 and 3100 at full throttle then you have experienced at least 98% of peak rated torque. If that isn't enough I guess you'll have to get a manual transmission!

needbigtruck
09-05-2002, 16:30
Some of the power that the engine is producing is going into heating the transmission fluid. Why do you think you have to have a transmission cooler? I say no, you don't get all of the torque the Duramax is producing.The fact that you have to remove the heat from the transmission that the engine is putting there is one of the top reasons I bought a 6 speed. I know that when I let the clutch out the engine is directly connected to the wheels. None of this TOW/HAUL mode stuff, or 5th gear lockup, I simply let the clutch out and use all the power my engine puts out.

afp
09-05-2002, 20:05
Don't forget one huge advantage an auto trans equipped turbo motor has over a stick. Turbo's need to be loaded to make power. On launch, the auto trans loads the turbo and keeps it loaded for the whole run. When a stick launches, the turbo isn't loaded. With each additional shift, the turbo is unloaded and reloaded. When the turbo unloads, power is greatly reduced.

For this reason--in NHRA drag racing--the auto equipped cars are quicker than the manual cars for each respective cubic inch to weight class. The same principal applies to our DMaxes.

If you are driving down the road in a fixed gear (not shifting) under WOT, then the stick would indeed put more power to the wheels than would the auto. However, under aceleration, the auto is quicker.

The raw HP and torque numbers are not the most important factor in making power. It's the HP and torque curve and how they are applied that's most important.

Blaine

TDIwyse
09-06-2002, 06:29
It was my understanding that the Allison in Tow/Haul mode had the torque converter locked up in 2nd through 5th gear. Does this not mean a direct, physical connection?

I've also got another question. I've seen stated on this site that the Allison will cause a loss of about 60 RWHP. Is this when the torque converter is not locked up? If it's locked up then shouldn't it transfer just as much power and torque to the rear wheels as the manual transmission? Is my understanding of transmissions severly limited?


(My first post to this site. I've had my 2002 2500HD D/A for about a month now and am really enjoying it. We are now a completely diesel family with our truck and our VW Jetta TDI smile.gif )

SoCalDMAX
09-06-2002, 10:19
TDIwyse,

Congrats on the new truck and welcome to the forum!!

As for your Allison question, the Allison loses approx. 18% of the power applied thru it, which happens to be close to 60hp at WOT. This is due to it's heavy duty construction and design, there are always compromises.

I believe you're correct on the T/H mode, it is locked in 2nd thru 5th. It is still losing 18% of the power thru frictional and pumping losses. (There's a big hydraulic pump in there.)

I second what Blaine said. To see it in action, just stomp on the throttle from a stoplight next to a RiCeb0Y. As your Allison tranny smoothly shifts thru the gears keeping the turbo WELL into the boost region, just listen to Wannabe Racer furiously rowing thru the gears, 4,000 to 8,000rpm every shift, as he falls farther and farther behind. All without spilling a drop of coffee... ahhh, it's a beautiful thing... :D

Regards, Steve

TDIwyse
09-06-2002, 12:01
Thanks for the welcome.

I'm still wondering about the transmission losses. From the EZamp web site they show a plot of the HP and Torque for stock and modified. The stock truck shows it's putting ~250 hp to the rear wheel with the Allison. This would be a total driveline loss of ~17%. Surely not all of this is due to the Allison (I thought rear wheel drives even with a manual transmission had drive line losses in the 16-18% range). If you dynoed the six speed are you saying that it would show more than 250 rear wheel hp?

Just curious about this. Has anyone dynoed the six speed to see how much more efficient it is than the allison when it's locked up?

c5dura
09-06-2002, 16:28
The generally accepted drive line loss averages are 15% for a manual and 20% for an automatic. This is for the entire driveline, not just the transmission.

You are correct that manuals typically dyno a little higher than their automatic counterparts, all else being equal.