M Needforspeed, on Dec 05 2010 - 04:21 PM, said:
I checked infos on the Chaparral gearbox, and crossed technical books and mags,thinking rather twice before posting this.
Beginning by the end, for those who had doubt, [u][b]the 2F selector wasn 't sequential.It was only a clutchless gear change system......
Hi Marcel and thanks for all this great information on the Chappie's "mystery transmission".
This agrees with the limited information we had at the time about the torque converter and how it worked. So the sports cars are modeled pretty well IMHO.
The only area that I'm a little sceptical about are the torque and horsepower curve charts. The 427 engine is shown producing about 580 horsepower which I believe is optimistic. In 1966, Ford tested a single overhead cam version of their 427 engine that produced 575 horsepower, but they never used it and stuck with the familiar pushrod design which produced 500 to 525 horsepower depending on which source you read. I suspect the Chappie's Chevrolet 427 engine was similar and AFAIK, it used pushrods too. So that's why we used 525 horsepower as the base power for that engine.
Of course, to simulate a torque converter in GPL, the only way is to artificially use a lower power engine which is what we did. On average, the Chappie 2D and 2F engine was tweaked to be about 80% efficient to account for the torque converter.
In one of your pictures, the torque converter is clearly shown.
FYI, here are my notes about how torque converters work.
Here are some notes on torque converters and how to simulate them in GPL.
A torque converter is a fluid coupling designed so that it can also act as a torque multiplier. The engine output shaft is connected directly to a rotor, called the impeller, and a second rotor, called the turbine, is connected directly to the transmission input shaft. Normally the two rotors are not physically connected and transmission fluid/oil is used to transmit the force from the impeller to the turbine.
When the two rotors are not physically connected, they are free to rotate at different speeds. The ratio of the turbine to impeller speeds is usually noted as V. V will vary from 0 when the turbine is not turning at all to 1.00 when the turbine and impeller speeds are the same.
The "stall speed" is defined as being V=0 and really has nothing to do with engine speed. It is simply the point where the turbine is not turning. Above the stall speed, the turbine turns and transmits torque to the gears. Passenger car torque converters are designed so that the engine can idle and not produce car motion; especially with the brakes applied. Racing torque converters are designed to have a stall speed that matches the engine's maximum torque rpm to produce maximum car acceleration.
At stall speed, the torque converter multiplies input torque by a factor of around 1.5 to 2.5 times. This multiplication factor decreases in a linear fashion with V and can go below 1.0 when V=1.00. Obviously, this is inefficient and passenger car torque converters have a mechanical clutch that activates around V=.90 which physically locks the impeller and turbine together so they rotate at the same speed. This immediately causes V to be 1.00 and torque multiplication to remain constant at 1.0.
The efficiency of a torque converter is measured as the ratio of output to input torque taking into account the torque multiplier effect. It is parabolically shaped, beginning at 0% at stall speed, rising to a maximum around 80% when V=.80 and decreasing as V increases to 1.00. If a mechanical clutch is fitted and engaged, the efficiency increases linearly from that point until it reaches 97% to 100% when V=1.00.
My research has turned up very little information on maximum torque converter efficiency before clutch engagement, but the few data I've seen show maximum efficiency between 75% to 90%. Close to maximum efficiency occurs over a fairly wide V range of .40 to .90.
The multiplier effect and efficiency probably vary depending on the input torque and impeller speed which are directly related to the engine. However, the only data I've seen and this discussion are based on a constant input torque and impeller speed.
For GPL, we don't have code to create a torque converter so the only way to simulate it is to modify the engine BMEP/torque curve and/or the engine's capacity so that less torque gets to the gearbox. Assuming a racing engine is usually accelerating or decelerating, we could also assume that the torque converter's V will typically be somewhere around .50 where efficiency is only slighlty less than maximum due to the relatively level shape of the parabolic curve at that point.
For the Chapparal 7.0L engine, Richard used an average of about 81% efficiency in the high rpm range where a racing engine normally operates which seems reasonable.
The Chaparral had what was considered at the time to be a "mystery" transmission. From my research today, it seems this was a torque converter coupled to a normal three speed manual transmission. I've found no data on the torque converter's efficiency or whether Chaparral used a clutch to couple the impeller and turbine rotors. I doubt they bothered with an impeller/turbine clutch as a racing torque converter is rarely at V=1.00 where the clutch could enhance efficiency. Also, one source says that Chevrolet didn't introduce the lockup clutch on their torque converters until 1979 so it's doubtful that the Chaparral had one.
Although less efficient than a manual clutch, the torque converter allowed Chaparral to do away with the clutch pedal and thus gave the driver the ability to operate the movable wing with his left foot instead. Obviously, Jim Hall thought the gain from the wing outweighed the loss from the torque converter.
Edited by Lee200, Dec 06 2010 - 08:33 AM.