Stump pulling and artificial dyno'ing

[fleabag]

Most people on this forum I would guess prefer Manual transmissions over Automatics and most would agree Manuals are more efficient and robust compared to an automatic. But in the case of Stump pulling, wouldn't an automatic fair better? And to test your car under various load conditions, wouldn't using an automatic be a better idea? The idea behind this is that an automatic uses fluid filled turbines while the manual uses a clutch. I'd have to imagine that one would burn out the clutch pretty quickly in these scenarios over an automatic transmission which to me, theoretically should last longer (IMO, if you can, please explain otherwise).

 

[Demon-Xanth]

For "stump pulling" an auto's advantage would be the ability to apply power without burning a clutch down. Though if you took my dad's manual which has a 5.62:1 1st gear, put it through the transfer case's low range which is 2.42:1, and then the rear gear which is 3.73:1, you end up with 460LB/ft of torque multiplied by about 50 and a top speed of about 2.5MPH, so once you got it moving you'd be able to keep moving it until the driveshafts hit the dirt. It just wouldn't be as impressive.

 

[Zenmervolt]

Originally posted by: fleabag
Most people on this forum I would guess prefer Manual transmissions over Automatics and most would agree Manuals are more efficient and robust compared to an automatic. But in the case of Stump pulling, wouldn't an automatic fair better? And to test your car under various load conditions, wouldn't using an automatic be a better idea? The idea behind this is that an automatic uses fluid filled turbines while the manual uses a clutch. I'd have to imagine that one would burn out the clutch pretty quickly in these scenarios over an automatic transmission which to me, theoretically should last longer (IMO, if you can, please explain otherwise).

A non-worn-out clutch doesn't slip unless you are riding the clutch pedal or there is something physically broken (or mal-adjusted) in the linkage. Because of this, the second situation, testing your car on a dyno under various loads, will not cause a clutch to "burn out" by slipping. Your engine would simply stall well before the clutch slipped.

The advantage of an automatic for towing and stump-pulling is its ability to apply to slip the torque converter and thereby apply torque when the vehicle is stationary. However, doing this for any extended period of time will overheat the fluid and can potentially damage the transmission which limits the usefulness mainly to simplifying things when one has to start from a stop while on a steep hill.

ZV

 

[LordMorpheus]

I thought automatics were regarded as being stronger than manuals.

A torque converter is more robust than a clutch. You don't see many stick shifts in pro drag racing.

 

[Demon-Xanth]

Originally posted by: LordMorpheus
I thought automatics were regarded as being stronger than manuals.

A torque converter is more robust than a clutch. You don't see many stick shifts in pro drag racing.

There's actually a number of examples to support both sides of the argument. But the primary reason why an automatic is often able to handle more torque than a manual is that an automatic's planetary gearset has three contact points while a traditional manual only has a single one. Gearboxes such as Lenco feature a manual shifting while using planetary gearsets.

A majority of pro stock drivers used manuals from Richmond, Lenco, and Liberty modified with air shifting until it was disallowed in the interests of being more towards driver skill and more fun. Now they have to use a lever instead of a button.

A number of my friends who were/are heavily into drag racing perfer automatics largely because they are more predictable going down the track. A manual's shifts cause more abrupt and less predictable changes to the chassis than an automatic's.

There is also a trend that an automatic will yield a better ET while a manual yields a better trap speed. And we all know what is used to light up a win light on the track. 🙂

 

[Zenmervolt]

Originally posted by: LordMorpheus
I thought automatics were regarded as being stronger than manuals.

A torque converter is more robust than a clutch. You don't see many stick shifts in pro drag racing.

A clutch with enough clamping force for the 8,000 horsepower that a top fuel dragster puts out would have a pedal that was just a trifle difficult to press down. However, you're probably not talking about Top Fuel cars since those have special, purpose-built transmissions that don't fall into the category of either traditional manuals or torque-converter automatics.

It all depends on what you mean by "robust". A manual transmission has fewer parts and fewer wear items. It is overall a simpler design and is significantly less prone to mechanical failure. This is one reason why most semis have traditionally been equipped with manual transmissions (though there are some automatics that have come onto the market in recent years).

However, manuals also mean a driver can miss a shift, etc. You also have to lift throttle between shifts, which is bad for drag racing speed. For dedicated drag racing, a specially-prepared automatic will generally be faster and will definitely be more consistent, but the modifications necessary make the same transmission less convenient for street use. The consistency (and the vastly reduced chances of missing a shift and blowing up the engine with an overrev) are huge reasons to use a modified automatic in drag racing, as well as the potential for faster shifts when set up accordingly.

Drag racing is a very specific setup where frequent component rebuilds are common. With a properly set-up automatic you can have the stall speed on the converter perfectly matched to the best launch, you can set the WOT shift points to the exact speeds where it is best for them to occur, and, if the transmission doesn't break, it will always hit those point perfectly, which eliminates a lot of potential driver error that may occur with a manual.

ZV