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Old 10-16-2002, 09:03 AM   #1
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Question Transmissions?

i dont know much about transmissions. i do know that they have a lot to do with towing though. why does it seem that automatics in small trucks are rated to tow more than the standards they put in the same trucks? is this because they assume that folks dont know how to drive with a load and will blow the standard transmission, or because the automatics are really beefier?

what is the deal with tranny coolers?

Trailer sway and lack of power aside, how does one go about getting the most from a smaller tow vehicle without damaging the vehicle?

for example, smaller engines are built to rev higher, but how much higher is really okay? what does the 'red line' on the tach really mean?

maybe i finally have something to call CarTalk about
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Old 10-16-2002, 09:43 AM   #2
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Probably a lot to do with development costs and how the vehicle is used. Automatics are better for towing. The torque converter is really a torque multiplier, a clutch slips to get the vehicle in motion. More auto trans are sold, more money to develop.

The shifting of the automatic can be controlled by design. It is built so there is a constant load on the driveline. The converter produces a gradual increase in torque, the trans can engage second gear before disengaging first, third before second, etc. A standard transmission completely unloads and loads the driveline for each shift, no gradual application. This is a lot of strain.

Heat is an enemy of auto transmissions. Factory coolers are built into the radiator so the trans fluid can never get any cooler than the temp of the engine. Auxilliary coolers are added after this and dissipate more heat before returning the fluid to the trans.

Red line is what the manufacturer considers the maximum safe operational rpm for the engine. The valve train is the weakest link, springs can't keep up. "Valve float" occurs, the springs can't return the valve to the seat fast enough. Some ECMs will limit engine rpm if you over rev. Higher rpm means shorter engine life and greater fuel consumption.

John
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Old 10-16-2002, 11:22 AM   #3
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In a manual transmission, the clutch is the weakest link. In order to pull a heavier load, a heavier (spring pressure=holding power) pressure plate is need. With out it, it is susceptible to slippage, heat, and failure. A heavy duty pressure plate requires more clutch pedal effort to disengage, not a real problem with modern slave cylinder set-ups. It basically takes more effort to drive, which most people don't want to deal with. As mentioned in another post, the manufacturer makes more money off an auto tranny. They don't want to put the resources towards another set of parts in their inventory that not many people want.
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Old 10-16-2002, 12:41 PM   #4
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clutch plate

sounds like replacing the clutch plate is a good place to start in trying to beef up the pulling capacity of my truck. (ending up with a stiffer pedal isn't a concern for me at all)

that doesnt sound too bad price-wise either. i have friends who have done clutch jobs themselves and would help me do it.

what is the weakest aspect of the transmission itself?

can i put a cooler on a manual transmission?
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Old 10-16-2002, 12:43 PM   #5
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Dave,
74Argosy24MH took the words right out of my mouth when he said that heat is the enemy of the auto transmission and correct again when he said that the tranny cooler used outside the vehicle radiator is better for lower temperatures. Some manufacturers such as Summitt Racing also offer a cooler/fan combination for additional cooling. Another way to reduce or prevent high fluid temperatures is to use synthetic ATF. It doesn't break down under heavy usage like the regular stuff.
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Old 10-16-2002, 03:02 PM   #6
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Manual transmissions cannot accept coolers, there's no pump or anything, not to mention 90W lube doesn't do a whole lot of movement. You might want to contact a couple transmission shops or dealers and get several sources of input before spending the cash. What about the rest of the driveline? i.e. engine displacement and rear axle ratio? Be a good idea to research it by trying to find what the towing capacities are with those other combinations of engine and axle ratio. My trucks owners manual lists them all.
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Old 10-16-2002, 03:15 PM   #7
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You might not be able to find a pressure plate any different than what you have. Most of the rebuilders are going to do a stock replacement, there won't be demand for anything better. Some of the specialty outfits might have a heavy duty version, but it is hard to do much with a 7 or 8" clutch as far as making it very heavy duty.

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Old 10-16-2002, 06:05 PM   #8
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Some truck/racing catalogs list the pressure plates as Stock, 20% and 30% for 20%/30% more pressure at the plate.
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Old 10-17-2002, 07:24 AM   #9
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After the clutch I would go for HD u joints. These probably will be another specialty manufacturer item, should be available from the same outfits as the clutch. The most common failure in manual transmissions is synchronizers or bearings. The rest of it is pretty much solid pieces of machined steel and hard to predict which part will fail first.

John
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