Car Talk by J. B. Griffin III.

Dear JB,
Can you tell me a little about how automatic transmissions work? I am specifically referring to the fluid coupler known as the torque converter. How does it convert torque and into what, may I ask? Why is there no

clutch pedal to push in and why doesn't the engine die when you are at idle in drive?

Dear Reader'

A torque converter works a little like a boat propeller with one major difference. Instead of your engine driving a propeller, as in a boat, your engine rotates a big drum of fluid called transmission fluid. The "propeller" part of the torque converter is also located inside this drum and it attaches to the input shaft of the transmission, so that the rotation of the fluid causes the propeller to turn. Another way to look at it is to think of wind turning a windmill. The reason that your car doesn't stall when you come to a stop is because at idle speed your engine

rotates the drum of fluid slowly enough so that there is very little force applied to the input shaft and it just slips through the stream of fluid. A side note here for you drag racers, the rpm at which the engine will begin to overcome the slippage of the fluid and turn the transmission shaft is called the stall speed. The higher the stall speed the higher you can idle the engine without overheating the transmission fluid. The

drawback to higher stall speed converters in street driven muscle cars is the great loss in fuel economy. To overcome this mileage loss the automakers introduced the lock-up converter. The computer commands a solenoid to lock up a friction clutch inside the converter so that the engine crankshaft is mechanically coupled to the transmission input shaft just as it is in a standard shift transmission. Sometimes these solenoids stick closed and the engine stalls when coming to a stop. This usually requires

your mechanic to replace the solenoid.

For your other question as to why it is called a torque converter I must say that baffles me as well. Torque is measured as turning force. In our case a turning force is being transferred through a body of fluid and applied to the input shaft as: Torque! I think a better term would be torque multiplier if you think about it.

Dear JB,

How many miles should I reasonably expect to go between oil changes if I use synthetic oil?

Dear Reader,

The same amount as with any oil, in my opinion, and here is why. The primary reason oil needs to be changed is contamination. This contamination comes from burning fuel in the engine. With each combustion cycle a tiny amount of carbon, unburnt fuel, water, and other oxides are carried down into the crankcase by the piston rings. Over time these break down your oil into sludge. A good oil filter will, at best, be able to filter out the largest pieces. The rest will begin to eat away at bearings, rings, cylinder walls, etc. The true benefit from using synthetic oil is its ability to withstand heat. If for some reason you overheat your engine there is a much greater chance you will not destroy vital moving parts.

JB Griffin owns and operates JB's Auto Sales and Service in Evensville, Tennessee. You may see him weekly on the WDEF-TV News 12 Morning Show every Tuesday at 7:10 a.m. where he will answer your car care questions live and on air. Visit him on the web at:  www.500overcost.com
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