| As a general rule of thumb a manual transmission is more efficient at transferring power through itself than an automatic transmission. This is primarily due to the oil pump in the automatic that takes some power to drive it. It’s not uncommon to do a manual to automatic swap and find the car loses some MPH in the ¼ mile due to this. Another factor that contributes to this is torque converter slippage; if it’s too loose it’ll really show up as huge loss of MPH. But if the converter is dialed in to the engine’s torque curve it should be capable of at least 95% efficiency on the top end and minimal loss of MPH much like what Eric (z324u) noticed with his TH400. If we do a comparison of parasitic power losses between automatic transmissions the differences will mainly show up in direct relation to differences in rotating weight. The trans with the lighter rotating assembly will typically accelerate faster than the heavier one. A four-speed auto has one more clutch drum than a three speed and these things easily weigh 15 to 20 lbs each. Therefore it has more mass and in theory will take more power to accelerate it at the same rate. This is one big reason why the Powerglide (a two-speed automatic) has been so popular in light weight cars with smaller low-torque engines. Lastly, manual transmissions suffer their own sort of internal power losses. The main reason is helical cut gear teeth that are designed to reduce noise in passenger cars, the second culprit is rotating mass. The most efficient way to transfer the power through a gear mechanism is with straight cut gear teeth mainly because they transfer 99.9% in the direction of rotation rather than a helical gear that tends to waste some power trying to screw itself forward (or backward) off of the gear it’s driving. I hope that answers your question. Sorry I jumped on you with snide remarks but after reading your post and the one that you replied to, I felt the TH400 was being misrepresented and all my hard work was being attacked.
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