Quote:
Originally Posted by Big Dave
The GV overdrive is a single planetary spur gear setup. Expect a 3% loss in driveline efficiency, but this will be more than offset by the lower engine RPM resulting in lower engine frictional HP.
What concerns me about t_vago's setup is reduced cooling in the automatic transmission due to the big overdrive ratio.
I had a GV on a 1983 GMC K2500 with a TH400 tranny and the wimpy old 6.2 engine. I destroyed not one but two TH400s in front of the GV. Once I raked up the parts from the Interstate, I took them to a transmission guy and he said excessive heat buildup.
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Hm...
Gear Vendors claims less than 1/400 of a HP for frictional loss. That sounds reasonable.
Certainly better than 25% through a transmission. Even granting hydraulic fluid at 100 psig flowing at 4 gpm with 50% pump efficiency, that's 1/2 a HP taken by the automatic's pump. Ecomodder's tools tell me that I need about 38 HP to push my truck down the highway at 72 MPH. This is supposed to represent the 75% of the engine output that isn't sucked up by an automatic. If this truly were the case, then that means that 12 HP is being sucked up by the automatic while cruising. If we were to assume that 1/2 HP is from the pump, that still leaves 11.5 HP (or about 8.5 kW) being consumed by the transmission. I can't think of anything other than heat frictional losses that could consume the remaining 8.5 kW during a steady cruise, and that amount of heat being lost would bring a gallon of transmission fluid from room temperature to boiling in about 45 seconds. So either the planetaries within the automatic are heavily dragging down the geartrain, or the clutch surfaces are doing a really lousy job of locking down the required combination of planetaries (as well as doing a truly hideous job of locking the torque converter). As we both know this isn't the case, I come to the conclusion that that 25% is not a valid figure to go by to determine losses through a transmission.
And before somebody mentions "radiator," let's run through the same set of numbers, using a 5-speed manual that was also offered for the 2000 Dodge Dakota, 15% losses, NO oil pump, and NO radiator. If 15% losses were truly what I could expect, I could also expect to have the transmission burst into flames 5 minutes into my 72 MPH cruise, or seize up from oil cookoff much sooner than that. This is what I mean by theory. Theory states 25% losses through an automatic transmission, while real world observation suggests something far less. The trick is to separate frictional losses from losses associated with angular acceleration (which is what you'd see on a dyno).
Big Dave does makes a valid point about reduced cooling, though. It would seem that the TH400 did not provide enough flow at reduced RPMs to carry away the heat that was being generated. I might have the same problem with a second overdrive.