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| Power Drive [message #327191] |
Sat, 16 December 2017 22:20  |
NextGenGMC
Messages: 146
Registered: December 2017
Location: Washington State
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Senior Member |
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Hello Everyone,
I'm a new member here on the forum, looking for some information. I'm a relatively recent owner of a beautiful 26' 1976 GMC Glenbrook. It is still pretty new to us and we are in the process of learning all its little quirks and peculiarities.
The coach has a 455 Olds and transmission with the PowerDrive unit. When I first learned about the PowerDrive, I thought it was a modification of some planetary gears in the transmission to modify output ratio and thus reduce stress on the internal transmission components. However, reading more about the TH425 transmission, I discovered that the PowerDrive is the modification to the chain drive part of the tranny (the power that goes TO the planetary gears). As I see it, the PD makes the engine rotate more revolutions in order to make the same number of revolution on the shaft going to the planetary gears. What is the exact benefit of this? Having PD and the 3.21 final drive ratio makes the engine run at much higher RMPs than I'm comfortable with - 2,700 RPM at 60MPH. Seems pretty excessive for the giant engine like 455. All my other cars (granted they are all newer) run at slightly above 1,500-1,700 RPM at the same speed.
When I try to visualize the power transfer in the transmission, it seems that the amount of torque transmitted through the internal components is actually higher with the Power drive than without PD. This is because engine makes more revolutions per same distance of travel (per same number of rotations in the transmission). To me that means that internal components (after the chain drive) will receive higher level of stress. Am I wrong about this? Was PD developed for the 403 engines? To help them with transmitting more power to the ground? Is this setup really beneficial on the coaches with 455? From reading info on various forums, nearly everyone recommends to have PD if you are towing a toad (which we do).
I would really appreciate your input and advice on this topic. Looking forward to learning form the combined wisdom of all. Please educate me 
Vadim Jitkov
'76 Glenbrook 26'
Pullman, WA
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