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Re: School me on overdrive
[Re: dogdays]
#1393260
02/27/13 02:54 PM
02/27/13 02:54 PM
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Joined: Jan 2003
Posts: 16,123
Grand Haven, MI
patrick
I Live Here
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I Live Here
Joined: Jan 2003
Posts: 16,123
Grand Haven, MI
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Quote:
At this point I must mention the Doug Nash 5-speed, which was an interesting design concept. If you study gear power losses, you come to the conclusion that a 4.56 rear gear wastes a lot of energy. There is a lot of friction in a hypoid gear setup. So Doug Nash designed a 5 speed transmssion with a 1:1 Fifth gear, and a 3.something to one First gear. It was intended to be used in front of a 3.07 or 3.23 rear. More gear reduction in the transmission starting out, with less friction in the driveline in Fifth. For some reason that idea never really caught fire.
R.
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One thing to watch out for with overdrives: Propshaft RPM. This can cause shaft to go over the critical RPM and blow up (regardless of torque), it also exacerbates vibration problems from balance and angularity defects. This is the biggest "bolt on" OD problem: The shaft is too short, U-joint angles go crazy.
Rick
I've never understood why OD's gained popularity for these reasons...
1) a higher (lower numerically) rear end is stronger, as the teeth are larger
2) a higher (lower numerically) rear end has less frictional losses
3) the most efficient transfer of power in direct drive (input and output shafts locked together), any overdrive is sending power through a countershaft of planetary gearset, which has frictional and inertial losses
4) the propshaft/driveshaft speed. there's a reason crown vic cop cars are electronically limited to 124mph....because of driveshaft speed.
I do understand why, from the standpoint of trying to base it on an existing product (i.e. 518 derived from 727, etc), but for a clean sheet design, a non-OD tranny with a deep 1st gear seems like it would make more sense....
Ideally, I'd like a 5 speed with say, a 4.5:1 first, 3:1 2nd, 2:1 3rd, 1.5:1 4th and 1:1 5th, and run a 2.2 or so rear end....
I like the gearing of the A580/NAG1 in the LX's, with their 3.59 first, 2.19 2nd, 1.41 3rd, 1:1 4th, and .83:1 OD...with my charger's 2.65 rear end, it gives an overall 1st gear the same as a 727/A518 with 3.88's, and OD gives an effective 2.2:1 ratio, about the same as an A518/42RH with a 3.20 rear end, but driveshaft speed is some 20% lower than the A518/42RH....
Last edited by patrick; 02/27/13 02:57 PM.
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Re: School me on overdrive
[Re: patrick]
#1393261
02/27/13 03:50 PM
02/27/13 03:50 PM
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Joined: Jan 2003
Posts: 25,747
Rio Linda, CA
John_Kunkel
Too Many Posts
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Too Many Posts
Joined: Jan 2003
Posts: 25,747
Rio Linda, CA
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Quote:
Ideally, I'd like a 5 speed with say, a 4.5:1 first, 3:1 2nd, 2:1 3rd, 1.5:1 4th and 1:1 5th, and run a 2.2 or so rear end....
Pretty close to the Porsche 928S4 which has 5-speed gear ratios of 4.07 first, 2.71 second, 1.93 third, 1.46 fourth and 1 fifth with a 2.20 axle ratio.
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