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Old 09-13-2004, 04:20   #1 (permalink)
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confused Diff Question!!

Hey guys,
How come when you convert from a smaller ratio diff. to a higher ratio diff., say 1:2.73, to 1:3.23, it increases ur RPM @ the same speeds? shouldnt it lower the RPM because it spins the wheels more with one turn of the engine? Could someone please explain to me! me and a mate are cofnused!!
thanks,
Rob
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Old 09-13-2004, 04:25   #2 (permalink)
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Re: Diff Question!!

Because you've got it the wrong way round.

Wheels : Driveshaft. So 1:3.23, the wheels turn once for every 3.23 turns of the driveshaft.
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Old 09-13-2004, 08:19   #3 (permalink)
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Re: Diff Question!!

the ratio is 3.23:1, well thats how its usualy expressed
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Old 09-14-2004, 06:31   #4 (permalink)
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Re: Diff Question!!

ok then, that makes sense, but why would you want to get a diff with a higher ratio then? wouldnt you want one with a smaller ratio so you dont need to rev as high to get the wheels to go faster? its so confusing! the more i try and work it out the more my head hurts!
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Old 09-14-2004, 06:54   #5 (permalink)
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Re: Diff Question!!

Mechanical advantage.

I'll assume by higher you mean lower (ie; 3.23:1 is higher than 3.45:1). Say you have a 1:1 diff and a 2:1 diff (for arguement's sake). Sure the 1:1 SEEMS more efficent, that the engine would spin once for each wheel spin (we'll forget about the gearbox), but that takes a lot of effort to get moving. With a 2:1 diff the engine is under half the load of the 1:1, it just needs to rev harder to achieve the same speed. But it'll take off way better than the 1:1 ever would.

Exact reason for gear boxes. Low gears are used for taking off, whereas the high gears are used once momentum has been gained.
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Old 09-14-2004, 07:47   #6 (permalink)
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Re: Diff Question!!

you can go too tall in diff ratio and suffer at 100km/h instead of help fuel economy
my 351 clevo had a c10 trans 2.77 diff = 2350rpm @ 100km/h 350km to 70l lpg
put a AOD trans(american FMX with overdrive) 1700rpm @100= 300km to 70l lpg(motor struggled to run efficienlty as cam was not in power range at these revs)(foot further down on accelerator for less revs) (CRAZY I THOUGHT ALSO)
put in 3.45 = about 2000 rpm @100 kays back to 350km to 70l lpg
conclusion FMX and AOD take a lot more power to run than a C10
should have left it alone maybe gone to 3.00 for a bit better take off.
this car also had 4 speed single rail till clutch died and got same economy as C10
4 speed got up quicker though(had 6 cyl ratios taller 1st gear on V8 box)
ran 16.1 calder on hot day, impco ontop of holley 650, gas cam, manifolds and 2.5 exhaust. ALL TORQUE NO REVS good DRIFT CAR 4500 redline for cam but reved okay to 5000
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Old 09-14-2004, 17:45   #7 (permalink)
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Re: Diff Question!!

ahhh k, that makes more sense! thats good, maybe now my head will stop hurting thanx guys for your help in clearing this up!
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