Rear end math
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Le Mans Master
Joined: Dec 2005
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From: Southeast TX
Rear end math
Or maybe a$$ math? If my rear end ratio is 3.07 and I'm thinking of going to a 3.73, could I figure the rpm I'd be running with the new rear with a little math? Ie 3.73 / 3.07 X old engine rpm (at any particular speed) equal rpm with the new rear at that same speed? If I'm doing 60 mph at 2000rpm, would that rear end swap yield 2430 rpm at 60mph?
Race Director
Joined: Jan 2005
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From: Slidell Louisiana
Originally Posted by old motorhead
Or maybe a$$ math? If my rear end ratio is 3.07 and I'm thinking of going to a 3.73, could I figure the rpm I'd be running with the new rear with a little math? Ie 3.73 / 3.07 X old engine rpm (at any particular speed) equal rpm with the new rear at that same speed? If I'm doing 60 mph at 2000rpm, would that rear end swap yield 2430 rpm at 60mph?
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If you have an A4 use the LT1 Camaro with A4 (700r4s 4l60E)
Mike
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From: Good health is merely the slowest possible rate at which one can die
St. Jude Donor '04-'05-'06-'07
Unless you plan to never use this car on the highway and don't care about mileage, I'd consider something in the 3.3 or 3.4 range maximum and go with a little more converter instead of a radical gear. If you're looking for more punch, I'd go with more converter first before even thinking about a gear change. The 3.07 is not a bad all purpose gear for the A4 cars. A 2400 RPM stall would wake up an A4 big time. 
Instructor
Joined: Jan 2006
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stupid question
I've been meaning to ask this of someone lately....can anyone explain to me how a TQ converter works/what it does, and how the 2400, 2600, 2800 different stalls would make an A4 car faster? And would it be faster just off the line, or from most any speed?
p.s. I got kindof some funny #'s from that calc, using the f-body a4, with 315/35x18 tires and a 3.45 rear gear. I put the redline at 7000, and it gives me first gear topping out at 53, second topping out at 99, third at 161....and fourth at 231?!!
p.s. I got kindof some funny #'s from that calc, using the f-body a4, with 315/35x18 tires and a 3.45 rear gear. I put the redline at 7000, and it gives me first gear topping out at 53, second topping out at 99, third at 161....and fourth at 231?!!
Last edited by YellowFvr1; Feb 12, 2006 at 11:13 AM.
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From: The Top of Utah
Originally Posted by old motorhead
Or maybe a$$ math? If my rear end ratio is 3.07 and I'm thinking of going to a 3.73, could I figure the rpm I'd be running with the new rear with a little math? Ie 3.73 / 3.07 X old engine rpm (at any particular speed) equal rpm with the new rear at that same speed? If I'm doing 60 mph at 2000rpm, would that rear end swap yield 2430 rpm at 60mph?
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From: The Top of Utah
Originally Posted by YellowFvr1
anybody?
I'm trying to figure out what an a4 would do with 315 35 18 tires in back, 7100 rpm redline. 3.45 gears....?
I'm trying to figure out what an a4 would do with 315 35 18 tires in back, 7100 rpm redline. 3.45 gears....?
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Racer
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From: Benoni Gauteng
I'm pretty sure it was ski_down_it (or was it Nathan?) who's normally in the scan and tune section who has put together a very usefull excel spredsheet for these calculations. All you do is put in the various options like wheel size and tranny and it works it all out for you. If you don't find him, pm me your email and i'll send it on to you
Last edited by SupaVette; Feb 12, 2006 at 04:43 PM.
Instructor
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so it looks like that calc wasn't too off......that excel sheet is priceless, though.
a4, 315 35zr18 tires, 3.45 gears, 7,000 rpm......
top of 1st: 53, 2nd: 99, 3rd: 161 4th:....????
If I get my 396 going and spin it to 7100 like I'd like, that'd add a mph or so to each gear
I like the shift points, although I wish first would be about 57, and that 4th had some sort of closed/shorter ratio so it would pull a bit harder
a4, 315 35zr18 tires, 3.45 gears, 7,000 rpm......
top of 1st: 53, 2nd: 99, 3rd: 161 4th:....????
If I get my 396 going and spin it to 7100 like I'd like, that'd add a mph or so to each gear
I like the shift points, although I wish first would be about 57, and that 4th had some sort of closed/shorter ratio so it would pull a bit harder
