#7276416 - 09/09/13 05:50 PM
Can someone please explain why shorter gears accelerate fast
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ReplicaR
Post Master Sr
Registered: 05/26/06
Posts: 7093
Loc: E.E.B.
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I would love to hear the actual reason why shorter rear end ratio, going from 3.08 to 3.55 final drive for example, would make the car accelerate faster? I used to think that it was because the car would get into powerband relatively quicker, but there has to be more to it than just that.
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'94 Mustang GT Acceleration....  -------+--  Handling......... -------+-- Speed............ ------+---
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#7276440 - 09/09/13 06:02 PM
Re: Can someone please explain why shorter gears accelerate fast
[Re: ScottStaypuff]
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ReplicaR
Post Master Sr
Registered: 05/26/06
Posts: 7093
Loc: E.E.B.
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I've got 3.08 right now. I've done some math, and realized that shifting at 6500, I'd get 121 out of 3rd and 161 out of 4th. Realistically, I am not going to hit 161 at any track around here. 3.55 gives me 140 at the top of 4th, which is right about right for some of the tracks around here. 3.73 would be running into rev limiter on some straights most likely.
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'94 Mustang GT Acceleration.... -------+-- Handling......... -------+-- Speed............ ------+---
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#7276471 - 09/09/13 06:19 PM
Re: Can someone please explain why shorter gears accelerate fast
[Re: ScottStaypuff]
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ReplicaR
Post Master Sr
Registered: 05/26/06
Posts: 7093
Loc: E.E.B.
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I've got an ACT Pro Lite Billet Steel. Weights as much as aluminum, and doesn't have any inserts.
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'94 Mustang GT Acceleration.... -------+-- Handling......... -------+-- Speed............ ------+---
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#7276611 - 09/09/13 07:20 PM
Re: Can someone please explain why shorter gears accelerate fast
[Re: ReplicaR]
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ReplicaR
Post Master Sr
Registered: 05/26/06
Posts: 7093
Loc: E.E.B.
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btw, I don't think we got deep enough into this. Anyone else would like to explain the gear ratio vs acceleration in more detail?
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'94 Mustang GT Acceleration.... -------+-- Handling......... -------+-- Speed............ ------+---
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#7276933 - 09/09/13 10:50 PM
Re: Can someone please explain why shorter gears accelerate fast
[Re: ReplicaR]
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UglyValiant
Post Master Sr
Registered: 10/09/02
Posts: 6364
Loc: Milwaukee, WI
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Acceleration is essentially force divided by mass.
The less your mass is, the greater your acceleration. The greater the force is, the greater the acceleration.
One thing to keep in mind though, is Horsepower is NOT a force. It's an oblique measure of force, but not an actual representation of it.
So what do shorter gears (higher numerically) do for you? Why, it simply increases the force applied.
Short of it is this...horsepower is not multiplied through gear ratio...torque, however, is.
...given that two engines have the same HP, then *in theory* with proper gearing, they should accelerate the same given the same vehicle weight, etc. So if I had a 2.5 liter i4 that made 300hp NA, but had to spin to 11,000 rpm to do so vs a 300hp 5.0 v8 that spun to 5,500 rpm, then the two should accelerate the ~ same if one had 3.08 gears and the other ~ 6.16:1 gears. (this of course, negates friction and rotational masss/etc).
Basically in the above scenario, the ~ 50% decrease displacement is of course assumed to result in ~ 50% less torque (and the 100% difference in power-band)...but then the acceleration difference is negated by the 50% difference in gearing.
In your case though, if your car is putting down 375ft-lbs of torque, then with 3.08 and 3.31 first gear, then it's putting 3823ft-lbs of torque to the pavement. But say you had 3.73's...
then you'd be putting 4629 ft-lbs to the pavement while in first.
Sure, it's for a shorter time, but the advantage continues through all your gears.
Now with your build, you did increase your torque substantially which improves acceleration...as well, the increase in HP by part increasing the rev range also helps your acceleration...which, no surprise, I'm sure you find your acceleration has improved dramatically. But as you allude, you can now go quite a bit faster in each gear...which allows you the luxury to alter the ratios and get similar top speeds in each gear. Do you do it? It'll result in faster acceleration, no doubt....
But going to far can be a hassle too.
If a drag number is what you want, go 4.10's. Me, I'd go 3.55's and split the difference...a healthy bit more torque multiplication without going to crazy....
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1979 Dodge Lil' Red Express Truck
1989 Shelby CSX (#500/500)
The most powerful production minivan money can buy.
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#7277688 - 09/10/13 10:58 AM
Re: Can someone please explain why shorter gears accelerate fast
[Re: Impulsive]
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ReplicaR
Post Master Sr
Registered: 05/26/06
Posts: 7093
Loc: E.E.B.
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Yeah, a lot of people suggested gears very early on, but since I was constricted to 5000 rpm range, I couldn't limit myself in speed that way, so I stuck with 3.08s as long as I had to. I think it would be a good idea to pop some new clutches into the diff as well, like you mentioned. The rest of the axle is actually pretty fresh.
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'94 Mustang GT Acceleration.... -------+-- Handling......... -------+-- Speed............ ------+---
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#7287739 - 09/15/13 04:18 PM
Re: Can someone please explain why shorter gears accelerate fast
[Re: ReplicaR]
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max
Poster
Registered: 05/18/09
Posts: 229
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I wouldn't go anything under a 3.55. What tracks do you frequent? If you're at the Horse Thief Mile often I think a higher (numeric) gear would be more suitable, less so if you're at Big Willow.
I kinda regret getting 3.73s. If I were to do it all again I'd probably go with 3.90s, but I have an extra gear to play with.
edit: if you're going in there, it might be worth upgrading to a Torsen ($600ish) or Torsen-esque differential like a Truetrac ($500ish).
Edited by max (09/15/13 04:24 PM)
Edit Reason: moar info
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#7288424 - 09/15/13 10:38 PM
Re: Can someone please explain why shorter gears accelerate fast
[Re: max]
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ReplicaR
Post Master Sr
Registered: 05/26/06
Posts: 7093
Loc: E.E.B.
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I can't stand horse thief mile, I've only done it once, and the car just felt completely out of place there. Bunch of hair pins, boring ass track. I do spend a lot of time at Big willow, love cal speedway, and planning on doing buttonwillow this year for the first time, all of which are fairly high speed tracks. I think 3.55 is the perfect gear, but I've got a shakedown at big willow in october, so we'll see where I trap at the end of straights. I'm going to guess mid 130s
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'94 Mustang GT Acceleration.... -------+-- Handling......... -------+-- Speed............ ------+---
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#7341716 - 10/17/13 11:27 AM
Re: Can someone please explain why shorter gears accelerate fast
[Re: ReplicaR]
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ReplicaR
Post Master Sr
Registered: 05/26/06
Posts: 7093
Loc: E.E.B.
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Well, gears ordered. Ford Racing 3.55s with gear install kit and Ford Racing diff rebuild kit (carbon fiber clutch packs). Hopefully will be installed some time in next two weeks.
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'94 Mustang GT Acceleration.... -------+-- Handling......... -------+-- Speed............ ------+---
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#7342568 - 10/17/13 04:51 PM
Re: Can someone please explain why shorter gears accelerate fast
[Re: ReplicaR]
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ScottStaypuff
Post Master Supreme
Registered: 03/05/00
Posts: 25530
Loc: Ohio
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Well, gears ordered. Ford Racing 3.55s with gear install kit and Ford Racing diff rebuild kit (carbon fiber clutch packs). Hopefully will be installed some time in next two weeks.
Saaaaweeeeet!
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#7354854 - 10/25/13 01:19 AM
Re: Can someone please explain why shorter gears accelerate fast
[Re: ScottStaypuff]
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ReplicaR
Post Master Sr
Registered: 05/26/06
Posts: 7093
Loc: E.E.B.
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Parts arrived.
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'94 Mustang GT Acceleration.... -------+-- Handling......... -------+-- Speed............ ------+---
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#7362997 - 10/29/13 09:37 PM
Re: Can someone please explain why shorter gears accelerate fast
[Re: ScottStaypuff]
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ReplicaR
Post Master Sr
Registered: 05/26/06
Posts: 7093
Loc: E.E.B.
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Got the gears in today. The car feels a LOT more responsive off the line, freaking love it. Coolant overflow bottle will be installed tomorrow, and once that is done, I'm registering for an event in two weeks at Buttonwillow. Looking forward like crazy to this.
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'94 Mustang GT Acceleration.... -------+-- Handling......... -------+-- Speed............ ------+---
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#7368128 - 11/01/13 12:29 PM
Re: Can someone please explain why shorter gears accelerate fast
[Re: ScottStaypuff]
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ReplicaR
Post Master Sr
Registered: 05/26/06
Posts: 7093
Loc: E.E.B.
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Coolant overflow is installed, I'll be leveling suspension this weekend, get it aligned again, clean it up, test fit the race wheels, and looks like I'm ready to go. I've signed up for the event already.
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'94 Mustang GT Acceleration.... -------+-- Handling......... -------+-- Speed............ ------+---
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