Rear end gearing for road racing/autoX - Ford Mustang Forums : Corral.net Mustang Forum
 
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post #1 of 6 Old 10-03-2010, 10:32 AM Thread Starter
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Rear end gearing for road racing/autoX

Ok so Im in progress of building my 01 cobra for road racing/ autoX

Current modds in my sig, but what I eventually plan on doing is a fully built NA motor 5.0 BB stroked to 5.3 P&P heads cams etc...

my question is what gears should I do? I want to say 4.10's but I have them in my auto GT, but now I want to try 4.30's in my Cobra. does anyone have 4.30's and runs autoX road racing?

What Im worried about is 4.30 being to large and when I do road racing I'll loose too much top end. suggestion greatly appreciated thanks in advance.


RACE CAR IN PROGRESS 01 COBRA,MM Race 3Valved bilsteins with front/rear Coilovers,ARH LT's,ACT Flywheel,SFI UDP's,EGR Delete,3.55's, JLT RAI. Dyno Tune302RWHP 298TorqueDD02 GT,Flowmasters,4.10,K&N RAI, 75mmTB/Plenum,MM rear LCA,Front LCA,Panhard Bar,Subframes,CC Plates.

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post #2 of 6 Old 10-03-2010, 11:26 AM
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For road racing it varies with the tracks you use. My 97 Cobra has a stocker 4V that is happy spinning higher revs all day. The 4.10's match very well with Thunderhill and Laguna Seca, the tracks I drive the most. Not many real long straights on the tracks I run, so I never hit 5th. 4.30's would force me to use 5th where I really don't want to.


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post #3 of 6 Old 10-03-2010, 12:51 PM
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For autox you want to avoid gear changes as much as possible, so ideally you'd have one gear that maxes out at just over 60 mph. Then you just run the whole course in that gear.

So you need to calculate based on your redline (or the rpm you are comfortable running the motor), gear ratio, and tire size.


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post #4 of 6 Old 10-03-2010, 03:55 PM
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I've autocrossed a 96 Cobra with 300whp, 4.30's and Hoosier A6's. Power delivery required quite a bit of throttle modulation/sensitivity......but it was QUICK!

However, 4.10's would probably be a best overall for autocross and track stuff.
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post #5 of 6 Old 10-03-2010, 05:05 PM
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I run a 4:10 on a two mile road course, but at redline in 4th half way down the straight. 5th does nothing, so I will be installing 3:55's that way I can stay in 4th without over reving down the straight, but all courses will be different.
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post #6 of 6 Old 10-04-2010, 12:09 PM
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Quote:
Originally Posted by leecea View Post
For autox you want to avoid gear changes as much as possible, so ideally you'd have one gear that maxes out at just over 60 mph. Then you just run the whole course in that gear.

So you need to calculate based on your redline (or the rpm you are comfortable running the motor), gear ratio, and tire size.
Exactly. I used the Gear Ratio Calculator here, https://www.corral.net/tech/gearcalc.html to find my top speed in 2nd gear for auto-x and 4th gear for HPDE. Although I'm limited to 3.55's per SP rules, they work out very well as they allow a 70mph ceiling in 2nd gear for auto-x events. This way I can stick it in 2nd and just drive..... which needs alot of attention.

You're pushing more torque and hp with your current set-up than my car. With the torque of the BB, I wouldn't think you should need more than 3.55's, depending of course on your final rpm cap.

What gears have you run at auto-x or O/T events with the 4.6?

Rob
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