btrokc

I recently put on a Paxton HO and am really having a blast enjoying what is like an all new car. I have 4.10 gears and an alum driveshaft so it wraps up real fast. First gear is just about useless except going up my driveway and in busy parking lots. I am considering going back to the stock gears but wanted to get some opinions first. I can start in second and it really hooks up nicely. But this does make my car a 4 speed. It saves me one shift thru the quarter and I still get the pulling power of the 4.10s. My questions are, is launching in second hurting any thing if I do not over work the clutch, is second and 4.10 a better gear ratio for drag racing than first gear with the stock gears. I am not sure what the math is on the ratios but I sure like way it drives with the second gear launch. It has been too cold to get to our local strip. Expect to get in some runs in Apr. Thanks.

Gearhead

I just ran these numbers:
Stock wheel size: 235mm 55% 17"
235mm / 25.4 inches/mm = 9.252"
Diameter = 9.252" * 55% * 2 + 17" = 27.177"
Distance traveled per revolution = 3.14159 * Diameter
= 3.14159 * 27.177 inches/rev / 12 inches/foot / 5,280 feet/mile = 0.0013475 miles/rev
Shift point at 6,000 rev/min * 60 min/hour = 360,000 Rev/hour
Best Cruise Speed is very roughly about 45% of peak torque speed for this engine/load, which is 4,500 rpm * 45% = 2,025 rpm
2,025 rev/min * 60 min/hour = 121,500 rev/hour
Speeds with 4.10:1 rear axle:
1st Gear: 360,000 rev/hour * 0.0013475 miles/rev / 4.10 / 3.38 = 35 MPH
2nd Gear: 360,000 rev/hour * 0.0013475 miles/rev / 4.10 / 2.00 = 59 MPH
3rd Gear: 360,000 rev/hour * 0.0013475 miles/rev / 4.10 / 1.32 = 90 MPH
4th Gear: 360,000 rev/hour * 0.0013475 miles/rev / 4.10 / 1.00 = 118 MPH
5th Gear: 360,000 rev/hour * 0.0013475 miles/rev / 4.10 / 0.675 = 175 MPH
Cruise speed @ 2,025 rpm = 121,500 rev/hour * 0.0013475 miles/rev / 4.10 / 0.675 = 59 MPH
Guess efficiency = 24.0 miles/gallon @ 60 MPH
Speeds with 3.73:1 rear axle:
1st Gear: 360,000 rev/hour * 0.0013475 miles/rev / 3.73 / 3.38 = 38 MPH
2nd Gear: 360,000 rev/hour * 0.0013475 miles/rev / 3.73 / 2.00 = 65 MPH
3rd Gear: 360,000 rev/hour * 0.0013475 miles/rev / 3.73 / 1.32 = 99 MPH
4th Gear: 360,000 rev/hour * 0.0013475 miles/rev / 3.73 / 1.00 = 130 MPH
5th Gear: 360,000 rev/hour * 0.0013475 miles/rev / 3.73 / 0.675 = 193 MPH
Cruise speed @ 2,025 rpm = 121,500 rev/hour * 0.0013475 miles/rev / 3.73 / 0.675 = 65 MPH
Guess efficiency = 24.5 miles/gallon @ 60 MPH
Speeds with 3.55:1 rear axle:
1st Gear: 360,000 rev/hour * 0.0013475 miles/rev / 3.55 / 3.38 = 40 MPH
2nd Gear: 360,000 rev/hour * 0.0013475 miles/rev / 3.55 / 2.00 = 68 MPH
3rd Gear: 360,000 rev/hour * 0.0013475 miles/rev / 3.55 / 1.32 = 104 MPH
4th Gear: 360,000 rev/hour * 0.0013475 miles/rev / 3.55 / 1.00 = 137 MPH
5th Gear: 360,000 rev/hour * 0.0013475 miles/rev / 3.55 / 0.675 = 202 MPH
Cruise speed @ 2,025 rpm = 121,500 rev/hour * 0.0013475 miles/rev / 3.55 / 0.675 = 68 MPH
Guess efficiency = 24.9 miles/gallon @ 60 MPH
Speeds with 3.31:1 rear axle:
1st Gear: 360,000 rev/hour * 0.0013475 miles/rev / 3.31 / 3.38 = 43 MPH
2nd Gear: 360,000 rev/hour * 0.0013475 miles/rev / 3.31 / 2.00 = 73 MPH
3rd Gear: 360,000 rev/hour * 0.0013475 miles/rev / 3.31 / 1.32 = 111 MPH
4th Gear: 360,000 rev/hour * 0.0013475 miles/rev / 3.31 / 1.00 = 147 MPH
5th Gear: 360,000 rev/hour * 0.0013475 miles/rev / 3.31 / 0.675 = 217 MPH
Cruise speed @ 2,025 rpm = 121,500 rev/hour * 0.0013475 miles/rev / 3.31 / 0.675 = 73 MPH
Guess efficiency = 25.0 miles/gallon @ 60 MPH
Specs on the Transmission Ratios are found here, on page 8:
http://www.fileden.com/files/2008/3/...ng%20Specs.pdf

Dixie_Flatline

I too have been wondering this... anyone with any real experience and knowledge care to answer, please? My completely uninformed guess is as long as the RPMs are there it's not much different from shifting at speed into 2nd from either 1st or 3rd - or 4th.... Maybe creating momentum from a dead stop puts more strain on the system and 1st is built more robust for the application? But that's a WAG on my part.

From all I've read on it, your trap speed in the 1/4 mile is your limit on gearing. You definitely don't want to have to shift into 5th, but you'd probably have to be hitting a fairly high mph in order to have to shift into 5th. In fact, the '08 Bullitt's increased redline and 3.73s has allowed people to do the 1/4 without shifting into 4th. 4.10s should help because they get you into the better performing areas of your RPM band sooner.

But please don't quote me on any of this, I'm just thinking out loud and have no drag experience whatsoever.

ski

-An internal combustion engine produces its max hp in the upper part of its rpm range
-An internal combustion engine revs into the upper part its rpm range quicker with gears that have a higher MA(mechanical advantage)
-1st gear(3.38) with a 3.55 rear has a MA of 3.38x3.55, or 12
-2nd gear(2.00) with a 4.10 rear has a MA of 2.00x4.10, or 8.2
-Thus, 1st gear with the 3.55 rear is better for drag racing than 2nd gear with a 4.10 rear(assuming the car does not smoke the tires)

bob

Naw, accelerating from 2nd in a car that has increased torque and torque multiplication via gears doesn't hurt anything, its more or less a ham-fisted approach to traction control (read torque management).

Heck when I've really got to pump up the MPG on my car (3.55 equipped), I start out in 2nd or 3rd and then just shift into 5th in between 1k to 2k rpm.

btrokc

This is what I was really looking for. So you would say go back to stock gears for a better launch ratio than staying with the 4.10. They have really be a blast but just do not seem like the best setup with the blower. I do plan to go to a drag radial. Believe I will wait until I get the new skins on the rear and see if I can get a decent launch. Thanks.

Five Oh Brian

I think that it has been widely accepted for quite a few years that supercharged Mustangs are best with moderate gearing - perhaps 3.55's or 3.73's at the most - as a supercharger generates more than enough tire-shredding torque without resorting to gears (which are torque multipliers). Naturally aspirated Mustangs are a bit shy on bottom-end torque, so they can benefit from the additional torque multiplication that gears offer, hence the popularity of steepers gears such as 4.10's or 4.30's for the NA crowd.

ski

You're welcome.
The best overall gear ratio/MA for launching is the one that can get the engine into its upper rpm range the quickest without the engine bogging and without tire spin. Drag radials may provide the solution to your tire spinning problem with your current 4.10 rear if you race at a well prepped track. But if the track is not prepped well or you do street racing(especially when ambient temperatures are cooler), then you may continue to experience excessive wheel spin with the DR's, and have to reinstall the stock 3.55 gears.

Rebel73

I'm going with 3:73s on my Whippled 08 GT/CS - best all around for my setup I think.