Well...the debate was on for some time...should I go with 3:73 or 4:10 on my Supercharged 2005 GT. I figured 3:90 would be a good compromise, but it turned out to be a $600 mistake ($200 for parts, $400 for labor...ugh). The only track within driving distance for me is an 1/8 mile track, so of course I want to gear my car to do well in the 1/8. With my factory 3:29 gears, I was putting out times of 8.9 @ 85mph (my supercharger LOVES 3rd gear!). After my gear change to 3:90 I improved a little to 8.8 but mph suffered 11mph as I was now running through the traps at 74mph. About 75 - 100 feet before the finish line, she shifts into 4th gear which takes my rpm all the way down to just below 4 grand, which is KILLING my 1/8 mile time.

My instinct tells me to go with 3:73 now because that should get me all the way through 3rd gear (which the S/C seems to love) and have me red lining right at the finish line. On the other hand, 4:10 might have me red lining in 4th gear right at the end. The only question is; will my S/C enjoy 4th gear as much as it does 3rd gear. Right now, with the 3:90s, when she shifts, the rpm go down to 5 grand; so 1st, 2nd, & 3rd gears all run from 5,000rpm to 6,200rpm (rev limited at 6.2k), but then 4th kills the rpm to about 3,900rpm. I am not really putting out boost until I get to 5k rpm so I basically run the last 100 feet of the race with no boost. The KEY question is: where will my rpms be when she shifts into 4th with 4:10 gears? If the answer is 4k, then 4:10s would be terrible idea, but if the answer is 5k, then I should go with 4:10. 3:73s would almost be a sure thing, assuming 1st, 2nd, and 3rd gears would continue starting out close to 5 grand like they do now...(except right now the 3:90s have me going through those three gears a tad too fast).

My latest time slips have me doing better in spite of my gear problem, but only because I did a pulley swap (2psi more boost) and had the car dyno tuned changing my rwhp from 327 to 411. Now, I am running 8.6 @ 82mph, but just imagine what that would turn into if I had the proper gear ratio. I am guessing a car with 411rwhp such as mine should be running closer to 8.0 @ ~ 86mph with proper gear. Now the debate is back on...do I go with the 3:73 gears, or the 4:10 gears??? Please Discuss...(and keep in mind trial and error in this business is expensive as I am sure most of you are aware :|

(Also bear in mind on the time slip below, my time is probably suffering 0.1 - 0.2 seconds from driver error...I am still learning how to take off from the line, and I think the psi in my Nittos were to high @ 19psi)
http://www.iowamustang.com/membercars/jonvanwyk/ <<< More info on my Mustang


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With a supercharger you want 3.73's the most. 3.55's are always good for supercharged stangs.

 

I don't really know what to say about the gears, I would never think that you would be at redline in the 1/8th with a 3.90 im third. Some questions. Whrn you go tthe nittos, they are the same height as stock correct? Where is your rev limiter set at?You also need to get that 60' down in 1.8 range. Personally I am a fan of big gears, they are what it takes to go fast. Assuming that the 411 hp is accurate you should be able to pull at least a 8.0-7.9 with a mph in the 88-90 mph range.

 

Thats actually a myth that seems to be rampant on this board, ask the people who have superchargers.

 

I agree my 60 ft times should be in the 1.8 second range (hence my disclaimer about driver error accounting for 0.1 - 0.2 second loss). Currently it is rev limited at 6,200rpm. Do you think it might be as simple as raising the rev limiter to, say...6,500rpm? I wonder if that would have me going through my first 3 gears just slow enough to be red line in 3rd gear in 1/8. Also, if that might be the answer, how does one raise the rev limiter?...that option is not on my diablosport predator tuner.

235/55/17 >>> Factory Pirelli on front
245/45/17 >>> Nitto Drag Radials on rears

I could not find a drag radial that was the same size as the factory Pirellis, so I felt these Nittos were as close as I was going to get without purchasing a set of 4 tires. Is it a big deal that they are not exactly the same? Racing season being over, I am putting the factory Pirellis back on the rears now.

 

Are the Nittos taller or shorter, this have an effect on you final gear ratio. You will need the tuner to raise the rev limit (such as SCT) but it needs to be a real tuner program and not one the hand held jobs. If it were me I would ditch the Nittos for either a BFG or the M/T drag radial. Nittos are nice but they don't work as well as the other two, and they are worse when you start to make some real power.did you ever post your dyno sheets? I would like to see them.

 

You think I should post all the dyno tests? The final one is posted on my site found above in my original post.

 

I am just interested in what the curves look like, and where peak power occures

 

the main reason I don't use 3.90's -4.10 in my s/c cars is because at 6500 I'm nearing 48k rpms on my s/c..seems like you need a taller tire to lower 3rd a few 100 rpms....