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Final Drive Modification


Flying V

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Hi guys! I'm planning to modify the final drive on my '99 GT3 MK1. Has anyone done the same to their car/s or has knowledge regarding this kind of mod? I'm curious what kind of gear ratio would be recomended and what to expect in accelaration performance likewise how much the top speed will be compromised? Leads on shops that carry such mod parts will be highly appreciated aswell!

Thanks and Godspeed!

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If you put in a shorter final drive, you will increase your rate of acceleration but decrease your top speed. In the Excellence Porsche mag there are ads from many tranny builders throughout the country.

I have yet to have my car over 140 mph. How often are you near the red line in sixth gear?. To calculate top speed, (engine rpm) /(final drive ratio) x (circumference of rear tire).

For the 08 911 final drive ratio is 3.44:1 so calculated top speed would be: Assuming red line at 7000 and 265 40 18 tire. Top, 6th gear has a ratio = 0.8:1 so the final drive gear is 4.3:1

= (7000 rev/min)/(3.44 engine revs/tire rev)X(82.74inch circ/12 in/ft)

= (14,030.5 ft/min)x(60 min/hr)/(5,280 ft/mile)

= 159.5 mph

If you would go to a final drive gear of (4.55:1) x (0.8) = final drive of 3.64:1your top speed would be;

= (7000 rev/min)/(3.64 engine revs/tire rev)X(82.74inch circ/12 in/ft)

= (13,259.6 ft/min)x(60 min/hr)/(5,280 ft/mile)

= 150.7 mph

If you were to shorten the final drive gear, first gear would be almost useless, I would look into shortening maybe 4th, and definitely 5th, and 6th gears. If you look at your owners manual, there is a chart of the gear ratios and speeds associated with them. You will notice that the curves on the graph are longer as you shift towards 6th gear. This is analogous to the time spent in that gear while accelerating, Shorten the gear, shorten the time to accelerate through it.

Jim Greer

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Jim is a smart man :)

There really is no need to change the final drive. We do EXACTLY as Jim suggested and shorten 4-5-6 :)

I can supply you with contact info for some of the best tranny builders here in the USA.

Send me a PM if your are interested. PRO people with 20-30 years of Porsche gearbox building.

B

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Jim, Brad,... thanks for the great inputs. It allowed me to further evaluate my situation and what I really want to achieve. I would just like to share with you guys the conditions that I live with in terms of how I can use my car ... and hopefully you can further give me your suggestions.

When I use my car for fun drives and track days, I hardly can push the car to its limits. Right now the race track we go to is quite a short track and we barely use the upper gears (I only go upto 4th gear in the main straights). During fun drives, due to limited nice roads, we end up doing short spurts and it is seldom we reach the top speed of our cars.

Based on the above, this is why I was opting to change the final drive of my car (to 4.00:1). Basically I want to gain better acceleration when I'm in 2nd, 3rd and 4th gears.

What you guys think?....

Thanks and Godspeed!

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Mr. V :)

I need to ask you a few questions about your driving style before I recommend gearing solutions :)

and here is why: 90% of the GT3 owners here in the USA have never experienced the rev limiter in any gear. This means they never experience 7800 and above RPM. I watch and see them at track events all around the USA (including AutoX's) rarely do they actually "use" the engine.

My concern: are you using your engine? You should be bumping the rev limiter in the lower gears even at your short tracks or spirited runs. The engines were never really designed to do anything below 5k RPM :lol:

Let me know how you drive, I'd hate to point you down a certain path, when you may not be utilizing what you have to the fullest extent.

B

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Mr. V :)

I need to ask you a few questions about your driving style before I recommend gearing solutions :)

and here is why: 90% of the GT3 owners here in the USA have never experienced the rev limiter in any gear. This means they never experience 7800 and above RPM. I watch and see them at track events all around the USA (including AutoX's) rarely do they actually "use" the engine.

My concern: are you using your engine? You should be bumping the rev limiter in the lower gears even at your short tracks or spirited runs. The engines were never really designed to do anything below 5k RPM :lol:

Let me know how you drive, I'd hate to point you down a certain path, when you may not be utilizing what you have to the fullest extent.

B

Hi Mr. B! ;)

If I lived in the U.S. (with my GT3)..then I would be part of the 10% :lol:

Yup...I make sure I use and maximize what the engine has to offer. In track days and fun drives, to bring out the true potential of what my tranny and engine can offer, I make sure I always hit the redline in each gear.

So what you think?

Mr. V

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I'm glad to hear it :) Let me get a quote together for you on just the gears. I have not priced them in several months and will need to see were they are right now price wise. The price doesn't change for different ratio's.

Now that I think about it, I'll get you a quote for building the tranny with gears also + shipping!

Check back here Monday night our time, or Tuesday :)

B

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