This topic is not discussed as much as it should and I was hardly able to get any information when I asked. I have run a search on Rennlist and although there is some data out there, nothing really comprehensive. I read claims about the Carrera ROW gearbox how better it is than the US gearbox, and how it "feels" like xx hp more etc.. But what are the FACTS?

Now that I have the chassis and engine sorted out, it is time for gearbox choices, to get the last fraction of a second or inch possible.

I have run over 200 computer simulations off all possible ratios and then tried to find a Porsche gearbox that would be the closest to the result so as to try to reduce costs. In my search, I kept the final diff ratio at 3.44 and as much as possible stuck with original Porsche's two only options for the first gear in 993s, 3.15 and 3.82. I will not bore you with details and numbers as I have hundreds.

Obviously a computer simulation is not the reality, but it works when you are comparing between different gearboxes with the same engine base. Also, a N/A engine might yield different results as the torque curve is so dramatically different.

Engine and gearbox builders do not have time to answer a zillion questions nor run hundreds of simulations, they will build it based on their experience, and they typically deal with race cars, on which the powerband is quite different.

Some difficult questions remain to be answered and I am puzzled by what is the optimum setup and the balance between 0-whatever and acceleration between gears..

Objectives:
1) Best acceleration possible 10-150 mph, rolling quarter mile runs, and fast track.
2) Solid gearbox to withstand 800NM of torque
3) Daily drivability

Some of the standard gear ratios I have analyzed are the following:
Carrera '96 USA (G50/20)
Carrera' 96 ROW (G50/21)
Stock 993TT (G64/51)
RUF 1997 CTR2
993GT2 (G50/54)
993 RSR 3.8 (G50/34)
993 Clubsport (G50/32)

The results are surprising, as I always thought that shorter gears are faster in straight line acceleration, they are not. Longer gears tend to improve the times related to reaching a certain distance, both in the quarter mile as a reference, and 0.625 miles (1000Mtrs.) where speeds reach around 175mph with the engine I am testing.

The winner in reaching both distances was the G50/20 Gearbox (infamous among the Carrera owners!) together with RUF, and both have the longest ratios.

0-60 mph , they are all quite close, with the stock gearbox beating and the others close behind. Difference between 1st. and 6th. is around 5 ft.
0-125 mph, again G50/20 beats everybody else, leaving the 6th. ranked, by around 20 ft. behind.

What is interesting is the speed between gears such as 30-50 mph, 50-70 and 50-90 mph in the higher gears , where the RUF is clearly behind, while the G50/20 is not too far from the 1st. (The Carrera Row and Stock 993TT).

From the above, I should be concluding that for my objective, the Carrera USA gearbox offers the best performance, and reaching top speed of around 200 mph, which can be taken to 220mph with a longer 6th. gear like the RUF.

The only caveat being the strength of the gearbox, which would need to have a stronger shaft and maybe steel synchros. All these tests were performed with the same engine and car specs (weight, tires, torque curve etc..)

Anyone wonder why RUFs are so quick? Other than a great engine, they do know how to get the best of a gearbox, great acceleration with little compromise for performance between gears.

Am I looking in the wrong direction and is there something important that I am missing in making my choice? I could not identify RPM drops between gears, but since the acceleration numbers are there, the drops do not mean much to me.

Let's hear the experts and also the less experts, share their thoughts and opinions!

 

Jean,

If you want to speak w/ an expert, talk w/ Brian Copans in Cincinnati, Ohio. His phone # 513-779-3302. He works for Porsche Motorsports of America and only works on Porsche gear boxes! He travels to all the Grand-Am races and rebuilds all the Porsche transmissions at the track. He has his own shop and can work on any Porsche tranny. He has any gear configuration that you could possibly want! He fixed my tranny when I dropped a synchro. When I want to move up a class in racing I would without a doubt let him pick the right gear ratios!! He as years and years of experience. You may also want to contact Glen Gatlin, he as some cup gears that would work in your car nicely! His rennlist name is Glen and his phone # I can PM you if you like? He knows these cars like no one else!!

Tim

 

Jean,
I'm looking forward to read this thread. I'm afraid I can't really answer your question, since I rely on people who know gearboxes. Interesting though, would be to know more about what type of engine (specs.) did you run your simulations on?

Cheers
Rassel

 

Tim,

Thanks, I have already spoken to Brian Copans, so I am running my own simulations and share the result with him for the rebuild just to save on telephone bills, using PW II gears.. He is looking for a GT2 gearbox for me as well. I read every post that Glenn posts, he is a wealth of information. I will contact him through PM.. Thanks for your help!

Rassel, the thread has picked up some steam on the 993 board. It seems hard to believe that a G50/20 with taller gears gets you faster through the distance than the TT box can, given all other things being equal. Many people with Carreras have gone through the swap for a ROW box for acceleraton purposes. Acceleration in between gears as mentioned 50-75 etc.. does suffer, very little though in a high output car. For Carrera owners the swap will be more beneficial, but not more than a 10-15 hp equivalent. Drivability will improve.

I was hoping some of our sponsors would chime in, it is not easy to defend with hard evidence the gear puzzle.

The simulations were run on a 3.8 Twin plug , Garrett based engine with an output of 600 hp using a real engine dyno curve.

 

Jean,

I've had this discusssion with Steve Beddor who races his RUF Ctr2, and I've seen his gear chart, which he was kind enough to share. Steve's gear box has lower ratios than the normal Ruf box. He runs different gear sets depending on the track which is somewhat contrary to what your findings are. Track conditions dictate that there is a more on and off throttle than justy plain acceleration hense the need for lower gear to get on boost sooner.

I've found that changing out 6th gear to a lower ratio, allows me to stay in the power band on long straights, rather than staying in 5th till a higher rpm. If your only after on throttle acceleration you might be correct. However, with on an off applictions of throttle, along with vehicle "feel" becomes important when cornering and braking. Did you simulations take into account different race courses?

 

JJB

I agree with you that there is no universal formula. Each application will benefit from different ratios. I was looking at the overall best combination. A Professional racer will benefit much more than an amateur or good driver since his shifting times are usually faster and they have short shifters, this will make a big difference. I assumed 0.5 seconds per shift and 0.2 secs for transmission engagement.
All of us race our cars at one time or another, we might not drag race, but we like to do some runs from rolling starts vs other 996GT2 and Corvettes etc.. For this particular application, taller gears will benefit you in reaching a distance faster, regardless of what the speedometer shows you, you will be ahead of the other guy.
Concerning the tracks, I have tested several, Nurburgring, Hockenheim, Laguna Seca, Le Mans etc.. the RUF CTR2 gearbox (Tall gears) fares consistently among the top 3, Nurburgring is probably the most representative, as it has many different types of corners, straight lines etc..
I have tested speeds 30-50, 50- 70, 50-90 and between all gears, some of the gears you fare beter , some others you don't since there will be shifting involved. Overall taller ratios will give the average to good driver a "faster" car. The CTR 2 gearbox is amazingly well balanced.

 

That's alot of research Jean, thanks for the info you saved me alot of time...

 

If anyone is interested in more reading on the subject, please visit the 993 board at the link

https://rennlist.com/forums/993-forum/173284-optimize-acceleration-gearbox-secrets-long.html

Having to repeat over and over again the same thing just became too time consuming, so I gave up but my gearbox builder agreed with the findings.. Net net, your best bet is to go for slighlty longer gears starting from 1st. with a 3.15. Only this option is quite expensive hence very few people do it. If you change 3rd. to 5th. you can have some improvement at a more reasonable cost..

You're welcome!

Did you get that membership done?