Hans Frisak

After last driving on track some weeks ago I got a rattling noise in the transmission. Two teeth had broken on the third gear. The 5th gear is also worn so it also needs to be replaced. Porsche had no new gearboxex availble, so overhaul of the old box will be done. All bearings will be replaced and steel sync-rings fitted.

Having to replace two gears I wonder if I should replace 3rd, 4th and 5th gears to shorter ratios keeping 1st, 2nd and 6th. The overhaul cost is high, but the cost for replacing 3 rather than 2 gears is "relatively low".
Most driving on the track is in 2nd, 3rd, 4th and 5th gears anyway and a long 6th is good for motorway driving. All sounds good - in theory.

My question is, have any of you any experience with this and if so any recommendations. The Workshop has contact with RUF for new gears. I do not know the ratios yet.

Jean

Hans, welcome to the club of damaged gearboxes

I would not touch the ratios. Based on your description and the videos you posted, I don't think you use 6th at all so I would not change it either. If you are reaching the end of the longest straight within your 5th gear rev limiter, no need to change anything, if you are having to put 6th gear before the breaking zone (for a second or two that is), then I would put a taller 5th.

Shortening gears will just make you change gears more often and loose time, these cars have great torque at 4K RPMs, you don't need more, traction becomes an issue.

All of this with the caveat that your track configuration has to be carefully analyzed on a datalogger, check out corner exits before the straights in order of priority, the longest first, do you start way off boost while exiting the turn before the long straight? How many gear changes do you usually do, and how would a different configuration change that.

For straight line acceleration , I would not touch my ratios, if anything, slightly taller 3rd and 4th.gears on a modded 993TT.

All of this is just an opinion, you need to do the homework. Cheers

Geoffrey

It is not uncommon to break the teeth off 3rd gear in a car that has been tracked quite a bit since it is the gear the car spends the most amount of time in. The gears do fatigue so I'd consider replacing 4th as well. If you use 2nd on a regular basis (I didn't in mine) then I'd consider replacing that too. I'd look at Guard gears since most of their 3rd gear ratios are thicker than the factory part and can withstand more torque for a prolonged period of time. They will require a special bearing and race from Guard for the 993 mainshaft, but if you are replacing the bearings anyway, it shouldn't matter.

Hans Frisak

Thanks for your thoughts guys, this is excactly what I was looking for. My car has maximum torque at 4500 rpm and a relative flat torque curve from 4000 to 6000 with peak power at 5700 rpm. I usually drive on two tracks where I use 2nd, 3rd and 4th on one track and 5th for å few seconds on the other track. With a shorter 3rd I would save one gearshift on both tracks (3rd instead of 2nd). I would possibly need to shift from 3rd to 4th during the exit of the corner before the longest straight on one of the tracks which is not good.
On the icetracks, a lower 3rd would be better, but again that is no competition (well apart from with my son which can be serious enough).
Recalling some of the corners of my frequented tracks, there are no corners where I am without boost. Traction is more an issue than power.
I am still investigating my options, but your input is most valued.

sparetireless

and leave the individual gears alone? Should be better acceleration with a top speed still plenty high.

TB993tt

My 3rd gear cracked a couple of years ago and now 5th and 6th are making noises so a rebuild is on the horizon - below is a "special gear" set which RS have specced for me for maximum acceleration up to 210mph......
I have not yet enquired into the costs of stock versus special gear sets and would be interested if anyone can throw some light of the relative costs since I really don't need special gears other than to post an outstanding KPG special 1 gearchange 60-130

Geoffrey

Toby, what do you mean by "special" gears? Do you mean non stock ratios, or do you mean aftermarket gears ie. Guard?

I have a mixture of gears in my trans.

TB993tt

I guess I mean "non stock" - I know nothing about gears, RS called the diagramm "TB spezial" so I went with it

I would be interested in a discussion, any information about the options, standard/aftermarket with associated costs/benefits etc.....

Geoffrey

It has been my experience that a 5spd is a better transmission for a track turbo due to the breadth of torque available. So I would gear 5th to the longest straight and sixth appropriate for the 1-5 gearset to give you an overdrive gear if street/track.

Toby, on your gearset, I'd like to see a lower RPM drop as you go up in the gears, meaning the 5-6 gear RPM drop would be less than 4-5, 3,4, 2,3, etc. This helps keep the acceleration up as you go through the gears since there isn't such as great a torque drop between gears. You can take your torque curve and plot axle torque against the gear and can determine the exact torque drop you will experience for a given gearset.

I would certainly recommend the stronger 2nd, and 3rd gearsset from Guard, especially with a higher HP turbo car, however, you'd need to ensure that someone familiar with using these gears assembles the transmission. Some of the gears are thicker requiring machining of the spacers, sometimes special bearings are used as well. Cost wise, you aren't going to see a large difference in price between the Motorsport gears and Guard gears.

JJayB

I geared my transmission for the track that I frequent the most. I've gone with a short 6th gear for long straights and it works quite well, just ask any Viper driver.
I know Ruf has a wide range of gear ratios that they've used in Steve Bedor's yellow bird.

Ima2nr

I spent approx 10k regearing my entire box...

1st and 2nd are longer...
3rd thru 6 are shorter...
(my gear box is now identical to a GT1) the longer 1st and second definetly help 0-60 wheel spin)

the acceleration is awsome... I had brian copans do the work... the major cost was in the parts...

Hans Frisak

Thanks for advice.
Here is an update of my project. I have considered your thoughts Jean, and I have also taken the advice of changing more gears that probably are worn. I have chosen to change gears 3-4-5 with Porsche Motorsport gear from GT2 evo. The gears are marginally more expensive than stock. They are made for motorsport and are shot peened to better withstand wear. Here is a table of available ratios.

Sorry about the table columns, they are pasted directly from excel.

911 GT 2 Evo motorsport gears.
(gear/number of teeth/number of teeth/ratio)
1st gear X 13 41 3,154

2nd gear 19 41 2,158
2nd gear X 20 40 2,000
2nd gear 22 39 1,773

3rd gear 21 33 1,571
3rd gear 23 35 1,522
3rd gear X 25 36 1,440

4th gear 26 34 1,308
4th gear 29 36 1,241
4th gear 30 36 1,200
4th gear X 30 34 1,133

5th gear 32 35 1,094
5th gear 32 33 1,031
5th gear X 33 32 0,970
5th gear 34 32 0,941
5th gear 34 31 0,912

6th gear 35 31 0,886
6th gear 35 30 0,857
6th gear X 35 29 0,829
6th gear 36 28 0,778

Ring/pinion X 9 31 3,444
Ring/pinion 8 32 4,000
Gears marked with X are series equipment for GT2 evo

I have chosen the following ratios compared to stock 993tt
993 tt/ GT2 serie/ Frisak
1 gear 3,82 3,15 3,82
2 gear 2,15 2,00 2,15
3 gear 1,41 1,44 1,52
4 gear 1,12 1,13 1,24
5 gear 0,93 0,97 1,03
6 gear 0,78 0,83 0,78
Final Ratio 3,44 3,44 3,44

This gives the following differences between gears:
difference 993 tt/ GT2 serie/ Frisak
1 - 2 gir 78 % 58 % 78 %
2 - 3 gir 53 % 39 % 41 %
3 - 4 gir 26 % 27 % 23 %
4 - 5 gir 20 % 17 % 20 %
5 - 6 gir 20 % 17 % 33 %

Speeds are as follows:
wheel 1970mm
Speed in gears @ 6800 rpm:
993 tt/ GT2 serie/ Frisak
1 gear 61 74 61
2 gear 109 117 109
3 gear 166 162 154
4 gear 209 206 188
5 gear 251 241 227
6 gear 301 282 301

It is on order and I hope to have the car back before easter.
I will keep you posted!

Stummel

TB, you can see some approx. costs on the albert homepage.

for example http://www.albertweb.de/Porsche/Getr...tzungenGT3.htm

it is very confusing homepage, you have to search a bit around.

Hans Frisak

I have now driven the close ratio gearbox on track. I improved the laptime with one second (1;02,62 compared to 1:03,75). Whether this is all due to the gearbox I do not know.
I felt the difference in ratios on the road, but to be honest I did not immediately feel much difference, it is 8 months since I last drove the car on a tarmac track.
The chosen ratios seem to be right, the power is instantly back on when gearing up.
I did not manage to keep a higher gear in some turns as I hoped, but in other turns the chosen gear was more suitable. There were no more gearshifts than with the old box.
All in all the car feels even more "on attack" than before. Looking at the power curve I end up about 25 - 30 hp higher on the curve when gearing up. This is ofcourse on a small part of the powercurve so that the average increase is lower. However the dynamic part of the powercurve benefits of higher revs when engaging a higher gear giving less turbolag in gearshifts.

My conclusion is, yes the close ratio gearbox it is better, but does not justify the additional cost.
Then again, the extra horsepower above 550 hp is quite expensive and the additional cost for GT2 gears is probably about equal to a equivalent increase in horsepower.
I am working on a video where you can see the car in action with the new close ratio gearbox.

Jean

Great feedback Hans, thanks.

I have two questions,

1- Where you able to find out where is it that you are gaining time on the track?
2- How many gear changes into 4th and 5th do you have on this track? Sorry if you posted it before.