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Eric
Just to correct the record, 60 is reached in 2nd w/the G50/21 well before redline.

Jean

Great insight Mark.
I might need to clarify some of my comments. Although my english seems good, it is not easy for me to explain myself clearly

I fully understand that shorter gear ratios make you accelerate faster, but within a certain gear. I have driven rallye cars and I can vouch for that.

What will change the equation for an acceleration run over a longer distance are a number of things, among which the number of shifts needed, RPM drops, and whether you keep your RPMs within the optimum torque and hp area in each gear change.

My simulations were run to find out which complete gearbox would take me faster from point A to point B , A to C, A to D etc.. regardless of the number of shifts needed, since these are factored in the model already. Across all distances, the G50/20 seems to be showing the fastest time, therefore factoring in the number of shifts and time lost for them, it still comes first.

What I meant by is that I am maintaining all the time my RPMs within the optimum powerband, torque and hp, due to the particularities of the flat curve of my engine above 4000RPMs, therefore RPM drops of 200-300 RPMS more or less will not make substantial difference.

Mark I agree that the ideal is to maximize the curve under the gear changes, which my specific engine does.

We should remember that I am using the same engine and all other parameters equal, and just changing gears to find out what balance works best between having shorter gears and shifting more often, vs longer and shifting less. The second option resulting more beneficial for time and distance all the way to 175 mph and across the whole distance. These benefits outweigh the slight performance shortage seen when accelerating between gears if I compare to the best result obtained. Therefore this gearbox seems to be the best balance for track (coming out of corners etc..) and acceleration.

If I read what I just wrote I would not understand a word

Edit: All of this applies to a (heavily) modified 993TT engine, and might not be true for a Carrera engine.

Jean

To put things into perspective, A Carrera US G50/20 with 300hp and 10 lbs torque more and a Carrera ROW G50/21 with 282hp, the US car will beat the ROW all the way up to 125 mph at least, therefore a gearbox swap US vs ROW does not represent more than 10-15 hp equivalent improvement at a maximum. aceleration between gears will be with mixed results.

Bill Verburg

I looked into that sort of thing when considering transmission gear changes and rear wheel sizes. There are obviously many ways of looking at this but I decided that potential forward acceleration which is directly comparable to forward thrust was the primary objective. This is one of the charts that I came up.

Comparison of available forward thrust w/ 275/40 and 265/35 tires.


The #s are the speed at which max thrust is available, rpm could as easily been plotted.

Note that there is a x-over speed at which the taller gearing from the 275/40(blue) tires offers better acceleration at a given speed

Best gearing also depends on the track, LRP and Watkins Glen have very different gearing needs.

Monique

The numbers don't sound reasonable to me. My RS is quicker than a US 282 HP car... independent tests state so.

Also, why would F1 teams shorten their gears to accommodate the slower circuits... before the quick shifting paddles came along.

What is the desig of the GT2 orig box? What were the ratios? Good luck.. I think your computer model is faulty.

Jean

Bill

That's a very good chart. Putting the smaller tire acts like shortening the gear, you will transmit more torque to the ground, and as you mention, comes a point that the taller tire will give you better numbers maybe due to bigger momentum.

Monique, you are comparing 2 different cars, yours is lighter and has different caracteristics than a carrera, you definitely beat it. You will beat as well a Carrera ROW specs if you are strictly accelerating. The difference is quite negligible however. On a high HP/Torque engine, the difference is more substantial.

Why would RUF put taller gears? I don't think the model is wrong.

Here are all the ratios:
1st. 2nd. 3rd. 4th. 5th. 6th.
Carrera '96 USA G50/20 3.82 2.05 1.41 1.11 0.92 0.78
Carrera '96ROW G50/21 3.82 2.15 1.56 1.24 1.03 0.82
Stock 993TT G64/51 3.82 2.15 1.56 1.21 0.97 0.75
RUF CTR2 3.15 1.79 1.27 0.97 0.76 0.60
993GT2 G50/54 3.15 2.00 1.44 1.13 0.97 0.83
993 RSR 3.8 G50/34 3.15 2.00 1.52 1.121 1.027 0.89
993 Clubsport G50/32 3.15 2.00 1.52 1.24 1.03 0.83

Hope it helps.
Thanks.
Edit: Sorry about the format, couldn't get it any better.

Bill Verburg

No, the torque applied as the input to a given gear is dropping past the torque peak, speed of the vehicle continues to increase because rpm continues to increase.

The result is that in a given gear there is a point at which the taller geaing can provide more thrust.

It is a similar situation to the need to shift up a gear to continue to accelerate even though the new gear is taller.

sonny1

Jean, what about a RUF EKS tranny?? no clutch pedal, just shift as fast as you can it doesn't miss a shift.,cheers,Sonny.

993RS

IMHO: the 0 - 60 mph times are sexy, but not really all that real life relevant. In everyday life, your car feels fast when you have power over the whole rev range and not just like racing engines which display a peaky torque curve. The closer gears give you this lower and mid rev power feeling. When racing you are always keeping your revs up high, so that the gear ratio differences do not make that much of a difference. In acceleration tests, I would expect, not much difference during the initial phases, then the closer gearing pulls away up to a certain speed where the longer gears catch up and, in their turn pull away. So, when racing, if the track does not have long straightaways, the closer gears should be better, and vica versa.

cojackin

I think the reason that RUF put taller gears are because thay have so much hp and tourque that short gears would do them no good.

Jean

Bill, I understand your point, very interesting. Thanks.

Phil,
Shorter gear ratios will obviously give you higher G's when you are in a certain gear and decide to smash the throttle, there is no debate, it is physics.

Where it all changes is when you are having to shift through the gears to reach a certain point. I am not talking about 0-60 only, I am talking about 0 - top speed, quarter mile etc.. It does not mean that you drag your car, it just puts things into perspective. At the end of the day, why do we modify our cars and gearboxes if it is not to improve performance.

Shorter gears definitely has its advantages, it also has its disadvantages on drivability as you have to change gears more often. On mountain roads like in Switzerland (I grew up in a similar context) the shorter gears will be very pleasant (forgetting fuel consumption and engine/gearbox usage)

Also, what I was saying was that a gearbox swap to ROW will not represent more than 10-15 hp equivalent with respect to acceleration across a certain distance. A similar US vs a ROW Carrera, the ROW will beat, add another 15 hp to the US Carrera and it becomes the other way around from 1st. to 6th. gear, not within a certain gear.

I am not rying to defend any of the 2 options, just having a healthy debate, in fact it is cheaper for me to keep my stock gearbox!
We have not heard from any tuners/ builders, I guess hard data and evidence is difficult to put on the table.

abar

Jean,
I suppose your initial simulations are using dyno output from a 993TT? If so, it's not suprising that given that amount of torque / power and hence thrust at the wheels can take advantage of longer gearing.

Do you have the data for a standard varioram 993 with the various g/boxes?

Jean

Abar

Yes, they are based on a 993TT, I repeated it often to avoid confusing the board.
As far as 993 Varioram numbers, I will share a few. Acceleration difference between ROW and US is not significant, a good driver shifting quickly vs an average one driving the ROW would offset that difference easily. Beyond about 100 mph, the G50/20 starts beating the ROW car (which might be in relation to Bill Verdug's great chart). Add 15 hp to the US car and it will be beat the ROW across all distances.
MPH

0-30 0-60 0-80 0-125 0-150

Carrera G50/20 US Carrera G50/21 ROW Feet Carrera G50/20 Carrera G50/21
This is a computer simulation that is using exactly the same parameters for both. Whuile the times might not be very accurate, the comparison is.

914und993

Jean -

I think you are spot-on with your gearing analyses. The overall acceleration advantages of shorter/closer gear ratios are badly degraded by the additional shifts required - unless as Mark has pointed out the gearing has been chosen to optimize the car to particular turns in a particular track.

Psychologically, the "Butt Dyno" ignores shift times and is only impressed by in-gear acceleration, which at normally experienced speeds the shorter/closer gears are more likely to excel at. This does make the car more fun, but of course that is not what you are looking at.

Chip

Carrera Mike

Jean, OT... did you make it at the GT Festival in Bahrain??? I missed that by 2 days.. I'm in Spain now.. good luck in your quest for the perfect gearing...