82

 

Thanks. I agree lower would be an improvement!

 

Since GTgears announced the 36:16 ratio for his new 2nd gear, I thought it would be interesting to compare it to stock 39:20, using engine data from the owner's manual. Below you can see acceleration data for stock (blue) and GTgears 2nd (red), assuming shifting is done at 7700rpm (100rpm below max). Other data included is engine power (bottom set of curves) and rpm (zig-zag curves). Vertical steps indicate shift speeds.
It is clear that there is higher forward acceleration due to the 2nd change, but also the shift into 3rd will occur at lower speed. Say, if one wants to accelerate to 80mph or beyond, does the change actually provide an advantage? In the 70-80mph range, the new gearing requires 3rd, whereas stock is still in 2nd! So, it's clearly faster to 70mph, but there will be a bit of lack-of-go up to 80 (all relative to stock, of course).
Hmm? Advantage?

 

Thanks for doing this! is that with a stock 3rd gear or a GT years lower 3rd gear? It would be cool to chart stock vs GT gears shorter 2-5 gears....

 

Having compared the GT4 stock transmission/power curve with a few scenarios, I believe that the stock configuration is a balanced street/track optimum. For example, the low speed acceleration can be improved, close to the 991.1 GT3 level and above the 997.2 GT3 level via the 981-2.7 manual transmission.


Of course the on-par GT3 is only there in 1st gear (not even all of it) and would only help to break loose and smoke tires, but not in "real life" above 40mph or so. In the end, engine performance wins.

That said, with a 981 2.7L transmission, the GT4 would get quite a bit closer to the 997.2 GT3, but there also, eventually higher engine performance wins out - not as much as with stock GT4, though.

FWIW, check the "long 2nd gear" in the 997 GT3. Vmax(2nd) is another 5% above the stock GT4, but noone is complaining about long gears there. It's the engine, fundamentally.

In summary, for track use, the GT4 would benefit from the 981 2.7L transmission at lower speeds and make the car feel & perform quite a bit more aggressively, with unchanged same performance above ~95mph., as well as same top speed. Slightly lower acceleration only between 42..47mph and 76..80mph. No big dip around 70..80mph as with the 16:36 2nd-gear-only. The latter is in the speed range that gets crossed essentially out of every corner on track.


What would the reliability of the 2.7L transmission be in a GT4? Anyone know?

 

Whoa, nice! I liked the idea of a 2.7 transmission swap - I think the clubsport transmission is actually a 2.7 pdk with the 7th gear locked out....

 

I don't see anyone who goes to the trouble of pulling the transmission and getting the GT upgrades done just doing "2nd" gear - as your charts show that simply would not make sense.

What does make sense is to change out 2nd - 5th to optimize performance, on and off the track - leaving the current sixth for freeway driving. At the same time I would expect most to add a GT LSD.

At that point you have a high performance, bullet proof transmission - albeit at a cost. But it will make the GT4 an even better car than it already is. Ultimately we are lucky we can optimize our transmission with a full build with a proven product that has had all the time and R&D put into it by a top Company.

 

That brings the question what the other ratios should be if the 16:36 is the starting point for the modification. Look at the clubsport, i.e. the 981 PDK (7th locked out). The PDK 2nd is 2.29 vs. 36:16=2.25 - only 1.8% different. Let's call that equivalent.
Using the PDK 1..6 ratios clearly results in a much more aggressive setup. About 18% more acceleration in 1st and 2nd. That is huge!
Keep in mind, though, that during a typical acceleration run, say 35mph-100mph, there is one more shift required than with the stock ratios. That will likely cost more time than what is gained from the ratio change. The latter doesn't apply so much in the clubsport case, because the PDK shifts so much faster.

 

Last comment, to (somewhat) conclude the subject for now.
Let's just say we'd assemble a custom GT4 box from:
1: from 981 2.7
2,3,4,5: same ratios as 981 PDK
6: stock
Final drive as is (common to all 981's anyhow).
What we'd arrive is basically a manual 991 GT3 at 10% lower speed across the range. Note that gear ratios 1-4 are the same between 991.1 GT3 and 911R.So this would give us essentially a "981R" spec performance car.
That sounds like a very fun proposal.
(edit: the original post noted gear 2 as being 16/36 from GTgears, the simulation picture below uses gear 2 from PDK)

 

81 MPH

 

cmosman, did you include aero drag? I expect acceleration to tend to zero at vmax.

 

Good observation.
The model here only includes engine, gear-ratio, vehicle weight and (rear) wheel/tire data, all as reported by the Porsche owner's manuals. Vehicle weight used is what is reported + 180lb for the driver + 50lb for additional equipment, not assumed in the factory number. Additional assumptions made were: 93% gearbox transfer efficiency and 98% loaded wheel radius.
All other forces that act on the car are not included, predominantly aero loss and rolling resistance. The cars are all very similar in cw*A and aero loss is very small in the gear 2 & 3 range. I believe the conclusions will not change if these losses were to be included. ..

 

IMO 93% efficiently may be optimistic.

 

Of course 3rd gear is going to suffer if you only change 2nd. It's a system and must be treated as such.

Since there's no actual listing of the PDK ratios here, just reference to them being good, I can't really comment one way or the other...

 

Just an update to let you know that BGB and Sharkwerks should each have a prototype mainshaft kit in their possession before Christmas.

We are planning to mate it with our existiing 1.60 3rd, 1.33 4th, 1.10 5th, 0.92 6th.