GTgears

Dunno Pete. It's just an excel file. Shouldn't be locked either. You can manipulate it as you see fit on your end before you save it.

Bill Verburg

It's probably a spread sheet, if so you need to convert it to a jpg or gif to post here. at least that's what I have to do.

Bill Verburg

To all the guys that want something different
What you see in the graph is a ss model of what is felt by thew driver when driving the car.
Inputs are engine rear wheel torque curve, max rpm(7800 here) trans gearing, wheel rolling radius(for a typical 295/30x20 tire), weight(3155# w/ driver), & aero characteristics(Cd .32, S 2.00). It is a model that is only as good as the input values which I tried to implement as accurately as possible.

It shows what you need to know, ie when to shift up, the actual shift points are where the tail of a lower gears curve crosses the next gears curve. You go a little long because of lags in the process but to remain in a lower gear after that is to have less acceleration available at wot,

the higher a curve the more acceleration is available from the car

stout

Thanks, Bill and Matt. Converted the xlsx file to a jpg and placed in the appropriate post above.

mark kibort

Tt doesnt really matter much at all due to the trade off of having to shift earlier and then being in a much lower rear wheel force range for the difference of the two gears in the higher speed.
In otherwords, there is a trade off.

as long as the car is in the max HP range, it will be accelerating with the same forces for a given vehicle speed, over all.

mark kibort

yes, that small change doesnt do much . (less than 2%) yes also that most owners dont rev to redline, but they should if they are wanting the most performance out of it. its a safe limit, especially how infrequently on the street you can run to it, and certainly not for any real period of time. so, now i understand your "barely 5000rpmm" comment.... still the GT4 upshift is 5500rpm plus, not 5250. if you run to redline.. and with the 2.7, yes, its much worse going to a 70% post shift RPM vs a GT4 72% post shift RPM...........and if you continue to use your "detuned" RPM.. (dont to that, its counter intuitive to performance of the car) yes, with the 2.7 gear box, the post shift is 5250 at a short shift of 7500... but who does that if they are wanting the most out of the car.
(edited for the detune matt used and the 2.7 vs GT4 stock comparison)

mark kibort

Good graphs again.. did you use the stock factory dyno output data or an actual run... the ones ive seen on the dyno dont seem to fall off that much at max RPM ranges, but if you go off the stock graph, there is some short shifting that is needed, but almost negated if you factor in the time to shift, which you mentioned. good stuff again!

Mark

Bill Verburg

Here's a different view w/ some additional info, of the same data, I use the actual rolling radius of the tire rather than the unmounted OD. The thing that pops here is the fuel economy nature of 6th and the erratic nature of the drops


Just for comparison here's what a pretty well geared race box, g50 /30, used in the 993 Cups would look like

GTgears

Bill,

The chart provided to Pete does NOT change the stock 6th because he has indicated that it is already too short. Furthermore, the version you have reposted is only changing 3-4-5 because of the mainshaft constraints and should be analyzed within those parameters. It is intended as a workable solution to DE drivers wishing to drive to and from the track on the highway.

GTgears

PS. What you really should be looking at is the 2nd chart, the one including a mainshaft proposal. The root issue Pete is chasing in this thread is the tall 2nd...

stout

All I'm looking for is how to replicate the effective ratios of 1-5 and 7 in a 991-1S gearbox within the GT4 transaxle with its stock R&P. I think that setup would be the bees' knees.

Btw, some "friends within" are watching this thread, and one noted that while the 2.7 gears may be strong enough, the rest of the 981 2.7 transmission is not. I am not sure what, exactly, is the issue, but I thought I'd pass this along. Curious as to Matt's thoughts.

GTgears

It doesn't have a cooler stock?

stout

I got the sense that it had something to do with the case—or something besides just the gears. The cooler can be added pretty easily, no?

GTgears

Quite easy to add a cooler and should be done on any car tracked regularly. The factory cooler is inadequate.

Skeptical about a weak case. Might just be marketing spin.

mark kibort

I still dont understand how the stock gear box isnt optimal, aside from the large second gear drop of RPM, which in most cases is a non issue . And if you want to shift more often, you can anyway, due to the high strung nature of the engine. aside from a brief period of missing the HP band out of 1st and into 2nd, for 5-10mph, (between 45mph and 55mph) you are right back in it and in it all the way til redline in 6th gear... no gear changes will optimize these other gears any better........ and i have seen and driven, the spacing seems to be optimal for most tracks. PLUS on the street , i would think the best advantage is the taller 6th,which gives less freeway whine at hyway speeds. otherwise, you are changing gears for the sake of changing and not really tightening things up or helping acceleration (full throttle ) anywhere in the RPM vs speed range. (except for that little area between 1st and 2nd).. so , is that worth the cost here of the change??? one could argue not.
just curious as the discussion continues.... by the way, Bill's spreadsheet is very nice having most of the data right there to analyse. my advice is to drive a car with the closeness in the lower 1-2nd gear range and see how awkward i can be. is i mentioed, i have a street car with the same gears as the GT4 and 1st is really low. (12.7:1) and 2nd is the same as the GT4 as well. its nice to get out of the starting gear and into the reall gears at a RPM and torque that doesnt spin the wheels as much and has the perfect spot to engage max HP ranges, which is around 55mph (after the first few hundred RPM of 2nd)
imho