Got the excel gear chart and my HP and Torque graphs.

So what is the ideal rev change gear to gear? I don't plan on changing gears for every track, just want to optimize the gears for my motor.

If there is good torque from 4000 to 6000, should each gear fall to 4K after shifting at around 6500?

Thats a start, since we dont have the luxury of swapping gears for each track, you have to com up with comprimises.. Think about what tracks you run at most often. Then pick the top speed you think you may reach, when you are driving at 12/10ths. Find which gear this is at redline.. Then choose your other gears..

For example I picked Watkins Glen to build my gear set. I approach 140 ( give or take) at the Bus Stop ( the fastest point on that track), I do have some rpms left on the table, so I CAN get faster if Im not a chicken shit and dont lift at turn 2.... anyway..

Since you have your HP and torque charts, pick your gears so that the shifts drop the RPM onto the rising part of the torque curve, As you grab higher gears you want to be moving up the torque curve. The car get exponentially harder to push through the air the faster it is going..

I have A GA K Q V in my 911, and A F M S X in my 914.. these are comprimises... At Watkins Glen or Summit I have an active tranny, at Lime Rock, Pocono, or NHIS I have a 3 speed gearbox

Oh, not sure what power you have in your car, but I shift at about 7500 and the RPM never goes below 4000

Dave, I'll help you. What do you have now??

I think Tim's advice about keeping in the powerband and also to first identify your "required" top speed and pick your top gear; then work down from there, is right on the mark.

I would add that if you are really gearing for a specific track that you drive most often, then you want to try to figure out what speed you can/should carry thru the most important turns there, and make sure that you will not need to upshift at an inappropriate moment in that turn.

Most of us are not intense enough to have different boxes for various tracks so you kinda just space out 3 & 4 to fall fairly evenly rev-wise between 2nd and 5th. As you know you can change 2nd if you go to a 904 mainshaft but finding gearsets may not be easy/cheap.

Lastly as you also know, tire & wheel size (overall circumference) has a significant gearing effect. If you have several sets of wheels you can actually use them as a fine-tuning tool by running different heights. IIRC in my car this makes a top-speed difference of >10mph.

Stock box of unkown origin.

I have a spare that I plan on rebuilding. First I want to run a couple races and see how she runs.

I'm in the throws of this right now, too.

Carol Smith has a lot to say about it, so if you have his books, check them out.

Short story, for us with not enough data to do it properly, is:
-Guess at top speed for a given track(s), I pick M-O and I know the fast EP guys hit 132 or so
-Pick your shift RPM (conservative estimate is ~200 RPM past peak horsepower)
-Since the 901 box can only easily change 3, 4 and 5, space them so that the drop in RPM is lower at higher speeds (just as it was stated before because of aerodrag). If you have more ratios to change you can do a little more to analyze it.

The tweaks from there are track specific and can only be done with more data on corner exit speeds and lengths of the straights.

Just in case you didn't see this:
http://www.coloradoscca.org/prodcar/viewto...opic.php?t=3523

I'm very excited about the new box I'm getting built this winter...

AJRS in grand junction, has been making a cool 5spd box for a few years now, with reverse removed and F down in the 1st gear location.....

This allows nearly a dual purpose box for our local tracks...

On our local short track you can run 2nd, 3rd, 4th
and on our local long track you can run 3rd, 4th, 5th

Its not a cheap conversion but it gives you the same benefit that a 6speed box would have.... (but with the loss of reverse gear)

I know this didn't really answer your question, but it might be something to consider.

What is said above is correct, take your top speed and then plot DOWN from there while trying to gap as evenly as possible between gears (oops.. taking into account the aero)

brant

Interesting gearbox you've got goin' there. Not legal for SCCA, though.

No offense Brant, but I don't think you want them spaced evenly, due to aerodynamic drag, which increase to the third with velocity.



TimT,
What's a active tranny?

some people call a tranny in which all gears are useable "active", just something I picked up somewhere along the way.

At Watkins Glen I use all 5 gears, at Lime Rock I use 2,3,4

Ooops..

I agree you probably don't want to space them out evenly.....

So does SCCA require reverse then?

brant

For some reason SCCA requires: "All transmissions will have a functional reverse speed/gear." There is a provision for a driver operated lockout.

So, TimT, you've managed to assemble a gearbox where you use 1st. Excellent. I've been trying to understand if people do that or not. Good to hear that someone does. Care to share any details? Is it a modified 901 or a Hewland? What R&P? Some have told me I don't want to be rowing through all of the gears, but I find that hard to swallow.

Kevin,

If you buy a 904 you can then buy and use 1st....
depending upon your tire size and redline, a lot of locals use a "C" 1st gear on rolling starts
b

Ignorance here....

Is the 904 a different layshaft? Does it require a different 3, 4 and 5?

No such thing as ignorance (except when I'm talking)...

The standard 901 tranny main shaft has 2nd gear fixed on it.

The porsche factory built some of these mainshafts with a removeable 2nd gear and presumably ran them in the 904 car... (I'm not sure if they ran them in that car or not)

but either way they are commonly called a 904 mainshaft and were used in a lot of racing boxes....

They are available mostly in reproduction, but they are out there... With the ability to change the 2nd gear ratio, then its realistic to change first also.

(3rd/4th/5th can all remain the same or change upon your need)

brant

Thanks, that helps.



Another ignorant question....

Is there any reason one cannot mix a transmission with dog rings and synchros?

For example:
Synchro 1st and 2nd, dog 3, 4, and 5

Heres a screenshot of the gears in my 911, notice how each shift point is a few hundred RPM higher that the shift before it? this is keeping the engine in the sweet spot.. I have the dyno graphs for my engine, I guess I could overlay the gear graph over the torque curve, with every shift the RPM drops to a stronger point in the toirque curve..

Its been a few years since I geared that tranny, but if I recall the torque peak was at about 6500 rom, its good to leave some RPMs on the table to allow for lessening loads as you drive, and maybe change in wind direction etc.