915 transmission question needing an honest answer

[dd74]

Honestly - Regardless of 7.31 or 8.31 r/p, are the gears in the 915 the best compromise available for street and track day driving? Or would you go shorter or longer?

Thanks.

[island911]

go shorter or longer only if you KNOW that you have a specific problem to be fixed (fixed only by going shorter or longer) like needing no shifts thru a particular section of track.

[shrug] people always trying to "fix" a problem they don't have...

[gtc]

I can't find it now, but one of the workshop manuals has diagrams for a bunch of different gearsets that were offered or suggested by Porsche for different types of circuits. Stuff like hillclimbs and airport circuits. I'm thinking it was in the early manuals though, so it might not be so useful to you.
Edit: I haven't tracked my car yet, but the 2nd to 3rd gear gap really bugs me.

[Walt Fricke]

DD

I don't think that "best" and "compromise" fit well together. For track use you want shorter gears, at least up above 2d gear. For the street you don't want that.

If you only drove on tracks which lacked a long straight, you could compromise by shortening 3d and 4th, and leaving 5th alone. I like putting a 7/31 into an 8/31 box - my wife wondered what I had done to the engine when I put that box in while working on the stock box. But others have complained that on long highway trips (as to far away race tracks) the shorter 5th was bothersome. So you could compromise on this. But what if you go to Daytona some day? Or Brainerd? Guys with 2.0L vintage cars often are geared low and say they have to sit just below the rev limiter toward the ends of straights.

The factory gear sets (offered only for the 901, I think, but the idea isn't model specific) were all optimized for a specific use - hill climb, airport, Nurburgring, etc. As opposed to a compromise for use under all circumstances.

The factory changed 915 gears some along the way. The '74s had shorter 1sts and 2ds (due to the shorter R&P), but made up for that with at least a taller 5th. Folks then called the next generation of 8/31 gears "EPA gears," because their higher ratios were thought to be in response to a need to increase the corporate average fuel economy. Along the way some Euro models had a higher 5th, doubtless in response to higher allowable Autobahn speeds.

I know how to optimize shift points - there is a formula based on the torque curve and the gear ratios. I don't see a way to optimize, scientifically, a compromise. Depends on how each individual values the different elements of the compromise.

My dual purpose SC is saved from some of this because it runs in a stock class, so I can't fiddle with the transmission. Of course, the full roll cage with door bars, and the big torsion bars and solid bushings, etc., are street comfort/convenience compromises, which I have gotten used to over the course of many years. Each addition to stiffness doesn't seem like much that way.

Walt Fricke

[dd74]

My trouble - if it is trouble - is I live in 4th gear. At my local track I never reach 5th gear in my '74's 915 unless I want to baby the car lap after lap as my engine limits at 7,300 RPM. On the street, 4th is almost sacrosanct because 3rd revs very high and 5th bogs the engine. It's just that on the track, it feels like I'm not getting everything I can out of the engine. I'd like more pop coming out of the corners.

Is there any truth to a claim I once heard that 4th in the 7.31 r/p box is an overdrive gear.

[Bill Verburg]

here is a comparison of many of the 915s, Jacks short geared race box is also included


I'm sure there are mistakes, let me know.

Shorter to a point is better on track, but its a dd, the buzz will get to you. Drive around in 4th, as if it was 5th someday

[Walt Fricke]

S'truth. 4th in the '74 box is a 27/25 (0.93) gear, up from the 26/25 (0.96) 4th in the '72/3, and even farther up from the 26/26 (1.00) installed in the later 8/31 boxes. So the pinion is being driven faster than the engine is turning in both 4th and 5th in your car.

Since 3d was the same from '72 through '86 (23/29 (1.261), and you have the 7/31, I wouldn't say that 3d is very high.

Pull your box apart and install a 1:1 4th, and a later, lower, 5th. 4th will pull better, and you can get into 5th and make good use of it. But you'll be droning along some on the highway getting to the track. Though if that includes U.S. 27, you won't be going fast enough for it to bother.

I assume you have carbs and a cam in that engine, so it is worth it to shift at 7,200? The performance shift point for stock CIS engines with a good exhaust (like your car came with) is somewhere between 6,200 and 6,500, depending on how you have your stock cam timed. You can't fool the torque curve.

Walt Fricke

[Bill Verburg]

Originally Posted by dd74

Is there any truth to a claim I once heard that 4th in the 7.31 r/p box is an overdrive gear.

Any time the numerical gear ratio is < 1, it is an overdrive gear.

[Bill Verburg]

Just an aside and FYI
Set up a spread sheet to compare different gear stacks that you might be interested in, then graph the results

That's what I did. g50/20 was my oem 993 trans, g50/30 is it's replacement

[dd74]

Well, here's what I'm considering.
1) 3.18 b
2) 1.83 HX
3) 1.32 (custom) NT
4) 1.00 (915/08 from later 915 box) QQ
5) 0.82 (915/08 from later 915 box) SN

limited slip - possibly.

shift points - between 6,500 and 7,000 RPM

All of the above vs. my stock box as is.

What do you guys think?

[Bill Verburg]

Originally Posted by dd74

Well, here's what I'm considering.
1) 3.18 b
2) 1.83 HX
3) 1.32 (custom) NT
4) 1.00 (915/08 from later 915 box) QQ
5) 0.82 (915/08 from later 915 box) SN

limited slip - possibly.

shift points - between 6,500 and 7,000 RPM

All of the above vs. my stock box as is.

What do you guys think?

You are describing a 915/44,61 or 63 (see above)

[dd74]

Originally Posted by Bill Verburg

You are describing a 915/44,61 or 63 (see above)

Correct. Just w/o the third gear. Now, two questions:
1) What would be the most apparent difference between the stock 3rd gear in the 915/44, 61 or 63 and the 1.32 gear. Would it be that noticeable? BTW: those custom gears like the 1.32, cost $700+ bucks.

2) How would a gearbox like the 915/44, 61 or 63 react to my 7.31 r/p if I were to install it?

[Bill Verburg]

Originally Posted by dd74

2) How would a gearbox like the 915/44, 61 or 63 react to my 7.31 r/p if I were to install it?

7:31 is ~13% shorter than 8:31 multiply all the speeds by .8749 to get the speeds w/ 7:31

Originally Posted by dd74

1) What would be the most apparent difference between the stock 3rd gear in the 915/44, 61 or 63 and the 1.32 gear. Would it be that noticeable? BTW: those custom gears like the 1.32, cost $700+ bucks.
?

speed in the gear drops ~ 4mph the drops become 42% 28% 24% 18% instead of 42% 31% 21% 18%

[dd74]

Speed I'm not as much concerned with as keeping the revs up.

Interesting that there's only a difference of 300 RPM drops when using the 1.32 3rd gear as opposed to the stock 1.26. Not that much for a dd/DE car. Hmmm....

Like I said, I always wondered if that extra $700 for the 3rd gear was worth it.

[Bill Verburg]

Originally Posted by dd74

Speed I'm not as much concerned with as keeping the revs up.

Interesting that there's only a difference of 300 RPM drops when using the 1.32 3rd gear as opposed to the stock 1.26. Not that much for a dd/DE car. Hmmm....

Like I said, I always wondered if that extra $700 for the 3rd gear was worth it.

But that's what a transmission does. It just trades speed either in revs or mph for accelerative thrust.

That is a small change, you will spend a little less time in 3rd and a little more more in 4th

[dd74]

Originally Posted by Bill Verburg

But that's what a transmission does. It just trades speed either in revs or mph for accelerative thrust.

That is a small change, you will spend a little less time in 3rd and a little more more in 4th

Well, I do want thrust.

At any rate, so in your view, this exercise as I present it with the various ratios, is somewhat moot?

[Bill Verburg]

The choice of 1.32 vs 1.26 is fine, either works well for most people most of the time. If it cures the specific issue you have then it is a good move. Is it worth the expense, only you can answer that one.

Most people w. daily drivers don't use more than 2 or 3 gears on most tracks most of the time. As I recall my stock C3 at LRP it was a 2 maybe 3 gears, if you used 5 it was for just a sec or 2 on the front straight. Lots of people will just rev a little higher rather than do the shift up then right back down.

[dd74]

Yes, you're correct. I think what I'll do is a season more of track flogging and reassess this decision. The great thing about the engine is it's truly very tractable as far as torque and hp. goes. It doesn't lose steam at 5500 like my old 3.0 did, but isn't so peaky that continual gear changes are needed. So, consequently, it does make good use of the gears I already have.

We'll see.

Thanks for the help.

[KTL]

At the tracks I currently frequent, 3rd is the absolute most important gear. I'd recommend you look at 3rd not only from a thrust and RPM drop or MPH perspective, but also from the perspective of where you will be on the track in 3rd at different locations around the tracks you frequent.

In my case, my gap between 2nd (2.059) 3rd (1.409) is rather large with the 9:31 R/P so the car lacks thrust at certain corner exits where the car is in the 3500-4000 rpm range. But switching to a 3rd that is overly short will cause me to run out of revs at other corners and require an upshift or short-shift.

When re-gearing a trans. for dual-purpose use (street/track) I think some important things to consider are:

-What's your top speed on the tracks? Select 4th gear to work best with your top speed. Leave 5th gear alone because you rarely use it on the track and want to keep the overdrive gearing for highway use. Keep in mind, if you visit small tracks and big tracks, you can't have your cake and eat it too. So you'll have to decide which is more important- short track or long track performance.

-Work backwards from your 4th and fit the appropriate 3rd and 2nd to acheive good spacing/rpm drops but also look at where those gears will place you at the different corners around the track(s). Having your gears charted (both existing and what you are proposing) and in-car video footage of your driving is helpful here because you can see where you currently are and where you will be rpm and mph-wise

And I think you'd regret not installing the limited slip. The benefits are worth it IMO. Corner entry understeer allows you to trailbrake more and the car is more controllable, predictable. Corner exit is improved because you can apply the power more effectively.

[Bill Verburg]

If you are compaing 915 7:31, 915 8:31 and g50 trans it's best to look at the overal ratios not the individual ratios, don't forget tires make a difference too