915 gearing options

[silverlock]

It’s winter, and that means winter projects!

I’m looking to re-gear the 915 in my 3.2SS + 993SS cammed + SW chipped SC. Should have done this when we rebuilt the engine & box in May, but you don’t know what you don’t know & the 10k miles since then have given me a better take on what I want out of the ratios. I’ve read through many other threads to help guide me to where I’m at now.

All gears except 5th (a longer 29:22) are currently stock for a 915/61.

Car is primarily a backroads/mountain rallye car. Lots of weekends & events in WV (sweepers and some hairpins), NC (tighter, more technical) and VT (a mix). I mix in some track days & AX where I can.

Problem to address: stock 2nd (1.83) to 3rd (1.26) is a big drop. 6.8k RPM in 2nd drops you to ~4.7k in 3rd, which is just below where the cam is really working. You’re trying to wring it out without bouncing off the limiter.

Going to have Steve Wong set the limiter to 7.5k (have the rods & valvetrain for it) but that’s a bandaid, not a fix. No more power up there, and while it’s nice to hang onto 2nd for longer vs upshifting in some circumstances, it’d be nice to keep the car in the sweet spot of the 993SS cams. The pull from ~5k to 7k is strong. They’re good from 4k+, but there’s a real step function from 5k+.

Current thinking: stock 2nd, shorter 3rd and 4th - “Option A” in the table below.

• 2nd: 18:33 (1.83) — stock
• 3rd: 21:29 (1.381)
• 4th: 24:27 (1.125)
• That shrinks the RPM drop from 2nd → 3rd from ~2,250 rpm down to ~1,770 rpm, and 3rd → 4th from ~1,490 rpm down to ~1,330 rpm.
• 2nd → 3rd is really what matters for me.
• I could go with a shorter 2nd: 15:29 (1.933), but then I'd need to find a matching 3rd to get back into that ~1,800 RPM drop window.
• My concern with the shorter 2nd is that it ends up too short, which means more shifting (2nd → 3rd → back down) than is actually useful.

Summary of options + links to charts:

Option A: Stock 2nd + short 3rd+4th

• 2nd: 18:33 (1.83)
• 3rd: 21:29 (1.381)
• 4th: 24:27 (1.125)
• 5th: 29:22
• Top speeds @7000 / 7500:
  • 2nd: ~75 / ~80 mph
  • 3rd: ~97 / ~104 mph
  • 4th: ~119 / ~128 mph
  • 5th: ~170 / ~182 mph
• Link:
  https://ratio.questionable.services/?g1=11%3A35&g2=18%3A33&g3=21%3A29&g4=24%3A27&g5=29%3A22&rp=8%3A31&td=24.9&mr=7500&sp=7200

Option B: Short 2nd+3rd+4th

• 2nd: 15:29 (1.933)
• 3rd: 21:29 (1.381)
• 4th: 24:27 (1.125)
• 5th: 29:22
• Top speeds @7000 / 7500:
  • 2nd: ~69 / ~74 mph
  • 3rd: ~97 / ~104 mph
  • 4th: ~119 / ~128 mph
  • 5th: ~170 / ~182 mph
• Link:
  https://ratio.questionable.services/?g1=11%3A35&g2=15%3A29&g3=21%3A29&g4=24%3A27&g5=29%3A22&rp=8%3A31&td=24.9&mr=7500&sp=7200

SCRS:

• 2nd: 15:29 (1.933)
• 3rd: 16:24 (1.500)
• 4th: 23:27 (1.174)
• 5th: 29:22
• Top speeds @7000 / 7500:
  • 2nd: ~69 / ~74 mph
  • 3rd: ~89 / ~95 mph
  • 4th: ~114 / ~122 mph
  • 5th: ~170 / ~182 mph
• Link:
  https://ratio.questionable.services/?g1=11%3A35&g2=15%3A29&g3=16%3A24&g4=23%3A27&g5=29%3A22&rp=8%3A31&td=24.9&mr=7500&sp=7200

Put the SCRS ratios there for comparison, but they’re almost definitely too short for what I want.

All of these look to be available via Guards (https://www.guardtransmissionllc.com/products/transmission-gear-sets/915-gears/) as well.

Thoughts? Feedback from folks with re-geared 915s appreciated.

[shoooo32]

I like option B. Option A (stock 2nd gear) would likely have you lugging hairpins in 2nd gear or shifting to 1st, and we all know how much 915s enjoy shifting to first. A shorter 2nd gear helps to avoid that shift.

[silverlock]

You're probably right. I guess I have the torque to sit in 4th so that cruising around isn't annoying.

An "Option C" with the shorter 1.933 2nd and 1.45 3rd would look like the below. 3rd goes from the 1.38 in Option B up to 1.45, and keeps RPMs up by ~200 or so. I don't know if I really get a ton out of 2nd here - in a 30MPH hairpin I'm at 2.8k vs. 3k, which is nice but still not fun. Any shorter on 2nd is even shorter than SCRS, which has the same 1.933 / 15:29 gearing as what I'm proposing here?

• Option C (short 2nd + matched 3rd)
• 2nd: 15:29 (1.933)
• 3rd: 20:29 (1.450)
• 4th: 24:27 (1.125)
• 5th: 29:22
• 2nd → 3rd @ 7200 rpm: lands at ~5400 rpm (≈1800 rpm drop)
• 3rd top speed: ~92 mph @ 7000, ~99 mph @ 7500
• Link:
  https://ratio.questionable.services/?g1=11%3A35&g2=15%3A29&g3=20%3A29&g4=24%3A27&g5=29%3A22&rp=8%3A31&td=24.9&mr=7500&sp=7200

[shoooo32]

I like that one the best. If 1st is stock and 4th gear is at a sweet spot of 120mph @7k rpm, 2nd and third seem to be optimized as best as the ratios allow.

Interested to see what Matt Monson thinks.

[Bill Verburg]

It would be informative to use the engine torque curve as an input

also what rear tires? That affects the speeds.

from what you posted
opt 1 has drops of
42% 25% 19% 33%
which outs you @ x in the next gear when shifted at 7500
x: 3918 5122 5540 4585






opt2 has drops of
42% 25% 19% 33%
which outs you @ x in the next gear when shifted at 7500
x: 4557 5357 6110 5057

using the torque curve for an input allows for this sort of comparison where you can easily se that 3-5 are a bit too short

[silverlock]

Originally Posted by Bill Verburg

It would be informative to use the engine torque curve as an input

also what rear tires? That affects the speeds.

from what you posted
opt 1 has drops of
42% 25% 19% 33%
which outs you @ x in the next gear when shifted at 7500
x: 3918 5122 5540 4585


opt2 has drops of
42% 25% 19% 33%
which outs you @ x in the next gear when shifted at 7500
x: 4557 5357 6110 5057

using the torque curve for an input allows for this sort of comparison where you can easily se that 3-5 are a bit too short

Good points!

Tire size is in the linked calculators - 225/50R16 / 24.9” dia (based on the RE71RS’ I’m running)

I don’t have an up-to-date dyno chart on this engine but an almost identical build (3.4 not 3.2) looks like this w/ 993SS cams and similar intake/exhaust/TB/porting mods - the blue line here:



Shift point here is much more ideal at 6.5-6.8k, not 7.5k. 3.2SS/3.4SS just can’t breathe enough to make any real power > 7k. 4.5k+ RPM on the torque curve is where I want to be, which also matches my experience here. Shifting into 3rd at (e.g.) 6.5k drops you down to 4.3k, right below where you want to be.

When you say 3-5 are a bit too short: expand? Which option?

[Jonny042]

I've been through this a few times. This is from page 50 of my "Heavy Metal" build thread. Below, I allude to the 2.00:1 2nd and 1.381 3rd I put in my 3.2 carrera (1985 w/915), and continue to detail the 1.933:1/1.381/1.08 I put in my 3.2SS SC.

I honestly don't think there's a better way to improve the performance of, and pleasure in driving, a 911.

Originally Posted by Jonny042

One thing I've been putting off for some time is the transmission. Hernia operation (minor, umbilical, no biggie, thanks for asking) in November precluded my lifting anything too big so this has been hibernating under the bench for a while. The plan was, originally, to put in the car and see how it shifts, but this project has evolved to be a nut and bolt sort of affair. Am I going to stick a used, tmu, unknown transmission in this car, after all the work I've done? Nah.

So somewhere along the way I started Autocrossing the Rot Rod and changed out 2nd and 3rd for shorter ratios. The thing is an absolute beast now. This little experiment was a success, and a learning experience, and it only reinforced my opinion that a gearing change to better suit a car for it's intended use is absolutely one of the best things you can do. Yes it seems a little pricey (more on that later) but it's such a huge advantage!

I called my trans for the Rot Rod an SC/RS/Carrera hybrid which is pretty accurate, as the final drive, 1st, 2nd, and 3rd are the same as the SC/RS and 4th and 5th are stock 3.2 Carrera ratios. changing just 2nd and 3rd meant it was a budget friendly proposition, and while I was in there I replaced the 1st gear dogteeth, the 1/2 shift collar, and the 1/2/3 synchros (remember the new gears come with fresh dog teeth). It was previously reluctant to upshift, downshift, or go from neutral into 1st at a full stop, but now it shifts like a dream instead of a nightmare.

One thing that I (predictably) don't love about the stock 4th gear is it's a bit doggy compared to third and the upshift does have a fair RPM drop but for the usage the Rot Rod gets it's a good compromise.

For this car I decided to go slightly longer for 2nd gear, with a 1.933 ratio compared to 1:2.000 for the SC/RS, the same 1:1.381 for 3rd, and a 1:1.08 for 4th (compared to the stock 1:1.000). The 5th gear for the late SC transmission is 1:0.8241 so it's a bit lower than the Carrera (but still geared for 165mph at 7000rpm!!). The 4th gear change gives me one more "good" gear for higher speed backroad driving, and retains the long 5th for cruising between special stages and driving home from the track

The results, in graphical form:



Comparing the before and after on the graph might make you thing there's little enough change that it's not worth the trouble, but after trying this out on the Rot Rod I know it's going to be fantastic! Nice close ratios from start up to 135mph, and a long fifth for relaxed highway cruise. Should be fun!

I should note, this is nothing groundbreaking, in fact pretty familiar territory for many on the forum. Bill Verburg and Matt Monson come immediately to mind for their tireless contributions to any and all gearing related discussions, gliding_serpent for sharing his though process in selecting gear ratios for his track car, Gordo2 did a 2nd gear swap with good results, and I am sure there are many more.

Of course let's not forget Peter Z. and his classic, the 915 tutorial, and Gordo's transmission rebuild thread. Thanks, guys!!!

[stownsen914]

When selecting gear ratios, remember to consider how useful the ratios will be coming out of turns - not just accelerating in a straight line. Which second gear ratio would be a more natural fit for your intended application and driving style?

[silverlock]

Originally Posted by Jonny042

I've been through this a few times. This is from page 50 of my "Heavy Metal" build thread. Below, I allude to the 2.00:1 2nd and 1.381 3rd I put in my 3.2 carrera (1985 w/915), and continue to detail the 1.933:1/1.381/1.08 I put in my 3.2SS SC.

I honestly don't think there's a better way to improve the performance of, and pleasure in driving, a 911.

This is super useful, thank you! Going to read through the rest of that thread today.

I see you were also playing with a gear calculator (I should build a comparison into my chart) — your 1.933 / 1.381 / 1.081 + longer 5th (mine is 0.758, longer still) mix is very similar to the two short-but-not-SCRS-short options I’m scoping out.

I have an email out to Matt Monson @ Guards (provided he doesn’t find this thread) to get his take + also confirm availability.

[Jonny042]

Originally Posted by silverlock

This is super useful, thank you! Going to read through the rest of that thread today.

I see you were also playing with a gear calculator (I should build a comparison into my chart) — your 1.933 / 1.381 / 1.081 + longer 5th (mine is 0.758, longer still) mix is very similar to the two short-but-not-SCRS-short options I’m scoping out.

I have an email out to Matt Monson @ Guards (provided he doesn’t find this thread) to get his take + also confirm availability.

I think Matt may favor the 2.0:1 2nd gear, and maybe a 1.429 3rd. I could see that being fun, and is probably the direction I'll go with the current build, with a low 4th and 5th as well. It's going to be a more performance focused build with a 2.7 tuned to be a bit more peaky than the 3.2SS.

Here's a video of me winding out the MFI 3.2SS to 7200RPM in 1st-4th. This is a really great 915 with a Wevo gateshift and Motul Motylgear oil.



And here's another one of the 3.2 Carrera, making sure to hit the (stock) limiter in 1st/2nd/3rd. It's a dog compared to the MFI 3.2SS! It's got swepco in it and doesn't shift nearly as well.

[silverlock]

Originally Posted by Jonny042

I think Matt may favor the 2.0:1 2nd gear, and maybe a 1.429 3rd. I could see that being fun, and is probably the direction I'll go with the current build, with a low 4th and 5th as well. It's going to be a more performance focused build with a 2.7 tuned to be a bit more peaky than the 3.2SS.

Here's a video of me winding out the MFI 3.2SS to 7200RPM in 1st-4th. This is a really great 915 with a Wevo gateshift and Motul Motylgear oil.

Love the video. Sounds fantastic too.

Matt just got back to me and mentioned your build. You’re also dead-on about that 2.00 2nd gear recommendation, and it’s pretty persuasive. Will be much better in low speed corners / tighter roads (North Carolina + AX) at the cost of shifting into 3rd / back down into 2nd more often (WV & less tight roads).

For those playing at home, Matt recommended a 2.00 (17:34) 2nd / 1.381 (21:29) 3rd / 1.083 (24:26) 4th - Matt was extremely thorough in his reply & shared a stock vs mine vs his chart and talked me through his rationale around the RPM drops vs torque curve.

I’m still tossing up whether to go to a shorter third (a 21:31 / 1.476) to keep things tighter in that 2-3 range where I do most of my spirited driving. Bumps RPMs by 350 at the 2-3 upshift though, which might be too short? 7k shift drops to 5.1k vs 4.85k.

Guards recommended option: https://ratio.questionable.services/?g1=11%3A35&g2=17%3A34&g3=21%3A29&g4=24%3A26&g5=29%3A22&rp=8%3A31&td=24.9&mr=7500&sp=7000

[Jonny042]

I'll have to admit to taking a bit of a summer break from the forum... do we have you to thank for this gearing calculator? It's awesome!!!

If you're looking for notes , I miss having a vertical line from the end of one gear to the start of the other, like the old quaife gearcalc program. It really helps illustrate the effects of the ratios and the spacing between gears. It might help inform the decision you're having to make.

It's starting to come back to me, the quandary I had between the 1.933 and 2.00:1 2nd, and its relation to the 1.381 3rd, and the RPM drops. To me a lot of it had to do with getting that pitch perfect sound and rythym to the shifts. I'll go ahead and claim a win on that.

If I was AutoXing (like you, and me, with the '85), I would almost definitely go with the 2.00:1 2nd, and if you're regularly doing courses with 3rd gear sections, the tighter 3rd ratio.

A valid argument for optimizing the shift drops in lower gear at the expense of the higher gears, is that every single time you drive the car, you use 1st gear, pretty much always use 2nd, etc. but you don't always get into 4th, much less 5th.

[silverlock]

Thanks for the feedback on the calculator! Need to spend some time this holiday weekend building in presets for common 915 configs (915/61, early mid-years, etc), a comparison mode and definitely agree that vertical lines from the shift RPM down to the next gear will help with assessing the gaps.

Nearly all of my fun driving is spirited, spicy group rallies (Ruckus, Ruchlos, Targa Carolina, Overcrest) and as many weekends with friends in NY, PA, VT or WV just ripping around. Most of that is 2nd-3rd — rarely am I ever shifting to 4th outside of a “get to the next fun bit”. It’s a commuter gear for me. AX 4-5 times a year when I can squeeze it in around work travel & everything else. Track days maybe 2-3 times a year at best (time consuming).

One of my favorite roads in NC had us hitting the limiter in 2nd, trying to balance shifting into 3rd without losing time shifting back down for the next corner, or keeping RPMs up at the bottom of 2nd on the really tight sections with a careful clutch kick to avoid bogging down below 3.5k RPM (with varying success). Getting the ratios right for those kind of roads is the goal.

[Bill Verburg]

IMO shortening gearing is more appropriate when the rev range has been extended, or there is some glaring deficiency in the existing gear stack.

Here's an example of a large engine power gap going through the same trans, the same gearing is appropriate for both, yes the more powerful engine accelerates better but that's not a trans issue

Here's a comp I did for an E/F series chassis w/, I think a 915/44 trans used in both cases, but it could have been a stock '73RS trans, I'd need to look that up, the 993 uses a slightly higher rev limit for shifting by ~2-400 rpm


In either case the gears could be shorted across the board if absolute performance was the main criteria, which it often is not.

[Matt Monson]

Originally Posted by silverlock

For those playing at home, Matt recommended a 2.00 (17:34) 2nd / 1.381 (21:29) 3rd / 1.083 (24:26) 4th - Matt was extremely thorough in his reply & shared a stock vs mine vs his chart and talked me through his rationale around the RPM drops vs torque curve.

I’m still tossing up whether to go to a shorter third (a 21:31 / 1.476) to keep things tighter in that 2-3 range where I do most of my spirited driving. Bumps RPMs by 350 at the 2-3 upshift though, which might be too short? 7k shift drops to 5.1k vs 4.85k.

I will add one comment that wasn't part of our discussion last night. I had mentioned that what I'm making/stocking these days is more limited than it once was, and we are making more "packages" of gears.

I left the 1.933 2nd because it was the gear you had requested. If you feel like the 1.476 stacks too closely on the 1.933 second, you can always go with the 2.00. The package that gear is actually most often used with runs a 17:35 2.06 2nd gear.

[silverlock]

OK, here’s where I’m at after digesting Matt’s feedback & recommendations, Bill’s advice, and thinking more about what I want out of the gearing in that 40-90MPH window.

● 2.00 2nd to make it more useful down low and bog less in tight corners. Was being too conservative here with 1.83 (stock) and 1.933 2nd as worried about the top end. Matt’s advice was to get that 1-2 shift closer + improve the bottom of 2nd in low speed corners.
● 1.476 3rd - the hardest part here. The shorter third keeps me a bit deeper into the torque curve at the speeds I care the most about, and more leeway to short shift if the conditions/road call for it. The challenge with the 2.00 2nd + 1.38 3rd is that it’s the same drop as the stock 2nd + 3rd pairing, which is the shift I want to tighten up.
● 1.08 4th - this works for me regardless of 3rd, but works a bit better with the shorter third as it drops you into 4th at 5.1k v. 5.4k. More useful at a track day (less common) vs a rally/backroads/group run (too fast). Less opinionated here.

It would be nice to have infinite-ish money and time to try it all back to back, and maybe something like a (non-existent) 1.43 (21:30) 3rd would be great, but we live in a world of compromises.

Critically: I am not trying to wring every ounce of power out of this car. I am mostly just trying to make it a bit spicier for the types of roads/events I enjoy the most + close a couple of gaps in the gearing that counter that.

Here’s what this looks like: gear calculator link

[Bill Verburg]

Originally Posted by silverlock

OK, here’s where I’m at after digesting Matt’s feedback & recommendations, Bill’s advice, and thinking more about what I want out of the gearing in that 40-90MPH window.

● 2.00 2nd to make it more useful down low and bog less in tight corners. Was being too conservative here with 1.83 (stock) and 1.933 2nd as worried about the top end. Matt’s advice was to get that 1-2 shift closer + improve the bottom of 2nd in low speed corners.
● 1.476 3rd - the hardest part here. The shorter third keeps me a bit deeper into the torque curve at the speeds I care the most about, and more leeway to short shift if the conditions/road call for it. The challenge with the 2.00 2nd + 1.38 3rd is that it’s the same drop as the stock 2nd + 3rd pairing, which is the shift I want to tighten up.
● 1.08 4th - this works for me regardless of 3rd, but works a bit better with the shorter third as it drops you into 4th at 5.1k v. 5.4k. More useful at a track day (less common) vs a rally/backroads/group run (too fast). Less opinionated here.

It would be nice to have infinite-ish money and time to try it all back to back, and maybe something like a (non-existent) 1.43 (21:30) 3rd would be great, but we live in a world of compromises.

Critically: I am not trying to wring every ounce of power out of this car. I am mostly just trying to make it a bit spicier for the types of roads/events I enjoy the most + close a couple of gaps in the gearing that counter that.

Here’s what this looks like: gear calculator link

The fat part of the torque curve appears to be from ~4500 to ~6000, all the shifts pu you there

It's also very nice to have the extra headroom so that you don' have to shift at an awkward time, like in the middle of a corner, or have to do a closely linked up then down on a straight that's just a tad shorter than the shifting likes

I don't see it in your charts but its also good to look at the cruising rpm in top gear

one way is to look at top speed in top if it's a ~170+/- then a 70 mph cruise will be ok, another way to look is to calculate the speeds in 5

[silverlock]

Good points!

I’m running this 29:22 5th now / for the last 10k miles and really like it. 3200RPM @ 80MPH is well suited to east coast highways and NY parkways. Car is not buzzy and the SSI+Dansk combo is happy there and not droning.

[silverlock]

Gears ordered from Mr Monson, who has been extremely helpful + responsive even during a holiday week. Wasn’t in a rush but at some point you can only deliberate so much.

2.00 (17:34) 2nd, 1.476 (21:31) 3rd, 1.08 (25:27) 4th. Will be a little while before they go into the car given winter & work/life, but will absolutely report back once they are. Should be late Feb + hopefully get some track time at Roebling outside of Savannah, GA to break them in ;-)

[silverlock]

These are going to stare me in the face until February when I get the car down to SC + have Andrew Eagan drop the trans.