Gearing 915 for Street Performance:

[White911SC]

Greetings,

I'm Finalizing specs for a full rebuild on 915/63 transmission. Box will be re-geared as close ratio (2-5 replaced) plus other upgrades for durability and performance (Wevo, Guard, LSD etc.).

Car is an 80' US 911 SC 3.0 with plans to rebuild as high performance 3.2SS as future project. Intended use is street/track DE (85%/15%) with focus on reliability but slightly high strung and scary quick with old classic racecar feel.

Seeking best compromise of tight gear spacing and big increase in rear wheel torque, while not making 5th gear high RPM too obnoxious for long distance highway driving. I drove across 5 states in the 3200RPM range and didnt find it objectionable at all.

I've narrowed my choices to the 2 options below which will ultimately be decided by the 5th gear choice (27/24 .889 or 33/30 .909). I want the short-ratio box and my thought is if you don't go to at least an .889 5th then the performance gain won't be worth the significant cost to re-gear. The .909 is only the next shorter but if the .889 is already on the edge then I don't want to overshoot...

If anyone has experience using similar gearing options on the street or input that can help me decide I welcome your input...

Questions to Consider:
1. Is an 8% increase in torque over stock a lot? how about 12%?
2. Is 3500 RPM at freeway speed obnoxious? how about 3800 RPM?

Ratio Options Considered:

[Bill Verburg]

Originally Posted by White911SC

Greetings,

I'm Finalizing specs for a full rebuild on 915/63 transmission. Box will be re-geared as close ratio (2-5 replaced) plus other upgrades for durability and performance (Wevo, Guard, LSD etc.).

Car is an 80' US 911 SC 3.0 with plans to rebuild as high performance 3.2SS as future project. Intended use is street/track DE (85%/15%) with focus on reliability but slightly high strung and scary quick with old classic racecar feel.

Seeking best compromise of tight gear spacing and big increase in rear wheel torque, while not making 5th gear high RPM too obnoxious for long distance highway driving. I drove across 5 states in the 3200RPM range and didnt find it objectionable at all.

I've narrowed my choices to the 2 options below which will ultimately be decided by the 5th gear choice (27/24 .889 or 33/30 .909). I want the short-ratio box and my thought is if you don't go to at least an .889 5th then the performance gain won't be worth the significant cost to re-gear. The .909 is only the next shorter but if the .889 is already on the edge then I don't want to overshoot...

If anyone has experience using similar gearing options on the street or input that can help me decide I welcome your input...

Questions to Consider:
1. Is an 8% increase in torque over stock a lot? how about 12%?
2. Is 3500 RPM at freeway speed obnoxious? how about 3800 RPM?

Ratio Options Considered:

How you gear depends a lot on the torque curve of you engine

just for example here is the acceleration curves for a car w/ 2 different engines. The car is other wise the same, ie same trans and wheels/tires.

w/ the stock 3.0 liter the trans is slightly under geared in 3 and 4(note the crossed tails) w/ the 3.6 gearing is just about perfect.

[Tremelune]

I rarely find myself accelerating with much urgency in 5th gear on the street. As such, if gears 1-3 are the same, I would surely go with a top gear that keeps the RPMs to a minimum. The down side is a slightly deader feel on the highway, but you can always downshift for torque.

I have close gears on one of my cars, and I find myself wishing for a 6th gear all the time. I wish I knew the ratios...

[gliding_serpent]

My poor mans method has been to study dynos of cars with a similar engine config to mine (i am rebuilding so do not yet have my own data, and i am rebuilding the tranny at the same time so i cant wait to choose my gears). Ideally you want to shift at peak hp, and have the rpm's post shift to have you land around peak torque. EDIT: I know better now. Many will advocate this (hey, I read it on the internet...) but they do not really know what they are doing.

Then balance that with ratios that are comfortable enough in the highway.

I will have a 3.4 ssi setup with a flat torque curve that should have a hp peak near stock at 6.1 or 6.2k. Peak torque is around 4.8k or so, thus i chose my gear ratios to try to get close to this rpm drop from second onward.

I chose 5th that gave me a 130mph speed at max rpm hp. 150mph in these cars would be scary anyway. This keeps my revs at 70mph in the sub 4k range... Which is liveable.

Tracking a car changes things a bit. Not for beginners, and it is not as simple as i will describe. In a simple form, the ideal is to be at max hp rpm in 5th at the end of the fastest straight, and you back calculate your ratios from there. I did not go this aggressive (we have a tight track, so doing this would render my car too high strung for the road). In reality you need to think about how fast you will be going, where you will need to shift, and balance time lost with extra shifts from a short ratio, with acceleration losses with a long ratio.

A track/street compromise might be to keep 5th stock for highway use, and choose 4th to max out at the fastest part of the track, and then back calculate your gears from there. Trick is not having made 4th too short. Better to go slightly longer than anticipated, as experience, tires, more power, and weight loss can change the game fast. Not for beginners to decide.

[White911SC]

ALL great input!

Bill V. I've studied in great detail a number of threads you've contributed your knowledge of gear ratio selection. That's how I got this far. Truth is my Option1 above is based exactly on one of your all time favorites, the 915/71 SC/RS std. (with the exception of a slightly taller 2nd).

With regard to your comment on the acceleration graph, "the trans is slightly under geared in 3 and 4"...are you saying those gears are too tall (numerically low) for the 3.0 engine to deliver adequate thrust at that speed?

[gliding_serpent]

Here is my current planed gear selection. Near your option #2.

11 to 35
15 to 29
20 to 30
24 to 28
33 to 30

RPM Drops Based on 6200 RPM shifts (estimated peak HP)

1st to 2nd=2437
2nd to 3rd=1381
3rd to 4th =1364
4th to 5th =1383

1st to second is a throw away, and I don't much care... but the remainder of shifts keep me between peak hp (~6.1-6.2k rpm) and peak torque (~4.8k rpm) with pretty even rpm drops, which will make heel toeing easier.

This is similar to when this car had a 7:31 R+P, and stock gears. The difference is that the 8:31 will be twice as robust, and the spacing between gears will be much better. It used to be a big rpm drop going second to third.

[White911SC]

I have seen people using shorter intermediate gears with a stock 5th for highway but the cost is a big RPM drop/loss of power between 4th and 5th and basically 3 speeds on the track (gears 2-4). My personal preference is to live with the higher RPM that comes with a street close ratio and stay with progressively smaller drops between gears as you go up the stack.

gliding_serpent, looks like you have done your homework on your ratios and nice description for selecting track ratios; Im no expert but I clearly understand what you're saying and the concepts involved. Your gearing looks like your car is intended for primary track/occasional street is that right?

I've been very conscious of gear spacings for street use which would see a wider RPM range than the track. You wouldn’t normally be going between max hp and max tq on the street, but I will follow up on your suggestion to look at some engine Dyno Charts to see how well they map to my ratios. Track is a secondary interest for me.

Since my use is primary street, my 5th gear is not being selected based on a desired maximum top speed per se (I sure dont want to go 150 either...anymore :-) but rather a maximum tolerable RPM at highway speeds (tolerable being the subjective variable...)

Hmmm...Maybe I should ask my wife how many RPMs she can tolerate as passenger? :-D hehe

[White911SC]

Any opinions on nuisance factor of long range cruising at ~3500 RPM @ 75mph or do you think this is pretty much a non-issue?


My 911 is in pieces so I can't test it out on that car....I tried it in my Cadillac CTS but hmmm...for some reason it doesn't translate well...

[gliding_serpent]

Long range at 3500 was fine by me... With near open exhaust. Ymmv. When i bought my car it had essentially the same ratio for 5th (~130mph top speed) with the 7:31 r+p. I chose my new fifth based on this because it never bothered me. at the time i did not even know the i had a short ratio as i had no basis for comparison.

I am building my car for it's intended use... A b roads car with occasional highway use to get to our local track a few times a year, which is tight.

I thought hard about building a 2nd to 4th for track use. It would minimize shifts which would save a lot of time, be easier to drive, and be easier on the car. But what is the challenge there? :-)

[Bill Verburg]

Originally Posted by White911SC

ALL great input!

Bill V. I've studied in great detail a number of threads you've contributed your knowledge of gear ratio selection. That's how I got this far. Truth is my Option1 above is based exactly on one of your all time favorites, the 915/71 SC/RS std. (with the exception of a slightly taller 2nd).

With regard to your comment on the acceleration graph, "the trans is slightly under geared in 3 and 4"...are you saying those gears are too tall (numerically low) for the 3.0 engine to deliver adequate thrust at that speed?

No, Look at the blue curves, 3 crosses 4 and 4 crosses 5, the gears are numerically too high. You are giving up torque for acceleration if you don't short shift or you are giving up rpm if you do.

The SC/RS is one of if not the best out there but short gears want higher rev limits and torque at higher rpm

[Matt Monson]

Given that the car will be mostly street, I suspect that short shifting 500-800 rpm short of redline may be a common practice, and as such the fact the ratios aren't perfectly optimized probably isn't a big deal.

[javadog]

Why not start with a taller 1st gear, make 1-4 pretty close, then add a tall fifth gear for cruising?

JR

[Matt Monson]

Taller first gear requires a mainshaft and adds $2500 to the build price.

I almost forgot to mention that cruising in a shorter 5th won't just be impacted by rpms. Aftermarket gears use race tooth profiles and make more noise than stock. Gear whine can annoy the wife, or you.

[Bill Verburg]

Here's a consolidated view of the trans choices discussed here, some of the speeds will be different from yours because I use the actual loaded rolling tire rather than the unmounted diameter usually used.

[javadog]

Originally Posted by Matt Monson

Aftermarket gears use race tooth profiles and make more noise than stock. Gear whine can annoy the wife, or you.

Is that true of all of the gear sets that you offer?

JR

[Matt Monson]

JR,
Yes. There's some commentary and photos on the latter half of page 1 of this thread:
3rd gear teeth gone! is this common?

[Matt Monson]

Just re-read that thread and see one thing I failed to point out is how we relax the helix angle. You can see it in the vs nos gear picture. The relaxed angle is yet another way to make it stronger. But it increases noise.

I am considering making some stock ratio 5th gears that will run quietly. Specifically SN,TL & TM. I'm starting to get calls on high mileage cars that did a lot of commuting and especially in the early gears nos stock is long gone and good used is starting to run out too. With changes in values, someone with a '73S often doesn't want to install the taller later gear.

[gliding_serpent]

My personal preference is to live with the higher RPM that comes with a street close ratio and stay with progressively smaller drops between gears as you go up the stack

What is the thought of having progessivly smaller rpm drops as you go up the gears? The factory does it. I presume the advantage is that it allows longer overall gearing, and the larger rpm drops are noticed less in lower gears as gearing advantage gives you more torque to play with.

[GaryR]

Or you could go to 15" wheels with 35 series tires...
Or put in a 7/31 R&P...

[White911SC]

Originally Posted by Matt Monson

Aftermarket gears use race tooth profiles and make more noise than stock. Gear whine can annoy the wife, or you.

I didn't know this was the case so I didn't consider it. On the Guard product page for 915 gears it mentions they are "Stronger, quieter, and longer-lasting" which I would now understand to mean in comparison to other aftermarket racing gears.

In this case increased gear noise would be a function of using aftermarket gears rather than the ratios selected.

Matt,
Were you just pointing out that increased gear noise is another compromise to factor in when using custom aftermarket gears? I'm not aware of any other option to build this box in the ratios I want...