Designing a 915 trans for the canyons

[moparrob]

Would appreciate any informed input on a re-gearing question if any of you are so inclined.

I have had my Euro spec 1978 911 SC (turbo look) for about 4 years now and drive it primarily through the canyons and mountain roads in the greater Los Angeles area, primarily Malibu and the canyons off Mulholland drive.





I run with a few guys who have newer and/or more powerful cars and I notice a few shortcomings of the stock gear ratios in the 915/61 trans on my 200 hp car. Mainly I have difficulty keeping the car in the proper torque range when pulling light grades due to the fact the car wants to either redline in 2nd gear or bog if I shift to third gear. The gap in the ratios in far from ideal for the driving I do.

So I did some research concerning what other gear ratios are available and have concocted a recipe for a new trans, changing around the first four gears. I am happy with the stock fifth gear, so I left that alone.

I also purchased a 1976 (magnesium case) 915/44 trans which I will use to build the new trans, so I can keep driving my car while the trans is being built. The new trans has the same ring/pinion (8:31) and same gear ratios as the 915/61 trans.

This is the spreadsheet showing the proposed ratios:





As you can see the stock ratios are listed on the right side and the new (proposed) gear sets are at the top of the sheet.

There is also a section at the bottom right showing the net change at 6000 rpm in each gear, which is my effective redline.


Given the above, as well as the plethora of other gear sets currently available on the market, my questions are

1) what do you think of my gear choices for my type of canyon driving, and

2) are there any other modification or choices you would suggest while I have the box apart?

Thanks in advance for the input.

[lateapex911]

1st, I'm jealous of your life! ha! (I'm visiting the Same canyon area end of march, will rent a Boxster, love those roads.)

So, if I drew the chart correctly (tough to read the chart here, lines are tiny), your speed spreads between gears looks like:
1-2nd: 6K > 3500
2-3rd: 6k > 4500
3-4th: 6K > 4600
4-5th: 6K >4600

approximately.....

Those certainly hit your tighter spacing desires.. sounds like you're concerned about the 1-2 shift and the 2-3?, based on your comment. What's your hp and tq curve look like?

The 1-2 spread is pretty big relative to your other gears. (2500rpm vs 1500rpm, or so)

And...while 1st takes you higher, it will also be slower off the line..

[moparrob]

Yes, I am most concerned about first being so short, and the gap from 2nd to 3rd being so big.

I was wondering if the 1-2 gap was too big but it only shows a 5.2 mph difference (over stock) at 6000 rpm. 1st seems so short anyhow...

This is the most recent dyno run, which was truncated at 5500 due to a lean condition in the CIS (another story, another project).






Does this affect your decision?

[Jonny042]

Just making sure you are aware - changing the ratio of first is very, very expensive compared to the other gears?

EDIT - I should add I am building a trans for my project: see post 991:

Project Heavy Metal - an all steel classic, remastered

[Bill Verburg]

Originally Posted by moparrob

Would appreciate any informed input on a re-gearing question if any of you are so inclined.

I have had my Euro spec 1978 911 SC (turbo look) for about 4 years now and drive it primarily through the canyons and mountain roads in the greater Los Angeles area, primarily Malibu and the canyons off Mulholland drive.





I run with a few guys who have newer and/or more powerful cars and I notice a few shortcomings of the stock gear ratios in the 915/61 trans on my 200 hp car. Mainly I have difficulty keeping the car in the proper torque range when pulling light grades due to the fact the car wants to either redline in 2nd gear or bog if I shift to third gear. The gap in the ratios in far from ideal for the driving I do.

So I did some research concerning what other gear ratios are available and have concocted a recipe for a new trans, changing around the first four gears. I am happy with the stock fifth gear, so I left that alone.

I also purchased a 1976 (magnesium case) 915/44 trans which I will use to build the new trans, so I can keep driving my car while the trans is being built. The new trans has the same ring/pinion (8:31) and same gear ratios as the 915/61 trans.

This is the spreadsheet showing the proposed ratios:





As you can see the stock ratios are listed on the right side and the new (proposed) gear sets are at the top of the sheet.

There is also a section at the bottom right showing the net change at 6000 rpm in each gear, which is my effective redline.


Given the above, as well as the plethora of other gear sets currently available on the market, my questions are

1) what do you think of my gear choices for my type of canyon driving, and

2) are there any other modification or choices you would suggest while I have the box apart?

Thanks in advance for the input.

the above sort of charts are informative but don't really give a feel for what it's like to drive the car.

You need to do a thrust or acceleration chart like this

Stock '78 SC vs shorter gearing 2 -> 5, every where the tails cross the next gear is wasted gearing, every where there is a gap from one gear to the next is an awkward bog


915/44 & 915/61 have the same gearing and are well suited to a stock 3.0

When rev limit is raised that is where shorter gearing shines

One of my favorite 915 gear stacks was the one used in the SC/RS w/ a 7000 rpm rev limit
35-11 3.182, 30/15 2.00 29-21 1.381, 27-25 1.080, 24-27 .889

optional w/ optional cams and 7600 rpm rev limit
35-11 3.182, 35-16 2.188, 32-20, 1.600, 29-23 1.261, 26-26 1.000

[Sboxin]

Just to add another set of gear ratio data points - this is what we use in a 78SC with 915 with both a 3.2L 200 rwhp and now a 3.8L 270 rwhp race car for mostly short tracks ( my son is the driver):
Custom built by CMS in Lake Havasu
1st: 35:11 3.18 Stock
2nd: 33:11 1.83
3rd: 30:21 1.43
4th: 28:23 1.21
5th: 26:25 0.96
Shift points usually around 6800 - but some straights 7000 to 7200(rev limiter)
Tire dia: 25.6 in rear

These are mostly 3rd and 4th gear tracks with one or two sharp 2nd gear corners -
probably very similar to your Canyon carving runs . . .

While you are in there:
You might consider adding a Guard Limited Slip to the diff.

Wish you the best in your project,
Regards,
Roy T

[Jonny042]

Originally Posted by Bill Verburg

One of my favorite 915 gear stacks was the one used in the SC/RS w/ a 7000 rpm rev limit
35-11 3.182, 30/15 2.00 29-21 1.381, 27-25 1.080, 24-27 .889

The SC/RS ratios are good advice, and have been the basis for my build:

Stock 1st
1.933 2nd (ever so slightly taller than the 2.000 to benefit 2nd to 3rd spacing)
1.381 3rd
1.08 4th
.8214 5th (stock for 78SC).

This should be a perfect all around box with a relatively relaxed 5th.

As a research effort I did the following in my autoX car (85 3.2 Carrera).

stock 1st
2.000 2nd
1.381 3rd
1.00 4th (stock)
.78(??) 5th (also stock)

This is a great box for autocrossing but for road use a lower 4th gear is in its future and also maybe the lower .8214 fifth which should be easy to find.

[stownsen914]

You probably won't get much bang for your buck in a street car by changing 1st gear, and maybe even 2nd gear. A good option for what it sounds like you are doing is to put in shorter 3rd and 4th gears. You might consider leaving 5th alone so you still have a good highway cruising gear, and just accept a larger gap between 4th and 5th.

[moparrob]

Thanks for all of the well reasoned input guys. Much appreciated.

Since my engine will likely retain its stock cams (and redline) I think I will be best off going with the stock 1st and 2nd gears and will change 3rd and 4th gears in line with the above suggestions. I will also go with a limited slip diff and perform a complete refresh of the bearings and other worn internal parts as necessary.

Now to locate some nice 21:29 and 25:27 gear sets...

[rgofast]

I would recommend CMS in Lake Havasu to do your build.

[Trackrash]

Get a 7:31 R&P, done.

[IROC]

Originally Posted by Trackrash

Get a 7:31 R&P, done.

This was going to be my suggestion. A 7:31 and a taller 5th gear. I actually bought these parts (the R&P and 5th gear) years ago to do this with my stock 2.7, but sold it when I went to the 3.2.

[Trackrash]

Originally Posted by IROC

This was going to be my suggestion. A 7:31 and a taller 5th gear. I actually bought these parts (the R&P and 5th gear) years ago to do this with my stock 2.7, but sold it when I went to the 3.2.

I was somewhat concerned when I changed to the warmed up 3,0, I have now, that I would be under-geared. Not so. Perfect gearing. Perfect for autocrossing and even big tracks and back roads. *except above 75mph on the freeway for any length of time. Yeah, I have the taller fifth. The only truly perfect solution would be a 6 speed with the correct gearing.

With a 3,2, I would probably be doing something like the OP.

[Bill Verburg]

Originally Posted by Trackrash

Get a 7:31 R&P, done.

Here's a stock '78SC except for a 7:31 cwp, not a very useful setup

[Bill Verburg]

Originally Posted by Bill Verburg

Here's a stock '78SC except for a 7:31 cwp, not a very useful setup

Just for a perspective here's alap at WGI , 3,4 & 5 are used, speeds generally 80 to 120+/-

915 w/ 7:31 would be in 5th 98% of the time

[betegh9]

I vote for swapping to a 7;31 ring and pinion specially on a 911 SC. Add the limited slip.

[RWebb]

a 1975 915 is not far off from the SC/RS set

[Bill Verburg]

Here's a comparison of various transmissions normalized for rpm & tire, in the real world both would be different and produce different results

[stownsen914]

I have a good 7:31 for a 915 for sale, in case you’re interested.

Scott

[pmax]

Sounds like a $$$ mod ...

but yes, I wish the 915 2-3 gearing were closer for normal spirited driving.