Gear Set for RS Spec 2.7

[Jandrews]

Hello,

I have the '71 911/01 transmission out while I am re-doing the bottom end of my motor. Thought I would go through and freshen up the synchro's, sliders and bearings as needed in the transmission while I was at it.

The gears appear to be in excellent condition, so I will likely be keeping them. I have always thought this gear set was particularly well suited to this screaming MFI RS motor, but thought I would run it by you guys for validation or further thoughts. The gears are as follows:

1st: A (11:34)
2nd: F (18:34)
3rd: N (23:29)
4th: V (27:25)
5th: ZD (31:22)

All of this of course, mated to a 7:31 ring and pinion. I have found that, according to the speedometer at least, this gear set yields approximately 30 m.p.h. in first gear, 60 m.p.h. in second, 90 m.p.h. in third, 120 m.p.h. in 4th and uncharted waters somewhere above 135 in 5th.

I guess I could calculate these given 7,300 r.p.m.'s and using the tire circumference as well. Does anyone have that formula handy?

Anyway, what do you think of these gears with this motor? Car is a '70E with the '73RS motor.


Thanks,

JA

[Early_S_Man]

Pretty close to the factory RS gearing ... a bit too tall for my tastes.

What tires ... brand, model, size at the rear end?

[Jandrews]

Thanks for the input, Warren.

Close to factory RS Spec, huh? Interesting. Wasn't this set also used in the 914-6?

For rear tires I am running Bridgestone Potenza 275-40/ZR 17.

I must say, I cannot imagine a shorter 1st or 2nd gear... they are fun! Third is fun too, but is is a little "longer". I could maybe see 4th and 5th being a little shorter, but overall this set seems to keep things right on cam.

What would you suggest, Warren?

Also, does anyone know the calculated speeds for the various gears given the above tire sizes?


Thanks,

JA

[Early_S_Man]

John,

The RS 915/08 transaxle with oil pump used the same ratios as production '74 models with the 915/06 version without pump.

1st -- 11:35 3.182
2nd -- 18:33 1.833
3rd -- 23:29 1.261
4th -- 27:25 0.926
5th -- 29:21 0.724

I will run it through my 16-year old DOS 'gears88.wk1' spreadsheet ... and print you a chart/graph.

Your speedo may be off or slow to respond ... 1st should be good for 37 - 39 mph or so!

The chart below was using 25.5" 185/70VR-15 tires ... yours are close at 25.7" ... per Tire Rack!

edit: I just looked at my spec book ... those aren't the 'stock' ratios for a 911/01 transaxle ... they are listed as:

1st -- 11:34 3.091
2nd -- 18:32 1.778
3rd -- 23:28 1.217
4th -- 27:25 0.926
5th -- 29:22 0.759

[Jandrews]

Very interesting, Warren.

I am looking forward to your chart. My initial calculations look suspect, as 5th gear works out to something like 177 m.p.h. at 7,300 r.p.m.'s. More than a tad optimistic for sure.

Also, looks like a little typo there in your note below regarding the stock 911/01 third gear. If stock third gear is really 1.217, it appears that my second and third gears are slightly lower than stock, while my fifth is higher, correct?

Thanks for the great info!!

JA

[TimT]

A F K Q V

short box

[Early_S_Man]

Here's the chart, John!

See what I mean about tall? I forgot to mentiopn that the RSRs used the same gearing as the RS!

Even the lightweight RS only got to 152 mph with that [similar] gearing when Frere tested it ... you need the power of an RSR 2.8 to get to 178 mph as the Road & Track crew did back in '73!

The basic question is what do YOU want the gearing to do ... track work, occasional blasts up to 120 mph, or autocross???

[Early_S_Man]

Here are a couple more charts some may find interesting when comparing gearing and doing some planning for the future!

Still for the 901/911/914 transaxle ... enjoy!!!

[Jandrews]

Outstanding Warren!!!

I am at work now, so only had a chance to glance through this. I will sit down tonight in my big leather office chair and digest this thoroughly. I truly thank you for taking the time to do this.

And yes, you are right, that little RS motor just doesn't have the guts to push the car that fast, despite the 5th gear's encouragement. I'll bet the 300 h.p. RSR could give it a go, though!

As for use, I would say my priorities are track work and occasional blasts to 120 m.p.h. Given that, I won't ever use my 5th gear, which I actually rarely do now. It does shut things down nicely for the odd highway drive, however.

Would you recommend some changes, or do you think my 1st through 4th are pretty well suited for my needs?


Thanks again!


JA

[Early_S_Man]

OK, John ... since you asked, I will give my 'standard' commentary about 'Donohue gears' which were hinted at in his famous test in Car and Driver of the 1969 Porsche lineup ...

... which, if you haven't read, is available, here, in PDF format for Acrobat Reader:

http://members.rennlist.com/emcon5/

To paraphrase Donohue ... he didn't feel the standard transaxle gearing was optimal for the 911E or 911S ... too tall for use on public roads, and not proving the best performance in the upper gears. He felt the gearing should make the car rpm-limited to about 120 mph to 125 mph, rather than power-limited to 140+ mph.

Over the years ... I analyzed his idea and decided that 'Airport gears' with a 'normal' first gear ... would do just what Donohue suggested with a 7300 rpm 'S' or 'RS' engine!

There are obviously several variations that can be built to satisfy the approximate upper four ratios of an 'Airport' gearset ... but, all are 'shorter' than what TimT suggested!

The 'basic' starting point would be this variation ... shown on the chart below:

1st -- 11:34 ... 3.091
2nd -- 17:34 ... 2.000
3rd -- 20:32 ... 1.600
4th -- 23:29 ... 1.261
5th -- 26:26 ... 1.000

I also included a slightly 'lower' 5th ... 25:26 ... 1.040 to show the difference to the next lower gear available ... just about 5 mph at 7300 rpm!

Anyway ... lots of people don't like this idea ... and will tell you so!

I am building this same transaxle with ZF limited slip from a '74 915/06 ... except 1st gear will be the 915's 3.18 ... 11:35 gearset. I also have on hand a 1.04 ... 25:26 gearset, and may even get a 1.08 ... 25:27 gearset to play with, as well!

If you need high-res copies of any of these charts to print at home for your records, drop me an email ...

[Jandrews]

Thanks again Warren!

I am starting to get the feeling that these changes you suggest would dramatically improve the "excitement" factor of this car. I thought 2nd and 3rd were fun before, but I can only imagine the exhilaration of the run to 80 in a quick 1,2,3 shift. Plus, I could still get to 60 without getting out of second, which seems to come in just a few seconds now.

Ok, you have me thinking!

Now, it sounds like you are playing around with some interesting combinations too. Do you know of a good source where I could find some differenct combinations for the 911/01? Do you happen to know the numeric codes that I would be looking for that would yield the ratios you decsribe below?

I must say that I am interested in the results I will get with this new motor and the gears you describe. This motor can put a smile on your face, and I think it might be downright shocking with these "airport" ratios.

Thoughts on sources?

Thanks again,


JA

[Early_S_Man]

John,

The 'basic set' is:

1st -- 11:34 ... 3.091 -- A
2nd -- 17:34 ... 2.000 -- E
3rd -- 20:32 ... 1.600 -- HA
4th -- 23:29 ... 1.261 -- N
5th -- 26:26 ... 1.000 -- T

optional 5th ratios:

5th -- 25:26 ... 1.04 -- S
5th -- 25:27 ... 1.08 -- R
5th -- 26:25 ... 0.96 -- U

JW has a bunch of 901 transaxles ... and may be able to supply some gears you would need. Carquip, Guard Tansmissions, and PowerHaus II all have some 901 gearsets, I believe.

You know .. it is funny how many 'so-called' high-revving current perormance cars have both 5th AND 6th gears that are major overdrive ... some can't even go as fast in 6th as in 5th!!! Really seems pointless to have 6 gears and two top ones are quiet 'cruising' gears! WRX is one, in particular ... $30K joke, actually!

[Jim Richards]

John and Warren, would you guys mind if I veer this thread a little bit? I was wondering, would an early 911 with 2.7RS motor and shorter tires (let's say 225/45X15 in back ~23") provide similar performance to your example? Or would it be wise to also revise the gearing? I'm thinking for track use with highway use as well. Thanks for your indulgence.

[Early_S_Man]

Jim,

I doubt John will mind adding another high-revving RS 2.7 early car to the discussion!

Did you mean 205/45-16 tires ... because those were the closest I could find specs for at Tire Rack? 23.2" for most I found ...

My original planning was done around 23.9" tall 285/40ZR-15 tires, but those vanished from the market a few years back! Next, I adopted the 245/45-16s as they were pn;y a bit taller at 24.6" ...

Only slight adjustments in the ratios would be needed for the shorter tires, IMHO ... it really depends on what top speed figure you would like to see.

[TimT]

This is what AFKQV looks like graphically



My experiences have taught me, that unless you have MEGA power, you wont be using all of your gears. I drive on some fast tracks...The Glen.,Summit, Pocono long.

I run out of gear at Pocono, at the Glen I top out in fifth braking for the bus stop

Im a summit noob but I love it and havent figured it out just yet.
Yet these short gears can work well on the street..

Another classic combo is MSX

You can build your gear box and install gears that will let you go stupid fast!!, but will you ever use those gears?

Given that, I won't ever use my 5th gear, which I actually rarely do now.

Figure out a gear set that will allow you to use all of the gears..(maybe except first) so inbstead if having a 2 speed gear box, you may actually have 4 gears to work with

[Jim Richards]

Warren, I was thinking of Hoosiers or Victoracers, I think they come in something similar to the 225/45 (or maybe 50 series?) X15. Regardless, I guess the critical detail is the height which as you pointed out will be around 23". I imagine 120-125mph will be plenty (ask me again after July 4th DE at the Glen ). Currently, at Summit Point with the 2.4E (growing into a 2.7RS this fall/winter) I'm just cracking 110mph. I'm also only using 3rd - 5th gears. Thanks again!

[Jim Richards]

Originally posted by Jim Richards
I'm just cracking 110mph.

Oh yeah, that's because I'm lousy...can't attribute that to gearing.

[TimT]

Im thinking of deleting the posts about 901 gearing. my experience and advice... are obviously for naught.

[Jim Richards]

Actually Tim, your comments about making 5th gear useable are very helpful, as well as pointing out that you top out in 5th at the bus stop at WG. Would you want a taller 5th, given that you also drive Summit? I'm not experienced enough to know what's good for me...just that I plan on driving Summit (multiple times), VIR, WG, Mid-Ohio every year and maybe throw in the occasional trip to Lime Rock or where ever.

I do have a 915, so I guess the gear choices are different.

[Jandrews]

Tim,

If you are referring to this thread, you are wrong...about your experience and advice all for naught! You have commented on A LOT of my threads, and I always look for your name when I post a question!

I have been at work and just now getting to go through these in detail. Tim, you will be happy to know that I have built a little comparison worksheet and actually have the gearset that recommended listed in there under "TimT".

I am just learning, and to have you and Warren offer opinions on these questions is VERY valuable to me. I think your combination looks EXCELLENT! As I compare it side by side with what I have now, it just brings 3rd, 4th and 5th down in a nice smooth curve. In fact, my 4th becomes your 5th!

The "stock" 911/01 ratios look just plain silly, as they are actually taller in 2nd and 3rd than what I have now, and just a hair lower for 5th. The 'Airport' set that Warren shows as a good basic starting point, looks REALLY short, actually using my existing 3rd for 4th, and with a 5th gear that is a fair amount shorter than my existing 4th! It is hard for me to imagine what it would be like driving something like this, but I do bet it would be a scream.

I am going to study this a while. In fact, I listed all the ratios from Warren's graph across the columns of a spreadsheet so I can view "relative" change from one set to another. I suspect I will contact JW at some point to see what he has, and whether I want to actually make some changes.

Tim, I hope you feel better. Your advice is VERY valuable, and I appreciate your input.


JA