Ring and pinion gears., Gear ratios. Options

[Keith914]

What are the practical (including cost) options to change the ring and pinion gears to "lengthen" all the gears so as to optimize a more powerful engine?

[IronHillRestorations]

IIRC there weren't options on R&P only gearing.

[forrestkhaag]

A one-day-newbie to gears with a more powerful engine, I can attest to the line of thinking Keith is pursuing. Yesterday was the first time I have driven my newly sorted 3.0. for miles (in breakin mode under 4K of course)... and I can tell you that first gear on a 901 box is useful if your hair is outta whack and needs a good inertial resortment toward the back of one's head.

I will spend a portion of today to understand this phenomenon better.

Sidebar: Perry's beautiful work on the harness went in without a hitch or spark / no need to tamper with that kind of work and support / same goes for his pal Rich Johnson. Excellent quality on the shift and accelerator linkage.

Happy Memorial Day and take time today to remember why we have the luxury of time behind the wheel of a 914. (IMG:style_emoticons/default/piratenanner.gif) (IMG:style_emoticons/default/piratenanner.gif) (IMG:style_emoticons/default/beerchug.gif)

[Mark Henry]

Most Porsche gearing changes are through the gears themselves, not the diff.
A good one for your app with a 901 is A, F, O, X and a flipped H gear in 5th spot.

915, 930, G50, etc can also be re-geared the same way but it gets spendy.
Example custom Guards gear sets (pair/set makes one gear) for a 930 are $1200 per gear (set).

[Mark Henry]

What is a more powerful engine? A /6? V8?

The 901 with stock gearing can handle any performance type 4 perfectly.
The only reason to change with a T4 is lower gears for track/AX.

I have stock 914-901 gearing in my 180hp type 4 in my '67 VW bug.

[forrestkhaag]

My early read is that it is as much an issue of the torque that a sex gives you over a fore....
(IMG:style_emoticons/default/piratenanner.gif) (IMG:style_emoticons/default/beerchug.gif)

[Steve]

AFAIK the 901 gear box only supports a 7:31 ring and pinion. 2nd gear is fixed to the mainshaft, unless you swap it out for a 904 mainshaft, which is overly priced on ebay all the time. With the 904 mainshaft you could change all the gears to get where you want to be. There are tons of gear charts out there. If your worried about breaking first, I have seen some rare 911 4 speeds, that only use 2nd -5, but the gears are more optimized for a four speed. The v8 and 3.6 guys just start out in 2nd and run a flipped H for 5th.
When it comes to the overpriced and harder to shift 915 gear box, from 72-73? It has a 7:31 ring and pinion. From 74-86 it has a 8:31 ring and pinion. I'm running an 86 915 gear box with my 3.2. I only changed 5th to the highest 29:21 for over drive on the freeway.
With a 29:21 5th. I can do 80 mph at 3k RPM's.
Here is a link to a 901 gear calculator. If there is a virus in it and it blows up your computer or innocent animals suffer, you've been warned.
https://cisco.box.com/s/9iw994c3qanzdmij21evrr1bghd9uuhd

[Steve]

(IMG:style_emoticons/default/icon_bump.gif)
For added Gear Calculator link in above post.

[Dave_Darling]

If you have a 915 gearbox, there were a couple different R&Ps to choose from. The early ones had a 7:31 like our 901s do; the later had something like an 8:31?

The cost for a different R&P in a 901 box would be astronomical. And it would take time to find someone who could manufacture the gear set for you.

The easier way to go is tires with a larger overall diameter. Most often done by going with larger-diameter wheels and not dropping the aspect ratio (sidewall height) of the tires to match. That effectively makes all of the gears taller.

Or change over to a 915-based transmission. Or some other trans.

--DD

[Montreal914]

What about the 7:27 R&P from the Sportomatic?
I believe there are the early and late Sporto. The early shaft is too short and the late is a little too long. Once cut, I think it might work. (IMG:style_emoticons/default/confused24.gif)

Here is a standard 914-901 pinion shaft (left) next to a late Sporto pinion shaft (right). I obviously have the matching ring.

Looking further into this is one of the many 914 projects I have lined up (I don't know when though... (IMG:style_emoticons/default/rolleyes.gif) )

[Steve]

Wouldn't you want a higher ring & pinion to raise the gears? With the 915 Porsche went from 7:31 to 8:31 to raise / lengthen the gears. That sportomatic ring and pinion would lower the gears.

[Montreal914]

Wouldn't you want a higher ring & pinion to raise the gears? With the 915 Porsche went from 7:31 to 8:31 to raise / lengthen the gears. That sportomatic ring and pinion would lower the gears.

I thought the OP was looking for options for a taller ring and pinion since he has a bigger output engine. Stock is 7:31, and Sporto is taller 7:27.

Easiest is what Mark Henri suggested with different ratios. I always think that A-F (first, second) are low for torquey engines, hence taller R&P, but I don't have experience with these types of engine. (IMG:style_emoticons/default/dry.gif)

[Chris914n6]

The easier way to go is tires with a larger overall diameter. Most often done by going with larger-diameter wheels and not dropping the aspect ratio (sidewall height) of the tires to match. That effectively makes all of the gears taller.

--DD

This. I stepped up to stock 205/55-16 Boxster wheels & tires.

2nd is machined into the shaft so can't do anything with it. As such changing first is near pointless, plus it sits on a smaller unsupported section of the shaft and snaps easily.

I have a 3.0 so starting in 2nd is easy, and nice around town as I don't have to shift as much. 3 and 4 are good city gears. Flipped H 5th for highway cruising at 3000rpm. With the Boxster tires that's 90mph (IMG:style_emoticons/default/biggrin.gif)

[Steve]

Wouldn't you want a higher ring & pinion to raise the gears? With the 915 Porsche went from 7:31 to 8:31 to raise / lengthen the gears. That sportomatic ring and pinion would lower the gears.

I thought the OP was looking for options for a taller ring and pinion since he has a bigger output engine. Stock is 7:31, and Sporto is taller 7:27.

Easiest is what Mark Henri suggested with different ratios. I always think that A-F (first, second) are low for torquey engines, hence taller R&P, but I don't have experience with these types of engine. (IMG:style_emoticons/default/dry.gif)

The goal with a bigger engine (more HP) is lower RPM's for the same MPH.
Example a 29:21 5th gear in a 7:31 Ring and Pinion 915 gear box is 70mph at 3k RPM's. The same 29:21 5th gear in a 8:31 Ring and Pinion 915 gear box is 80mph at 3k RPM's.
If we could increase the ring and pinion from 7:31 to 8:31 in a 914 gear box, we could lower the RPM's in each gear and make all 5 gears taller.
A 3.0 six with webers produces enough HP and Torque to make 1st gear pretty much worthless. It winds out way to quick.
Starting out in second, makes for a tall 1st gear.
Play with the gear calculator link. Change the ring and pinion from 7:31 to 8:31 and watch 5th gear increase by 10mph in 5th at 3k rpms.

[Montreal914]

Wouldn't you want a higher ring & pinion to raise the gears? With the 915 Porsche went from 7:31 to 8:31 to raise / lengthen the gears. That sportomatic ring and pinion would lower the gears.

I thought the OP was looking for options for a taller ring and pinion since he has a bigger output engine. Stock is 7:31, and Sporto is taller 7:27.

Easiest is what Mark Henri suggested with different ratios. I always think that A-F (first, second) are low for torquey engines, hence taller R&P, but I don't have experience with these types of engine. (IMG:style_emoticons/default/dry.gif)

The goal with a bigger engine (more HP) is lower RPM's for the same MPH.
Example a 29:21 5th gear in a 7:31 Ring and Pinion 915 gear box is 70mph at 3k RPM's. The same 29:21 5th gear in a 8:31 Ring and Pinion 915 gear box is 80mph at 3k RPM's.
If we could increase the ring and pinion from 7:31 to 8:31 in a 914 gear box, we could lower the RPM's in each gear and make all 5 gears taller.
A 3.0 six with webers produces enough HP and Torque to make 1st gear pretty much worthless. It winds out way to quick.
Starting out in second, makes for a tall 1st gear.
Play with the gear calculator link. Change the ring and pinion from 7:31 to 8:31 and watch 5th gear increase by 10mph in 5th at 3k rpms.

Yep, exactly what I was trying to express while replying to the OP. He was asking about a taller ring and pinion option.
The Sporto ring and pinion that I'm hoping to fit into a 901-914 box is 7:27 (0.259) vs 7:31 stock (0.225) (IMG:style_emoticons/default/stirthepot.gif). I have heard it has been done before (IMG:style_emoticons/default/confused24.gif)
Your 8:31 option on the 915 happens to be 0.258, pretty dam close to the Sporto 0.259!
This could render both first and second on the 901-914 box better suited for torquey engines, the rest of the gears are open as usual.
Other option, redo the whole 901 box box with a 904 shaft, as previously mentioned, and select 1-5.
No matter how, all fun projects (IMG:style_emoticons/default/smile.gif) (IMG:style_emoticons/default/smash.gif)

[IronHillRestorations]

I thought the '73 stock gearing worked pretty good for a 3.0. The only one I'd have changed was 5th which could be just a bit taller.

[Mark Henry]

A healthy 3.0 can start in 2nd no problem on everything but a steep incline. Think of 1st as a granny gear or a get her rolling gear.

The 901 Achilles heal is 1st gear, 2nd-5th it can handle all the 3.0/3.2 NA power you can throw at it, but if you had a usable 1st gear you would smoke the box PDQ.
Also you can't change the 2nd gear ratio, it's fixed on the shaft, so even if you make a taller 1st then 2nd isn't tall enough.

My tranny is set up A, F, O, X, HA (flipped). HA is one tooth short of H.
I choose HA because I had it and I'm cheap, also it's almost but not quite as tall as H, so for me a bit more in line with typical highway speeds here.
I'm using a hot street 3.0 that should have at least 250.hp

[rick 918-S]

Don't forget that part of the cooling on an air cooled engine is fan speed. You want to run the RPM's up a little higher. specially on hills or hotter days. Water cooled engines on the other hand are a different machine. With 255/50/16 rear tires and an HB 5th gear I run the freeway about 2800 RPM's at 80-85 and lots of torque to pull hills or stay in 5th at 2000 RPM's. I use 1st gear to get the car rolling because its easier on the clutch disc but never abuse the gear. I have a custom length 901 case. If I explode it, it will a setback to get the car back on the road

[Steve]

Good point Rick. Even though my speedo says 80 mph at 3k rpms, I'm actually only doing 75 due to the 245x40x17" tires. Pretty close to a stock 5th in an 86 gear box.
I always keep the rpm's at 3k or higher.

[Keith914]

Thanks for all your responses. I hope to have my carbureted 2.7 911 engine ready for installation this week and will then consider this matter further. Options clearly are limited (IMG:style_emoticons/default/dry.gif)