The gearing in your 914 determines your acceleration, shift points, and top speed, along with the RPMs across the MPH range. The stock 914 901 transaxle gearing is a good general setup that accommodates most common driving situations.
NOTE: Most 914 owners will never need to consider re-gearing.

However, you may want to consider re-gearing if you:

• Change Tire diameter
• Change to a Power Plant with a different torque curve and/or RPM range
• Desire to tune your ratios to match your driving style: Autocross, Road Courses, etc.

Here are some symptoms you may experience that can be resolved with re-gearing:

• Uncomfortable RPM at cruising speed, e.g. you are ‘in between gears’ at your cruising speed because you have too high RPMs in one gear, or too low in the next
• You don’t have ‘the right gear’ for your autocross or road course corners, e.g. you are ‘in between gears’ in the corner because you either have too high RPMs in one gear, or too low in the next
• You don’t have ‘the right gear’ for the road course straightaway, e.g. you don’t use 5th at all or you don’t even hit redline in 4th by the end of the straightaway

The 914 901 Gear Ratio Workbook helps you do “what if” models to see how changing gears affects your RPM at different speeds as well as your shift point entry/exit RPMs for each gear. This spreadsheet has four worksheets (Tabs):

• Tab 1 - 901 Gear Ratio Model – Provides graphs of the stock gearing as a baseline and allows the user to enter their tire diameter, redline and gear selections to do ‘what if’ models.
• Tab 2 - 901 Gear Ratios & RPM-MPH table – Table of all 901 gear ratios along with a RPM/MPH table to assist with gear selections. RPM/MPH is based on your tire diameter entered on Tab 1
• Tab 3 – 901 3rd & 4th-5th “FLIP” Chart – Some 3rd gears can be used as 4th/5th gears and vice-a-versa. This table shows which gears will interchange.
• Tab 4 – 901 Transaxle Codes & Gear sets – Different model 901s had different stock gear sets. This table helps you decode which 901 you have or are looking to buy.

914 901 Gear Ratio Workbook

As a baseline, the Stock 914-4 gear ratio chart is below. Note that 1) Porsche uses letter codes for its gear sets (“A” thru “ZD”) you’ll see this notation whenever 914 901 gear ratios are discussed, and 2) The letter codes refer to the number of teeth on each gear of the pair, e.g. “A” = 11:34, this is important since some gear sets have letters stamped on them that are not related to the actual ratio. The stock 914-4 901 gear set is “A”-“F”-“N”-“V”-“ZD”. The complete gear ratio table, along with teeth counts, is included in the 914 901 Gear Ratio Excel sheet.

Figure 1. Stock 914-4 901 Gear Ratios, with 205-50x15 tires and a 5500 RPM redline

Some thoughts on how to use the 914 901 Gear Ratio Workbook follow. This is definitely not the only way to approach this project, probably not the best way, just the way that has worked well for me.

General approach

In general, it’s hard to create a ‘perfect’ set of ratios and you’ll probably have to settle for some trade offs. While it’s usually not possible to have ‘the perfect’ gear for each corner, here are some things to think about to get you that good compromise.

Road course example

Things to consider in order of priority:

1. Corner onto the longest straight

2. Top speed on the longest straight

3. Shift points of the ‘in between’ gears

Corner onto the longest straight

Start with the corner preceding the longest straight. The acceleration coming out of this corner and your resulting top speed at the end of this straight has the biggest impact on your lap times. For more info on this, refer to “Driving in Competition” by Alan Johnson, available at your favorite book vendor.

It ‘s critical that you can take that corner as fast as your handling setup will allow you, and also be at a point in your engine’s power band affording the best possible acceleration onto the straight.

Let’s assume you can take the corner before the longest straight at 40mph. Now refer to figure 1 and you’ll see that you are either at your shift point in 2^nd @ 4800RPM or @ 3200RPM in 3^rd. This isn’t the best case for a 40 MPH corner. Now look at figure 2, the example Road Race setup which is a “A”-“HA”-“KA”-“Q”-“V” setup. Now at 40mph in 2^nd gear you are at 4100RPM eliminating the ‘in between gears’ situation and allowing good acceleration onto the straight.

Note that 2^nd gear is part of your main shaft and the options are “F”, “GA”, and “GB” (see the gear table on the 914 901 Gear Ratio Workbook) if you need more options for 2^nd, you can go to a 904 main shaft which allows you to exchange 2^nd gear ratios just like 1^st, and 3^rd-5^th. But be forewarned, 904 main shafts are not cheap! Alternatives to a 904 main shaft for 2^nd gear changes are discussed in the Summary section at the end of this article.

Depending on your tire diameter, engine power band, and your favorite track’s configuration, you’ll have to determine the best gear for you on the corner before the longest straight using the 914 901 Gear Ratio Workbook. Start with the MPH you take the corner then find the gear that puts you in the RPM ‘sweet’ spot to catapult you down the straight.

Top Speed on the Longest Straight

The second consideration is your top speed on the longest straight. The stock “ZD” fifth gear doesn’t help on the track as it’s an overdrive gear and acceleration is barely achievable in this gear (great for gas mileage on the highway though!) On many tracks a stock geared 914 will never use 5^th and many times not hit redline in 4^th. The objective here is to accelerate as fast as possible by keeping the engine in its power band with no bogging between gears and also not hitting 5^th gear redline too early. Best case is, you’re just reaching redline in 5th when you reach your braking point for the next turn. To help zero in on the correct top gear, take notes on your MPH or RPM at the end of the straight with your current gearbox. From there you have a good starting point to select a top gear. For our example, let’s assume with the length of the straight away 85MPH @ 5000RPM in 4^th gear is your top speed. Now you need to factor in that having a close ratio gearbox you’ll accelerate faster and have a higher MPH at the end of the straight. As a ballpark assume you’ll gain 5% top speed. So from 85 you’ll probably go up to 90. Now to figure 2. In this case a your stock “V” 4^th gear is perfect as a fifth as it will let go you to 90mph at your redline of 5500. Best case is this happens right as you reach the braking point for the next turn.

Depending on your tire diameter, engine power band, your favorite tracks configuration, you’ll have to determine the best top gear for you using the 914 901 Gear Ratio Workbook.

Shift points of the ‘in between’ gears

In the example above, we’ve talked about 2^nd & 5^th so let’s look at 3^rd & 4^th. Ideally you’d like to have a perfect gear for every turn on the track so you could set them as with 2^nd gear and use the remaining corner MPHs, but you also have to think about efficiently getting from 2^nd to 5^th down the straightaways. You probably can come pretty close to a good gear set by selecting your 3^rd & 4^th to give you the desired RPM drop when up shifting. Again the goals is to drop the RPMs from redline into the heart of your engines torque band when up shifting. In the attached example my RPM entry points are as follows: 2^nd to 3^rd 4359 RPM, 3^rd to 4^th 4391 RPM, and 4^th to 5^th 4527 RPM. Here are the charts from the 914 901 Gear Ratio Excel sheet.

Figure 2. Example Road Race setup, with 205-50x15 tires and a 5500 RPM redline

The setup in this example “KA”-“Q”-“V” is also one of the economy short gear sets:

• 3^rd – your stock “ZD” 5^th is ‘flipped’ to make a “KA” and moved to 3^rd

• 5^th – your stock “V” 4^th is moved to 5^th

• 4^th - You need to find a “Q” gear for your new 4^th

See the “901 3^rd & 4^th-5^th “FLIP” Chart” on the 914 901 Gear Ratio Workbook for more info on ‘flipping gears’.

Another popular combination for longer tracks is “M”-“S”-“X”. Of course what will work for you is dependent on your tire diameter, engine power band, and the specific track you are tuning for. That’s where the spreadsheet comes in handy to do your “what if” calculations to model your RPM and MPH at your shift points before buying and changing out any gears.

Autocross Example

Autocross setup is similar except the courses are lower speed and you usually need to tune 1^st-3^rd. Also, many cars are driven to autocross events so in those cases a reasonable highway cruising RPM needs to be retained, unlike Road Race setups where that setup would be detrimental to your straightaway top speed.

Here is a setup that requires no new gears to be purchased. By ‘flipping’ the stock 3^rd & 5^th gears you get a lower 3^rd gear and a 5^th that is still acceptable for short trips on the highway. See the “flip chart” on the 914 901 Gear Ratio Workbook:

• 3^rd - Stock “N” becomes a “KA”

• 5^th – Stock “ZD” becomes a “Z”

Figure 3. Example Autocross Setup, with 205-50x15 tires and a 5500 RPM redline

You can see from the graph that this setup give close ratio gears for 1^st - 3^rd and leaves 4^th & 5^th for getting to and from the event. The next step for your Autocross setup is to dial in your 2^nd gear. If the “F” 2^nd isn’t working out for your typical autocross MPH, try the “GA”, or “GB” main shafts in the model. If they don’t fit the bill either, you’ll need to a) find a 904 main shaft so can you can interchange gears same as with the 1^st and 3^rd-5^th positions, or b) consider the alternative for other 2^nd gear options noted in the Summary section below. Of course what will work for you is dependent on your tire diameter, engine power band, and the situations you are tuning for.

Street Example

Determining gear ratios for the street isn’t quite as involved. The issue is usually that the changes made to the car: tire diameter, power plant swap, etc., has resulted in less than acceptable RPMs at commonly driven speeds. For example, our 914-6 street/track car is running the “M”-“S”-“X” setup and driving at 70mph on the highway is only OK for short trips. A higher 5^th would definitely be needed for longer trips… The same holds true for in city driving, you may be ‘in between’ gears at your normal cruising speed.

For this application, use the spreadsheet to model the speeds you typically drive at and change out the gears until you get your desired RPMs at your desired MPHs. Remember to pay attention to the RPM exit & entry points so you also end up with acceptable ‘steps’ between the gears. Also make sure you end up with a 5^th that is appropriate for your highway trips.

Summary

The 914 901 Gear Ratio Workbook allows you to model and evaluate different gear combinations without buying gears or rebuilding your transaxle. While this provides valuable information, I recommend that you always talk to others who have already taken on this project, especially those that have a similar car setup and similar objectives. If possible try to get a ride in, or better yet a test drive a car with the modification you are considering before embarking on the project. Always take advantage of the experience of other 914World members before spending money!!

Where can I get gears? How do I change out gear sets?

Once you’ve decided to go forward with this project and have worked out your ‘perfect’ gear set. There are 914World members that provide products and services for your 901 transaxle.

Dr Evil provides a 901 information/diagnostic/rebuild service which includes re-gearing.

Racer Chris provides products & services to provide more 2^nd gear options beyond “F”, “GA” and “GB”, without having to buy a 904 main shaft.

Rich Bontempi provides products & services including gear set planning consultation, gear-sets, 901 diagnostics and rebuilding services.

Or, for the DIY crowd, 914World member contributions from the “Lapuwali Classic Thread Forum”:

DNHunt -- There sure are a lot of parts in a 901 box, now I'm going to try to put them back in

Bondo -- 914 transmission disassembly, Step by step, so it can go back together

Search for related discussion threads here:

914World Search page

Member Gear Set Library

Lastly, but probably most importantly, I've created a "library" of member gear sets used for Autocross, Street, and Track to provide a consolidated repository of 'what works for who-where' along with associated notes or discussion threads to help you with your research for that 'perfect' gear set.

Important Note! The ‘right’ gear set for you is particular to many factors. To list a few, the car’s weight, suspension, engine, and DRIVER! Don’t take the gear-sets listed for a particular application as your ideal setup. Contact the owner, compare corner speeds, suspension setups, engine characteristics, driving skills, etc. All of these factors play into what will work for you.

The “Member Gear Ratio Library” can be viewed on the 914 901 Gear Ratios, Spreadsheet, Member Gear Set Library thread in the Paddock.

If you’d like to contribute, please post on the 914 901 Gear Ratios, Spreadsheet, Member Gear Set Library thread and include: Applicable Category, Track (if applicable), Engine (cc & cylinders), HP, Redline RPM, Tire Diameter, Weight w/driver, 1st-5th gear codes and any short comment that you’d like to pass on, or better yet provide the link to the applicable 914World discussion thread.