How do our Porsche transmissions work?

There have been a lot of questions lately about transmissions, mostly brought on by winter when the older style Porsche transmissions sometimes become a bit difficult to shift, a condition I like to call ‘balky”. Shifting those transmissions is a little like getting a teenager out of bed in the morning. Once you get them going things are fine but it may take a few tries to get their attention. From the early days when Porsche was using a pure VW transmission until now, the principles that govern power conversion and transfer are still the same.

Your engine converts chemical energy to mechanical energy that is output by the crankshaft and directed into the transmission. The transmission takes that rotational energy and, depending on your driving needs, changes the speed and acceleration of the car by directing energy through the transmission (with gears) to the wheels of the car. We are going to talk about gears for a bit.

At the core of a transmission is a simple machine component called a gear-pair. Our Porsche transmissions are essentially a cluster of different diameter gear-pairs and we should understand how they work. The following discussion applies not only to manual but also to automatic transmissions since the engineering differences between them are their shifting and clutch mechanisms. Think of it this way. There are two things going on in your transmission:

1. Transmission of power, accomplished by gear-pairs and shafts and…

2. Control systems

a. Shifting mechanisms, which change the ratio of input to output speeds by selecting different gear-pairs

b. Clutch function, which momentarily “disengages” the engine from the transmission long enough for the shifting mechanism to change gears-pairs.

Other “mini-clutch” mechanisms exist as well such as the synchronizers, synchro hubs, and shift sleeves etcetera, but we needn’t cloud the discussion with that detail for now. For this article we’ll concentrate on the first function, power transmission and how gear-pairs do their job to get us “up to speed”…. I couldn’t resist.

Transmission of Power
A gear is nothing more than a circle with teeth where the teeth force a positive engagement of two wheels at their perimeters. They are meshed together to transmit motion and force. In any pair of gears the larger one will rotate more slowly than the smaller one, but will rotate with greater force because of the longer lever-arm. Another characteristic is that each gear in a series reverses the direction of rotation of the previous gear.

Gears
A gear when in operation, may actually be considered as a lever (between the axis and the perimeter of the gear) with the additional feature that it can be rotated continuously, instead of rocking back and forth through a short distance. One of the basic relationships for a gear is the number of teeth, the diameter, and the rotary velocity of gears. Figure 1 shows the ends of two shafts A and B connected by 2 gears of 24 and 48 teeth respectively. Notice that the larger gear will make only one-half of a revolution for every revolution of the smaller gear. That is, the ratio of speeds (velocity ratio) of the large to the smaller is as 1 to 2.

Figure 1, Gears

If the shaft A is rotating at 2 rpm (revolutions per minute) the ratio of these two gears demands that gear B rotate at 1 rpm. So for every revolution of A, B will rotate ½ a revolution. A transmission must constantly be shifted into higher gears when accelerating. This happens simply because the engine rpm range is not big enough (with a single gear-pair) to take a car from zero to 150 mph, or even to highway speeds. If internal combustion engines could do that easily then we wouldn’t need gears at all and a single shaft would do the trick. Of course gas-mileage would really stink and it would be a bit noisy! Now I know what some of you are thinking…. We won’t talk about jets and turbines until one of you puts one in your 914.

We’ve said that your transmission is a cluster of gear-pairs, usually from four to six of them, plus reverse. The number of gear-pairs is generally referred to as the number of “speeds” and transmissions are considered as “four, five or six-speed” varieties. Reverse, by the way, is not considered one of the speeds. Lets take a real-world example of a 1995 Porsche G-50/31 transmission. Figure 2 shows a cutaway side-view of the G-50 manual transmission. The two large shafts running right-to-left are the input (upper) and output (lower) shafts where the engine would be on the right side. The clutch, pressure-plate and flywheel (and other stuff) would be in the cavity on the right mounted on the input shaft.

Figure 2, G-50 Transmission

In Figure 2 the ring-and-pinion are “bevel” gears that appear on the lower right side of the sketch as a tapered gear (the pinion) against another bevel gear (the ring) that looks like a white circle. The ring-and-pinion gear-pair not only changes the speed of rotation (little pinion, big ring) but also changes the output direction by 90 degrees. This allows power and motion to go out to your wheels and finally to the road.

The gear ratios in the table below show all six “speeds”. Z1 is the number of input-gear teeth (input shaft from the engine) and Z2 is the number of output-gear teeth (to the ring-and-pinion, or final-drive) and then to the wheels. Z2/Z1 is the ratio of how many revolutions the input gear will have to turn for every revolution of the output gear.

Gear Z1 :_Z2 Z2/Z1

1^st gear 13 : 41 3.154

2^nd gear 20 : 40 2.000

3^rd gear 23 : 35 1.522

4^th gear 33 : 41 1.242

5^th gear 41 : 42 1.024

6^th gear 39 : 32 0.821 ( This is lower than 1:1 also known as overdrive)

Reverse gear 14 : 40 2.857

Final Drive 9 : 31 3.444

Now these gears can be changed (and often are) in racecars to get either more power or higher speeds in certain engine rpm ranges on various tracks. Car manufacturers try very hard to tune these gear combinations between power and speed for use on public roads. They have to take into account the power and torque of the engine at various rpms, as well as the weight of the car and proposed tire diameters. It’s not much of a stretch to imagine that radically different tire diameters are essentially the same as “final drive” gear changes!

In our transmissions the gears are always in contact with one another. This is referred to as “constant-mesh” which prevents us from grinding or shearing the teeth from the gear. We’ve mentioned some other components called synchro-hubs and synchronizers. They are there to allow us to engage and disengage the input and output shafts from one another while gear-pairs are shifted. I’ll not go into that here but suffice it to say that if the actual gears were engaged and disengaged, our transmissions would be in need of overhauls every week!

Our Porsche 901, 915, 916 and G-50 transmissions are wonderful machines. To keep them in tip-top condition you need only to follow a few simple guidelines.

1. Observe the maintenance schedules on fluid levels and changes

2. Get your clutch adjusted when needed

3. While driving with a manual transmission, make certain that the clutch pedal is completely depressed to the floor while shifting. This is VERY important since two things will happen if you don’t depress completely. Over time your clutch will wear from never really disengaging. The synchronizer components will wear as well, which will eventually make your transmission difficult to shift.

Keep ‘em healthy folks, our cars like it that way

Keep the Shiny Side up!

Ron Mott