This article is one in a series that have been released in conjunction with Wayne's new book, 101 Performance Projects for Your Porsche Boxster. The book contains 312 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 950+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Boxster owner's collection. The book is currently available and in stock now. See The Official Book Website for more details.
One of the easiest tasks to perform on your manual transmission Boxster is to change the transmission oil. The Boxster transmission is what is known as a transaxle. It includes all the standard components of a normal transmission, plus an integrated differential. This design is possible because of the mid-engine design of the Boxster. The transaxle design is more compact and theoretically lighter in weight since you don't need a dedicated differential.
The differential and the transmission both share the same lubricating fluid. It's very important to make sure that the fluid in your transmission is at the proper level, otherwise your transmission will experience significant wear. The synchro rings and sliders all depend on a slick surface in order to match speeds when shifting. If your transmission is low on oil, the wear on these components will accelerate significantly. In addition, shifting the car will be more difficult. One of the first things that you should check on a Boxster that is having problems shifting is the level of the transmission oil. Keeping the differential and all the associated gears well lubricated should also help increase your fuel mileage.
The transmission oil also helps to keep temperatures down inside your transmission. The engine is one of the primary sources of heat for the transmission as it conducts and radiates through and around the points where the engine and transmission are mounted. The transmission also creates heat itself as the gears and synchros turn within its case. Keeping the transmission fluid at its proper level helps to mitigate heat problems. Having a large reservoir of oil to spread the heat throughout the transmission helps to keep temperatures down. On some of the higher performance Porsche transmissions, there is even an external transmission cooler that operates similar to the engine cooler.
I recommend that your transmission fluid should be changed every 30,000 miles or about once every two years. This number is a rough estimate, and may vary depending upon your use of your Boxster (track vs. street). There are many moving parts in the transmission, and they have a tendency to drop small microscopic metal particles into the oil. Specifically, the synchro rings wear down slowly over time, each time you shift. While the transmission bearings are not as sensitive to oil contaminants as the engine bearings, they can still exhibit wear from these particles in the oil.
The Boxster transmissions have two plugs for filling and emptying the transmission oil, located on the side and the bottom of the case. The five-speed transmission requires a 17mm hex socket to remove the filler plug, and a 16mm triple-square anti-tamper socket (Hazet 2567-16) to drain the fluid. Both are available in the online catalog at PelicanParts.com. The six-speed plugs are a bit more generic: a 10mm hex socket is all that is required to remove both the drain plug and the filler plug.
The first step in checking or filling your transmission is to gain access to the plugs. Jack up all four corners of the car (Pelican Technical Article: Jacking Up and Lifting the Boxster on Jack Stands), making sure that the car is perfectly level with respect to the ground. Then remove the front plastic engine covers, the lower diagonal aluminum braces and the metal transmission cover (Pelican Technical Article: Changing Automatic Transmission Fluid).
If you are simply checking the level of oil in your transmission, start by removing the filler plug on the side of the transmission. This is the plug that you add fluid to. For the five-speed transmission, take a large paper clip bent it at a right-angle, stick it inside the hole pointing towards the ground, and measure the fluid level on the paper clip. This "mini dipstick" should register transmission fluid at about 11mm below the lowest edge of the filler hole. Make sure you do this when the car is cold, and parked on level ground. If the level is lower than 11mm, then you will need to add some fluid. For the six-speed transmissions, it's a bit easier. Simply stick your finger in the hole and see if you can feel fluid at the bottom level of the hole. If you can feel the fluid level with your finger, then your fluid level is about right, or perhaps will need only a little topping off.
If you cannot feel the fluid level, then you will need to add transmission oil to the case. If you are planning on changing the oil, then remove the plug on the bottom of the transmission case. It's a wise idea to try to empty the transmission oil when the car is warm, as this will make the oil more viscous and it will flow out easier. Make sure that you have a drain pan capable of handling at least 4 quarts of transmission oil. Check the fluid in the pan to see if you see any unusual metal pieces, or grit in the oil. The five-speed transmission holds about 2.25 liters (2.4 quarts), and the six-speed transmission holds 2.8 liters (3.0 quarts).
While the fluid is emptying out, you can use this time to clean out the drain and filler plugs. The bottom drain plug should have an integrated magnet in it that traps metal debris. Using a cotton swab or a paper towel, carefully clean out all of the black debris and particles that may have found their way in there.
Replace the bottom plug on the transmission, but don't tighten it too tightly (18 ft-lbs or 25 Nm maximum). These plugs do not have a tendency to leak (transmission oil is thicker than engine oil). If it does leak later on, you can always tighten it a little more. Now, add transmission oil to the case. The best method of doing this is with a hand operated oil pump. These are available from most auto parts stores and attach to the top of the plastic transmission oil bottle. They work very similar to the liquid soap dispensers you find in most bathrooms. Pump the transmission case full of fluid until it just starts to run out the filler hole. Replace the filler plug and clean up the few drips that might have run out of the hole. Tighten down the filler plug in a similar manner to the drain plug.
The automatic transmission cars also have a differential built-in to the transaxle. This differential uses the same type of fluid as the manual transmission and must be checked and filled in addition to the automatic transmission fluid. On the automatic cars, there is no drain plug however, so the gear oil must be drained by loosening the outer differential cover (see Project 39 and Figure 5). Or, you can possibly get around this by inserting a fluid vacuum pump into the fluid fill hole and sucking out all of the old fluid. You top off the fluid and fill the differential in a similar manner to the six-speed transmission. Simply unscrew the plug and fill until the fluid starts flowing out of the hole. The automatic transmission uses the same fluid as the manual transmissions and takes about 0.8 liter (0.85 quart). Tighten up the drain plug to 22 ft-lb (30 Nm).
In many cases, generic transmission gear oil that meets or exceeds SAE 75W90 will suffice perfectly fine. Also very effective are the Porsche factory lubricants (typically manufactured by Shell Oil), or Mobil Delvac Synthetic Gear Oil 75W-90. In addition, if you have a Limited Slip Differential (LSD), be sure that you get transmission fluid that is appropriate: using a fluid that is too slippery can reduce the torque bias effects of the differential and make it less effective at distributing torque.
This photo shows a typical 5-speed Boxster transmission that has been removed from the car. You need a special hex socket in order to remove the filler plug located on the side (17mm for the 5-speed and 10mm for the 6-speed). The inset photo shows the 17mm hex socket tool along with the special 16mm triple-square anti-tamper drain plug removal tool required for the 5-speed transmission. The 6-speed requires only a 10mm hex socket. The lower inset photo shows the transmission being filled using a hand pump.
Here's a photo of the five speed transmission from my Boxster being reinstalled. With all of the exhaust components out of the way, it shows a clear view of where the drain plug is located in relation to the rest of the transmission (yellow arrow). The inset photo shows a close-up of the wacky plug that requires the triple-square removal tool. I'm not sure what the motivation was for Porsche to use such a specialized plug here.
The five-speed transmission should be filled approximately 11mm below the bottom edge of the fill plug in the side of the transmission. Use a paperclip as a mini-dipstick to measure the level inside the transmission, as shown here in this diagram.
Shown here is the side of the automatic, Tiptronic transmission for the Boxster. The green arrows point to the differential cover bolts that need to be removed in order to loosen the cover and drain the differential gear oil. If you can't empty the differential by simply loosening the cover bolts, then you need to remove the center bolt and pull out the stub axle. The red arrow points to the fill plug. Top off the fluid to the edge of the bottom of the fill plug, just like on the six-speed transmission.