One of the more daunting tasks is the 928’s timing belt. The timing belt is the Sharks aorta. If it pops… your shark is likely a goner. The 928 has the dubious distinction of having one of the longest timing belts of any production car in history. Measuring over six feet in length, the 928’s timing belt can be a bit of a challenge. It is generally recommended to change the timing belt every 60,000 miles or 5 years. I personally think that’s a little too often, but I’m always doing something on my car, so I’m always looking at it anyway.
There are several write-ups on how best to tackle the timing belt on a 928 including the factory workshop manual. My personal favorite can be found at this web address: http://tinyurl.com/po88q9y. In this procedure, the author re-uses the stock tensioner and gears. We have found that the stock tensioner design is less than optimal and very antiquated. We’ve also found that the gears are often failing or on the verge of failing. There is also a preference for the brand of belt. Let’s take each of these, one at a time.
The stock 928 tensioner is a bit of a Rube Goldberg contraption. It is used to set the initial static tension at a particular level, and then it has a stack of 20 “Bellville” bi-metal concave washers (see picture). The idea is that as the engine heats up and expands, the concave washers flatten out with heat to allow the tensioner to deflect and de-tension the timing belt. This overly complex system requires frequent checks and is prone to failures, which is why the factory put a warning system inside the car.
Finally about 4 years ago, a smart guy in the pacific northwest developed a replacement tensioner that, aside from the bracket, uses off-the shelf Audi parts to provide a dynamic tensioning system for the timing belt. It is known as the “PorKen-sioner” and can be purchased from 928s-R-Us in Texas (tell them I sent you). This setup has over a million road miles in various climates and has shown to be extremely reliable. All the local 928 owners are converting their cars to this system. I just converted my “Coke Car” over a few weeks ago. The best part is that it costs less than a rebuilt factory tensioner and about a quarter of a new Porsche tensioner which is now around $1,200!
Water pumps don’t generally fail too often on 928s, but when they do, the results can be catastrophic. Therefore it is generally recommended that you replace the water pump when replacing the timing belt. I think that’s a good idea if you are having someone else do the work, but if you’re doing it yourself, I think every-other time is fine. Just be prepared to dive in should you find an ominous leak or see strange engine temperatures.
There are a number of water pumps available from $89 to $1,000. You get what you pay for in my opinion. I think it is probably best to ask a reputable 928 parts vendor what they currently recommend as supplies go in and out. Whatever you do, do buy it off eBay with high expectations.
TIMING BELT GEARS:
Each 928 has two cam gears and one oil pump gear that are made from an aluminum-magnesium alloy. They come from the factory with a hard-anodized coating. We have found that after about 80,000 miles this coating begins to fail. Once the coating is gone, the gears can wear very quickly and start to cut into the belt.
Replacement gears are about $1,000 for a set of three gears. We have found that if you catch them early enough you can have them recoated with a dry-film lubricant that will withstand the long-term abuse of a timing belt. The cost to recoat the gears can range from $40 to $120 but can save a significant amount of money. However, if the original coating is too far worn such that the profile of the gear has changed, then no amount of coating will fix them and they will need to be replaced.
There is a lot of chatter in the 928 community about which brand of timing belt is best. It is widely thought that Gates is the supplier to Porsche for their belts, and the only other known maker is Continental. Continental belts are perceived as being stretchy and compliant. This is a quality that is not favorable in a timing belt as it can affect cam timing. The Gates and Porsche brand belts are highly regarded and are less prone to stretching.
With some of these simple tips you can take a $3,000+ job and do it for less than $600 (if you don’t replace the water pump) by doing it yourself. If you need to have the work done, contact me or come to one of out beer nights and we can point you in the right direction.
About the author: Andrew Olson
I'm a long-time 928 enthusiast. I like long walks on the beach and a soft shoulder to cry on... where's Brian?