Prevent Damage to the 5HP18 Transmission in Your BMW 325i When Racing

Posted by on Friday, July 18th, 2014

The 5HP18 is a five-speed automatic transmission built by the German company ZF Friedrichshafen. It is designed for use with engines mounted in a longitudinal position and rear-wheel drive. It comes from a large family of transmissions that include the 5HP19, the 5HP24, and the 5HP30. This transmission was first used in 1992 and was quickly phased out of production in 1999. It could be found in many popular BMW vehicles, including the E32, the E34, the E36, and the E39. It was also used I the BMW 325i.

The BMW 325i came with the 5HP18 automatic transmission.

The BMW 325i came with the 5HP18 automatic transmission.

The 325i is a compact car and was available is a two-door coupe, a two-door convertible, a four-door sedan, and five-door tour model. It came with both four and straight six-cylinder engines, including a twin cam version. In the rear of the car it used a multilink rear axle. This car carries the typical German high quality, and in fact was named one of Car and Driver Magazine’s 10 Best Vehicles.

Because these cars are very light and have good weight-to-horsepower numbers, many have been turned into race cars. Currently, you can pick one up off the used car market for a fraction of their original cost. If you do indeed plan to race your 325i, precautions should be taken to protect the automatic transmission. Here are a few ways to prevent damage to the transmission if you plan to race your 5HP18-equipped 325i BMW.

First on your list should be the addition of an aftermarket transmission cooler to supplement the factory transmission cooler. Why, you ask? Simple – over-heating is the number one cause of automatic transmission failures, and the easiest way to keep your transmission cool is with an additional transmission cooler.

Ideally, you want to keep your transmission temperature under a maximum of 200 degrees. For every 20 degrees you go beyond this limit, you risk cutting the lifespan of your transmission by a factor of two. In addition, at temperatures as low as 240 degrees, your transmission fluid can turn to varnish, at which point it loses all lubrication properties and the total failure of your transmission is just around the corner.

Switch over to a synthetic transmission fluid. Synthetic transmission fluids have much more stable properties than their oil-based, organic cousins, and will not break down as easily under extreme hot or cold conditions. Synthetic transmission fluids also have much better lubrication features, and will help extend the life of your 5Hp18 automatic transmission.

You should also install a deep sump aluminum transmission pan. Deep transmission pans usually hold an additional 2 – 3 quarts of transmission fluid. This will help your transmission run cooler. Aluminum pans also dissipate heat better than the stock stamped steel pan, and will provide additional cooling benefits. Aluminum pans are also much more rigid than stock steel pans and will help strengthen the overall transmission case.

Sooner or later, due to high mileage or abuse, you will be faced with the decision to replace the 5HP18 transmission in your BMW 325i. We highly recommend doing so with a re-manufactured transmission over one that was simply rebuilt. A re-manufactured transmission will include major internal modifications and upgrades designed to make it both stronger and more durable. To learn more, contact ETE Reman today!

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