Porsche at the Salt Flats

928S4 Sets Two Speed Records

VW&Porsche magazine
January 1987, page 50

The word is starting to come out of Porsche, and Porsche wants to make sure everybody knows about it: The 928S4 is one hell of a performer. As part of their attempt to spread the news, Porsche asked Al Holbert Jr. to take a 928S4 to Bonneville and see what it would do. Accordingly, in August of 1986 Holbert arrived at the Bonneville Salt Flats with a 928S4 and a heavy right foot to explore the limits of Porsche’s flagship automobile under the auspices of the United States Car Club (USAC), who would be certifying the records on behalf of the Automobile Competition Committee for the United States (ACCUS) and the Federation Interationale de Automobile (FIA).

According to Berdie Martin of ACCUS, the 928S4 was entered in Category A, Group 2, Class 9. Category A is the special automobile category, Group 2 is for non-turbocharged reciprocating engines, and Class 9 is for engines displacing between three and five liters. Category A differs from the purely stock category in that a streamlined body and other modifications are allowed. In a stock category, not only are no modifications allowed, but the testing procedure is extremely complex and expensive. As a result, most manufacturers choose to run in Category A.

In keeping with the Porsche philosophy, modifications were kept to a bare minimum. The car was run with outside mirrors, catalytic converter, stock wheels, stock transmission, stock engine, power accessory belts, and unleaded gasoline. The only modifications were shaved tires, lowered springs, stiffer shocks, and a piece of plexiglass blocking off the air passage between the rear spoiler and the rear window. The only other change was to wire open the computer-controlled radiator cooling ducts, which made the car less slippery but helped the motor run cooler.

In an exclusive interview with Al Holbert, the first thing we wanted to know was what it was it like driving the 928S4 at top speed. The car hunted a little due to the limited traction available on the salt, said Al. For lack of a better word, it was fishtailing.

The fishtailing, caused by tire slip at the rear wheels, was clearly limiting the top speed of the car. The first thought we had was that we might get more traction by running tires with more tread depth, states Al. So we did the next run with a set of 944 Turbo wheels and tires we borrowed.

Even with more tread, the tire slip persisted. We were using the European transmission in order to have the motor operating at its range of maximum torque at the speed we were shooting for. With all the tire spin we were getting, we figured we might be getting too much torque, so we changed over to a U.S.-spec transmission. This cured the tire slip, but because the engine rpm at speed weren’t at the torque peak, the top speed remained the same.

For the fourth run they decided to try to minimize the surface area of the car by folding the mirrors flat against the body. For the fifth and last run, they removed the mirrors completely, and filled the holes in the doors.

Surprisingly, these changes resulted in less than one mile per hour difference from the first run with the stock car. At the end of all the testing, a stock 928S4 had set two FIA international speed records: 171.110 mph for the flying mile, and 171.926 mph for the flying kilometer. Porsche intends to submit these records to Guinness for inclusion in their Book of Records.