A couple of recent pieces in the motorsport press have raised separate issues over the legality of the Brabham-BMW which won the 1983 World Drivers' Championship in the hands of Nelson Piquet. Gary Watkins's Autosport article re-considered the exotic, ex-Luftwaffe fuel brew used by BMW in the latter stages of the season, while Mike Doodson's Motorsport article elicited the following admission from then-Chief Mechanic Charlie Whiting that, "All I will say is that we always, um, attempted to make the car as light as possible."
Indeed, and not just in qualifying it seems, for Gordon Murray rather gave the game away earlier this year with the following comment:
"Whenever we planned to stop - we could go without on street circuits - we tended to do 60-70 per cent of the race on the first set of tyres because that meant we could run very close to, or under, the minimum limit before adjusting the weight with the amount of fuel we put in," (Motorsport, May 2013, p86).
That's a pretty unambiguous admission that the Brabhams ran under the legal weight limit in many 1983 Grands Prix, and used fuel-weight as ballast. If you go through the races in 1983 you'll see that Piquet's Brabham was almost always the last of the leading runners to pit for fuel, and that he sometimes pitted 5-10 laps or so after the Ferraris and Renaults, his championship competitors. At Hockenheim, for example, Prost's Renault stopped on lap 20, while Piquet stopped on lap 30.
Why would Brabham want to stop after everyone else? Well, in the era of refuelling, a car on empty tanks and worn tyres was generally faster than a car which was fuelled-up on fresh tyres. Moreover, in 1983 the Ferraris were shod with bias-ply Goodyear tyres, and thereby tended to suffer greater tyre warm-up difficulties than the Michelin-tyred Brabham. Thus, it was in this 5-10 lap window that the Brabham would often make hay.
At Spa, Tambay's Ferrari was running ahead of Piquet until stopping on lap 21; Piquet stopped on lap 24, and jumped ahead of the Ferrari, forcing Tambay to re-pass some laps later. Similarly, at the Osterreichring, leader Arnoux's Ferrari stopped on lap 28, Piquet on lap 31, after which Piquet emerged ahead, forcing Arnoux to re-pass some laps later.
So why, then, wouldn't everyone schedule their pit-stops at 3/4 distance? Well, that extra fuel-weight costs lap-time, and it costs you lap-time on each and every lap that you carry the extra weight around on your back. If you started a race with about 20kg of extra fuel, that would cost you about 0.6seconds of lap-time, which over 30 laps would mount up to a very substantial 18 seconds or so.
However, if your car was 20kg beneath the legal weight limit when drained of fuel, you could start the race at the same weight as your competitors, but with the ability to run 5 laps or so further. You would suffer no disadvantage in the early stages of the race, and you would also be able to jump ahead of your competitors by running longer. Perfect.
In fact, just about the only time Piquet didn't run long was in the final race at Kyalami, when he shot off into the distance on a light fuel load, pitting on lap 28 of 77, able to resume without having lost the lead. It's possible that the Brabham started the race over the legal weight limit, but was running underweight for a significant portion of this stint. Moreover, in the late stages of the race Piquet slowed considerably, sacrificing what appeared to be an easy victory. Contrary to the explanation given on the day, that Piquet was merely trying to ensure the reliability of his car, he might also have been minimising fuel consumption to ensure the car was actually over the legal weight limit at the end of the race!