Here is what Mike Smith said about it last year

"Where we simply couldn't live with Andy R was on the power circuits like Silverstone - he was running way more power than us or any of his customers. In July 89 we found out why, when a crate of Garrett turbos destined for AR were delivered by mistake to us. We saw that they all had identical welded-up slots in the worm. They had been cut into, scooped out and then welded-up. We sent them back - but informed the scrutineers. Sure enough, Andy's car got sealed at Silverstone GP and by Brands Hatch 2 weeks later he had lost his points. But he appealed and won - because he claimed that the FIA Yearbook rules in French permitted repairs (whereas the English didn't). Clever bugger. He was ruled correct by the FIA. There was no way that these slots were repairs - and I'm afraid that's when I realised what we were up against".

and an ex race mechanic on the same forum claimed similar about DJR when they came to Silverstone for the TT. This is what he said

"Going slightly OT here, I was told by someone who worked for DJR at the time, the REAL reason for their DNF during the 1988 Silverstone TT. Although DJR claimed "vapour lock" after the first pitstop as the reason for withdrawal, there was actually nothing wrong with their car. They wanted to go out and kick everyones butts (which they were) but were worried about getting scrutineered so withdrew the car (citing the "vapour lock" reason) My friend who told me this said it was his job to send the turbo exhaust housings away to be laser cut in two, he would then port the hell out of them, and they would be then welded back up. Apparently this was good for a lot more power (hence the reason the cars had more straight line grunt at that event than the other RS500's) I believe this "tweak" also caught on in the UK amongst the cheaters. I have watched the DVD of the Silverstone TT race, and this story does fit. The car appeared to be going perfectly normally, up to the time of its withdrawal. There were no external signs of any problems, such as coolant loss, engine smoke etc. As they say, it is only cheating if you get caught!"