Evo magazine did a test on a Golf GTi a few years ago and the test results were interesting. The test car was run on each fuel until its onboard computer showed a range of 3 miles or less, filled with 10 litres or more of the test fuel, then run down to a range of 3 miles or less again and filled with 20 litres or more of the same fuel.
The quoted figures for the Golf GTi are 197 bhp and 207lb/ft. Tests were done at WRC technologies in Silverstone.
Some results:
Shell Optimax: 209 bhp, 242lb/ft.
Shows useful gains compared to Shell 95. Compared with other high octane fuels, it's on a par with Tesco 99 low down but lacks its mid-range (and that of Bp ultimate) with torque fading slightly earlier. Driver notes: (Tried after the 1st batch of bp ultimate) The Golf suddenly discovers a top end - you want to rev it right to the redline. Of all the fuels the GTI felt the crispest and most fun to drive on this. It would be my choice.
Tesco 99: 212 bhp, 242 lb/ft.
Highest octane rating of the pump fuels didn't quite give the top results. Massive gains over our sample of Tesco 95. Doesn't deliver like the BP ultimate low down but then gets ever stronger matching its power and staying stronger for longer. A fine result, and its part bio-ethanol. Driver notes (After Shell optimax) Mega mid range but feels a little laboured to the redline. Somehow less clean feeling. Sound was less positive too.
BP ultimate: 212 bhp, 250 lb/ft.
Level pegged with Tesco 99 on peak bhp but significantly out performed it and every other pump fuel in terms of torque (and even matched the race fuel into the mid-range) Very strong right from the off and stays ahead of the others right up to 5000rpm. Drivers notes (Tried after a 95) Explosive low down - really quick on boost and sharp response too. Wooly top end though - the last 1000rpm is a struggle, strange.
Some other quotes from the article:
It's worth noting that in WRC's experience some cars respond better to Optimax (turbocharged Subaru's for instance) and some to Bp ultimate (Mitsubishi Evo's) so its possible your car will prefer one or the other, or Tesco 99.
Drivers perceptions are interesting: Some fuels make the engine FEEL a lot keener to fly to the redline.
The next test they did was on an E60 M5 and the results are interesting. One note is the sophisticated ECU and the way it adapts timing using the spark plugs!!!!!!
I think after reading the outcome of the M5 article where they said putting 95 RON in a car designed for 98 RON and there being no difference really does throw a spanner in the works but it could be down to simple things like batches of fuel. Sometime even though you are putting 95 RON into a car it is minimum 95 ie not going to be 94 but could be 97RON. When BMW tested it in their labs they noted 501bhp on 95 RON and 515bhp on Optimax, hardly a 20% gain but certainly something which would be noticeable by most drivers. The one thing which should be true is that:
If you put 95 RON into a production car designed to run on 97 RON it will detect knock and change the timing to compensate. You will get less power / efficiency.
If you put 99 RON into a production car designed to run on 97 RON, it will not notice that there is no knock and change the timing until detonation starts to occur.
But as Cat says above it's a fine art and there really are no proven or reliable stats out there. I know one thing,my seat dyno tells me if the old girl is going well so I always use BP Ultimate and that's my choice. It's up to you guys and gals