I used my magnetic downforce gauge (see here to make your own) to measure the downforce on the rear of the BMW 125 and the Civic R from my Touring Car Battle set.
BMW 125 - 25.3g of downforce
Civic R - 39.4g
I didn't touch the existing magnet - the chassis on these BTCC cars is pretty fragile. I added a small Slot.it neodymium (ie shiny silver) bar magnet on top (it fits under the motor shaft) and took a measurement of 47.3g. Too much...
I then took two small round neodymium button magnets (from an Auto World HO car, but similar to the Micro Scalextric traction magnets) and placed one to the side of the motor shaft. I took a reading of 31.1g. Adding another button magnet to the other side of the motor shaft, the reading went up to 35.9g. The two cars then had a similar 'pull' when I picked up the rear of them on a piece of Scalextric track.
If you, like me, have magnets lying around from other projects, this is easy to experiment with. If not, I'd try 5mm diameter x 1mm thick N52 neodymium button magnets. Or 3mm x 1.5mm. Anything bigger (more volume) may be too strong. I see a pack of 5 of the 3 x 1.5 buttons for £0.99 on eBay. Two of those should be perfect to boost the BMWs to compete with the other BTCC cars.
Certainly without the magnets, the BTCC cars are nicely matched. We did this 20 minute test session with five different cars/drivers on a big digital track:
1. BMW125 Priaulx - Tyres: Paul Gage - Fast Time: 10.186 secs
2. MG6 Goff - Slot.it P6 - 10.506 secs
3. Honda Civic R Shedden - Slot.it P6 - 11.015 secs
4. BMW125 Tordoff - Paul Gage - 11.312 secs
5. VW Passat Plato - Scalextric tyres - 11.917 secs
6. BMW125 Collard - Scalextric tyres - 11.968 secs
In race conditions and on a slower, more technical track, we got these fast times:
1. BMW125 Priaulx (Paul Gage) 11.44 secs
2. Civic R Shedden (Slot.it P6) 11.755 secs
3. BMW125 Tordoff (Paul Gage) 11.78 secs
4. VW Passat Smith (Paul Gage) 11.921 secs
5. MG6 Goff (Slot.it P6) 12.15 secs
6. MG6 Cook (Slot.it P6) 12.852 secs
7. VW Passat Plato (Slot.it P6) 12.932 secs
8. Civic R Smith (Paul Gage) 14.551 secs