Tuning a Scalextric Porsche 911 GT3R for GT2 Racing

By Dave

I am preparing a group of cars to create a category for the less experienced drivers that race on my track. Cars that run in the ALMS Series GT-2 category should be ideal for this class. The Scalextric Porsche 911 GT3R will be the first car that will be set up for this class.

The power of the motor is going to be reduced to make these cars more drivable for the less experienced drivers. I will use the original tires, since rubber tires in this lap time range are about three to four tenths a lap slower than silicone tires on my wood track

The Jet Alliance Porsche 911 GT3R is the first car that I purchased for the GT-2 class. After inspecting the car, I oiled the axles and armature, and cleaned the tires. I ran the car about 20 laps in each lane. The car bounced under hard acceleration and cornering. These are the average lap times and Scale MPH of the new car. 9.000 seconds @ 175 SMPH.

The car was disassembled and the tires were removed from the wheels. I trued up the front wheels by spinning them with a Dremel sanding drum held at a 10 to 20 degree angle to the wheel.

I then reinstalled the front tires. The sanding pad was removed from the drum and the Dremel was mounted in the vise.

The front tires were spun by holding them against the drum as the Dremel ran at a moderate speed. 600 grit sandpaper glued to a piece of plastic card was used to remove the high spots and contour the tires.

The rear axle was removed and a .010" bronze thrust washer was added to the outside of both axle bearings to decrease the lateral movement of the axle assembly. The pinion was pressed further onto the shaft to give the pinion side thrust washer clearance to the spur gear. The chassis was mounted on my tire truing fixture and the rear wheels were trued up with a file.

The rear tires were installed and the tire fixture adjusted so the high spots on the tires were just touching the sand paper. The car was run for about ten minutes at 6 Volts. Then the fixture was adjusted so the remaining high spots made contact with the sand paper. I kept repeating this process until the high spots were almost gone. White chalk was applied to the spinning tires and they were adjusted a little closer to the sand paper. After a few minutes I stopped the car and checked the chalk pattern on the tires. I keep repeating this process until the tires are close to being finished. I put a small radius on the inside and outside edges of the tires.

The magnet was repositioned to the front mounting hole to move the weight forward and then I reassembled the car. The car still did some bouncing under hard cornering. These are the average lap times and Scale MPH after partially truing the tires. 8.300 seconds @ 177 SMPH.

To stop the chassis from flexing while truing the tires, I mounted the assembled car on the tire fixture to finish truing the tires. I placed a piece of printer paper between the tires and the sand paper and adjusted the fixture for a light load. The paper shims were removed and the car was run at full power for ten to fifteen minutes to finish truing the tires.

Getting the tires perfectly round and concentric is the most important part of setting up a slot car. The bouncing caused by the rear tires was completely eliminated. These are the lap times and Scale MPH after finishing the rear tires. 8.000 seconds @ 183 SMPH.

I removed the pickup plate and repositioned the pickup braids to remove the double layer of braid between the guide and the track. This gives more clearance and allows the pickup braids to flex. The pickup braids were bent so they are flat and just touching the track tape, these adjustments removed all the front end bounce.

The magnet was moved to the center mount to improve the balance of the car, 32 grams front, 51 grams on the rear. The car now runs perfectly smooth and fast. These are the average lap times and Scale MPH for the completed tune up. 7.700 seconds @ 183 SMPH.

My fastest Scalextric GT1 car, an Aston Martin DBR9 with Indy Grips runs lap times of 7.500 seconds @ 187 SMPH.

To get the Porsche within ALMS guidelines for GT-2, the speeds should be below 180 MPH. I was going to rewind the motors to reduce the power, but it is much easier to add diodes to the motor leads. Each 2.5 Amp diode reduces the Voltage to the motor by .73Volts. For testing I used three diodes in series for multiple Voltage drops. To maintain the braking effect I installed one diode in parallel and reverse direction.

These are the results of adding diodes to the motor. I ran the car 20 laps in each lane, these are the average lap times and Scale MPH.

0 diodes 7.700 seconds @ 183 SMPH
1 diode 7.800 seconds @ 171 SMPH
2 diodes 8.000 seconds @ 159 SMPH
3 diodes 8.250 seconds @ 148 SMPH

I also tested the car at 12 Volts on the chassis dynamometer, these are the results in Stall Torque and Maximum RPM.

0 diodes 87g/cm sT 22200 RPM
1 diode 81g/cm sT 20100 RPM
2 diodes 76g/cm sT 19500 RPM
3 diodes 71g/cm sT 18000 RPM

For the final setup I decided to use two 1 Amp diodes in parallel, one forward, the other reverse. The 1 Amp diode drops the Voltage 1.2 volts under full load on the dyno, and returns to a .73 Volt drop as the load decreases. This reduced the acceleration a little, but kept the top speed close to what I wanted. These are the average results of the this setup. 8.000 seconds @167 SMPH.

I now have a car that should be drivable by most drivers, and it is also very quiet, smooth, and fun to drive.

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