In our last installment, Project Street S MINI received a substantial increase in horsepower with Mini Mania's Stage 2 power upgrade kit. Project MINI was already fast--much faster than a stock specimen. It has become even faster since then.
In stock trim at wide-open throttle, it was as though the MINI was playing Mozart--so smooth, so elegant. Now, after our modifications, stab the throttle and the engine roars something more like Quiet Riot's "Bang Your Head" as it thrusts you and your co-pilot's noggins into the headrests and your passenger's eyes are opened at how a car so cute can pull so furiously.
The car was now a completely different animal, but it was clear from the dyno's fuel trace that the engine would have produced more power. With the air-fuel ratio dipping below 10:1 at wide-open throttle after 4000 rpm, this car was spraying far more fuel than the engine needed. Sure, it's nice to have fuel available, but too much isn't the answer for optimum performance. Even in this overly rich state, Project MINI was seeing 177 hp at the wheels, just 23 hp shy of our goal.
The positive displacement blower used on the MINI Cooper S, and even some Mercedes-Benz and Jaguar models, produces a relatively flat torque curve. Project MINI was falling off more than it should have at the top end. It needed less fuel after 4500 rpm to maximize power through redline, so we went back to evosport.
evosport is known for its tuning of BMW, Mercedes-Benz and Ferrari automobiles. evosport is able to design and sell parts that actually make horsepower, in part because it does extensive testing on its 248C Dynojet chassis dynamometer. So far, all testing on Project MINI has been performed on evosport's dyno, which we plan to continue for consistency's sake. Rather than install more hardware, we asked the evosport technicians to do some custom software tuning to make Project MINI's fuel mixture leaner and its torque curve flatter.
evosport got right into the brains of the car and started tweaking things. After a few adjustments, Project MINI was once again strapped down to spin the big dyno roller. This time, the air-fuel ratio was near perfect. At 4500 rpm, rather than dip off the charts below 10.0:1, it was maintained in the low 13:1 range, eventually dipping into the lower 12s by 5000 rpm. At a little past 6000 rpm, the fuel mixture was maintained at a safely rich 11.8 to 11.9:1 air-fuel ratio. At this point, there were no signs of detonation. The 91-octane pump gas had been able to fend off pre-ignition.
The leaner air-fuel mixture produced substantial power gains, the maximum increase registering 20.4 hp and 17.6 lb-ft of torque at 6100 rpm. evosport's tuning raised peak power to 187 at 7000 rpm, the result of maintaining the torque curve throughout the rev range. evosport could have leaned out the mixture a little bit more up top and gained a few ponies, but conservatism is key when tuning street cars--we wanted to keep the air-fuel ratio in the upper 11s to be safe. Even with 186 hp at the wheels, Project MINI screams.
Looking at the graphs, the curve was still breaking up in the 6000- to 7000-rpm range. evosport installied a fresh set of Denso Iridium spark plugs, which did a surprising amount of good. The jagged output was smoothed and horsepower rose to 191 at 7000 rpm. From 6900 to 7100, the car gained 4 to 8 hp, with a peak maximum increase of 9.4 hp at 7000 rpm. Although the surprisingly high peak gain corresponds to a small dip in the previous run, these plugs proved that the stock ones weren't providing optimum performance.
Since we began, Project MINI has seen tremendous improvments with the Mini Mania Stage2 kit, evosport tuning and Denso Iridium spark plugs: 38.4 hp and 30.2 lb-ft at 6700 rpm representing increases well over 22%. Power is increased throughout the rev range, with torque gains of 20 to 30 lb-ft from 3000 rpm to redline. That's none too shabby for a few thousand dollars worth of parts.
As it sits, Project MINI's 191 hp at the wheels is still shy of our 200 hp goal. Stay tuned to see if the next installment can get us there.
|At a Glance|
|Estimated time:||1/2 Day|
|Upgrade parts:||$754 Labor (included software/plug installation): $90|
evosport's custom tuning involved leaning out the air-fuel mixture. The still-safe 11.8:1 air-fuel ratio helped the car sustain torque through redline, which translated into peak increases of 20.4 hp and 17 lb-ft of torque at 6100 rpm.
evosport's custom tuning involved leaning out the air-fuel mixture. The still-safe 11.8:1
When the Denso Iridium plugs were added, the upper rev range was smoothed out, returning 9.4 hp and 7.1 lb-ft at 7000 rpm.
When the Denso Iridium plugs were added, the upper rev range was smoothed out, returning 9