Grassroots Motorsports
Issue link: http://grassrootsmotorsports.epubxp.com/i/397024
Grassroots Motorsports 113 PROJECT CAR: NISSAN 350Z We'll be using these 275/35R18 BFGoodrich g-Force Rivals on 10.5-inch wide TSW Nurburgring wheels. But we wanted to know which of the competitive STU cars were putting the highest demands on their rubber. So we did a little math. Predicting whether a car will be a suc- cessful autocrosser is guesswork at best, but we can apply a few metrics to at least make educated guesses. First, let's look at the cars that are currently competitive in the class. The 2006 Mitsubishi Evo, the 2006 Subaru WRX STI and their contemporaries claimed the bulk of the trophy spots at the Solo Nationals in 2013, so they'll make good benchmarks. Now we can look at power and gear- ing. All three cars have listed horsepower and torque fgures that put them roughly in the same class. They all sort of hover in that "well above 250 ponies and firting with 300" category, even though they get there in very different ways. If we fnd a few representative dyno charts for stock examples, we can assume a broad sweet spot for the class's preferred operating range. 2006 Mitsubishi Evo: In the case of the Evo, the torque peaks early–just over 3500 rpm–but the horse- power climbs all the way to 6500 rpm and drops off very little all the way to the 7500-rpm redline. Above 4500 rpm, the Evo climbs to more than 200 ponies and stays well above that number all the way to redline. What can we extrapolate from this data? The Evo is probably comfortable operating in the 4500-7500-rpm range. With a 25.7-inch-tall tire, a 1.94:1 second-gear ratio and a 4.53:1 fnal drive, the math tells us that this rpm oper- ating range stretches from about 39 to 65 mph. Those speeds seem good for the typically open and fowing courses at the Solo Nationals, although tighter courses may cause the Evo to struggle a bit. 2006 Subaru WRX STI: If we take a look at the specs on the STI, we see a comfortable operating range of 4000 to 6500 rpm. With the same tire diameter as the Evo, a 2.375:1 second gear, and a 3.90:1 fnal drive, the STI has a pace of about 33 to 54 mph. That's still a good autocross range, but the car may lag behind, or require a shift to third on longer, more open courses. It seems very well suited to tighter courses, however, thanks to that more aggressive second gear ratio. 2003 Nissan 350Z: Now for the Z. Its VQ35 V6 engine has a much more traditional power curve than the turbocharged powerplants of the Evo and STI. The Z's power grows from idle to near redline, and the torque curve is typical-V6 fat, trailing off just a bit above 5500 rpm. From 4000 to 6500 rpm, however, the Z produces more than 200 ponies at the wheels and over 200 ft.-lbs. of torque until about 6300 rpm. From these numbers, we can discern a very comfortable operating range of 4000 to 6500 rpm for the Z. Combined with a 25.6-inch-tall tire, 2.32:1 second gear and 3.54:1 rear-end ratio, this translates to speeds of 37 to 62 mph. It neatly splits the difference between the two seemingly most competitive cars in the class, putting the Z in the right ballpark when it comes to power on the ground at autocross speeds. ANALYSIS: IS THE 350Z AN INHERENTLY GOOD AUTOCROSS CAR? 200 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 210 220 230 240 39-65mph 190 hp tq rpm 200 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 210 220 230 240 37-62mph 190 hp tq rpm 200 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 210 220 230 240 33-54mph 190 hp tq rpm Now back to the tire question. Earlier we mentioned that the 350Z scored some addi- tional tire width when it moved from STR to STU. It used to top out at 255mm-wide tires on 7.5-inch-wide wheels. With the move to STU, wheels are now unlimited and max tire width is a steamroller-like 285mm. And here's the punch line: The AWD cars that dominated the class last year are relegated to a 245mm-wide tire. Yes, power to all four wheels affords them extra trac- tion, but a twisty autocross course turns the competition into a war of contact patches. So let's do a little more speculative math to fnd out which car has the most rubber dedicated to lateral grip. We'll divide the curb weight of each car by the width of the contact patch in inches to come up with a "pounds per inch" fgure. We'll use published WHERE RUBBER MEETS ROAD