Where's The Power?
SCC has always prided itself as one of few publications that regularly measures wheel horsepower. Usually, it's a straightforward task that's proven to be consistent and reasonably repeatable. Our yardstick has always been the Dynojet 248C two-wheel dynamometer using a standard correction factor to compensate for changes in air density. But in recent years, measuring power repeatably has become more and more difficult.
Mazda's third-generation MX-5 should have been simple. After all, it's rear-wheel drive, naturally aspirated and our test car had no electronic ninnies interrupting power between the crankshaft and the rear wheels. However, during testing for this story, power peaked between 134 and 137 hp over six consecutive pulls-not good for a 2.0-liter engine rated at 170 hp. Did Mazda pull another RX-8?
Thankfully, no. How do we know? Mazda showed us reliable dyno data proving another MX-5 made 145 hp at the wheels-six months earlier when the air was 13 degrees cooler. The hot summer air during our test caused the newer and smarter ECU to use a less aggressive ignition map.
So we logged ignition timing-once on the dyno with 90-degree air temperatures and later that night when the air was 77 degrees F. Base ignition timing changed more than a degree between our tests. In the 77-degree air, the exact same car made 140 peak hp. So why can Mazda claim 170 bhp?
According to SAE's Standard J1349, "Engine Power Test Code-Spark Ignition and Compression Ignition-Net Power Rating, power measurements must be performed under set test conditions for ambient temperature and pressure. Intake conditions must be 25 degrees Celsius and 100kPA pressure (77 degrees F and 14.5 psi.) The only provision for ambient conditions is that they range from 15 to 35 degrees Celsius (59 to 95 degrees F.)" Problem is, our test conditions are almost never the same as the SAE's standard.
After more research we discovered the MX-5 ECU includes a function that allows it to run lean for a few seconds when conditions are favorable-like in the cooler 77-degree weather. Our mid-summer dyno test was just outside the above range, which contributed to a richer condition and less power. This function and the difference in ignition timing were enough to make up for the 10-hp discrepancy.
So, if you're going to race against an MX-5, do it when the air is hot. Just don't blow up your own car in the process because there's a good reason Mazda's engineers were willing to sacrifice power for this margin of safety. - Jay Chen
Editor's Choice
2006 Mazda MX-5
The new MX-5's superiority over its predecessors can be summed up in one word: cupholders. Anyone who's owned or spent a reasonable amount of time in any Miata will understand what I mean. This isn't some sissy American commuter's perspective validating the new car, but having the option to hold my U.S.-spec Big Gulp is a plus.
That aside, the new MX-5, though different, improves virtually every aspect of the previous cars. It's a true front mid-ship engine layout with better chassis engineering, ergonomics and above all, a better engine.
Though this early production car underperformed against the two not-so-run-of-the-mill Miatas, the MX-5 has the tuning potential to be the best yet. When the new torquey engine is combined with a well-tuned enthusiast suspension, the MX-5 is my choice for the street, road racing, autocross and all-around ass-hauling. - Jay Chen
2006 Mazda MX-5
Estimated Price: $22,935
Engine
Engine Code: MZR
Type: Inline four, aluminum block and head
Valvetrain: DOHC, four valves per cylinder, variable intake cam timing
Displacement: 1999cc
Bore x Stroke: 87.5mm x 83.1mm
Compression Ratio: 10.5:1
Claimed Crank Hp: 170 hp @ 6700 rpm
Claimed Crank Torque: 142 lb-ft @ 6000 rpm
Measured Wheel Hp: 134 hp @ 6830 rpm
Measured Wheel Torque: 124 lb-ft @ 5400 rpm
Redline: 7000 rpm
Drivetrain
Layout: Longitudinal front engine, rear-wheel drive
Transmission
Gear Ratios
1: 3.815:1
2: 2.260:1
3: 1.640:1
4: 1.117:1
5: 1.000:1
6: 0.832:1
Final drive: 4.100:1
Differential: Conical clutch spider gear, limited-slip differential
Chassis
Exterior dimensions
Curb Weight as Tested: 2,459 lbs.
Weight Distribution F/R as Tested: 52/48
Overall Length: 157.3 in.
Wheelbase: 91.7 in.
Overall Width: 67.7 in.
Track F/R: 58.7 in./58.9 in.
Height: 49.0 in.
Suspension
Front: Upper and lower control arms, anti-roll bar
Rear: Five-link, anti-roll bar
Brakes
Front: 11.4-in. vented discs, single-piston sliding calipers with ABS
Rear: 11.0-in. solid discs, single-piston sliding calipers with ABS
Wheels and Tires
Wheels: 17x7-in. aluminum
Tires: 205/45R-17 Michelin Pilot Preceda
Handling
Lateral Grip (200-ft. skidpad): .88g
Slalom Speed (700-ft. slalom): 70.7 mph
Making the Mazdaspeed MX-5:
A Crash Course In Chassis EngineeringEver wondered about the engineering involved in creating a works-tuned suspension like the one you can buy from TRD, NISMO or Mazdaspeed? So did we, and this month Mazdaspeed gave us the rare opportunity to join its engineers during a test session of the soon-to-be-released MX-5 and RX-8 coil-over suspensions. One of Mazdaspeed's many roles is to develop factory supported aftermarket products for Mazda's production vehicles.
Now that Mazdaspeed is no longer a strictly Japanese venture, products specifically for the North American market are developed by domestic engineers to meet American tastes and road conditions. This gives Mazda engineers the chance to show off their stuff. However, like aftermarket companies, Mazdaspeed's scope of re-engineering is limited to changing suspension specs, alignment and wheels and tires. The new MX-5 equipped with the Mazdaspeed coil-over suspension shows just how drastic those changes can be.
Mazdaspeed will offer three kits: a conventional lowering spring for use with the base model and the Bilstein dampers of Sport Package cars; a height- and damping-adjustable stainless-steel coil-over; and a spec-series coil-over designed for use with stiffer cars using a roll cage. The spec suspension uses stiffer spring rates, independent compression and rebound adjustment, and external reservoirs. It won't be available through Mazda dealers since it's considered a race part.
The design process begins by determining spring rates-a multi-stage process that requires determining the ratio between chassis flex and suspension travel. Stiffer springs mean the suspension moves less and the chassis flexes more. Mazdaspeed used the MX-5's torsional stiffness as a reference point since all cars are more flexible torsionally than they are longitudinally.
Torsional stiffness is measured by placing the car on four contact points with the suspension removed. The chassis is loaded through the suspension pickup points until visible deflection occurs. The chassis stiffness is calculated as the force required to twist the chassis divided by the distance it was deflected.