Jan 12, 2006 update to the Mazda STS2 Miata project car

Power Analysis

Last installment we established a competitive target weight for our project from a handling perspective. In that analysis we found that we could get our Miata to a legal weight of about 1950 lbs with some straightforward modifications. Today, we’ll look at some power predictions to see if the drivetrain is up to snuff.

The STS2 engine allowances are fairly similar to those allowed for Spec Miata (SM). For each, the motor internals must remain stock, as per the factory service manual. Externally, each is allowed any cold air intake system which utilizes the factory airflow meter. SM is allowed any clutch and may remove the catalytic converter. STS2 requires those parts to be OE-equivalent, but instead allows for any exhaust header. STS2 also allows for any ECU programming within the OE box, while SM optimizes fuel and ignition through timing bumps and airflow meter calibration. Given the tradeoffs of each, power output should be relatively similar.

One of our project partners,Applied Racing Technology, provided us with a dyno graph of one of their typical SM motors. We then took a similar graph from our fully-prepped STS Civic and overlayed the torque curves. One caveat is that the SM test was done on a Dyno Dynamics eddy current dyno, while the STS session was from an inertial DynoJet. To normalize the curves, the Civic numbers were converted downward using typical conversion estimates. We hope to redo the test in the future on the same dyno once our STS2 Miata motor is complete.

Looking now at the graph, there are three mostly horizontal torque curves. One is a bone-stock Miata motor, the similar but higher one is the Spec Miata, and the one with the hump in the middle is the Civic (ignore the two HP curves). You can see that the SM makes similar peak torque to the Civic, but offers a much broader torque curve especially in the range from 5000-6000 rpm. It also does a bit better from 3200-4500.

One cause for concern is that the Miata drops off very quickly once it gets to 6700 rpm, while the Civic revs on nicely. ST Hondas typically have their rev limiters raised in the vicinity of 7800-8000 rpm to avoid costly shifts to 3rd gear (and back). If the Miata wheezes too hard, we may have to shift more which could be a hit against it. Our hope is that this can be helped out by installation of a good long-tube 4:1 header in the Miata since there is plenty of physical room for it prior to the cat. More on that in a future installment.

Now let’s look at gear ratios. The Miata’s first two gears are 3.136 and 1.888, with a final drive of 4.3. Multiplied out that gives 13.48 and 8.12. The Hondas have 3.250, 1.894 and 4.25, respectively giving 13.81 and 8.05. So in typical 2nd gear acceleration off the turn, its essentially a dead heat. The Hondas do enjoy an slight advantage off-the-line in 1st gear, but that is probably negated by the superior launch of the RWD Miata.

So, who’s faster in a straight line? Since the CRX is potentially 100 lbs lighter than the Miata, and since they both make the same peak torque, one would think the CRX would be the winner, right? Well, not really. Its actually the area under the torque curves for each that is important. 100 lbs is about a 5% weight difference. And from 3200-5800 rpm, the Miata produces 3-6% more torque, excluding the Honda “hump” zone. So it makes up for lower weight with higher torque throughout much of the range. And given the level of accuracy of our paper analysis, its really too close to call.

So enough analysis, let’s get to work! Next installment is a visit to the engine shop.

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