What interests a gearhead the most? Getting more juice out of the motor, of course. So lets drill deeper into B6ZE's potential of producing more than 136Nm/116hp - in naturally aspirating form.
Q1: Why only 136Nm and 116hp?
- In 1600cc engine that makes 85Nm and 72,5hp per liter. That is not the poorest result if you see the big picture. Late 80's engine management tuned for "what ever gasoline", low compression ratio (9.4:1), super silent stock intake and exhaust, mild cams and relatively standard valves. Clearly a healthy dose of performance potential is hidden somewhere there.
Q2: Why so small torque?
- Displacement is only 1600cc, so it will never be a torque monster of any kind. Compressio ratio (CR) is only 9.4:1 and stock ignition timing is far from wild. Adding 4deg ignition timing can produce 5-8Nm extra torgue below 5500rpm (with basic intake and exhaus upgrades). Check dyno graph: http://www.mymazdamx-5.blogspot.com/2007/05/tehomittaus-dynamometriss.html
Q3: Why torque peak is in relatively high rpm, 5500?
- Mazda wanted to make a revhappy motor. That was basicly the only way to make such a small motor enjoyable and at the same time produce decent performance, with the right gearing of course. Stock intake manifold and cam specs are the main reasons for torque peak to occur at 5500rpm. Intake manifold's runner lengths are tuned so that the resonation takes place at around 5500rpm. Stock cams make the airflow (edit: not airflow, but volumetric efficiency, Ve) reach it's peak between 5500 and 6000rpm. It is intetersting that this can also been seen in boosted motors with original intake manifold and stock cams.
Q4: What restricts the top end power, essentially the air/exhaust gas flow?
- Through the whole rpm range there are two basic restrictions: intake (air box and filter) and exhaust (original cat back exhaust pipe). Torque curve come's up about 5% from bottom to top with each of the basic mods: high flow cone filter and high flow cat back exhaust. Import Tuner magazine has done excellent dyno testing that can be found here: http://www.importtuner.com/powerpages/impp_0808_1993_mazda_mx5_miata/
NOTE that baseline is not showing true stock motor rear wheel performance because catalysator was busted. Guys put a new one in but did not bother to make a new dyno run. So, I would say that estimated stock crank hp in that particular motor was about 112 hp or so (89-90 rwhp with 20% drive train loss).
- Flapper door style air flow meter (AFM) is small and can flow about 165CFM (cubic feet per minute) in atmospheric pressure = naturally aspirating motor. Randy Stocker has done some flow testing (http://www.solomiata.com/airflowmtr.html) and with 90% volumetric efficiency at 6500rpm the peak intake flow rate becomes 165CFM (http://www.mk5cortinaestate.co.uk/calculator3.php). This is enough evidence for me to come in the conclusion that stock AFM restricts power to about 105rwhp (about 130 crank hp) as seen in Import Tuner's dyno testing: http://www.importtuner.com/powerpages/impp_0808_1993_mazda_mx5_miata/total_cost.html.
- Stock header get's small above 6500rpm if all the other basic bolt on mods are made, including larger AFM. Graph shows significant gains from 6500 to redline (pictures are really poor, sorry...): http://www.mymazdamx-5.blogspot.com/2008/05/tehopenkitys-nro-2.html
- Unbroken stock cat can support at least 155 crank hp. It's highly recommended to get rid of it, but I would do that last.
- And last there is The Flow: the cylinder head, valves, cams, throttle body and intake header. More about these later.
No pictures at the moment, only hard core text. Links should give you something to watch for a while. Better quality graphs, torque curve comparison and further analysis coming up in Part 2!
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