THE WEAK LINK
(analysis of MCP dyno day results)
by: Henry "Hitman" Co

The Club had its first full scale Dynamometer day at Autoplus EDSA. The Dyno Results are posted at the Technical page of the Miata Club Website. Revealing and humbling, the activity generated a lot of questions.

One of the principal issue raised was the accuracy of the readings and
specifically to the fueling air to fuel graphs generated. The consistent results of the Dyno runs indicated a near to Theoretical Stochiometric Air to Fuel ratio of 14 parts air to 1 part fuel by weight or slightly leaner and slightly richer on the top end for the stock Philippine 1.6 Liter Miata. As well as similar bias to leanness for the modified Miatas was disclosed. Based on what we already know from the specs of the stock Miata and similar studies made elsewhere and previously by others like Jason, the
results display fidelity to the lean bias in fueling. As such, the Dyno fueling. results are up to expectations.

As a consequence, the issue of being too lean and setting ourselves up for
destructive knock and detonation or pinging arises. To the credit of the Autoplus people, they alerted club members to the dangers of extracting power on lean
fueling. The matter at hand left remaining is on the interpretation of their comments thereof.

Mazda tuned the stock Miata for General Road use, giving due consideration
to emissions and fuel economy concerns. The concentration on near to theoretical Stochiometric (14.7 : 1) and slightly lean thereto on the lower revs and slightly
richer in higher revs conform to this mindset. Hard to clean-up Carbon
Dioxide and Carbon Monoxide gasses are produced in unmanageable numbers when the fuel is set too rich so sticking to stochiometric ideal minimizes these
gasses. The higher amounts of Nox generated in the leaner fueling. can be handled by Catalytic Converters. Fuel economy is also improved to satisfactory levels with such settings.

For general road use, inclusive of the safety margins in the oil sprayed pistons and engine block strengthening, the fuel set up is acceptable. This state of fueling. however is too lean for Racing usage.

Race engineers, Race technician, Race mechanics, have long known that to
provide power under constant Race condition loads, the Stochiometric Air Fuel Ratio they should target is 13.3 : 1 or richer. Racing fuels call for even richer fueling. Racing fuels normally start with 12: 1 and richer set ups. Methanol as an example is
usually set up with 11: 1 ratio. The boys of Autoplus, having a mindset tuned to Drag Racing, prefer fueling. in the range of 13 : 1 to 12: 1 centered on the dragsters accustomed target of 12.5 : 1 air to gasoline vapor ratio. Thus for the racers among us this would be the guide to go by. The principle that producing more power needs more intake rich fuel to burn holds valid as always.

Finally, as to suggested means of enriching the mix for power generation:

In using enlarged Fuel injectors, we have to consider the principle of the weakest link on the chain. It should be the overall principle in all modifications for car performance improvements. Is injector size really the culprit at this level? (All engine system components impose their power generation limit on overall performance but the idea is to deal with the weakest link first or the most immediate factor that is limitting power at a particular stage of tuning.)

The easy short cut is to examine the potential horsepower the stock 1.6 injectors can generate. The stock injectors are 205 cc/min (this is an extension of the old carburated days where specifications of CFM flow rate of the carbs and jets determine the potential HP that can be generated on that set up). Divide this by the factor of 5 (This is determined by the mathematical conversion from flow rate to HP) and you will get 41 HP per injector. Multiply this by 4 injectors and you will have the sum of 164 HP which is the potential maximum HP on the stock injectors.

The Dyno Results show HP figures way below 164HP so we know that this is not
the "weakest link" yet on the chain for tinkering in this study. Neither is it the stock fuel rail at the stock 40 psi of the Fuel Pressure regulator as the fuel rail can flow up to 80 psi. Increasing fuel pressure would help but still at this stage the resultant increase in fueling. richness indicates that this is still not the "weakest link". At this stage, the weakest link would be Alfie's favorite...the stock ECU and the stock Fueling. map programmed therein.

Solutions range from the the more expensive modification via programmable
ECUs to Autoplus' suggestion of adapting the Apexi Super Air Flow Converter which is a piggy back fueling. computer that can enrich the fueling. map; and to the
cost reasonable addition of an extra enrichment fuel injector somewhere in
the intake tract as was done in the winning ride of Miguel Bicharra at the recently concluded Miata Cup races at Subic. Again, budget constraints and the mindset of
the tuner would determine which solutions would be optimal.

It is my delight to note that Club members now understand more of the technical matters covered in these tech topics. They have obtained a good grasps of tuning optimally and not be driven too enthusiastically to over do set ups beyond optimal.
I hope that in some small ways, we have contributed to this. I have a healthy respect for the intelligence of Miata owners and as much as possible, I would want each and everyone to understand and make informed choices on their mods and care for our beloved rides. In the final analysis, nobody should know best in caring and tuning for the Miata than its owner.