AFR Guage Article 2013 Brett Becker

[Dshaw340]

I understand the factory O2 sensor sends a signal to the computer to adjust the amount of fuel or as in the article, dump more gas to cool the chambers.  So doesn’t the factory O2 sensor defeat the purpose of an adjustable fuel regulator?  I did not use the factory sensor and used the Bosch sensor in its place for the AFR guage and am able to adjust  the AFR with my adjustable fuel pressure regulator to around 13.0 AFR at wide open throttle in 4th gear.  Am I losing power by not using the OEM sensor?
 

Edited March 16, 2022 by Dshaw340
Used the word 'guage' instead of 'regulator'

[Brett B.]

It's been so long since I thought about this, let me see if I can offer a cogent explanation. 

The adjustable fuel pressure regulator used in Spec Miata is something of a blunt instrument. The regulator we use is mechanical, so all it does is control maximum pressure.

If memory serves, the factory fuel curve drops to something like 11:1 above 4,000 rpm, which is done to cool the chambers as you point out. We are simply lowering the overall fuel pressure to lean the mixture out. The rule of thumb is 12.8:1 at 6,500 rpm. In theory, we have better cooling systems to deal with the leaner mix. 

If you were to look at the factory fuel curve and the one that results from the use of the mechanical fuel pressure regulators we use, the FPR merely moves the curve up or down in the rpm range. The shape remains the same. Genuine software tuning changes the shape of the curve, which is the better way to do it, but that's against the rules. 

We are typically running less fuel pressure than it would in stock form above 4,000 rpm. If you look at your AFR gauge on your cool-down lap, when you're not at 10/10, you'll see that the ratios are wildly lean because we have moved the curve, but not changed it's shape. 

I don't know what year car you have, but suffice it to say that the hardware varies between NA6, NA8 and NB cars. The method is essentially the same.

The AEM UEGO gauge comes with a sensor. I used the factory sensor in the factory bung, and added a bung for the sensor for the gauge. To answer your question, I don't believe you are losing power by not using the OEM sensor as long as you are tuning the fuel pressure properly. 

Others may chime in here, too. 

Edited March 16, 2022 by Brett B.

[alexwilliams9811]

Interesting point. While it's true the factory O2 sensor provides feedback for fuel adjustments, an adjustable fuel regulator allows for more direct control over fuel delivery. By bypassing the factory sensor and using a Bosch unit with an AFR gauge, you’ve essentially taken manual control of fuel mixture. However, without the factory sensor's feedback, the ECU might be operating in a less-than-optimal mode. It would be interesting to see how the car performs under different conditions and compare it to the factory setup. heardle

Edited August 3 by alexwilliams9811

[Brett B.]

For the 1995 Miata in this story, we did not bypass the factory sensor. It still fed information to the ECU. We added the Bosch wideband that comes in the AEM gauge kit. 

Again, if you look at the factory fuel curve and the one that results from the use of the mechanical fuel pressure regulators we use, the FPR merely moves the curve up or down in the rpm range. The shape of the curve remains the same because the factory O2 sensor still provides input the ECU, which still adjusts the fuel curve with injector pulse and duration. The factory MAP sensor also contributes to maintaining the shape of the factory curve.