by Keith Quistorff
I did some testing of a product used by many MR2 owners, the HKS FCD, and thought the results might be worth posting on the website.
The inevitable problem of fuel cut arose recently when I decided to go after extra horsepower by installing a simple mechanical boost controller, aftermarket boost gauge, and aftermarket air intake system on my MR2. Although the Zener diode solution is real attractive, I hesitate to completely eliminate fuel cut for fear of catastophic overboost. After reading differing information about the aftermarket FCDs, it was unclear whether they would change the fuel cut boost level or eliminate it. I contacted HKS to get the answer, and one of their techs informed me that the HKS FCD raises the fuel cut boost pressure to about 17 psi when HKS's recommended setting ("7" for MR2s) is used. He said all bets are off if another setting is chosen. This sounded good, so I ordered one. After receiving the unit and looking over the (very sketchy) instructions, I had to contact HKS again for clarification. I spoke with another tech who contradicted the first one by telling me that fuel cut would be completely eliminated. If this is true, the Zener diode is just as good, so I decided to return the HKS FCD for refund. Before returning it, I decided to test the unit to see exactly what it does to the boost sensor output signal. With assistance from an Electrical Engineer friend, we set up a bench test to check input voltage vs. output voltage at the recommended setting (7) as well as two other levels, 5 and 9. For each setting, the input voltage was varied through the 0 to 5 volt range, and corresponding output voltages were recorded. I compiled and plotted the results (see the attached EXCEL files below).
The results are interesting. Clearly, the HKS FCD modifies the boost sensor's voltage seen by the ECU. Depending on the setting used, the severity of voltage modification (and fuel cut pressure) can be varied. For example, the recommended setting, 7, results in 4.08 volts seen by the ECU when the boost sensor is outputting 5 volts. Setting #5 results in 3.69 volts to the ECU with 5 volts sensor output, and setting #9 results in 4.44 volts to the ECU with 5 volts sensor output. According to information provided on MR2.com by Dr. Chryssos, fuel cut happens somewhere just above 4.25 volts to the ECU, and I've also read that the recommended Zener diode is 4.3 volts. From this, I would assume fuel cut happens at around 4.4 volts to the ECU. If this is accurate, setting #5 would cut fuel at ~6.4 volts from the boost sensor, setting #7 would cut fuel at ~5.6 volts from the boost sensor, and setting #9 would cut fuel at ~4.9 volts from the boost sensor. Again, referring to information provided by Dr. Chryssos, MR2 boost sensor's maximum practical output is around 4.9 volts, which corresponds to a boost pressure of about 17.4 psi. From this, it appears that HKS FCD setting #9 would result in fuel cut around 17.4 psi, and settings #7 (recommended by HKS) and #5 are off the chart, requiring boost pressures well above the sensor's practical output (4.9 volts, 17.4 psi) to engage the fuel cut mechanism.
Based on these results, I decided to keep the HKS FCD and use setting #9, which should change the stock fuel cut to around 17.4 psi boost. Although this is conservative compared to HKS's recommendation (setting #7), I like the idea of having a failsafe at non-destructive boost levels.
Date: Sat, 23 May 1998 22:51:56 -0400 From: "D. Heath" (dheath@caribsurf.com) Subject: FCD Info I was reading an article on mr2.com on the FCD and it's voltages relavant to boost, and where it puts the fuel cut. The setting for the MR2 and Celica is 7 according to HKS, but as Keith Quistorff explained in his article 9 seemed to bring the boost cut to a safer limit (around 17.4psi), so I decided to test this in real life. I set the FCD to 11, and then set the HKS EVCIII to 1.2 bar, and the result was that at about 1.2 bar (17.5psi) the ECU fuel cut was hit on the first attempt to accelerate, which register a check engine light, and wouldn't allow the car to boost again until the ignition was switched off and back on again, it also registers, as expected, a code 34. The point to all this is that the 11 setting will garuntee fuel cut at boost pressures above 17.5psi. During the course of next week, I will be trying settings 9 and 7, and see where they cut fuel, however I don't intend to run the car over 1.2 bar, so all I will be able to say is whether it happens or not at those settings. If you are interested in hearing what happens at the other two settings let me know. Dennis 1990 Toyota Celica GT-Four