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Detonation and ECU tuning

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I have a 1990 non-turbo 300zx. A few years back I had some extractors installed and at the time of installation the EGR was removed (the extractors did not accommodate the EGR). Since then, the car has suffered some light detonation at light throttle or cruise conditions in hot ambient temperatures (more so when the aircon is on, I assume because the aircon puts an extra load on the engine). The exact same spot where the EGR is normally active. Now, I have spent the last couple of years checking everything on the car (fuel pressure, fuel flow, spark plugs, compression….. everything that can cause detonation) and everything is fine. So it seems that the detonation is caused by the high temperature of the incoming air, and since the cooling effects of the EGR are no longer present, this allows detonation to occur. I only use 98 octane fuel (the highest we have in australia is 100 octane, but that contains ethanol), but the detonation still persists. If I use a bottle of octane booster, the detonation is gone. Using Nissan Datascan I have been able to track at exactly what location in the timing and fuel map the detonation occurs. In the fuel map, it is in the closed loop section, between 2000-3000 rpm, light load, where the A/F ratio’s are quite lean. In the timing map, the detonation occurs at an area where the ignition timing is quite high (42 degree’s). I have been able to combat this problem to a degree by reducing the ignition timing in that area to about 37 degrees (5 degree retard). However, on really hot days (35 celcius +) the pinging is still there. Very feint, but quite noticeable if driving past a wall or barrier where the engine noises are reflected back. More so if traveling up hill. And using the aircon on these days is just impossible as that makes the pinging much worse and very noticeable.


I am reluctant to take out more timing in that area as it may affect the driveability of the car. I haven’t noticed any difference between having the ignition timing at 37 degrees compared to 42 degrees, but was wondering what would be the safest and lowest that I could go with the timing in those light throttle / cruise areas?


Also, I am planning to add more fuel to the effected areas to try and cool the combustion chamber. I will need to convert those cells from closed loop to open loop, and increase the A/F ratio from 14.7:1 to maybe 13:1??? Is that too excessive?? I know this will effect my fuel economy, but I don’t mind as I have a device which enables me to alternate between 2 separate ECU tunes. So I would only use the “non-economical” tune on hot days. Once again, is there a limit as to how much fuel I should add in this area??


If anyone has any advice or suggestions to helping me tune out the detonation, or see a problem with what I have listed above, please let me know.



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