Daz, fuel trim and pressure

[blitzracing]

Following on from various posts from "Daz the plumber" on mixture control I thought Id experiment a bit more. We know the ECU automatically trims the overall mixture at tickover (long term trim) to get it near Lambda 1, and then cycles the mixture up to 3400 rpm to keep the catalysts at peak performance. Above this point the ECU stops cycling the mixture and goes open loop. The odd bit here is if the basic mixture has been trimmed, is the just the basic trim value applied to the open loop area? After winding the fuel pressure up and down to see the effect, it appears that the answer is NO, and you are now totally reliant on the fixed part of the fuel map and fuel pressure as the only mixture control. So a change of around of around +/ - 2 PSI will signifficantly affect the full power mixture. This is not a huge pressure change either way, but can make the car run seriously lean, or over rich, although driving it you would not realise. I then thought to minimise the amount of correction the ECU has to do, I removed the Lambda inputs, reset the ECU and altered the fuel pressure until I reached Lambda 1. Problem was this happened at around 32 PSI, and if you then run full power the mixture becomes very lean, so it appeaars that some basic trimming of the mixture has to be done anyway at the correct fuel pressure. If you have significantly high or low fuel pressure, the car will also snatch as the ECU tries to compensate, although TVR's tend to run lean at low RPM.

Mark

[daz the plumber]

ive not managed to take the car to the dyno centre,ive been a bit busy .i hope to get the readings next week .