Basically If you have done anything that can change your fuel rail pressure, or MAP pressure, you either need to put a sensor on it and correct the inferred logic values, take an educated guess, or copy someone else's tune hoping they did it right. Its always best to use knowns, known good MAF curve, known good injector data, ETC.
Cams change MAP.
FI changes MAP.
Bigger injectors/ higher RPMs and more fuel demand can change fuel rail pressure.
Return style and regulated rail pressure changes rail pressure, you wouldn't guess and set this with out a gauge.
If you are at 8psi boost ~23PSi MAP, and are seeing 2.0 load, you either have error in your MAP(From SD) or rail pressure. Its not so bad as the ECU is being overly cautious as its farther down the tables than it should be. This can also go the opposite way which is more dangerous as the ECU is commanding timing for cylinder pressures thinking its getting .7-.8 load when its really getting to 1.05 load. This can lead to situations where you are getting 29* of borderline timing when your engine is actually only going to tolerate 25* before detonation.
The fuel trims will correct the load, so if you get a manifold, intake tube leak, or any other think that can effect MAF frequency. You are not going to end up at a lower load and a dangerous situation. The problem I'm describing comes from people seeing fuel trims and just moving the MAF curve to get rid of the error when there is error in their inferred MAP or rail pressure they are not even looking for as they don't have the sensors and don't want to do the right thing and put sensors on the car. The stock SD does very well as long as the MAF curve is accurate giving it an accurate air mass and not compensating for the fuel side of things being wrong.
.70 load just means the cylinder filled to 70% of Baro pressure. To do this you would need around 70% Baro as MAP. You are not going to get 70% in the cylinder when the MAP is only 50% baro, cylinder filling through the heads is just not that efficient on any engine. Also keep in mind airflow is driven by high to low pressure, with very slight dynamic airflow variance that you don't need to worry about as thats not the normal. You can say the intake tube will be a higher pressure then the manifold and the manifold will be higher than the cylinder. Pressure is then modified by compression and power stroke and it continues this trend through the exhaust. If you only get 70% in the cylinder when the MAP is 100%, it is possible but only at very low RPMs and very bad cylinder filling. As MAP gets above Baro into boost, the air is just moving slower and slower making the cylinder filling less and less efficient so assuming 100% cylinder filling is less and less likely the higher you go. Four valve engines do breath at high RPMs and high boost very well compared to two valve engines, but they are still governed by compressible gas physics.
Reply With Quote
Originally Posted by murfie
