Originally Posted by CSt6804

MAP is pressure. 0 MAP is 0 pressure. 100 MAP is atmospheric pressure which is approx 14.5 psi. A cylinder can't fully fill and achieve its maximum VE potential if the throttle is partially closed and there is only say 50 kpa of pressure or approx 7 psi in the intake. Obviously there is more to it then that but thats the general idea. Diesels are a different story as they don't throttle the air. Nor do they tune for AFR. But that's not a discussion for here. Anyways The ECU uses the ideal gas law to calculate fuel requirements at any given moment in time. MAP is one of those variables. The ECU needs to calculate VE before it calculates fuel. It starts with number given in the VE table and modifies that value according to a set of variables or multipliers. Air intake temp, Engine temp, Engine speed and MAP being major players. The easiest way to understand it is high MAP = high pressure. High pressure = better cylinder fill, Low MAP is less pressure so less cylinder fill with everything else being equal. And if 14.5 psi isn't enough there is always forced induction. In which case you should be tuning with MAP and not TPS. That's as simple as i can make it. The problem with TPS based VE tables is that there is no correction for engine load. It assumes that if the VE is say 80 percent at 1500 RPM with 15 percent throttle, then that's the fuel you get with no correction for load in the equation. Correction does occur after the fact with the 02 sensor, but only if you are in the closed loop portion of the fuel map. Thundermax uses widebands so you get the bigger closed loop window for correction. The Cats clean up the rest. Well that's the theory anyways. A 4 gas machine will say differently. But obviously TPS VE tables do a good enough job to meet current EPA requirements on a Harley V Twin. Would you notice a difference in performance or fuel mileage with a MAP based tune ? Probably not. Its just that MAP is the only way the ecm can determine load under all conditions. TPS just can't do that. All that being said, even though most of us only get TPS based VE tables to work with, Im not convinced the Delphi ECU completely ignores MAP for fuel delivery. I say that because MAP is a variable required by that magic ideal gas law equation. But getting that information is near impossible even though there are at least two guys on this thread that would likely know.