The excerpt from an SCC article:

http://www.sportcompactcarweb....part2

shown below says something about the MAP sensor not compensating for I/H/E very well. Can somebody explain this a little further since I'm not 100% clear on what they mean? The article suggested a fuel pressure riser? I thought these were only needed for forced induction. A fuel pressure riser would increase fuel delivery, but by doing so, would it be reducing fuel economy and increasing emissions? What can possibly go wrong if I don't get a pressure riser? I'm looking for long term reliability, fuel economy, and low emissions with a small power increase with I/H/E. Am I better off with or without a fuel pressure riser and why?

"A few popular imports use what is called a speed density system. Honda and Acura are the main examples. These cars use a MAP or Manifold Air Pressure sensor to help the ECU determine how much fuel to inject for a correct mixture. Unfortunately, since they must calculate airflow based only on manifold pressure and rpm, their calculation must be based on the pumping efficiency of the stock engine. If you improve the pumping efficiency of the engine, most speed density systems do not compensate for modifications very well.

All is not lost, however. Add a device called a fuel pressure riser. This is an adjustable pressure regulator that goes in the return line to the gas tank from the injectors. With a fuel pressure riser, the fuel pressure to the injectors can be controlled, a process allowing an increase in the pressure to add more fuel to make up for the additional air being drawn into the engine."

 

My understanding is that the article suggests because Honda use MAP sensors they have to work out the amount of air getting pumped through the motor. To do this they've assumed that a certain intake / header / exhaust is attached, causing on average a certain level of airflow restriction. Therefore, they can simplify their calculations to something like -

Total air = Theoretical max air flow through stock motor - restriction from stock IHE.

Hence changing anything can lead to a rich / lean condition due to changes in the motor flow characteristics without any change in the preset fuel delivered (ECU mapping). However, what the SCC article fails to mention is that when you're running closed loop the stock ECU uses the oxygen sensor reading to adjust the air-fuel ratio. It also doesn't mention that there is a long-term average that is used to compensate too.

It's only under open loop / WOT conditions will the ECU revert to the built-in maps, which is what I'd say is referred to in the SCC article. Even then I suspect the long-term correction factor is used so it's not as bad as it's made to sound.

At the end of it all, adding an FPR will just shift the base fuel delivered up / down and would still need to be tuned with something like a VAFC / standalone for best results.

I'm sure someone else will add their bit if I've missed / incorrectly stated something.

 

Thanks for the detailed reply! More replies are always welcomed.