The Basics of Promming
Promming is not some Voo Doo magic that ought to be scarry.  in short it is changing a few values in the prom in the ECM of a car so that the ECM can run properly with the changes that were made to the engine. 

For now we are going to talk about factory cars that have modifications, High performance aftermarket stuff is another subject for another day.

What neccesitates a change to the prom is a significant change to the engine configuration that the ECM can not compensate for.  For example, if you put a set of mufflers on your car & a K&N air filter, you are not likely going to have to change anything.  BUT, if you increase your compression, put in a lump cam, remove the cats, put in a free flowing exhaust on your car....... absolutely.  Now if you are somewhere in between, the answer is Maybe.  how does the car run?  is it setting any codes?  before you jump into promming I would stronly encourage you to check the basic state of tune on your motor.  promming will NOT fix a broken car.  what it will do is to take a motor that has been changed from the factory configuration & make it run like it should run ..... nice, smooth & with all the HP is should have.

How does it work?  Quite simply.  When the manufacturer of your car put it together it had a target in mind.  They put a certain piston, cam & head in your motor & then had to burn a prom that was going to run with that setup.  They took a basic prom configuration (Speed Density or Mass Air Flow), & modified the tables in the prom to make the car run properly.  For each configuration that motor is going to have a certain charactersistic.  The prom that they make is going to fit that charactersistic so that the car runs well, gets good mileage (well........... maybe not good, but OK) & has some amount of power.  If you change some significant items in the motor you are going to change the charactersistics of that motor, so the motor will not run as well as it should with the prom that it has.  this is the situation that is going to need a prom change. 

As a general rule, a motor is an air pump (No new news here right?).  Idealy when the piston draws down on the Cyl, it would take in 100% of the volume of that Cyl.  realistically, it cant.  The efficiency of how well it takes this air in, can be thought of in terms of Volumetric Efficiency (or VE).  for the sake of argument lets say that you had a 100% efficient Cyl.  if you were running down the road, you would want to have 14.7:1 Air to Fuel.  that is that for every 14.7 parts of air you want 1 part of fuel.  this is what is typically considered the best efficiency for an engine.  

We are going to use tables in the prom to give the right amount of fuel for the amount of air that we have going into the engine.  If we make modifications to the amount of air that an engine can take in (change the VE of the engine), then we are going to have to make changes to the prom to keep this 14.7:1 ratio. 

What does it involve to change the prom?  Quite simply this just involves changing some values in some tables for that engine.  Now I know that is very vauge, but bear with me we will get into more detail.

There are two basic types of EFI systems out there the first is what is called Speed Density & the second is MAF. 

The MAF type of EFI systems use a Mass Air Flow sensor.  This device has a wire in it that tells the ECM exactly how much air is going into the engine.  This is GREAT for those folks that want to modify the engine, becuase the MAF will tell the ECM how much air is going into the engine & the ECM will give it the right amount of fuel.  The down side to this system is that it is not hard to max out the MAF.  once you have done this, the ECM cant compensate & put the right amount of fuel in.  this is where all those expensive aftermarket MAF's come into play. 

The second system is Speed Density.  In short the ECM has to calculate how much air it is taking in my the density of the air (Manifold Air Temp, MAT), & the Pressure in the manifold (Manifold Air Pressure, MAP).  Using this information it can caluculate how much air is going into the engine & then calculate how much fuel it needs.  The advantage of this is that there is no MAF to max out.  What limits how much air goes into the engine is how well it breaths (heads & cam) & how much pressure there is to force the air in (Barometric pressure).  The barometric pressure can not get above 30 or so inches of mercury (or 100 KPA), so the MAP can not max out.  Now I know there are those of you that are thinking about blower motors & such, we will get into that later.  So what limits this application is the infinate combinations of VE's that this setup can have.  This type of engine is primarily run off of a VE table that dictates how much fuel & when.  When you modify the engine, you change the VE so now you have to change the table.  the good side to this is that you can make serious modifications to the engine & the ECM can keep up (As long as you have enough Fuel injector to feed the engine), with the demands by the modified VE table. 


a brief overview of the aftermarket stuff.