Sorry for the late reply. I've been thinking on how to answer this in depth. For the short version see
this thread.
Homer hit the nail on the head. The engine is an air pump, making it breath better is where you get performance increases. Always has been, and as far as I can tell always will be.
Over the past several years, several changes have been made in increasing complexity. These changes were primarily necessitated by environmental protection laws, which made the manufacturers dig deeper to provide performance and meet EPA restrictions.
But all through that, decreasing the air to fuel mixture ratio to a rich condition (less than 14.7:1 == stoichiometric) is good for acceleration but you don't want to run rich at constant speed. Plugs foul in rich mixtures and then start to perform poorly and when that happens your back to no HP gain or even loss.
Now I'm not certain of the sequence of technology and my memory has a lot of holes and this is by no means complete, but will take a stab. People chime in and correct me, I know I'm missing a lot here, perhaps even misstating some things. I learned this stuff 15 years ago and am going by poor memory. It's really a lot more complicated than this...
O2 (oxygen) sensors and catalytic converters. The catalytic converter is typically a honeycomb ceramic impregnated with a catalyst (I believe at one time it was platinum?). As the catalytic heats up it burns off the excess hydrocarbons (unburned fuel like running rich). Most O2 sensors used back then and I think still widely used today toggle around .5 volts indicating rich or lean air fuel mixtures, see note***. The key is they toggle, they don't give a reading that can be interpreted as a precise mixture ratio. So these sensors are tied to computers that are essentially a bang-bang controller (with time averaging) that adjusted carburetors or injectors fuel flow. So if the O2 sensor detected a lean condition, the computer would tell the carb or injectors to allow more fuel, etc. But this is no use for accelerating. During acceleration, the computer would go to a preset rich air fuel mixture. Ignition timing needs to be advanced too, and you detect the operator wants to accelerate by vacuum, throttle position sensors, Wide Open Throttle WOT sensors versus rpm sensors etc).
Obviously that helped reduce the amount of unburned hydrocarbons to a great degree, but the allowable limits were tightened. So, enter air temperature (ambient outside) and flow sensors (flapper doors). Air temperature and flow was used as a guide to determine air mass. If the air mass was greater, then the fuel had to be adjusted accordingly...but for acceleration only, the O2 still serves for constant speed driving. So the "fuel map" was developed that was basically a table or algorithm that said for this mass use that fuel. That way, the car would produce less hydrocarbons over a wider range of air temperatures. MAP sensor plays in here too as well as altitude sensors etc. Air temperature and flow is one means of determining air mass but it does not take into account other factors such as barometric pressure, altitude, and humidity, all of which change air mass/density and the optimum (performance versus environment) rich air fuel mixture for accelerating.
So obviously, that helped reduce hydrocarbons but we love our environment so...stricter standards again. Now enter the Mass Air Flow (MAF) sensor. Which to me is essentially a hot wire anemometer. You pass current through a wire as the air flows over the wire and cools it (mass effect), you have to send in more current to keep the wire hot. So the MAF directly determines the air temperature, flow and most importantly mass. So once again it looks at the fuel map table or algorithm and computes the right amount of fuel to inject for acceleration, all things considered (performance and environment).
Ah but we're not done...we love our environment but we also love performance. The MAF sensor and fuel map does good but the map was fixed and does not take into account other things, not sure of all but examples to me would be the gas itself, differences in engines and engine wear. So the fuel map basically has to change or adapt. Enter adaptive controls one I'm familiar with is called the Kalman filter. It allows the computer to sense through it's MAF, O2, temperature, and various sensors and adjust the fuel maps accordingly. This is part of the reason why the ECU for the Z has been so difficult to crack and why performance mod attempts have resulted in initial dyno run small gains but then back to no gains or even loss on subsequent runs. IT LEARNS!
So, I suspect highly, but don't know for sure, that this resistor trick that used to work in the old days won't after about 20 minutes (what we have seen in the Z forums as the ECU "learning curve"). And worse, might even throw a code or the Check Engine Soon light. In any event, you don't want to run rich forever, just for acceleration. Even today, constant speed air fuel ratio mixture is still determined by the O2 sensor(s).
***There are some O2 sensors that yield measurable readings that equate to an air fuel ratio, not just bang-bang. But these sensors used to be really expensive. I suspect they still are and aren't used to a large extent. But before the mass air flow stuff, you could build a circuit around one of these to set your air fuel ratio and fool the computer. I highly suspect that won't even work today.
Hydrocarbons are only part of the environmental story but is what you talk about when running rich as this $9 mod suggests it does ... :bsflag: .
As the air to fuel ratio increases you run "lean". That is greater than 14.7:1 or stoichiometric. In lean conditons the engine produces other bad things, if memory serves NOX??? At any rate, the curves oppose each other and the lowest emissions considering hydrocarbons and NOX is right at stoichiometric. So all things considered, the objective of the computers is to run as close to 14.7:1 as possible for all conditions and variations. Meaning running lean also is a bad thing. Now running lean in extreme cases can cause severe engine damage, not just spark plug. As the mixture gets lean, the spark plug can't ignite the mixture as well and results in non-uniform combustion inside the chamber and can create hot spots that can burn through the piston (doesn't ignite the whole chamber). But slightly lean mixtures can produce better gas mileage depending on a whole lot of stuff I forgot. I do remember the pre-combustion chamber though. Essentially a small combustion chamber that was run slightly rich and then expelled itself into a large combustion chamber that was run slightly lean. Resulted in great gas mileage and the hydrocarbons were low but the NOX ouput would be higher.
I heard a story (perhaps urban legend, never was able to confirm this) from a friend about the pre-combustion chamber. Honda had developed a pre-combustion chamber engine, mass produced it, sent it to the states, congress passed a law before the cars left the boats, honda turned the boats around, took the engines out and put in engines without pre-combustion chambers but WITH catalytic converters. Anyhow, thought it was interesting even if it's urban legend!
Ok you folks with better memories and knowledge than I have...chime in here and correct me.
