[C3H8]

Olympiadis, I appreciate you answering Arragonis for me. you did a pretty good job at explaining it. better then I could anyways.

arragonis, thanks for jumping in with a relevant question. I'll expand on Olympiadis's answer and it is quite relevant because using a potentiometer on the tps is close to the same thing as using it on the coolant temp sensor. It works pretty much how Olympiadis said. My car Uses tps, among other things, to determine spark timing. there are a few exceptions, however If i depress the throttle on my car, the ignition timing goes down in value. things that make it increase are the base timing table, (that is rpm dependant) the map sensor, the tps, and the intake air temp sensor, as well as coolant temp sensor. they all play a role in determining how far to deviate from the base (plus or minus) ignition timing table in the computer.

In a real world situation if i am driving steady state at 50mph my ignition timing is about 33 degrees. I depress the throttle petal 1/2 inch, according to the scan gauge the ignition timing retards about 10 degrees, putting me around 23 degrees timing. And at that instant the engine is still at the same rpm and i am at the same mph. Now I also start to accelerate because of increased throttle opening. as i accelerate the ignition timing slowly comes back up.
In another situation, if i am again traveling at 50mph with 33 degrees of timing, and I release the throttle petal, I can watch my ignition timing go from 33 degrees to 34, 35, etc until it peaks about 38-39 degrees, until the rpm's drop enough for the timing to again start backing down.

It's the computerized version of the old fashioned dizzy's that used counterweights, cams and springs to determine timing, along with vacuum advance and retard diaphrams. It's the same thing.

So my goal with the potentiometer on my tps is to 'trick" the computer into thinking my throttle petal is JUST off of idle position. This is where my obd2 scanner shows me that I get maximum ignition timing for my car at pretty much any speed between 0 and 90ish. (and believe me i have thoroughly testes this to be sure because I do not want to make a modification that would result in less mpg) And that is exactly what I did. Now my ignition timing reads about 37 degrees when I am again doing 50mph under steady load. Only difference is, I am getting a bit better mileage now. I noticed that as i slowly turned the pot, my ignition timing slowly went up, and so did my miles per gallon.

keep in mind that you can go too far with ign timing. I was able to make the car ping like a dozen marbles in a tin can if i cranked the tps down too much. I also got the same effect when I messed with the MAP sensor and went too far. also it is possible to go too far with ignition timing, and not expierence pinging, yet you can expierence DECREASED fuel economy with too far advanced timing by having the combustion pressure peak too early in the cylinder's cycle. I don't quite remember exactly where combustion should peak in terms of degrees of rotation for maximum efficiency, but i am guessing it is somwhere around 15 degrees after top dead centre.
I have not got my timing to go that far, yet. My car just pings when it goes too high for the load I am putting on it.

for instance, at night when it is 30 degrees cooler, I am able to run a bit higher ignition timing and get away with it without pinging or decreased mileage. That is too finisky for me though, so I generally keep the timing set to daytime use and leave it there. I might screw around with drastic timing changes when winter comes, though.

I have heard of other brands of cars which have different timing needs. Me mum's honda crv has over 40 degrees of timing right out of the factory under steady load cruising around and does not ping at all. And i have not screwed around with the crv's timing by the way. It also does not have egr but it does have higher compression ratio then me dodge. It also has a drastically higher horsepower to CC rating them my engine too. So I guess the honda is a better design. Fancy that.

My point is, different engines need different ignition timing settings.





Edit: One other thing I forgot to mention. I have heard of certain cars that do not use some sensors for setting ignition timing. Most noticably would be the air temp sensor. I have seen some turbo mitsubishi's that do not care what the air temp sensor says concerning timing. they use that sensor for other things I guess. I believe that OBD1 miata's only use tps to tell the computer if it should be in open loop or closed loop and have nothing really to do with timing) but I could be wrong. My point in this edit is that although MY engine appears to use air, coolant, map and tps for timing, not all engines do. so if one of you guys decide to screw around and try to increase timing and it DOESN'T work, try another sensor.

If you're screwing around causes a change in fueling, the oxygen sensor will probably correct it shortly. the obd2 cars have short and long term fuel trims. short term is quick. My car can add or subtract up to 25% of the fuel from being injected almost instantly. (I tested this with a big propane torch).
My engine's long term fuel trim is also capable of subtracting an ADDITIONAL 25% of the fuel. I also determined that with propane, but it was from an earlier test via literally flooring the motor with a 20lb propane tank while driving down the highway. I just wanted to see what the computer was capable of dealing with, and yeah it can actually subtract about 50% of it's injector duty cycle and still maintain proper o2 sensor voltages (dancing from 0.1V to 0.9V) but thats another subject for another time...