SOME MATHEMATICAL STUFF
Finding the TP
Once you have reached the point that you can half-way understand everything up until now, you are ready for some deeper mathematical stuff to help you understand how the ecu comes up with these numbers.
First is the equation for TP: (VQ x K Value / CAS Value) / Number of Cylinders
VQ is the value taken from the VQ Table. CAS is the rpm / 50 x 256.
Here is an example:
On stock injectors at a light throttle and 2500rpms, the MAF is sending back a 2.56v signal (example only). That voltage falls on the 32nd block of the VQ Table. That value is 1B39hex or 6969dec. So our equation will look like this:
[ 6969 x 288 / (2500rpms x 50 / 256) / 6 cylinders = TP ]
So 2007072 / 12800 = 156.8 - and since this is the total amount of fuel needed for the whole engine, we divide it by the number of cylinders (156.8 / 6cyl). That gives us our TP Value of 26dec (or 1Ahex) . How significant is this??? If we know the MAF voltage and the rpm we can calculate the TP and find the exact points being accessed on our Timing and Fuel Maps! So lining up the TP value of 26dec (1Ahex) and the rpm of 2500 we can find the corresponding block (or block of 4 points if it does not fall exactly on 1 point) on the maps under those engine conditions. With a data logger like a Conzult we can quickly find and adjust or fix points on the maps. Even without data logging we can still find points using a good voltage tester and remembering rpms.
Finding the Pulsewidth in MS (milliseconds)
In the example above you can see that a TP of 26dec or 1Ahex at 2500rpms falls on a lean AFR (~BC) closed loop area of the Fuel Map. With such a short pulsewidth it is best just to let the ecu take care of it..which makes it sorta uninteresting for this lesson.
Lets say that at 7000rpms you are sucking in enough air to hit the last value (58hex) in the TP Scale Fuel. 58hex is 88dec and this TP value when multiplied with by 0.125 will equal the pulsewidth in ms. 88 x 0.125 = 11ms.
So now we know that 11ms at this engine load and 7000rpms, the engine will have the proper amount of fuel for a 14.7 AFR. Of course a 14.7 AFR at this load would be muy baddo for your engine so the next step is to reference the Fuel Map. Since the stock RPM Scale Fuel is limited to 6400rpms we know that we are on the bottom row. A TP Scale Fuel value of 58 hex is the final column so now we know that we are accessing the final lower right hand value which is 3Chex. 3C is roughly 10.1 to AFR but it is also a percentage of enrichment. Here is how you calculate it:
3C is 60dec and based on a scale of 128, 60 is an enrichment value of 47%. (60 / 128 = .46875)
So we increase the 14.7 AFR value of 11ms by 47%. (11ms x 1.47 = 16.17ms) So the total equated pulsewidth required to make 10.1 to 1 here is 16.17ms.
Finding the Injectors Duty Cycle
Continuing on the example above we have a 16.17ms pulsewidth at 7000rpms. What does this equate to in a percentage of duty cycle? Our duty cycle is the injector pulsewidth divided by the maximum injector opening time per revolution. So lets figure what the maximum open time at 7000rpms.
Since we have a 4 stroke engine, two up and two down, one of the down strokes is for combustion and the other is for the intake of air and fuel. So at 7000rpms only half or 3500rpms can be used for the injection of fuel.
7000rpms / 2 = 3500rpms
3500 rev per minute / 60sec = 58.333 revolutions per every 1 second
58.333 rev / 1 sec (now flip it) = 1 sec / 58.333rev = .01714sec /revolution
.01714sec x 1000ms = 17.14ms and this is 100% duty cycle.
The actual pulsewidth of 16.17ms divided by 100% duty cycle of 17.14 will give us the actual duty cycle:
16.17 / 17.14 = .9434 or 94.34% duty cycle.
Considering that some injector shops dont recommend going over 80% duty cycle, you can see that our stock injectors are already at their limits. In this example we used 7000rpms to calculate the duty cycle, at higher rpms the maximum open time for the injector is even less so it quickly reaches 100% duty cycle. We also have to consider other enrichments like cold start, throttle position, etc that may add a bit more pulsewidth pushing our injectors even further. This is why larger injectors are so valuable to our cars.
Be the first to like this post.
Originally Posted by unijabnx2000 
Back to top