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# Calculating Injector Pulse Width

### Tech Articles

Discussion and questions related to the course EFI Tuning Fundamentals

Hi,

The example given for calculating the injector pulse with presumed WOT conditions @ 6000RPM.

How can we modify the calculation(s) for part throttle operations?

Vincent

Good question. Depending on the change in throttle opening, the cylinder filling may not change significantly, so it really depends on the flow potential of the throttle at a given engine speed, throttle angle, and pre and post throttle pressure.

Keep in mind many engines may achieve similar filling at 60-100% throttle opening, so the result would be about the same until you really close the throttle down a lot. As the throttle closes further, airflow becomes more restricted and the air mass in the cylinder is reduced.

The relationship between throttle angle and cylinder filling varies with engine speed and other factors as well, since for example at half the engine speed (3000 RPM), there's twice as much time to fill the cylinder, so reduced throttle angle may not have as large an impact on air delivered as it does at 6000 RPM.

Also if you replace a 50mm throttle with a 75mm throttle, air mass delivered per throttle angle changes as well. Hopefully this explains why throwing throttle angle into the equation makes things significantly more complex and introduces additional variables.

Mike,

I think I muddied the question by using "throttle"... what I really mean to ask is how to vary the Injector Pulse Width calculation for different manifold pressures.

In stating this, I think I have answered the question on my own... all the calculations start with airflow then to air mass then to required fuel mass... so that any intersection of manifold pressure and rpm begins there.

The base reason for the question is how to use a spreadsheet to calculate a base fuel table before jumping in the vehicle to begin tuning... effectively pre-calculating a base fuel map before any compensations are applied... the specific vehicle is a FD Rx7 fitted with an Apexi PowerFC... in order to validate my understanding of the material presented in the EFI Fundamentals course I want to manually calculate the fuel map to see if my numbers are near enough to those of the original tuners'.

Am I on the right track?

Vincent,

Since most modern ECUs use volumetric efficiency and fuel injector characterization data rather than an injector on time table, I'll give you a way to think forward a bit, while working with an older product.

If you calculate peak fuel on time where peak torque will occur, you can then estimate how to offset values at higher and lower RPM (still at full throttle) based on a typical torque curve for that engine. For example if peak tq is at 4500 RPM and torque falls off 20% by 7500 RPM in semi linear fashion, you could take your peak tq injector on time, multiply by 0.8, place it at 7500 RPM, then interpolate between those points.

Once you ballpark your full load values at various engine speeds, you can fill in lower MAP values based on the reduction in air mass associated with the pressure change.

For example if you calculate on times at 2 bar MAP, then half those values for 1 bar MAP, half again at 0 bar MAP, and interpolate between those pressure axis points, that will give you some quick ballpark values.

This won't account for injector non linearity, change in exhaust pressure etc. so you'll certainly still need to make other tweaks, but I'd expect the engine to start like that as long as cranking fuel, post start fuel, and idle air for start/idle are reasonable.

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