LPG2CV wrote: ↑Fri Jan 01, 2021 5:28 pm
As tuning starts from idle and progresses through the rev range, why not start with the smaller injectors and see how you do. They will be easier to tune, and you should soon get an idea of their suitability by the required VE. Don't forget that at the end of the day, the RF - VE is just math, so you can set what you like with it. Duty cycle is the key thing. If you start getting past 85%, then larger injectors may be required. But smaller in the first instance may be easier the manage than larger.
^^ This. Your calculated injector flow may be generically valid, but they can be limited by time available to inject each cycle within 75-85% of the time available. However, that limit may be further modified by reality with need for added fuel flow (WUE, plus AE, etc) on-top of that. Those aren't limitations to the ECU servicing all 4 injectors, but separate considerations.
So, I would agree to either randomly try the smaller set and judge based on logged DC results where to go from there. Or else make the calculations to determine that threshold injector size, i.e., the flow to inject one cycle of fuel for one cylinder @3bar, within (time of one cycle - dead time), multiplied by cycles per minute. If the rpm is not limiting then you'll get 155cc or similar, or larger if it is. Then-up-size again with the potential added fuel % based on your experience and data from prior projects.
These are common reasons some high-rpm or high-power setups use additional injectors to carry those extra burdens, in order to hold the main injectors as small as reasonably possible, allowing them better fuel control in their range of flow under most-common conditions. The secondary injectors then handle any demands exceeding that range. We can read that as easier and more effective tuning, with better results through better control. Using this concept, you could begin with 155cc (or even smaller) injectors and add whatever secondaries are then predictably needed through real-world data analysis. Note there are other schemes useful to flow control, such as large single injectors with rising-rate fuel pressure for expanded control range. Lots of fun ahead!
David