[Power Hungry]

One of the big concerns with any larger nozzled injector (160/80 Stage 2, 200/100, 238/100, 250/200, etc) is not just the relationship of timing to injection flow (which is significant, of course) but also the significance of ICP to the entire combustion event.

If you follow any of the timing related maps, you can very easily see that as ICP increases the SOI decreases. This is due to the fact that increased ICP causes 1) an inherent change in timing do to the charge being introduced into the cylinder faster and 2) increased ICP causes a "hotter" spray and advances the SOC (start of combustion) point of the fuel.

The major concern with tuning for many of the windowed blocks that I've analyzed, was not pulse width... it was excessive timing and ICP at moderate RPMs. Ever wonder why folks would say "Never had a problem when I drove the truck hard, but threw a rod just driving down the street"? There are very specific reasons for that and we need to take a logical look at exactly why it happens.

In any engine, as RPM increases the SOC point (spark for gas engines or SOI for diesels) needs to be advanced as well so that the highest point of the cylinder pressure occurs shortly after TDC. This provides the most efficient use of the thermal expansion of the air in the cylinder. With gasoline engines, the combustion point is easy to define and the only significant influence to the combustion process is how fast the flame front propagates from the ignition point. Diesels are much harder to pinpoint since there are several factors that not only influence the combustion point, but also the propagation of the flame front. Some of these are:

1) Base Fuel Temperature
2) Engine Temperature
3) Air Temperature
4) Static Cylinder Pressure (PSI vs. Crank Angle)
5) Air Charge (Boost)
6) Injection Pressure (Droplet Size and Spray Pattern)
7) Injector Nozzle (Droplet Size)
8) SOI

Changes in any of these factors can significantly affect the SOC. Of course, the easiest to control are the SOI and ICP, but considerations must be made for Engine Temp and Boost.

One thing we continue to try to determine is when the SOC will occur, and that is a much harder thing to do than it sounds. Take, for example, you are running 2200 RPM with 1800 PSI ICP and that you have an SOI event that occurs at 30º BTC and the resulting SOC event occurs at 4º BTC. Raising the ICP to 2600 PSI could change the SOC to about 12º BTC, which in itself may not be a bad thing but when you consider that you are also increasing the fuel volume nearly 40%, you end up with a significantly higher Cylinder Pressure by the time you reach TDC. This is the phenomenon that causes engine failures... Achieving peak Cylinder Pressure BEFORE you reach TDC.

So what's the solution? The reality is that there is no clear-cut, singled answer for this. However, one important guideline to follow is to use the highest reasonable ICP possible for the given RPM range. The reason ICP is low at idle at because of the minimum pulsewidth limitations of the HEUI injectors. They don't respond well below about .75 ms (in fact, the minimum PW is .6 ms) so they ICP needs to drop to keep the fuel volume low enough to achieve idle. ICP is ramped up quickly off idle to help achieve both improved atomization and higher spray volumes while still maintaining PW in the 1.5 to 2.5 ms range. With this in mind, ICP has more to do with the fuel volume injected than the pulsewidth does. Again, higher ICP will have an inherent advance in SOC so always be sure to balance your SOI with your ICP curve.

I'm sure that this may seem like a bit of an oversimplification, but once you got your head around the full relationship of SOI and ICP, it really starts to make sense.

I hope this helps.