Round II Fellas

[Cadash]

OK....After reading the injector article more than 3 times it looks like the 175/146 single shots will be fine (Stage 1.5) on my PMR truck. Just a tiny bit more than stage 1. So Hybrids are a no go on a daily driver due to the high oil pressure requirements?

Bill, can the 175/146 single shots (if I choose to get them) be tuned to safely give me a bump in power, tow 11,000lbs+ , with minimal risk of hurting my baby? I think thats it....

[Power Hungry]

Dan,

Here's the deal... We can tune injectors down (within reason) to any level that we need to maintain good driveability characteristics and keep the engine power output levels within the capabilities of the hardware (block, rods, etc.).

175's on your truck will be absolutely no problem to tune and shouldn't be any more of a concern than stock PSD injectors. In fact, for upper power levels you're better off going with the larger injectors because the timing demands to achieve power output on smaller injectors are much more aggressive and can ultimately lead to premature engine failure.

Unfortunately, injectors have gotten a bad rap because a few tuners have gone with the mentality that many of the parameters would remain the same and hadn't looked at what was really going on with the timing curves versus the amount of fuel being injected and how fast it was being injected. Consider the following examples:

Stock injectors will deliver ~135cc in about 3.5ms. Using the following table.

RPM - Deg/ms
2000 - 12.0000
2100 - 12.6000
2200 - 13.2000
2300 - 13.8000
2400 - 14.4000
2500 - 15.0000
2600 - 15.6000
2700 - 16.2000
2800 - 16.8000
2900 - 17.4000
3000 - 18.0000
3100 - 18.6000
3200 - 19.2000
3300 - 19.8000
3400 - 20.4000
3500 - 21.0000

...you can see that even at 2000 RPM, the crankshaft is rotating about 42º in the time it takes to fire the injector. When switching to a larger nozzle and reducing injection time to about 2.5ms, you're looking at about 30º to complete the injection, even for the same volume of fuel injected.

So what does this mean? It's pretty simple if you stand back and look at it like this (forgive the crudeness of the graph)...

* = crank rotation
[ = start of injection
; = spray time
] = end of injection
: = burn time
! = TDC

Stock injectors @ 3.5ms of spray time
|***************[;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;]:::::!:::::

Modified injectors @ 2.5ms of spray time
|***************[;;;;;;;;;;;;;;;;;;;;;;;;;]:::::::::::::::!:::::

As you can see, larger nozzled injectors will complete the injection and start the full burn process significantly sooner than a stock nozzled injector. Of course, this is just a rough idea of what's going on, but I think it illustrates the point that if you aren't making significant timing considerations, you are missing a large part of the picture and are putting an engine at risk for damage.

The bulk of the engine damage occurs when the flame front causes the rapid rise in cylinder pressure before the piston and crank are at the proper location to utilize the energy. What ultimately happens is the rise in cylinder pressure becomes a downward force against the piston at a time when the piston is trying to still rise in the cylinder.

This early pressure can significantly increase both linear and torsional loads on the connecting rod because not only are both the piston and crankshaft pushing against both ends of the rod, but the crankshaft position has the rod at an angle and the rod is not able to properly handle the forces being exerted on the piston. Also, because the pressure on the piston is actually being applied to the wrong side of the skirt, this can lead to excessive piston wear and fracturing of the piston and rings.

We've talked to a large number people that have "windowed" or "ventilated" their engines and there are some very significant similarities in nearly all the cases of engine failure. One curious similarity is that contrary to what you would expect, the engines would often fail while just cruising down the street and NOT while actually under extreme load, which is where you would expect. There are some very good reasons for this.

First, consider that the time it takes to combust a certain amount of fuel is relatively constant and for the sake of argument, let's say it takes 1ms. Also, we'll keep this extremely simple and assume a base SOI offset of 10º. Using the previous graph, at 2000 RPM the crankshaft will travel 12º in 1ms while at 3500 RPM the crankshaft will travel 21º at 1ms. In most of the files we've seen where engine failure has been observed, there is usually about a 3º difference in the SOI point with 2000 RPM being about 9 degrees of timing and 3500 RPM being about 12 degrees. Looking at these values, the 2000 RPM event will begin at 19º BTC and complete 12º later at 7º BTC. Conversely, the 3500 RPM event will begin at 22º BTC and complete 21º later at 1º BTC.

Again, this is an oversimplified example used to illustrate some of the basic concepts and issues that arise when tuning for injectors. It doesn't take into account variables such as timing offset caused by ICP changes and other similar factors. However, it should help clear up a lot of confusion. When you stand back and look at what is going on from a mathematical standpoint, everything becomes much clearer as to why these engines fail when they do.

We've been saying for years that timing kills these engines, but I guess nobody ever actually sat down and properly explained the reasons behind it. I hope this makes sense to everyone and not just me.

Take care.

[Cadash]

Excellent write up Bill!! Well, after reading that about a million times I think I got!! I will be giving you a holler sometime this week!! Thanks again!!