You say there is no 5.0V fuel cut?
I'm not sure I agree with that. I have seen evidence that the injectors do shut right down when MAF=~5V. I've seen A/F maps when using 5th injectors without a 5V MAF signal hold where the A/F ratio takes a dump at ~4000-5000rpm no reason. We believe it was the stock injectors shutting down.
The guys at DiabloSport told me that the reason is at 5.0V MAF signal, the computer switches over to a 1 dimensional fuel map. The table just looks like 5.0V=X injector duty cycle. There is no rpm variation.
And it just so happens the X value is 0. They were supposedly able to run some turbo engines past 5.0V simply by putting a value in that location. You lose all rpm variability, but at least you're getting fuel.
Chris F posted a dyno graph a while back from the 5th injector setup with A/F tracking.
I believe that at say 4.1V, the A/F was about 12 or 13:1. When the 5th injector starts firing, it jumps to like 11:1. It holds around there as rpm and airflow climb, but when the MAF signal gets pegged at 4.95V, the A/F starts to rise as the rpm rises and the airflow goes up with no increase in fuel. At peak HP, the ratio is like 12.5:1.
The injector is a compromise in that you have to have it large enought to supply enough fuel on top of the stock system to hit 12.5:1 at make HP, but not too big that it drowns the motor when it first kicks in.
One very interesting part of the whole thing is that the engine needs it's richest setting at maximum cylinder fill: Peak torque. This is roughly when the 5th injector has the highest A/F ratio.
Some people bag on the 5th injector for no reason, but it actually works quite well. It's a bodge, but it works well at lower power levels like the AC.
I wouldn't want to do it with Tom's setup. But for 200fwhp, for $200, it can't be beat.
The only reason I don't want it is because I do a lot of lapping, and I can't have my injectors at 100% for 20 minutes...
You talk about the disadvantages of 5th injectors vs. varying MAF, but it works fine for lower power levels.
At 3000rpm, the turbo is making full boost, and the computer is happily doing the job just fine, holding at around 12.5:1.
As the rpms pass 3500rpm, the engine is approaching it's peak torque, which means peak volumetric efficiency, peak cylinder fill, and peak combustion chamber pressure. This is when it needs a rich A/F ratio the most. This is when the 5th injector kicks in. The ratio drops to 11:1, the engine is happily getting a nice fat fuel supply to avoid detonation.
As the RPMs pass 4500rpm, the SFMS holds the MAF voltage at 4.95V. So as RPM goes up, airflow goes up, fuel remains constant, so the A/F goes back up. But the volumetric efficiency is also dropping, so cylinder pressures are going down, so fuel requirement is going down. By 5300rpm, the A/F ratio is about 12.5:1. Which is fine.
Past 5300rpm, the airflow actualy starts to DROP. (contrary to what some people think). Past this point, A/F ratio is going down again.
And the proof is in the pudding. Some have spent thousands of dollars on stand alone engine management systems and dyno time, and gotten no increase in peak hp, just some mid-range torque.
The only thing wrong with the SFMS for 200fwhp, is for those of us who are lapping.
I've actually thought about just working with him on a custom SFMS that holds the voltage at 4V or something like that, to keep the main injectors at 80%. Then, use a variable rate 5th injector.
Are you planning on developing software for computer codes other than JFA8? I have a 2001, and if I'm looking at $600 for a chip, $200 for a MAF, and $??? for a JFA8 computer... then a stand alone engine management system is getting more and more attractive.
