Larger injectors for more fuel.

Larger injectors for more fuel- Sounds like a great idea doesn't it? Car is running lean from running extra boost we will just install bigger injectors... Well the problem is the computer is stump dumb and has no idea you have installed larger injectors so you run pig rich all the time. Lets look at the way the system works and ways around this problem.

Chryslers use a speed density fuel injection system- speed (rpm) x density (map sensor input) The computer KNOWS the displacement of the engine because it has tables for how much air the engine should be using at any given rpm, map value and throttle opening. So what's this all mean? The computer KNOWS how much air the engine is using based on sensor inputs and tables and it KNOWS how large the injectors are and what pressure they operate at also because of tables in it's programming.  

So if you change the engine size (to a larger engine for example) the fuel mixture will be to lean because the computer has no way of telling the engine is larger! It's just not in it's programming. The computer can compensate a little for engine variations but it's a small percentage for fuel flow at best (less then 3-5% I'm told)  By the same token if you install larger injectors the computer doesn't know it, nor can it compensate very much either. Aggravating this of course is the fact that most of you are fooling the computer into not seeing over 14 psi of boost so the computer has no way of richening up the mixture over 14 psi even IF the computer had the right size injectors. 

So what's the trick? Well there is 3 or 4 ways....

The S-60 Computer.

The S-60 SMEC is the only 3-bar ECU that Chrysler ever produced AND it's calibrated already for 52 pph injectors and 3-bar map sensor- Such a deal right? Well its considered a race only part so it has no warrantee if you drop the bucks for one, it's calibrated for a 2.2 engine, it will only work in 88-89 SMEC based vehicles AND it has no O2 feedback.  Not to mention it's part of a package the includes a high flow head and turbo. Remember what I said about computers being dumb? Same thing here- If you install a S-60 computer, 52 pph injectors and 3-bar map sensor in a car with a stock engine it won't run right.

For some people it's the best way to go because it's fairly idiot proof installation and the map shut off is 21-22 psi which for most people is good enough. Some fine tuning can be done with a adjustable fuel pressure regulator for engine variations.

Want to read a detailed report about the S-60 package from the man who help design it? check this out.

Computers with non-Chrysler calibrations.

I have not seen but one or two really good 3 bar/52 pph injector calibrations for our cars.  

I would not go down this route. The factory spent lots of time and money making optimum & safe calibrations for these engines. I would not purchase a calibration from someone unless they had experience equal to the people at Chrysler.

Rising rate regulator.

Ahh the best! It works and it's fairly foolproof. So you can't reprogram the computer to do what you want so why not reprogram the injectors? Make the injectors act small when your in vacuum & low boost and BIG when you need REALLY need the fuel. This was covered on this page but if you missed it....

1) Install your 52 pph injectors (assuming your using the 52 pph units from Mopar Performance which is a screaming good deal at $150)

2) Install a adjustable fuel pressure regulator where the stocker was.

3) Install a Rising Rate Regulator as shown in the diagram.

Lower the fuel pressure using the a.f.p.r. to about 20-25 psi (stock is 45-50) Now when you start the car it will idle fairly normal and run fairly normal- look at this.

33 pph injectors at 55 psi = calculated flow of 28.05 pounds per hour.
52 pph injectors at 23 psi = calculated flow of 28.50 pounds per hour.

As far as the computer is concerned the injectors are the same size because it has no idea the injector has been changed nor that the fuel pressure is lowered! AND your fuel pump isn't working as hard at the lower pressure so it will last longer.

But Gary if the big injectors are acting like little injectors am I not right back where I started?  AHH HAH! that's Where the R.R.R. comes in! It's not like a normal FPR, instead of rising the fuel pressure 1 pound for every 1 pound of boost it can rise it TWO pounds or even THREE or FOUR pounds of fuel pressure for every pound of boost! 

So at 20 psi of boost the rising rate regulator will have taken over the fuel pressure, lets assume we now have 55 psi in the fuel rail...

33 pph injectors with 55 psi of fuel at injector pintle are flowing =  28.05 pph
52 pph injectors with 55 psi of fuel at injector pintle are flowing =  44.05 pph

Jeff Chojnacki has a great program for figuring all this stuff out, it's on my downloads page.

You now have enough fuel to support the boost your running AND the RRR is adjustable so you can control the rate of gain (that is how many pounds of fuel pressure you get vs. psi of boost) It's not the cheapest setup out there but it is pretty darn foolproof and flexible from a tuning standpoint. 

Gus Mahon has a great page covering this as well- head on over!

2.5 engine with 2.2 computer and 42 pph injectors.

Ahh the one special case!  Mopar Performance sells a 42 pph injector- It flows exactly 20% more then a stock 33 pph injector used in the 2.5 engine.  Now from what I understand the 2.5 is 11% larger in displacement then a 2.2 BUT it's fuel curve is 17% larger... so what does all this mean? If you install a Turbo II 2.2 computer on a 2.5 engine the calibration will be 17% lean... unless you install a the 20% larger 42 pph injectors at the same time! This is the setup I ran on my K-car and it works pretty well and the extra 3% of fuel will get you up to 16-17 psi.  Some cars seem to need a little tweaking with the fuel pressure and others have no problem at all. This of course lets you swap a 2.5 into a car like a GLHS that never had a 2.5 computer available for the application from the factory... you get the idea..

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