Thread: Yes, I do feel like a lightning rod - MY OWN TUNE.
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Old Jan 11, 2009 | 11:04 AM
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waldig
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From: VA
Talking Yes, I do feel like a lightning rod - MY OWN TUNE.
How Iam tuning my engine fuel system, unraveling the mystery.

I cause a lot of my own "attention plus and Minus" but as an engineer I have the need to question the status quo rather than just accept the standard dogma. Having addressed the inner child's need for acceptance let me get to it.

I have considered the tune for my car and have systematically been gathering data. The data logger I got proved that I was running lean above a boost pressure of 18 PSIG. This was independent of the RPM and that’s a critical finding. This what caused me to resist drinking the convention cool aide, why was the fuel mixture lean?

The answer is a bit complex. As the s/c induces additional oxygen into the engine with the higher boost levels, the fuel required to accomplish this increases. This is the obvious effect that most people would point out.

It took me a good bit of time to recognize the second and more important effect that was leaning the motor under boost. The fuel system was not able to compensate for the boost and above a level like my 18 PSIG boost became inadequate, again WHY.

Having auto crossed a 4 cylinder Daytona with a super sixty turbo doing 18-psig boost; in the back of my mind (what’s left anyway) I knew that it had fuel pressure modulation. The fuel pressure at the injectors was adjusted by the boost level, rising 1PSIG of fuel pressure to 1 PSIG of boost. I really should have been quicker on the uptake of this concept as boost is boost be it turbo or super charger. I finally parked the Daytona because a standard transmission, front wheel drive and a turboed 4 cylinder just could not out run a Mustang GT that I was competing with.

The physics of the deal is that the fuel delivered by an injector is controlled by several factors. These include the width of the pulse driving the pintal to open, the fuel pressure behind the injector and TAAAADAAA the boost.

We have a boost in the cylinder during the injection portion of the intake cycle. That’s obvious but what is at work here is that the BOOST subtracts from the fuel pressure. That’s right, the injector has fuel pressure on one side and boost pushing against it on the other end. As the boost increases; the fuel flow for a given pulse width DECREASES with increasing boost. At an impossible condition of 60 PSIG fuel and 60 PSIG boost there would be no pressure for fuel to flow out of the injector. . . . . . . TAA DAAAaa.

Numbers: What does it all mean. I have gotten a bevy of data points and here are a few, though you guys will ask a good question that will remind me of another. At Idle [ This is my car readings ] the fuel line runs at 62 psig. Under boost it is regulated and pretty much stays at 60psig. If you figure that at 60 psig the fuel delivers some volume, lets call it 1.00 delivery ( 100% ). If I change the pressure the new delivery is a function of the square root of the new pressure divided by the original (60 psig). As an example if I set the new pressure to 120 psig or twice the fuel pressure; 120 / 60 is 2 and the fuel delivery would increase to 1.414 or the square root of 2. That is too drastic to be realistic but gets you thinking along the right path.

In our cars (mine ) Iam getting to 20 PSIG of boost with my set up and the 178 pulley. The effective pressure is now, 60 – 20 which is 40 psig. The square root of 40 / 60 is about 0.816 or 81.6 % of the fuel without boost. This may not seem like much but the fuel deliver down by over 18% lower due to the boost “back pressure effects”. Remember the effective fuel pressure is DOWN 1/3……Aarguu

The higher rpm allows less and less TIME for the injector to stay open. This is occurring at the point that the engine is asking for more fuel. The combination starves the engine of the fuel at some point and the power drops accordingly.
Turbo cars have a boost compensator that regulates the fuel to some value and INCREASES the fuel line pressure as the boost rises. This eliminates the reduced delivery of the injector under boost, as the pressure ACROSS the injector is held constant. Our fuel pressure is held constant as boost rises and thus we need to go to other means to adjust the fuel deliver.

The classical TUNE adjusts the map that controls the timing of the injector against the RPM. This is not aware of the boost level and unless you are under full boost the fuel delivered may be TOO much depending on the non-adjustable map value. That is less than optimum and less power overall. What is needed is a fuel pressure regulation system that is BOOST COMPENSATED.

Fuel pressure regulators are commonplace and at Summit for example a Magnafuel MP9925-B has the needed range of operation of 85 psig. This could regulate in place of the factory unit to give us up to 80+ psig at the fuel rail. That would be 60 psig plus about 20 additional psig to compensate the 20 PSIG of boost. Now as boost increases, the fuel pressure is compensated to the actual boost level at that particular instant. I call that DYNAMIC compensation.

Better yet it works as I just got back from testing it. My fuel increased to 70 psig with 20 PSIG of boost. Not ideal but I got it to work and my car ran like never before. There is work to do, as I have to understand why my simulated operation on the lift gave me a full 80 psig. The deal is that I was able to run wide open without drooping or lean missing. The NON-data logger f/a gauge (in car) was indicating 12.8 at that point.

I will get better information as to the fine-tuning, but it was kick **** for the first time.

Woody
 
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