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

 

I agree 100000%.... The Crossfire is my first crack at a S/C car. I have had 2 previous modified turbo cars and rising rate fuel pressure is a must in high boost applications. I have asked the question if the stock fuel pressure system is 1:1 rising rate and it looks like you have finally given me an answer. The great thing about our fuel system is that there is a test port at the end of the rail, a perfect place to add rising rate regulator and reroute fuel return. Or inline before the rail for that matter as well.

 

I will go one better, have done the TUNE tongue in cheek without any parts?????????????????? Thats right I used a piece of hose, though the routing is still up in the air.Woody

 

EDIT~ Nevermind, re-read the end of your original post

 

Sounds great but what do you have done to reach 20psi with the 178?

 

You raised the fuel pressure without buying any parts? Do tell.

 

I get map codes all the time with my 181mm.... If the conditions are right, I can see where 20psi at the top of 4th is possible with a 178

 

SRTpowa I have the dual Needswings intake, clipped secondary cats, and gasket matched passages on the intake - head. Freer breathing intake.

I have gotten 20 psig only on the lift, 10 -12 on the road. Still have to check setup to see why may be pump capacity, in process.

Woody Be happy

 

Damn now this is interesting!!

 

This could be the breakthrough us 185 guys need to take advantage of the extra boost too. Good job as always woody.

 

Quote:
Originally Posted by waldig
I will go one better, have done the TUNE tongue in cheek without any parts?????????????????? Thats right I used a piece of hose, though the routing is still up in the air.Woody

Does this mean the stock regulator will compensate for boost?

Nevermind, I just looked at it, and it does not. pressure is regulated by quantity returned to tank, via a diaphram.

 

Correct, but adding a rising rate regulator should be fairly easy. When I get my car out of storage for the winter I will tackle this project. Im sure waldig will be done way before me but Ill post my steps as well.

 

James at InMotion actually mentioned this solution during a phone call with him. Woody thanks for putting this into action. Now how are you only getting 10-12psi with the 178 on the road? Maybe it could be because no tune? I get 15-18 psi at WOT with mine. James further mentioned that you could really personalize a tune to your liking with an adjustable regulator inline. With datalogging you can make incremental adjustments as needed. Looks like woody gets the woody award once again.

 

Iam getting up to 20 PSIG boost, 10 - 12 is the gain in fuel pressure, 60 to 72 or so which is not to my liking as I want 80 psig like I got on the lift. In process

Here is some of the backup info that I used for reference, it is GOOD READING

http://www.stealth316.com/2-fpr_upgrade.htm

Enjoy, WOODY

 

Another thought-provoking thread from waldig. Who’d have thunk it?;-) Valid points all, IMHO.

It’s my (frequently faulty) understanding that our fuel supply system is a returnless or deadhead type, and that the fuel rail’s pressure is regulated by the fuel filter’s internal regulator. If the pump is able to supply sufficient pressure – and its commensurate volume during peak load/RPM/boost – from where are the gains you speak of being derived?

Would you kindly enlighten us with the specifics of your “dynamic compensation” arrangement? TIA.

 

If this is the case (and I think it is) adding a returnline is fairly simple. SRT-4 Neon guys (including myself) do it all the time. Adding an AFPR to the test port on the rail and running a braided line back into the tank or filler pipe is all that is needed. Plus modifying the stock regulator to push continues pressure.

EDIT~ Screw it, Ill go play with the car next weekend and see what I can do for an AFPR.

 

Quote:

Originally Posted by ImportLabSRT

If this is the case (and I think it is) adding a returnline is fairly simple. SRT-4 Neon guys (including myself) do it all the time. Adding an AFPR to the test port on the rail and running a braided line back into the tank or filler pipe is all that is needed. Plus modifying the stock regulator to push continues pressure.

Yep. I understand about adding a return line. Unless one does away with the OE filter on the supply side, the fuel rail will still only reach its regulated maximum pressure.

Perhaps there’s more to this modification than has yet been shared…

 

Fuel pressure can be regulated after or before the rail. Look at it this way.... Fuel, like air, is lazy. It will travel the path of least resistance. Most vehicle adjust post rail if a return system, where as returnless system use fuel pump modulation on a diaphragm type filter to change pressure. Im just 100% sure what out system is from the factory. Obviously if its a return system its easier to modify.

 

OK, I read all of that and still don't have a clue. Someone wanna give me the "for dummies" version?

 

The idea is to increase fuel rail pressure, as boost pressure on intake increases. To give the injectors more fuel. thus keeping fuel/air ratio inline