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Project 420a Sleeper, Part 5

So it's time to start preparing the sleeper for its destiny with the turbo gods. What we need to focus on first is the most crucial piece of the puzzle, the fuel supply. It's really easy to just slap a turbo onto your engine and to shove tons of air down the throat of your 420A. Having the additional air is only half of the equation and too many people focus on the turbo setup without taking the same time and consideration when designing the fuel system to keep up with the demands presented by the turbo system.

I will explain how the most basic fuel system works and try to point out the advantages and disadvantages to this design.

Stage 1 Fuel System



The simplest way to add the additional fuel required is with a FMU. An FMU is basically a second FPR(fuel pressure regulator) that is installed between the stock FPR and the return fuel line heading back to the tank. As boost pressure increases, the fuel pressure will increase above the set limit of the factory FPR at an increment determined by the size of the FMU you are using. FMU's generally are available in sizes ranging from 12:1, 10:1, 8:1, 7:1, 4:1, and 3:1. The most commonly suggested size FMU to use on our car when using the stock FPR and fuel injectors would be the 12:1 ratio unit (this isn't necessarily the best choice, read on). What this means is that for every 1 lb. of boost pressure, the fuel pressure will rise by 12psi above the static fuel pressure set by the stock regulator.

With the stock FPR factory set to roughly 55psi static pressure (0psi of boost present) and our system running at a maximum of 8psi of boost, that would top out our fuel system at:

55psi + (12 x 8lbs. of boost) = 151psi!! Yikes!

In reality, it generally doesn't reach this high of a level due to pressure losses from the stock fuel injectors having to be opened for such long durations and the fuel pressure limits of the fuel pump. Most people have seen good results with a maximum fuel pressure of around 105-110psi while running 8-9lbs. of boost on an otherwise stock setup. With that being said, why don't people use a 7:1 FMU instead?

55psi + (7 x 8lbs. of boost) = 111psi

This seems perfect so why does everyone suggest the 12:1 FMU? There are a few reasons why. The biggest reason is simply for extra insurance. Most FMU manufacturers will suggest this size because they honestly have no idea how efficiently your engine responds to forced induction and there are too many variables to consider with each customers turbo setup. All in all, this would be a safe choice to suggest to the general public. It's better to have too much fuel when running a turbo than it is to run too little. The problem with the 12:1 FMU is that it isn't really regulating the fuel pressure if you are running more than 5lbs. of boost, its simply closing off the return line to the fuel tank. With just about any aftermarket high pressure fuel pump, 100-110psi is the maximum fuel pressure it can deliver while still delivering any significant volume of fuel. Exceed this level and you are simply plugging the return line and reducing the volume of fuel being delivered, not to mention you are stressing the HECK out of your fuel pump!

The "Proper" Size FMU

Okay, this is going to be a bit lengthy because I am about to explain this more thoroughly than I have seen anyone else do, I apologize in advance. After talking for countless hours with FMU manufacturers and Turbo kit manufacturers for the non-turbo Ecipses/Talons, I have finally got down to the truth about what size FMU is correct for our application.

The great thing about our car is there has been quite a bit of research and development by major turbo kit companies, like Hahn Racecraft and Star Performance. Thanks to them, we have a general idea of how efficiently our engines respond to a turbo setup. Not only that, but we also have a target range to start with for our maximum fuel pressures. Granted, the size and efficiency of the turbo comes into play when fuel pressures are involved but most people decide to use a turbo that produces roughly the same CFM range as the turbos supplied in the Hahn and Star turbo kits. If you are going with a much larger turbo than they offer.... I sure hope you aren't using this type of fuel system!

Back to the FMU situation. Like I mentioned earlier, at 8-9lbs. of boost on an otherwise stock engine you would want to achieve a maximum fuel pressure of roughly 105-110psi. This should provide a slightly rich fuel mixture, which is good to prevent detonation (also the number one killer of most turbo engines). When choosing an FMU you always want to aim a bit higher than your target fuel pressure. If running 8lbs. of boost you would try to achieve 110psi so lets figure out what size FMU we would need:

110psi - 55psi(maximum static fuel pressure of the stock FPR) = 55psi

So we need the fuel pressure to increase 55psi from 0lbs. of boost
(static Fuel Pressure)
to our maximum boost pressure of 8psi.

Now divide the difference by the maximum boost pressure.

55psi / 8lbs. of boost = 6.875

So you need to increase the fuel pressure by 6.875psi for every 1lb. of boost pressure. This is pretty darn close to a 7:1 ratio. Since each vehicle is different, there are always slight variances in the maximum fuel pressures seen. With that in mind, I would suggest the use of an 8:1 FMU when running 8-9lbs. of boost. This would give you a maximum fuel pressure of roughly:

55psi + (8 x 8lbs. of boost) = 119psi

Air Bleed Valve


Vortech Engineering offers an air bleed valve HERE which you install inline with the vacuum source to the FMU which allows you reduce your maximum fuel pressure down a full 2:1 ratio. This will give you the ability to adjust your FMU range anywhere from 8:1 down to 6:1 so you can dial in your maximum fuel pressure for the best possible air/fuel ratio.

For people who think that this is too low of a ratio, just look at all of the users who run the Vortech S-FMU unit. The maximum ratio it is capable of is 7:1 and there are countless success stories of people who have used this setup for many, many years with the stock injectors. People have recommended the 12:1 FMU because that is what some tech support guy told their friend, who told their friend, who told their friend to use. Do you want the universal answer or the one that pertains to your car? Hmm?

For anyone choosing to run only 5-6lbs. of boost, thanks to Hahn Racecraft we know that a maximum fuel pressure of 85-90psi is generally needed. Using the calculations above we can see that we'd need a FMU ratio of 5.83:1, roughly a 6:1 ratio. In this instance I recommend the 7:1 FMU with the use of the air bleed valve to dial in your maximum fuel pressure.

I always recommend having your car tuned by professionals on a chassis dyno for a safe air/fuel ratio of 11.0:1 to 11.5:1 while turboing the stock engine

Fuel Pump

Pump Installation:

Where to buy?:

The stock fuel pump wasn't designed to ever exceed 70-80psi of fuel pressure or deliver the volume of fuel that is necessary so an upgrade is in order. The fuel pump is located inside the fuel tank which can be accessed simply by removing the rear seat cushion and removing a service plate. Since the fuel pressures required are so high, not just any aftermarket fuel pump will do. The most widely recommended fuel pump used by 2G owners has to be the WALBRO 255lph(liters-per-hour) pump assembly. Walbro makes 2 models that are a direct replacement for our cars, one being the standard 255lph model and the second being a 'high pressure' version. Both flow exceptionally well but the standard model can't sustain the fuel flow needed at such a high fuel pressure we are required to have with the FMU. The 'high pressure' model (part # GSS-342) is up to the task, though we are still just squeaking by with our fuel flow requirements at 8lbs. of boost. Honestly, the high pressure model is the better candidate no matter what fuel system you plan on going with so I recommend the high pressure model no matter what setup you decide to use.

The higher pressures the fuel pump has to achieve, the less VOLUME of fuel it can deliver. The main reason you see people setting their turbo kits and fuel systems to a maximum of 110psi has more to do with the fact that 110psi is the fuel pressure where the amount of fuel flow begins to decrease dramatically. At roughly 275-300hp, the fuel pump would need to deliver roughly between 18.8-
21GPH (Gallons Per Hour) of fuel. If you look at the chart above, you will see that you are just BARELY shy of producing the proper amount of fuel. Its a little too close for comfort for most people so they opt to rewire the fuel pump to supply a more consistent, higher voltage to the fuel pump.

The chart above is at the factory rating of 12 volts and the pumps are rated at a fuel pressure of 43psi. As the voltage supply to the fuel pump increases, so does its flow capabilities.

Here is an example of how much can be gained by increasing the voltage supply to a fuel pump. The factory wiring which powers the stock fuel pump is sufficient for a stock engine, but it isn't up to the task of powering the new Walbro fuel pump.

The simple solution is to rewire the fuel pump like is shown

or HERE:

My only suggestion is
Soldering directly to a relay causes far too much heat to enter the super thin coil wires inside which can be damaged and ruin the fuel pump over time.

The best bet is to get a RELAY SOCKET from Radio Shack and solder all connections to the wiring on the socket. This generally will increase the fuel pump voltage from 11-12 volts to 13.5-14.4 volts! At these voltages, you will have just enough fuel for the FMU setup.

Fuel Injectors

The next risk is with the fuel injectors. Again, with such high fuel pressures present, the fuel injectors simply cannot maintain the same spray pattern as they can at the factory suggested fuel pressures. Not only does the spray pattern suffer but the injector can be forced to stay open for longer durations (duty cycle). A fuel injector is designed to run safely up to an 80% duty cycle. After you exceed a duty cycle of 80% you run the risk of the fuel injector sticking open which would basically pouring gobs of fuel into the engine continuously.

We tested the actual duty cycle of the stock injectors under the conditions presented by the stock system [i]
[/i] the FMU system and found that in both cases the maximum duty cycle recorded was between 78-80%. The ECU is designed to never exceed this limit (to prevent the possibility of injector failure). Just because we are adding more air into the engine with the turbo kit doesn't mean the ECU will keep trying to add fuel. The ECU has NO CLUE there is more air! The 'MAP Sensor' and 'Throttle Position Sensor' are the 2 primary signal sources which the ECU uses to calculate how much fuel is required when driving WOT(wide open throttle). Once the MAP sensor reaches 0lbs. of pressure.... its maxed out, and since the gas pedal is to the floor its maxed out too. As far as the ECU is concerned, you just floored a completely stock engine.

The ECU is in open loop mode (oxygen sensor is not used and the air/fuel ratio is controlled solely by a fuel map built into the ECU) once you exceed 80% throttle position or a calculated load amount derived from multiple sensor readings. While in open loop mode, the ECU is set to run a rather "safe", rich air/fuel mixture mainly to prevent thousands of warranty issues from blown engines that were running too lean. Since they know its running overly rich at 80% duty cycle, there was no need to have any sensors monitor the air intake to increase the duty cycle any further.

With that all said, you do not run the risk of your stock injectors getting stuck open during operation. I definitely suggest replacing your stock injectors with new injectors or having yours cleaned,blueprinted and balanced by a reputable shop like RC Engineering. Cleaning services usually run about $100 for a set of 4 injectors and the price includes new o-rings, a detailed before/after data sheet and full ultra-sonic cleaning of your fuel injectors so they literally look and perform better than new.

Things to Know

There are risks to using this setup which should be taken into consideration. With fuel pressure exceeding 100psi, things begin to fail. The first item to fail would be the incorrect choice of rubber fuel lines when installing the FMU. Standard rubber fuel line cannot safely support these pressures so you must use rubber fuel hose specifically used for FUEL INJECTION systems. It will be labeled directly on the side of the hose so if you don't see it, its probably not what you need. The most popular and safest bet is to use flexible stainless steel braided fuel lines and fittings. Its rather inexpensive and looks and performs far better than the standard rubber hoses.

Stage One Summary

All in all, the Stage 1 system does a fair job of adding the additional fuel needed. As long as you do not exceed 8lbs. of boost in this setup, it does indeed work. What the system lacks is overall control of the fuel system, which the other fuel stages address.


Simple Installation
Low Cost


Extremely limited fine-tuning adjustments
Taxes the fuel injectors
Insanely high fuel pressures
Robs potential horsepower

That's it for now! I will try and update and create more articles down the road for more fuel setups.

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Document statistics: Last modified on 2009-03-11 12:54:00 by adamzty

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