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Mixture Modifier

Enhancing your turbo car's top-end mixtures the easy way...

By Michael Knowling

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This article was first published in 2004.

Have you added a few basic breathing mods to your late-model turbo car? Does it now run bulk rich - perhaps even blowing black smoke out the exhaust while under load? Do you realise this excess richness is robbing power?

If the answer is "yes" we've got some important news for you.

Instead of making expensive engine management changes to achieve leaner high-load mixtures, there's now the option of the Fuel Controller from Adelaide's RPM Performance Centre. Costing a relatively cheap AUS$220 it shapes up as a very attractive proposition...

A Close Look at the Fuel Controller

The Fuel Controller is designed to lean high-load mixtures on any EFI turbo car equipped with a fuel return line leading back into the tank. This includes Nissan 180SXs/Silvias, WRXs, Lancer GSRs and most other popular go-fast turbo machines.

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The Fuel Controller must be plumbed into the hose connecting the intake manifold to the fuel pressure regulator. The purpose of this hose is to send the manifold pressure signal to the fuel pressure regulator, the regulator reacting to this pressure by varying fuel rail pressure so that the same 'headroom' over manifold pressure is always maintained.

Once installed, the device bleeds (to atmosphere) a portion of the boost pressure sent to the fuel pressure regulator. The less boost the regulator sees, the less is the fuel rail pressure. This equates to a leaner mixture for a given injector pulse-width.

And what happens to the fuel that would otherwise pass through the injectors? It's returned to the fuel tank.

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Here are the internals of the Fuel Controller. As you can see, the bleed passage is equipped with two adjustable valves - the check valve and the flow control valve.

The check valve - the first in series - lets you set the boost pressure at which the Fuel Controller starts taking effect and, therefore, leaning mixtures. This activation point can be adjusted from around 2 to 10 psi boost.

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The check valve system comprises a steel ball that's forced down onto a seat by a coil spring. Spring preload is externally adjusted to vary the boost pressure required to lift the ball off its seat. Once this occurs, boost can escape through a passage to the second adjustment mechanism - the flow control valve.

The flow control valve lets you vary the amount of boost pressure bled from the fuel pressure regulator signal. Enough pressure can be bled to achieve high-load mixtures up to as lean as 13.0:1.

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Note that, despite serving different purposes, the operation of the check valve and flow control valve influence one another.

Tim from RPM says there have been instances when the flow control valve is fully opened and there's still not enough boost being bled from the fuel pressure regulator. The answer is to release some preload on the check valve spring. In addition to altering the activation pressure, this serves to increase the volume of air making its way to the flow control valve. This enables more boost pressure to be bled from the fuel pressure regulator line.

Click for larger image

Finally - in addition to the two adjustment mechanisms - the Fuel Controller contains a fixed airflow restriction inside both of its in-line ports. These restrictions allow an adequate amount of boost to be bled.

Note that the restrictions inside this particular Fuel Controller were only 0.35mm in diameter. This restrictor size apparently suits most Nissans, but Subaru WRXs are said to go very lean due to the transient pressure drop across such small orifices. Larger diameter restrictions (0.5mm) are available to suit these vehicles.

To overcome this awkward situation, however, Tim says a variable restriction system will soon be incorporated into one of the in-line ports.

The Fuel Controller in Practice

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Our testing of the Fuel Controller was performed on this imported R33 Nissan Skyline GTS25t.

Sporting a 3-inch cat-back exhaust, pod filter, front-mount intercooler, oil cooler and 70 kPa boost ceiling, it's a typical example of a vehicle in need of some mixture enleanment. As Tim says, "a lightly modified GTS25t runs mega-rich through the top-end."

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Using its standard fuel system, the R33 generated a steady 172kW at the back wheels (as measured on RPM's Dyno Dynamics chassis dyno). The red plot shows power delivery is quite linear, except for the drop-off at an indicated 125 km/h (in third gear).

Boost pressure maxed at 70 kPa, while fuel rail pressure reached 340 kPa. Air-fuel ratios hovered surprisingly lean for a modified R33 - about 11.3:1 all the way from mid-range rpm to redline.

Next came the fitment of the Fuel Controller.

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Installing the device is a 2-minute exercise; the ¼-inch hose connecting the intake manifold to the pressure regulator is cut and the Fuel Controller is inserted in-line (there's no specified flow direction).

The attractive anodised alloy body of the Controller can be mounted using the small hole through its top section, but we didn't bother for the sake of our test.

Now comes the delicate task of adjusting the Fuel Controller to give the desired air-fuel ratios. Note - whoever you get to perform the necessary adjustment, make sure they realise how easily engine damage can occur if they're not careful!

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Tim begins the process with the Fuel Controller's two adjustments nearly fully tightened (ie closed). This serves to produce a very small mixture variation seen only at high boost; a good - safe - place to start.

Next, a dyno run is performed with close attention payed to mixtures; this gives a guide to how much more adjustment is needed. Depending on the amount of enleanment tolerable, the Fuel Controller adjustment mechanisms are then individually altered in search of the maximum power output. A dyno run follows each adjustment.

It's very much a suck-it-and-see exercise.

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This graph shows the sort of benefit you can expect from a carefully adjusted Fuel Controller - the pink plot reveals that the test R33 gave a definite improvement beyond mid-range rpm. Up to 6 percent extra power is released, though peak power is up only around 3 percent. A 3 percent gain is a little disappointing, but this is partly due to the relatively lean mixtures that this particular vehicle already had prior to Fuel Controller fitment.

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And how were the all-important air-fuel ratios affected? Well, adjusted to give the aforementioned power gain, we saw an average mixture of 12.2:1 (compared to 11.3:1 in standard form). Fuel rail pressure was as high as 300 kPa (down from 340 kPa in standard form).

Leaner Still?

For interest's sake, we got Tim to adjust the Fuel Controller to bleed the maximum amount of pressure from the fuel pressure regulator hose.

So adjusted, the R33's air-fuel ratios leaned out to 12.6:1 and fuel rail pressure peaked at 280 kPa. Quite a difference over the 11.3:1 starting point.

Note, however, power output went backward with these maximum lean settings.

At light engine loads, the Fuel Controller has no effect on fuel rail pressure and, therefore, mixtures. This is because the check valve is effectively a one-way valve - only positive (boost) pressure can force it off its seat and allow air to be bled off.

Furthermore - even if the Controller did have some effect on light-load fuel pressure - the engine management's closed-loop oxygen sensor feedback would adjust the injector duty-cycle to maintain standard mixtures anyway.


RPM's Fuel Controller certainly does its job.

If you want to lean your high-load mixtures, this is one of the cheapest off-the-shelf ways to do it - it's certainly a lot cheaper than making any engine management changes.

Note that the relatively small power gain shown in our example shouldn't distract you from the potential benefits of the device. If your car is running rich enough that there's a lot of power to be unlocked by leaning mixtures, the Fuel Controller can certainly do it.

For AUS$220 you can't go wrong.


RPM Performance Centre
+61 8 8277 2266

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