Justin Sane Twin Turbos a GMC 2500 HD with 6.0 LS Part 2

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We move on to modifying the fuel system to support a turbocharged 700-800 HP engine.

The Justin Sane Racing Products Parts Truck , as we’ve already learned (Part I) came equipped with a 6-liter LS engine backed by a 4L80E automatic transmission. Craig decided to install a twin set of his company’s turbos on it, so it needs some modifications to the fuel delivery system in order to support the power those turbos are going to help produce. We’re going to follow along here as Craig does the work to show us how you can make these modifications at home.

The first thing we’re going to look at is how he converted the stock OEM fuel pump to a dual fuel pump system. Next we’ll take a look at how he converted from the OEM regulator-in-the-tank setup (return-less fuel system) to a return-type system with the regulator on the fuel rail. Since he used a shaved truck manifold with no vacuum ports, we’ll also take a look at how he created his own vacuum ports for the regulator and wastegates and wired in the new sensors required for the turbochargers.

Fuel System Modifications Start with Removing the Truck’s Bed

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For easier access to the fuel system,Craig removes the truck’s bed.

Before we get into this, I’ve got to point out that if you aren’t making these upgrades to a truck, you can’t remove the bed to get at the top of the fuel tank-you’ll have to drop the tank, or maybe access it from a cutout hatch in the trunk. The truck’s bed is removed by removing the 4-6 mount bolts and then unplugging the lighting wiring harness.  Gain access to the top of your fuel tank in whatever way your vehicle allows.

No matter how you get at the top of the tank, you want to take a light wire brush to the top and brush and then vacuum off all loose dirt and grime to keep it out of the tank when you pull the pump assembly out. Also no matter how you get there, you need to be sure to disconnect the fill and overflow hoses before either lifting off the bed or lowering the tank.

Removing the Fuel Pump Assembly Cover and Fuel Pump Assembly

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This is the OEM pump and regulator assembly.


Squeeze the tabs on the two electrical connections (the fuel tank MAF sensor and the pump/gauge power connector) and wiggle them lose. Next, use two small screwdrivers or picks to pry the locking tab on the fuel line up and the pry the tabs locking the lines from the charcoal canister to the fittings, wiggling the connectors while pulling to remove them.

Craig uses a screwdriver to rotate the retainer ring that locks the pump assembly in place. I don’t really recommend this; I prefer to use a brass punch because brass won’t cause sparks if it slips. Start off with light taps against the ring in a counterclockwise direction and build up the force you use. Whaling on it right off is going to bend the locking tabs and make the lock ring worthless. Use only enough force to work the ring around enough to unlock it.

Run the vacuum and wire brush around the top of the pump assembly one more time to be sure you’ve removed everything that can drop into the tank and contaminate the fuel.  Once you’ve done that and got the locking ring unlocked, pull the pump basket assembly out.

Installing Twin Walbro Fuel Pumps to Keep Up with Fuel Demand

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The new twin Walbro fuel pumps almost ready to reinstall.

The upper and lower basket portions are separated by unscrewing the bottom of the posts from the lower basket, giving you access to the pump/regulator assembly. This assembly is removed. Craig also removed the evap system check valve, but I don’t recommend doing this because doing so could make it so your vehicle doesn’t pass smog. Craig removed the OEM pump and regulator assembly and used two three to five-inch worm clamps to secure the new Walbro 400 pumps into the fuel pump basket. He then used a drill at low speed to open the hole in the bottom of the basket to assure adequate fuel flow to the pumps.

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Craig couldn’t find a brass “T” with two 3/8-inch inlets and a 1/2-inch outlet, so he made one.

Craig used a stepper bit to carefully drill out the supply line fitting from the top of the assembly to upgrade to ½-inch fuel line from the 3/8 line that was used. Craig bought a ½-inch brass “T” fitting and two 3/8-inch barbed to threaded fittings. You should be able to find a brass “T” with two 3/8-inch barbed hose fittings and a ½-inch barbed fitting somewhere like Lowe’s or Summit Racing so you don’t have to buy the extra adapters, cut the barbed nipples off ,and braise them to the ½-inch  fitting on the “T.”

Once the 3/8-inch lines from the pumps were teed and the ½-inch fuel line was fed through the new hole in the cap, Craig soldered the power leads together and pinned the ends of the wires to the OEM connector to connect to power. Once this was done, the pump assembly was put back into the tank and the locking ring secured. Craig ran the ½-inch supply to the front of the truck along with a 3/8-inch line for the return from the rail-mounted regulator. He then secured one of his JS Racing fuel filters to the bottom of the charcoal canister shelf and, using barbed to 8-AN adapters, spliced the fuel filter into the supply line. You can use any high-capacity fuel filter you want.

Converting from Return-Less to Return-Type Fuel Pressure Regulation

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The 99-00 fuel rail assembly installed on the shaved truck intake.

We’re installing twin turbos on this 2004 GM truck, so we need to get rid of the stock return-less fuel system. We’ve already modified the tank end of the system, so now we need to modify the fuel rail side. Craig went with a return-type fuel rail from a 99-00 Chevy with an LS engine and “smoothed” LS-type truck intake. This intake required some extra work that yours might now require because all the vacuum ports were smoothed over and the regulator and wastegates need manifold vacuum to work.

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Craig’s shaved intake manifold has no vacuum ports, so he created his own.

On the back left side of the intake as you face the front (See image above), there’s a sealed vacuum port. Use a drill bit to carefully drill the plug out. Craig then press-fit a ¼-inch brass vacuum “T.” One side of this is used to send vacuum to the regulator, while the other side is teed again to supply vacuum to both wastegates. Obviously, when installing a single turbo only one wastegate will need to be supplied with vacuum.

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Craig’s Siemens-Decca injectors required longer bolts. Make sure you get the right size and pitch, as well as the correct length.

The Siemens/Decca injectors that Craig used were taller than the stock Delphi injectors, so he had to head over to the local hardware store and pick up a package of spacers which he then cut down to the correct height. These were inserted between the intake and fuel rail clamps. He also picked up a package of Grade 8 bolts in the correct size/pitch (M6-1.00) and length ( As you can see in the image above, he used 30 mm, but you should measure to be sure.).

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The gap caused by the new and taller injectors.

Craig then installed adapters on the supply and return ports on the fuel rail. The supply side got a ½-inch click-lock to 8 AN male adapter. The end of the ½-inch fuel line received a ½-inch barbed to 8 AN female adapter and was then attached to the supply side of the fuel rail. The return line received a 3/8-inch barbed to 6 AN adapter, which was attached to a 6 AN to 3/8-inch click-lock adapter attached to the return port from the pressure regulator.

The Odds and Ends

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Craig picked up this GM-Delphi IAT from ebay.
engine swap twin turbo
Once the ribs on the IAT are shaved down some, it slides right in with a little silicone and help from a rubber mallet.

Craig says that the OEM MAF sensor isn’t sensitive enough for a boosted engine, so he pulls the MAF sensor and installs an intake air temperature (IAT) sensor instead. Engines equipped with a MAF sensor will have a five-wire connector. Two of those wires, black and tan to the right looking at the bottom of the connector (the locking clip will be facing up) , are for the IAT, while the other three are for the MAF. Those three can be cut back inside the loom and taped off, while the IAT wires are soldered and heat-shrinked to the leads for the IAT connector that comes with the GM/Delphi IAT sensor. The Delphi IAT is labelled “A” and “B.” The wire in the stock harness furthest to the right (Black wire) is “A” and the yellow wire is “B.”

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The MAF sensor harness is cut down for the new IAT. The two wires on the right in this image will be used while the other three will be cut back.

There’s a port on the front of the intake manifold where the IAT will go. However, the ridges on the IAT have to be carefully ground down to almost flush with the body to fit that port. Once the ridges are almost gone, you can use either a small dead blow or rubber hammer to slap it into place. It is perfectly positioned when it is bottomed out on the upper ridge as seen in the image above.

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Craig upgraded to a Corvette ZR1 brick-type MAP sensor due to space constrictions.

Craig decided to go with a three-bar MAP Sensor out of a ZR1 Corvette. You don’t have to; you can stay with the OEM MAP sensor and not have to worry about modifying the MAP sensor harness/connector. If you do go with a new MAP, remember to keep the wiring straight, “A,” “B,” and “C” must be soldered to the corresponding wires in the harness. The wire labelled “A” on the new pigtail will connect to the orange with black stripe and “C” will go to the grey wire. The light green is the MAP signal wire and is connected to the “B” wire on the new connector/pigtail.

One other item about the MAP sensor: If you stick with a stock manifold, you shouldn’t have to modify anything. Craig only had to modify the harness and connector because the manifold he used isn’t a stock manifold. For the vacuum signal to the MAP, he simply teed into the vacuum source he created in the section above and mounted the ZR1 MAP to the bulkhead behind the engine.

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The new injector connectors soldered and heat-shrinked into the harness.

Craig also used a different set of injectors from the stock ones. This required him to change the connectors for the injectors. Since injectors are nothing more than simple coils, he advises to cut the stock connectors off and solder and heat-shrink like colors from the harness to the connectors-dark brown to black and pink to red. If you aren’t changing the injectors when performing an upgrade like this, obviously you can skip this step.

Stay tuned. In Part #3 we go through the valve train modifications/upgrades that a 700-800 HP boosted LS engine needs to stay happy.

About Mike Aguilar 388 Articles
Mike's love of cars began in the early 1970's when his father started taking him to his Chevron service station. He's done pretty much everything in the automotive aftermarket from gas station island attendant, parts counter, mechanic, and new and used sales. Mike also has experience in the amateur ranks of many of racing's sanctioning bodies.

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