How to Convert a Gen IV GM Truck to Flex Fuel for More Power
Powered by Corn
Remember that old joke about imports being “powered by rice.” Well, what if that was literally true? What if their cars ran on fermented and distilled rice spirits? In the United States, we don’t have many rice paddies, but we do have fields and fields of corn. And its high sugar content has made it the crop of choice for many legal and illegal moonshiners for decades. It should come as no surprise that corn is the most popular ingredient for U.S. refineries that produce ethanol. The ethanol is blended with gasoline to produce E10, E15, and E85 fuel sold at our local pumps.
Despite ethanol’s shortcomings with cold-starts, water absorption, and energy content, ethanol has two huge advantages when it comes to producing power. Ethanol has a higher resistance to knock (or detonation) than typical gas station swill, and it has the added benefit of the latent heat of vaporization (it absorbs heat from the incoming air). At one point, we thought Sunoco 94 was the bee’s knees? Now, pump E85 is reportedly 96 to 100 octane.
Those of you who own Flex Fuel GM trucks may already be muttering that E85 is terrible. Yes, it does get significantly worse gas mileage. And although it often costs less per gallon, you will wind up spending more money per mile. But, for the purposes of this story, we are simply talking about its ability to produce power. When it comes to making power, E85 is actually a cost-effective means. How? Compare the cost of 100-octane race fuel to E85. On average, you are looking at $7 or $8 per gallon for race fuel. As I type this, the national average was below $1.90 per gallon for E85, and below $1.50 at my local pump. So, is it starting to make sense why you might use E85 in your GM truck?
As any builder or tuner will tell you, you can’t just dump 100-octane fuel into your truck and have it instantly make power. A stock 6.0L L76, which includes ’07 to ’09 Suburbans, Avalanches, Silverados, Sierras, Yukon XLs) and LY6, which includes ’07 to ’16 Silverados or Sierra HDs, does not need 100 octane. However, a stock LS2 with its 10.9:1 compression just might gain benefits (more on this in a second). Lately, the Gen V ’14 to present GM engines have been showing power gains, nearly bone stock, with a performance calibration, and a tank full of E85.
We set out to see what would happen with a stock ’08 Trailblazer SS, and in the process, how to convert a Gen IV GM truck to Flex Fuel. (A follow-up is already in the works to see how changes to the combination make E85 more or less effective in adding power.)
Tuning is the essential component to creating more power with E85. A high-compression or force-inducted engine is extremely limited by how much spark advance the engine will take before it knocks. (If you don’t know, engine knock is also called detonation or pre-ignition.) This is what happens when the air/fuel mixture ignites spontaneously, before the spark plug fires. Typically, the more timing you can add, the more power it will make, up to a point.
This is exactly why higher-octane fuels are available. Octane is a measure of how much compression the fuel can take before igniting, or its resistance to knock. A higher-octane fuel is slower to ignite, and will therefore, accept more spark advance before ignition. If you had an 11:1 compression engine with 11 pounds of boost, you may only be able to run 12 to 15 degrees of timing with 93-octane pump gas. Meanwhile, that same engine might have had 28 degrees naturally aspirated.
The essential component to running E85 safely is to perform a full Flex Fuel conversion. In the past, some GM tuners have claimed to offer Flex Fuel setups, especially with Gen IIIs, but they are essentially letting the knock sensors tell the ECM whether to use the high- or low-octane tables. Your pistons are going to be very unhappy with you while the ECM figures this out. That’s one reason using a GM Flex Fuel sensor is the only way to go.
In addition, the ethanol percentage from the pump can vary greatly by location and season, which drastically alters the desired air/fuel ratio and spark timing. KraftWorks puts together custom setups for GM cars and trucks that plug into the fuel line and fuel rail in the engine bay. The sensor actually measures the ethanol content of the fuel, relays that information to the ECM, and the ECM adjusts the fueling and spark based on its internal calibration tables. Thankfully, these tables for stock Flex Fuel trucks are readily available, giving a good tuner like Jay Healy at KraftWorks a reference to work from. It’s just a matter of enabling the sensor in the tune and modifying the tables.
The final component, the one everyone dreads, is providing additional fuel supply. If using E85, you require roughly 30-percent more fuel, and thus, need a fuel pump and injectors to handle it. In the case of our ’08 Trailblazer SS, we solved this by calling Lingenfelter Performance Engineering (LPE). The company set us up with an E85-compatible, high-flow fuel pump module (PN L710101108) and stock GM Flex Fuel injectors. The 416-liter-per-hour capacity of the fuel pump was more than capable of handling the higher flow. And by using the module, it was a complete drop-in affair. According to Jason Haines at LPE, this pump supports up to 625hp to 650hp naturally aspirated and about 100hp less than that with forced induction. Past that, Haines suggests using a fuel-pump voltage booster or a custom dual pump setup. The stock Flex Fuel truck injectors would be good to 460hp to 500hp naturally aspirated, Haines advises. Because they are factory injectors, they are very reliable and easy to tune.
KraftWorks in Delray Beach, Florida, was commissioned to handle all the part swaps and tuning on the Trailblazer SS. Owner Jay Healy has a nearly identical model, except it has a pretty potent little engine combo that is awaiting a set of twin turbos. If it runs half as well as his customer’s cars, the master engine builder, fabricator, and tuner will have quite the tire-burner. Though the focus of this story is on how to convert your truck to E85, we also planned to hit the dyno once KraftWorks had wrapped up the install.
Thankfully, Titan Motorsports was gracious enough to loan us some dyno time. The Orlando-based shop is known for building some of the fastest Supras in the world, not to mention all-wheel drive Porsches, Evos, Subarus, and GTRs. Which is why the shop has one of the few all-wheel drive dynos in Florida, to say nothing of the manner and professionalism with which they operate the Dynojet 424 LC2. The last thing you want is some hack who blows up your engine or transmission on the dyno.
Follow along to see how to do a Flex Fuel conversion on your Gen IV GM truck, and stay tuned for more updates.
To ensure enough fuel supply to handle the extra requirements of E85, Lingenfelter Performance Engineering sent us a complete E85-compatible fuel pump module with its high-flow, 416-liter-per-hour pump (PN TI-TU453-1). This exact unit also works in an SSR and supports up to 625hp to 650hp naturally aspirated, 100hp less with forced induction. Use a fuel-pump voltage booster to squeeze more out of it, or create a custom dual pump setup for additional volume. The pump is available by itself, but the kit negates having to swap pumps, and it includes the wiring harness, convoluted fuel line, fuel filter, terminals, Oetiker clamps, and O-ring seal.
To install the new fuel pump KraftWorks, tech Jason Bleen unhooks the fuel lines, fore and aft, from underneath the truck.
The straps are unbolted while this hydraulic cart helps support and lower the fuel tank. According to Bleen, the hardest part of the process is the fuel filler.
Using a chisel, the plastic ring is broken free to release the fuel pump.
The OEM module is carefully worked out of the tank. It is very tight, and you have to angle it just right.
Bleen took the bucket apart to show the stock pump and sock. LPE’s aftermarket pump not only flows more, but it is designed specifically to work with E85. Many materials like aluminum, brass, zinc, natural rubber, polyurethane, certain plastics, and fiberglass are not compatible with E85. It is best to stick with an E85-specific pump for this reason.
Purchase the whole module, and you don’t have to install the pump in the OEM module or replace the filter. Simply drop the new one into the tank with the new O-ring, and you are good to go.
Bleen used a chisel and mallet to spin the ring tight to the tank with the LPE module in place.
In addition, LPE sent these OEM ’07 to ’13 GM Flex Fuel injectors (PN 12613412)—good for 460hp to 500hp naturally aspirated. The box may say they are rated to 40 pounds, but that is incredibly misleading. The flow can change drastically, depending on a number factors like fuel pressure. Jason Haines at LPE assured us the OEM LS3 and LS7 injectors (supposedly rated at 42 lbs/hr) cannot supply enough flow for E85. The beauty of using an OEM injector is they are reliable and the tuning specs are already available, eliminating the guesswork. Plus, these plug right into the fuel rails and harness without adapters.
To swap over the injectors, Bleen used the GM fuel line tool to unhook the quick release connection from the fuel rail.
After removing the air intake, engine cover, and harness, we removed the stock fuel rails. Though they are rarely a concern for performance, the boxy rails are just plain ugly.
It took some convincing—a twisting motion is key—but the O-rings eventually let go of the rail and the excess fuel dumped into a pan.
Oil was used to lube up the supplied O-rings and on the new injectors before inserting into the fuel rail and clipping them on. After that, Bleen bolted the rails back down and plugged in the injector harness.
KraftWorks put together this custom Flex Fuel sensor setup that plugs into the factory fuel line on one end and the fuel rail on the other. It is essentially some hard line, fittings, and a factory GM sensor.
Bleen loosely installed the Flex Fuel sensor while running the wiring.
The #10 fuse for the ECM was tapped to have a key ignition source for power.
A ground wire bolts up to the inner fender.
The ECM was unbolted and unplugged from the wiring harness, so the sensor can tap in.
Most LS engine harnesses have a blank pin on one of the connectors for Flex Fuel input. The 2008 Trailblazer SS uses the E67 controller, and this blank pin (#5) is on the X1 connector (the top plug).
Healy shows the blank pin he pushed out from the connector.
A wire is inserted into that pin, which runs up to the Flex Fuel sensor.
Bleen covered all the soldered connections with hea-shrink tubing and ran wire loom the length of the entire wire for a better than OEM look and protection. A couple zip ties secure it and keep it away from hot exhaust and moving parts.
With the install complete, it was time to calibrate the ECM. Healy is the owner and expert tuner at KraftWorks. He typically uses EFI Live software to dial in LS engines. He started on the existing tune in the E67 ECM by adjusting the Injector Flow Base for the new injectors. The LMG Silverado uses these injectors, which made that the starting point. The LMG uses a different ECM and operating system, and you can’t quite copy over the same numbers. He begins with values that are close and uses the interpolate function to populate the rest of the table.
After altering a few other tables, Healy clicks here to activate the Flex Fuel sensor in the calibration. On Gen IVs like the E67 or even a ’14-plus Gen V, that’s really all there is to it. It’s much more complicated with the Gen III computers.
This is where the power lies. The timing is dictated by the fuel’s ethanol content—higher ethanol means higher octane. Healy used the ’13 Silverado (LMG) tune seen here as the basis for adding timing, which dictated an additional 5 degrees in the top part of the table. There is another spark multiplier table besides this one, which increases with the percentage of ethanol. Sound complicated? You are not alone. That’s why they’re called KraftWorks!
Last, but not least, Healy turned on the sensor’s diagnostic. Using the scan tool, you can see the current tank of Florida’s finest E10 registered 9.8-percent ethanol. Before handing over the keys, Healy did some street tuning to adjust the fuel trims, which now adjust for the ethanol content because ethanol requires a different air/fuel ratio. We did not have an all-wheel drive dyno locally, this was about as far as we could get. Ideally, you’d want to do any tuning on a load-bearing dyno.
A few weeks later, I drove over to Titan Motorsports to see what difference the additional timing would make when using E85. Several pulls were made with standard 93-octane E10, and just over 16 gallons of E85 topped off the mixture. Doing some quick math, that should be about 66-percent ethanol because the mixture varies little in Florida, unlike colder states. Given the mixture, cool ambient temperatures, and stock engine, I was surprised to discover the LS2 picked up more than 15 horsepower, and nearly the same in torque. On a hotter day, with more tuning, and closer to 85-percent ethanol, I’d expect an even greater power difference. Ethanol might not be all it’s cracked up to be as an alternative fuel source, but it does well as a race-fuel alternative and even in a stock daily driver.