After deciding not to upgrade to a new model truck, I decided it was time to do some performance improvements. After much research, the following is the list of mods I settled on. All were sold and installed by DI Performance (aka Diesel Innovations).
I did all of these modifications in February 2007, and together they've made a HUGE difference in the engine's performance - and improved the truck's fuel economy. Economy was my driving force behind the mods, but I have to admit, it is kind of fun to have the extra power. As mentioned above, I believe my highway fuel economy has increased to around 18.5 mpg if I keep my foot out of it.
I have not run any dyno tests, but I believe with the chip in the race setting that my engine is cranking out somewhere around 330 horsepower and probably around 675 ft lbs of torque. I did these mods at 111,000 miles, and I didn't know the old girl had it in her!
I believe for straight performance improvements, I'm done. However, I will have to make a couple of changes down the road. Primarily, I will have to upgrade my clutch. With all the extra horsepower and torque the engine is producing, the stock clutch (and mine is currently the original clutch) can't quite handle it. It will slip if I really get into the throttle. So I'm looking at either a South Bend or LUK replacement clutch, either of which is rated to 350HP. And if you can believe it, I can get one of these clutches installed at DI for less that my regular mechanic will install a stock OEM replacement.
ZooDad and Intake Sound Control
The only problem with the aFe Stage II intake is that it completely replaces the factory air box with this metal tray. In other words, the filter is out in the open. This is great for the engine to breathe, but it isn't so good on sound. Why? The turbo's inlet turbine is basically right there through, oh, a couple feet of wide-open plastic tubing. The other problem is that there is a factory-designed hole on the interior of the fender right next to where the air filter goes. The end result was that the turbo noise was coming down the filter, through that hole in the fender, and right into the passenger cabin. Pretty noisy when you've got your foot in the throttle and the turbo's really breathing. The cure to the problem was two-fold.
The first order of business was to plug the hole. I'm not a metal worker, but I do know how to work with wood. I cut and molded a piece of 1/8" thick masonite to plug the fender hole. I then sanded it and applied three coats of Verathane polyurethane sealant to keep the water from damaging the masonite. I then screwed it into place and sealed it with silicone. There are some other smaller holes in the fender for sound to travel through, so the noise isn't completely gone. But it has been drastically reduced. It is kind of hard to see the plug with the intake fully assembled, but here's a picture nonetheless.
That presented another problem. The whole point of the Stage II intake is to allow more air into the turbo. And the colder the air, the better. With that fender hole plugged, the only place for the engine to breathe from was the surrounding engine compartment, even with the new mounting tray. That air is typically hot - not good. The solution was simple. There is a modification you can make by simply cutting a hole in the fiberglass divider between the radiator and driver's side headlight assembly. This in effect is a ram air inlet straight out the front of the truck. This is called the ZooDad mod, named after a guy on TheDieselStop.com with the handle ZooDad. Here are some pictures of ZooDad's truck and his namesake modification. His hole is a lot prettier than mine, and his procedure is much more well documented.
Update January 2010
So I know I wrote a while back that I had an appointment for the regulated return fuel system and all that. Well, that got put off for quite a while and I just had it done. I had DI do their regulated return fuel system, in-tank and pre-pump mods, and the high pressure oil crossover hose. They also tried to track down a pesky (and slooooowwwww) oil leak that had gunged up the bottom front of the engine, and they helped me diagnose a vacuum leak that prevents the 4x4 from automatically engaging. First, the fuel system mods.
The regulated return fuel system has two primary purposes: get the air out of the fuel flow, and properly feed the injectors. All of them. The problem with the stock fuel system on the 7.3L Powerstroke is the two injectors at the back (closest to the firewall) are at the end of the fuel rails. They just dead end back there. Sometimes, especially under heavy loads, a lot of what makes its way back to those injectors is foamy fuel. Air kills injectors, so foam isn't good. The RRFS essentially fixes the dead ends on the fuel rails and turns the system into a "through" system. That means that all of the injectors - including the two on the rear of the block - get good fuel flow.
The in-tank and pre-pump mods essentially fix the somewhat leaky fuel uptake issues. This in turn reduces - practically eliminates - air in the fuel system.
The result: well-fed injectors, plus no air in the system, means longer injector life, slightly increased economy, and 35 more horsepower. However, the biggest thing I've noticed: man, the engine runs quieter! It idles smoother and quieter, too. I didn't have a decibel measuring device to test before and after results, but I'd be willing to bet that ambient noise at cruise has been reduced by a couple of decibels. Pretty impressive!
As for the other issues they looked at (oil leak and vacuum leak), the news was good for the oil leak. I was scared it was going to be the front main or something else really espensive. Turns out that an o-ring around the oil cooler is the likely culprit. They cleaned the gunge off and watched for the leak for a day, and nothing happened. So the leak is really slow and not something to be super concerned about. As for the vacuum leak, the news wasn't so great. After troubleshooting it, we discovered that the vacuum seals inside the hubs themselves are leaking. Everything pulls a strong vacuum until the hoses are attached to the knuckles. Damnit. That means either new hubs or rebuilding the hubs to install new seals. Front hubs are expensive (about $900/side installed), and the rebuild of both hubs would run me about $600. I don't use the 4x4 often enough, so I think I'll just live with it until the hubs need to be replaced. I can always hop out and manually engage the hubs if I need to go trundling around in the muck.
I still have a couple more minor mods to do, but I think I'm done with the major engine changes/modifications. I'd like to install a coolant filtration system, a 203-degree thermostat, and an aluminum thermostat housing (so that it never corrodes again). With the exception of the RRFS just installed, I've had the modifications on the truck now for 3 years and almost 50,000 miles and they're awesome! Great extra power, and the goal of increasing the trucks fuel economy has definitely been achieved.