IDI Exhaust Upgrades
Factory exhaust systems are restrictive, so a replacement is a simple way to unleash a little extra power and even drop EGTs. There are not many kits on the market for these older diesels, but a few companies do offer them. A 3.5" or 4" kit with a straight through muffler will work just fine. If you don't mind the noise, these engines have a very distinct, pleasant exhaust tone when straight piped. If you are working on a tight budget, just cut out the muffler and weld in a section of exhaust tubing (it should cost you under $20). If you'd like a more complete setup, pay attention to pipe diameter and curvature near the exhaust manifolds; the factory tubing has tight bends and relatively small diameter tubing in this area.
The IDI Soup Bowl
Most 7.3L IDI and some 6.9L IDI engines have what's been nicknamed the "soup bowl" integrated into the lid of the air cleaner assembly. The purpose of the soup bowl has been debated, but the primary function seems to be to cut down on engine noise (you'll figure this out if you choose to remove it). Trimming and removing the soup bowl increases air flow, but only marginally. Other than some additional engine noise, it doesn't hurt anything. It's easily removed with a cut off wheel and while you won't feel any major improvements, the general consensus is that you'll build a few extra ponies on the top end. For those 6.9L IDI engines that don't have a soup bowl, they have a restrictor plate on the intake manifold that can also be removed by grinding away the spot welds. This is best done with the intake removed, but if previsions are taken to keep debris out of the intake, it can be done without removal.
To the best of our knowledge, there are no complete air intake kits on the market specifically designed for the 6.9L or 7.3L IDI. Most owners who desire such a system fabricate it themselves. This can be done by using anything from PVC pipe fittings to miscellaneous tubing and "build it yourself" cold air intake kits. Alternatively, we've been extremely satisfied with the results after installing an open element air cleaner. Although this may draw in warmer air, it is the least restrictive option and we've experienced both lower EGTs and more pulling power after making this upgrade on our own 6.9L IDI project.
IDI Injection Pump Tuning
Before you touch the injection pump, install a pyrometer. With that said, the Stanadyne DB2 injection pump is adjustable. With the small access cover on the pump removed, the engine can be "bumped" to reveal a small allen head fastener. Turning the fastener clockwise will increase fuel flow, while counter clockwise rotations will reduce it. This should be done in small increments, one "flat" at a time (one "flat" is 1/6th of a turn at a time since there are 6 flat sides to an allen head). A pyrometer is important because you'll need to adjust the pump and then drive the truck, carefully monitoring EGTs and visible smoke. In extreme cases, injector pump adjustments will cause the truck to smoke excessively under load, and reduce driveability do to high exhaust gas temperatures.
For the 1993 and 1994 model years, the 7.3L IDI was offered from the factory with a turbocharger. For all other 7.3L and 6.9L IDIs, the engines are naturally aspirated. Many companies offer complete aftermarket turbocharger kits, though support has shrunk as the market continues to degrade with time. Turbocharging an IDI is the most effective means of significantly increasing performance, but the rules are different for IDI engines. Because of their high compression ratios, don't expect to run a lot of boost. 15 psi is considered high for an IDI, and a more realistic range is 10 to 12 psi. The cylinder heads on the 6.9L were not designed for the application of a turbocharger and the head bolts are smaller than those on the 7.3L IDI, so consider head studs to improve reliability.
Changing injectors in an IDI for more performance is generally not acceptable, with the exception being turbocharged engines. Even then, options are extremely limited. Since injector upgrades deliver more fuel, they are only sought after once airflow upgrades are performed. For a naturally aspirated IDI, installing larger injectors would be the equivalent of turning the injection pump up beyond a usable point. As such, don't go looking for a set of performance orientated injectors for your naturally aspirated IDI.
Water Methanol Injection for IDIs
Water methanol injection increases performance and reduces EGTs by creating a cooler, denser intake air charge and introducing methanol as a supplemental fuel. While entirely compatible with an IDI engine, tread lightly when it comes to water or water methanol injection. There are no kits engineered specifically for the 6.9L or 7.3L IDI engines, which means a universal system is your only option. The problem with water methanol injection in an IDI is the high compression ratio and, since there is no turbocharger, cooler air intake temperatures. In a water methanol injection system, the mixture is converted from a liquid to a gas before entering the combustion chamber. Limited space in the combustion chamber at top dead center means that if any liquid enters, you risk damaging the engine. At top dead center, your IDI's combustion chamber is only slightly larger than the thickness of the head gasket. Use water injection in an IDI at your own risk; if EGTs are a problem, consider tuning down the injection pump.