I have a '96 Ford Explorer and would like to know if there are any aftermarket parts to enhance the engine performance without voiding my warranty. It's a 4.0 liter with a 4-speed overdrive automatic.
There are a couple of modifications you can do to your Explorer without damaging the warranty, or the engine. Technically, the manufacturer cannot void a warranty due to the installation of a power amplifying accessory, unless they can prove it damaged the vehicle, or has an adverse affect on emission control devices and/or emission output.
The first thing I'd suggest would be swapping the stock air filter with an aftermarket unit which does not restrict the intake air flow quite so much. K&N makes a quality high-performance filter (part# E-0995) that is kind of expensive at $40 or $50, but it's designed to be cleaned, reoiled, and reused so it can last the life of the engine.
Reprogramming of the Powertrain Control Module (engine's main computer) is another option. A software company that specializes in this area is Hypertech. They produce a complete line of so called "Power Modules" that will fine tune the spark timing curve and fuel delivery from the original factory settings, and safely attempt to boost the engine's performance. The instructions will direct you to use a 93 octane fuel in order to coincide with the timing changes. The power increase varies from engine to engine, and please don't expect any miracles out of your 4.0-liter V-6. Hypertech swears the Power Module will not cause any harm to pistons, valves or the ozone layer, and I have yet to hear different.
If you want to spend some more money, custom exhaust systems from companies such as Borla help open up the exhaust flow from factory restrictions.
That's about as far as I'd go on an Explorer, unless you plan on racing.