First, to clear up a point of common confusion: the TD04 and TD04L turbo is an MHI, which stands for Mitsubishi Heavy Industries. IHI is a separate turbo manufacturer that didn't produce the TD04L housing, but did produce a number of other, similar turbos that can be used in place of the TD04L. Regardless, the TD04 is one of MHI's most popular turbochargers, found in a number of variations on several different vehicles. The TD04L is one of those variations.
The two most common applications for the TD04L were the Dodge Stealth/Mitsubishi 3000GT and the Dodge Neon SRT-4. Subaru also used a variation of the TD04L in lieu of the standard TD05; the smaller TD04L didn't produce as much top-end horsepower as its larger counterpart, but its quick-spooling nature made TD04L-equipped Impreza WRXs drive more like a naturally aspirated car than a turbo-enhanced ride.
Mitsubishi's turbo classification system can be a bit confusing, but it's pretty straightforward once you get the basics down. "TD04" represents only part of the turbo's full designation; the turbo's full name may be TD04-13G or TD04-15T. The first series of numbers and letters -- TD04 -- designate the turbine housing/wheel design and diameter, The second series -- 13G or 15T -- represents the compressor housing/wheel design and diameter. In the MHI system, letters typically designate a particular design and numbers designate a size. For example, a TD04 uses the same basic design as a TD05, but is a little smaller. A 15G compressor wheel/housing is about the same size as a 15T, but uses a different design.
The TD04L differs from the standard TD04 only in that its exducer is about 0.15-inch larger in diameter. A compressor and turbine wheel have two basic diameter measurements, the inducer -- where air goes in -- and the exducer -- where air goes out. In an exhaust turbine wheel, air goes in through the larger diameter portion and spins out through the smaller diameter. An compressor wheel is the other way around. The TD04L uses a turbine wheel with a 1.86-inch inducer and a 1.62-inch exducer, up from the TD04's 1.57-inch exducer.
An exhaust housing size and design is only half the equation, playing a large role in determining how fast the turbo spools up and the maximum capable rpm, but having little to do with how much air the turbo can actually pump. The compressor size and design determines how much air the turbo can pump and ultimately how much boost it can produce. The TD04L turbo came with compressor housing/wheel trims including 09B (US-Sprec Mitsubishi 3000GT VR-4/Dodge Stealth) 13T (Subaru WRX), 13G (Japanese-spec 3000GT VR-4) 16T (Volvo), 18T (Saab), 19T and 17G (Subaru WRX).