by Chris Monahan, C. Asaravala, and Greg Hall

History
The 2003-2004 Mustang Cobra debuted in late 2002. Code named 'Terminator' by Ford, it was the most powerful Mustang ever released due to it's Eaton supercharged 4.6L four-valve per cylinder engine. The factory rating of 390 horsepower and 390 torque was conservative, and the engine was primed for easy power gains. Enthusiasts quickly found that adding an open element air filter and less restrictive exhaust system yields as much as 50 horsepower, as they made well over 400 horsepower and torque on rear-wheel chassis dynos. Naturally these sorts of gains created a resurgence of the horsepower wars remminicant of the late 60's. Cobra owners quickly looked, and continue to look, for ways to increase the bench-racing ante. As many guys soon learned, increasing the boost pressure was the hot ticket. This was easily achieved by swapping pulleys (Fig.A) to spin the blower faster.

The Problem
Boost junkies quickly learned the limitations of the Eaton blower, giving it the nickname Heaton for the amount of heat it added to the air with increasing boost pressure. This however is a characteristic of a positive displacement, "roots" style blower like the Cobra's Eaton M112. Such superchargers operate on the principle of positive displacement. Internal rotors move a fixed volume of air with each revolution - the increase of rpm does not directly result in compression (boost pressure) but does increase the rate of air volume forced into the engine. As a result pressure builds in the intake manifold. Centrifugal superchargers, on the other hand, work as compressors and build boost within the blower unit itself.

Adiabatic Efficiency
One cannot discuss super or turbo charging without a basic understanding of adiabatic efficiency - a thermodynamic concept of heat gain of a gas as it is pressurized. A 100% efficient supercharger would be one where the temperature of the inlet air is the same as the outlet air. This does not exist (intercooling does not count for it introduces pressure loss.) The most efficient superchargers, generally centrifugal style units, can obtain efficiencies of 80% at high rpms. Roots style blower however exhibit the lowest efficiency at around 60% due to the manner in which they supply uncompressed air into an area of compressed air. The compressed air in the manifold works against the gearing which in turn generates heat.

In a nutshell adiabatic efficiency becomes a balancing act between the gains of compressed air charge versus the losses from hotter, less-dense, (and detonation prone) air. Air temperature after the blower and even after the intercooler can be as much as 100 degrees hotter than ambient at full boost.

Efficiency equals power.
It's clear then that the goal isn't simply to obtain a higher reading on the boost gauge. Rather, the objective is to increase efficiency and thus reduce the heat. A colder charge of less pressure will yield more gains that a hotter temperature at a higher boost reading.

The challenge for the Cobra owner (or even Lightning owner) seeking more power is how to go about obtaining more efficiency. While pulley swapping does yield impressive gains the Eaton taps out around 15 lbs of boost and roughly 460 horsepower. However those wanting to push the envelope into the high 400 and beyond range have simply opted for a more efficient blower design - twin-screw (such as Kenne Bell), or Centrifugal (ala Vortech and ProCharger.) These are hefty mods, more so in cost than in complexity, and likely not in the cards for the average 03-04 Cobra owner just wanting to pack a bit more punch than the other average Snake owner.

Bob Stiegemeier next to just a few of the over 1500 Eaton blowers he's ported over the years.

That is where Stiegemeier Engine Air Flow comes in. Bob Stiegemeier, better known as DynoBob, has developed a cult like following in the world of 03-04 Cobra owners. In actuality he has amassed a loyal following amongst owners of just about every Eaton-equipped vehicle. Be it a Cobra, Lightning, Jaguar or even MiniCooper, if it came with an Eaton then Bob is the go to guy for extracting more power. For as little as $350 and the cost of shipping your blower to and from his shop he'll port and polish you on your way to 30 plus horsepower. We teamed up with him to see how much his Stage IV port work on our Cobra's Eaton would gain. The Stage IV not only includes a full porting of the blower internals, but significant porting of the stock plenum and throttle body. All together Bob has measured gains of 60 or more horsepower from the $745 Stage IV package. He also explains that this amount of port work results in the maximum obtainable power from the M112 when run at 15.75 lbs of boost. More on that later.

Stiegemeier Engine Airflow addresses some of the issues with the Eaton's efficiency limitations with extensive port work to get the air mass moving through the blower at a faster rate and with less heat production. Shown is the stock 2003-2004 Eaton M112 blower outlet underside.	When any blower arrives at Stiegemeier it is first fully disassembled, inspected for rotor, bearing and case wear. Then it is precisely ported to the maps they've established through flow bench testing. The unit is then high temperature and pressure washed before assembly. This is the Stiegemeier ported blower. Note the increased passage size.
The Stage IV porting package for the 03-04 Cobra includes porting of the intake plenum and throttle body. Note how the plenum bore diameters are enlarged and the bosses which protrude into the air path have been ground away.	A view into the Eaton's inlet side reveals the detailed porting and shaping that Stiegemeier puts into the blower. Overall flow improves over 55%, from 950cfm stock to 1452cfm after the StageIV treatment.
Another example of the attention to detail by Stiegemeier. This casting indentation in the plenum protrudes into the cavity restricting airflow...	Stiegemeier cuts it out, welds in aluminum and then ports it to a smooth radius.

(Removing the stock Eaton)