Naturally Aspirated vs. Forced Induction Part 2 – Forced Induction Fox Body Mustangs

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Having previously looked at naturally aspirated 5.0L Fox Body motors, let us now unravel the potential behind forced induction and adding boost to a Fox Body Mustang.

Fox Body Forced Induction Applications

Supercharged Fox Body Mustang

Naturally Aspirated Mustang Engines--A Quick Refresher:

a naturally aspirated engine works by using air drawn into the cylinders by atmospheric pressure. Recall the air to fuel concept? If you have more air (technically, more oxygen molecules), you can then mix more fuel with it to make more power. A supercharged or turbocharged engine works by forcing air into the engine, above what would be naturally drawn in.


Here’s a simple analogy. As I child, whenever I was presented with a bowl of grapes, I would always test how many I could fit into my mouth. After 8 or 9 grapes, my mouth would be full. However, never content to stop there, I could always force in a few more. In doing so, the other grapes would squish against each other and my teeth, compressing them. So, despite already being unable to talk with 9 grapes in my mouth, a few more could always be forced in, bringing my total grape capacity to 12. The same concept is at play with a turbo or supercharged engine. Air fills into the cylinder, but once full, it doesn’t stop there like a naturally aspirated engine would. Instead, the power adder continues to cram air in, compressing it and giving a greater oxygen density. The result? More air, add more fuel, spark it and BOOM! More power!

Making More Power With Forced Induction

As explained above, a forced induction engine makes power by forcing more air into the cylinder, at a positive pressure called ‘boost’, measured in pounds per square inch. How much power can you make? That invariably depends on how much boost you are running. More boost is more power, but also greater stress on the engine!

Turbocharged Fox Body Mustang

Supercharging/Turboing a Stock 5.0L Mustang's Motor


So, what if you were to turbo or supercharge the stock 5.0? Well, you may actually be a little disappointed. The factory Fox top end (cylinder heads & intake) are major choke points, even if when being force fed! Gains of around 80HP over stock are to be expected if just simply slapping on a charger. That doesn’t sound like much… in fact, if you were to spend the same on upgrading the 5.0 but keeping it naturally aspirated, you would most likely have more power with the NA engine. Does that make a naturally aspirated engine the clear horsepower winner? Not at all. Remember, a key to making power is moving air into (and out of) the engine. What if you upgraded the top end with better cylinder heads, intake manifold and a camshaft designed to work with a power adder, then slapped on a charger? Now we’re talking! In a scenario like this, the amounts of power you can make is insane! So insane, in fact, you may have to dial it back a bit otherwise you risk splitting the block!

Fox Mustang Engine Internals and Hardware

Making power with a forced induction motor is not a problem. The problem is containing it without ripping apart the motor. Ford used forged pistons from 87-92, whereas in 93 hypereutic pistons were the factory option. Forged pistons are the key here, as hypereutics are known to crack under the strain placed on them by a charger. Furthermore, the stock cylinder head bolts won’t hold up under boost, either. Thus it is highly recommended when going forced induction to upgrade to head studs, otherwise it is very easy to lift a head and/or blow a gasket.

The stock roller is too considered a weak point from a forced induction perspective. It is very easy to get carried away cranking up the boost and splitting the block, which happens at around 500HP. If you plan to go big with boost, you may want to consider an aftermarket block.

Custom Tuning Computer Chip

Boosted Mustangs - Fuel & Ignition

Here is where things start to get tricky. Compared to a naturally aspirated engine, a forced induction engine has to have very precise fuel and ignition delivery. Due to the nature of the increased pressure and temperatures imparted by a charger, forced induction motors typically need higher octane fuel and precise timing to avoid premature or late detonation. In order to control these important parameters as best as possible, it is highly recommended to get an aftermarket computer chip and tune to manage the all the variables (air to fuel ratio, timing etc). And since we’re forcing in a lot of air, we need to be able to dump in enough fuel to match. Thus boosted motors need very high capacity fuel systems, generally running two pumps (one in-tank and the other in-line) as well as large capacity injectors (42lb +).

Does Forced Induction Affect a Fox Body's Reliability?

So far, this article may have given the impression that forced induction motors are more complex and stressed than a naturally aspirated motor. This is entirely true, but does not mean they are unreliable. A properly built and tuned forced induction engine should be able nearly as reliable as a naturally aspirated motor. I say nearly, not because they will leave you stranded, but because a boosted motor requires more maintenance. Adding any type of charger is like adding another entire system to the engine, and that system needs to be maintained as well. Fluid changes, bearing and impeller rebuilds etc, are needed at regular intervals – just like regular vehicle maintenance.

Bottom Line

Forced induction motors cost a pretty penny. First, they require a good base motor (i.e: capable heads, intake and cam). Then there is the cost of the actual system itself. But boy, when all is said and done, charged motors are fun! Feel the tickle for more power? No need to go back to the drawing board… simply turn up the boost. As a plus, with today’s cam technologies, boosted cars are entirely streetable across the whole RPM range.

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