Naturally Aspirated vs. Forced Induction Part 1: Naturally Aspirated Fox Body Motors

Which power adder is best, you ask? Well, the short answer isn’t really an answer at all. Rather, each setup has their own distinct advantages, disadvantages and requirements. With that being said, however, we can go through the specifics of each setup to better aid you in your decision.

A naturally aspirated engine, as the name implies, breathes on its own without any external help (external help being a power adder). Moving on from that great insight…

An engine, at the most basic level is essentially one large air pump. The more air you can get into it, the more fuel you can dump in. Then, provide the two with a spark and BOOM, work will be accomplished. At what rate said work will be accomplished (aka how much power is the motor producing) is reliant on how big we can make that boom. Right, enough physics. Let’s dig into the topic at hand.

In terms of making power, the key is not only to get as much air flowing into the engine as possible, but to use it efficiently. With a stock 5.0L Fox Body engine, the best way to do this is to do what is referred as an HCI swap (heads, cam, intake). The stock cylinder heads and intake manifold are major restriction points.

Slapping on some aftermarket cylinder heads with matching intake from any of the big names such as Trickflow, Edelbrock or Ford Racing can have some momentous gains. The stock HO cam is actually a good piece and will work fine, but an aftermarket bump stick will maximize the fresh top end.

A proper aftermarket street grind cam can see gains of up to 30HP over the stock cam in conjunction with a better flowing top end. Overall, a street driven naturally aspirated 302’s top out at ~380 HP (at crank). Furthermore, properly done, a naturally aspirated engine will be able to get up and go at virtually any point in the RPM range.

• A Naturally aspirated engine(n/a) breathes on its own without being forced air from a power adder, like a turbocharger or supercharger
• A key to making more power with any engine is to get as much air into the motor as efficiently as possible
• When adding horsepower or increasing the engine's airflow, your n/a engine will also need a larger supply of fuel
• A naturally aspirated motor is hard to beat in terms of all-around reliability

Looking for more than that? No problem! Ever hear the age old adage ‘no replacement for displacement’? Well, with a naturally aspirated motor, this is irrefutably true. If you need power beyond the 380HP available from a 302 cu motor, stroke it! Longer cylinders (which is what is created by stroking a motor, you are changing the stroke length) can pull and fit in more air, allowing for more fuel and consequently, more horsepower.

Rolling out of the factory with a cast crank, forged pistons (hypereutic were used in 1993) and rods, the stock 5.0 internals are ready to handle whatever you can throw at it. In fact, the block will let go before the internals do, which usually occurs at power levels around 500 HP. The only need to dig into the bottom end is if stroking to a larger displacement, or you wish to change the compression with a set of aftermarket pistons.

Of course with more power comes a need to supply more fuel. Luckily enough, with a naturally aspirated motor, the fuel system needn’t be complicated.

Up the stock 88 liter per hour pump and 19 lb/hr injectors and that’s pretty much all you’ll need – dead simple. Best yet, most natural aspirated engines can run on regular gas, saving you cash at the pump.

Same goes for the ignition – make sure the plugs, coil, cap, rotor and wires are in good shape and you should be set. Maybe throw in a digital module to aid spark generation at higher RPM, but it isn’t necessary. If you want to squeeze out every last pony possible, you can opt for a custom chip and corresponding tune, but the stock EEC-IV does a fine job too.

Make sure the timing and fuel pressure is good and that’s it, no need to drop some more Benjamins on a custom tune.

In terms of reliability, a naturally aspirated motor is hard to beat. Relatively low stress is placed on the internals and if all parts are correctly installed, expect a long life. Maintenance remains the same – regular fluid changes and the odd ignition and fuel system tune-up will see a naturally aspirated engine last many a mile.

All in all, a naturally aspirated small-block Ford V8 is a great choice for pretty much any application, be it track or street duty. Overall build and maintenance costs will be lower as compared to a forced induction car and you won’t need to dig into the bottom end. At the end of the day, the naturally aspirated motor is always going to be the iconic motor for any muscle car. It is a passed the test of time and boy, they do sound perrrrrty.

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!

• The stock valvetrain and intake is suffocating, but letting the engine breathe means more power and more risk of damage
• Froged vs. hypereutetic pistons: forged are stronger for forced induction, but not all Fox's came stock with them
• Utilize a tune to dial in air/fuel ratios, timing, among a number of other things to maximize power
• Keeping on top of maintenance can handle the "reliability" issue

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!

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 hypereutetic pistons were the factory option.

Forged pistons are the key here, as hypereutetics 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.

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 ).

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.

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.

• Stock E7TE heads are heavy and don't flow as much as aftermarket heads
• An aftermarket camshaft increases valve duration and lift, allowing for more air flow and unlocking power
• Aftermarket intake manifolds have larger ports to mate well with aftermarket heads and camshafts
• A new exhaust system will flow better and sound better while adding extra style to your Mustang's back end

The stock E7TE heads the Fox’s came equipped with leave a lot to be desired. For one, they are cast-iron, so they’re quite heavy. However, the biggest drawback is the flow. They are simply anemic as far as flow goes. Some would suggest porting the stock pieces.

In my opinion, they are better off replaced. Trick Flow, Patriot, AFR etc. all offer aftermarket heads that will outflow the stockers and pack on a nice bit of horsepower. Not to mention there is a significant weight savings if you switch to a set of aluminum heads.

Just keep in mind, changing the combustion chamber size will effect compression. If you’re sporting forged internals, and want to boost compression slightly, opt for a smaller combustion chamber.

If you plan on upgrading your mustang’s cylinder heads, tackle the cam while you’re at it. By leaving the stock cam in place with aftermarket heads, a lot of horsepower potential will be left on the table.

After all, with the heads supplying an increased amount of air, a cam that holds the valve open further and longer is going to make a significant difference. Be sure to upgrade valve springs if necessary.

Ford Racing, Comp Cams, and Anderson all make great cams. Camshaft replacement is a great time to replace the timing chain and timing gears. Opt for a quality double roller timing set.

The factory 5.0 intake is pretty restrictive. After all, it was designed for use with the poor flowing factory heads. The aftermarket intakes offer larger ports and longer runners.

They are generally quality aluminum pieces. Not only do they perform great, they look pretty awesome as well. Trick Flow, Edelbrock, and Holley are a few companies that make nice intakes for the fox body mustang.

Needless to say, the factory exhaust manifolds don’t have much to offer in the way of horsepower. The small primaries and collectors are very restrictive. Usually aftermarket heads are made with larger intake and exhaust ports.

So, a header with a larger exhaust primary is going to be more beneficial. There is always going to be pros and cons with long tubes vs. shorty headers. Basically, if ease of installation is the main concern, go with the shorty headers.

If maximum horsepower is the goal, long tubes are where it’s at. Don’t overlook the header-back exhaust. Switching to a 2.5 inch dual exhaust set up is going to perform and sound much better.

When changing these components, it is far better to replace all components together. For example, just upgrading the camshaft and exhaust isn’t going to net a lot of power gain. It’s all about the components working together. The camshaft, exhaust, and intake will all work together to compliment the heads.

Aftermarket heads have the potential and capability to flow a lot of air; the rest of these mods will compliment them perfectly. Keep in mind, on EFI engines, a tune is necessary if you want to reap the benefits of engine mods. This will result in a huge horsepower gain, and will wake up your Fox Body’s 5.0 engine.