Muscle Mustangs & Fast Fords
Boost Pressure on 5.0L Applications - Big Bad Boost
Effect of boost pressure on different 5.0L applications.
Adding boost to any application, but especially the original and venerable 5.0L Mustang, has been a time-honored tradition of power improvement. Back in the day, MM&FF made performance history by getting an otherwise bone-stock 5.0L Mustang to dip into the 10s with nothing more than a single power adder. After the installation of an intercooled turbo system, the 5.0L Mustang in question ran 10.90s using nothing more than big boost!
Today, we focus on efficient boost, as too much or inefficient boost can bring bad results. So how on Earth can boost be bad when adding boost pressure from a blower or turbo? The answer might surprise you, as boost (especially big boost) does indeed have many negative attributes. To illustrate why big boost is bad, we first need to understand the nature of boost pressure and the elements associated with pressurized performance. From there, we can illustrate examples of how to actually make more power with less boost.
Whether supplied by a blower or turbo, boost is additional airflow supplied to the motor that it could not ingest of its own accord. Basically, a turbo or blower is used to force-feed the motor. A related and common performance misconception regarding forced induction is that boost is actually a measurement of power output. How often have you heard statements like, "That guy is running 15 psi of boost," or "These forged pistons will handle 20 psi of boost?"
Obviously, there is a relationship between boost and power output, as more boost will almost always yield an increase in power (to a point). The problem is that generic statements related to boost tell only a fraction of the story.
At best, boost should be considered more of a multiplier than an absolute indicator. If you apply 15 psi of boost to a stock 5.0L (or any stock motor), the results will be considerably less power than subjecting a stroker and/or heavily modified 302 the same boost pressure. The question now becomes whether the pistons, rods and crank in question were designed to handle the power output of 15 psi.
Often referred to as positive pressure, the reality is that boost is a measurement of backpressure or, more accurately, the indication of a flow restriction in the motor. Basically, boost pressure indicated on the boost gauge is the amount of flow supplied that the motor is unable to process. Lucky for enthusiasts, this build-up of pressure has a positive effect on power production, but the power output would actually be much higher if the same flow through the motor came with no pressure.
The easiest way to illustrate this is with a few examples. As we have tried to stress time and time again, the best route to an exceptional forced-induction motor is with a powerful normally aspirated combination. Building power in the normally aspirated combination is a function of what we like to call shifting the torque curve. The laws of physics dictate that for any given torque output, the horsepower production is a simple matter of the engine speed at which the torque is produced.
Suppose we have a 302 that puts out 300 lb-ft of torque at 2,000 rpm (an impressive number amount given the minimal engine speed). This torque production would correspond to a horsepower output (at 2,000 rpm) of 114 hp. The formula used to calculate this is: horsepower = torque x rpm / 5,252). Using this formula, we see that shifting the 300 lb-ft of torque to 3,000 rpm equates to a hair over 170 hp, while 4,000 rpm will up the power ante to 228 hp.
Stepping up the rpm scale to 5,000 rpm means the torque numbers are nearly matched by the horsepower numbers since the mathematical equation relies on 5,252 rpm as the constant. This means that the horsepower and torque curves (for any motor ever produced) will always cross at 5,252 rpm. At 5,000 rpm, our 302 will equate to 287 lb-ft, and the same torque output at 6,000 rpm will allow our 5.0L to produce 343 hp. The higher the engine speed of a given torque output, the greater the horsepower production.
Because this is a simple mathematical equation, the inverse is also true. If our 5.0L produced 400 hp at 6,000 rpm, this would equate to 350 lb-ft. Producing the same 400 hp number down to 5,000 rpm would yield 420 lb-ft, while dropping it further to 4,000 rpm would produce 525 lb-ft (an output obviously not possible with a normally aspirated 302). Combining a given horsepower with lower engine speeds will yield greater torque numbers.
Taking this scenario to the extreme, we see that same 400 hp produced at just 3,000 rpm would unearth 700 lb-ft of torque and an astounding (and probably rod-bending and piston-smashing) 1,050 lb-ft down at 2,000 rpm. This is, of course, modified turbo diesel territory, but it is important to show the relationship between horsepower and torque as maximizing the horsepower or torque outputs may require rethinking where you want the motor to make max power.
This shifting of the torque curve can be accomplished with the installation of a wilder cam, a different intake design, or even a set of ported heads, and as we shall see, these gains become even more important once we apply boost.