Richard Holdener
July 8, 2013

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.

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As indicated previously, boost can best be thought of as a multiplier of power. The reason this works is that any normally aspirated combination is already subjected to what is known as atmospheric pressure. It is atmospheric pressure (roughly 14.7 psi at sea level and a given temperature) combined with a pressure drop in the cylinders when the piston draws down the cylinder that allows air to pass the valves and fill the cylinders.

A properly sized blower or turbo artificially increases this pressure differential (between the air in the manifold and the cylinder on the intake stroke). Using the author’s power/boost formula (boosted horsepower = NA horsepower x pressure ratio + 1), we can reasonably predict the power output of nearly any boosted combination with reasonable accuracy. All it takes is knowledge of the normally aspirated power output and the supplied boost pressure.

Using a 350 hp normally aspirated 5.0L as and example, if we supply 14.7 psi of boost (basically doubling the current atmospheric pressure) it is possible to double the power output of the 350hp 5.0L to 700 hp. The formula works at lower and higher boost levels, as 7.35 psi (1⁄2 atmosphere) should increase the power output by 50 percent to 525 hp. Adding 10 psi should increase the power output of our 350hp 5.0L by 68 percent to 588 hp, while 20 psi will yield an increase of 136 percent to 826 hp.

From the examples, it should become apparent that big boost is not the only route to big power, especially for street motors. If we apply 7.35 pounds of boost to a 300hp 5.0L, we increase the 300 hp by 50 percent to 450 hp. If we increase the power output of the normally aspirated combination from 300 hp to 400 hp using ported heads, a cam, and revised intake manifold, the same 7.35 psi will increase our 400hp motor to 600 hp. Improving the power output of the normally aspirated combination by 100 hp resulted in a gain of 150 hp once we added 0.5 bar (7.35 psi) to the boost.

The gains increase even more as we further increase the boost. You see, the power gains on the NA combination are actually multiplied by the boost pressure, so it's easy to see why starting with a powerful normally aspirated combination is so important. Given the problems associated with big boost pressure (elevated charge temps; increased detonation and possible engine damage), a more powerful NA motor with lower boost is a better combination.

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Now it's time to get to the good stuff. Theories are all nice and convenient, but nothing compares to actual dyno results. To properly illustrate the accuracy of the power/boost formula, we assembled three different test motors and subjected all three to some positive pressure.

Test mule number one consisted of a stock 5.0L 302 that had been subjected to a few internal upgrades. In anticipation of boost from a (new) single-turbo kit from CX Racing, the '91 5.0L motor was fortified with a SCAT forged steel crank, matching 5.090-inch forged connecting rods, and 0.030-over JE forged flat-top pistons. The short-block was assembled using new rings from Total Seal, and fresh rod and main bearings. Topping the forged rotating assembly was a set of fresh E7TE heads (valve job, surface, and new seals), a stock 5.0L stick cam and production H.O. upper and lower intake. With the exception of the forged rotating assembly from SCAT and JE, the 302 was otherwise stone-stock.

This stock motor would serve as the baseline to verify the accuracy of the power/boost formula. The idea was to run the motor in stock trim, then again with a new turbo system. We would follow by subjecting a modified 5.0L to the same test at the same boost levels.

The stock 5.0L was run on the engine dyno with Hooker headers using a Holley Dominator EFI system and 36-lb/hr injectors. Originally rated at 225 hp by Ford, our test motor produced peak numbers of 261 hp at 5,100 rpm and 321 lb-ft of torque at 3,400 rpm.

The long-runner 5.0L intake combined with the small-port (and valve) heads and mild cam to produce a torquey output, bettering 300 lb-ft from 2,500 rpm to 4,400 rpm, but these components also limited power production higher in the rev range because the restrict flow as the rpm increases. These numbers pale in comparison to the 450-plus horsepower (on the dyno) produced by the modern 5.0L Coyote using the same test procedure.

Now it was time for some boost. The CX Racing turbo kit featured dedicated tubular exhaust manifolds, a cross-under pipe, and T4 turbo flange. A wastegate was used to regulate boost, while a massive air-to-air intercooler would minimize charge temperatures. The kit featured 3-inch polished-aluminum tubing on the cold side to maximize flow to the motor, while spent exhaust exited through a 3-inch down pipe. In addition to the turbo kit, CX Racing also offered a twin-turbo kit, which we plan to test soon.

The single kit comes with a 72mm turbo standard, but upgrades are available up to 76mm. Looking to test the kit on stock and modified 5.0L motors, we stepped right up to the 76mm turbo. Since boosted motors generally require additional cooling, CX Racing also developed a bolt-in aluminum radiator upgrade for the 5.0L applications.

Adding the turbo kit to the stock 5.0L required drilling and welding a drain hole in the stock oil pan. After bolting on the exhaust manifolds in place of the header, we ran a oil feed line to the 76mm turbo. The kit included the necessary stainless steel V-band clamps and silicone couplers to connect all the tubing from the turbo to the intercooler and then to the throttle body. We mounted a fan in front of the intercooler to supply a cold-air source while dyno testing.

With the supplied wastegate spring set for 7 psi, we tuned the Holley EFI system to provide safe air/fuel (11.8:1) and timing curves (22 degrees). The peak power number was up to 391 hp, while torque jumped to 471 lb-ft. Using a manual boost controller from Turbo Smart, we stepped boost to 10 psi, which brought 429 hp and 542 lb-ft of torque, while 14 psi took us just over 500 hp with 615 lb-ft of torque.

Though turbo response was artificially enhanced on the engine dyno (stationary loads help spool up), this stock 5.0L application would be better served with a smaller 72-, 70-, or even 60-series turbo. It is at this torque output that you worry about the strength of the production 5.0L block.

After testing on the stock 5.0L, we moved to the second turbo application. This test was run with a single-turbo system from HP Performance, but utilizing the same 76mm turbo from CX Racing. As with the stock 5.0L, this modified 302 was run both normally aspirated at identical boost levels to the stock 5.0L. The modified 302 also featured a forged reciprocating assembly from SCAT and JE, but further enhanced with CNC-ported aluminum cylinder heads from Procomp Electronics, a Crane hydraulic-roller cam (0.542/0.563 lift, 224/232 duration, and 112 LSA), and Holley SysteMax upper and lower intake.

Run in normally aspirated trim, the modified 302 produced 413 hp and 389 lb-ft of torque. Compared to the stock 5.0L, power production was higher in the rev range, though there was no loss in power as low as 3,000 rpm. The heads, cam, and intake simply offered more power everywhere and allowed the modified motor to make peak power 1,000 rpm higher than the stock combo.

The extra 152 hp offered by the modified motor over the (mostly) stock 5.0L carried over once we added boost. Running the same 7 psi, 11.8:1 air/fuel, and 22 degrees of timing, the turbo combination belted out 601 hp and 570 lb-ft of torque.

Duplicating the testing with the stock 5.0L, we adjusted the boost level supplied by the 76mm turbo to 10 psi, and the modified 302 produced 688 hp and 656 lb-ft of torque. At this point, you might be expecting the results of the modified motor run at 14 psi, but the results would likely be less than stellar for two reasons. First, the 76mm turbo was nearing its limit. Though we have exceeded 750 hp on one occasion, calculations told us that the modified 302 combination might exceed 800 hp at 14 psi. Second, not only was this beyond the flow limit of the turbo, it was also well beyond the strength limit of the production block. We were taxing the block at just 10 psi (and 656 lb-ft), so we didn't risk stepping up to 14 psi with this combo.

Looking strictly at the math, we see that the extra 152 hp offered in normally aspirated trim by the modified motor translated into 210 hp at 7 psi and 259 hp at 10 psi.

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Since we didn't get a chance to run the modified 302 at 14 psi, we decided to compare it to a test run previously done by MM&FF. Needing the appropriate strength to crank up the boost, test-motor #3 checked in at a full 363 ci. The 302-based stroker featured a 4.125-bore Dart block stuffed with a custom turbo-grind roller cam from Cam Research Corp. Feeding the 363 stroker was a set of CNC-ported 225 heads from Airflow Research and a Box R intake from Trick Flow Specialties.

In normally aspirated trim, the stroker produced 519 hp and 469 lb-ft of torque. Running a larger 76mm turbo from Precision Turbo, it produced 759 hp at 7 psi and 858 hp at 10 psi. Things got serious at 14 psi—the 363 stroker topped the 1,000hp mark with a peak of 1,003 and nearly 900 lb-ft of torque.

If you're keeping score, this 363 stroker produced twice as much power as the stock motor with the same boost level. If you are looking for a powerful 5.0L, make it big and bad before adding boost.

22. The combination of the healthy stroker and a properly sized turbo from Precision Turbo yielded impressive results. Run at 7 psi, the stroker produced 759 hp, while 10 psi brought 858 hp. The strength of the block and internals allowed us to crank up the boost on this application to just over 14 psi, where the turbo stroker produced over 1,000 hp! Which 14-psi motor do you want, the stock one that makes 500 hp or the stroker that makes 1,000 hp?