Muscle Mustangs & Fast Fords
Boost Pressure on 5.0L Applications - Big Bad Boost
Effect of boost pressure on different 5.0L applications.
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.