The Big Easy: Boost vs the Block

In the battle of boost vs. the block, the little 5.0L block is always the runner up.

In the battle of boost vs. the block, the little 5.0L block is always the runner up.

Words and Photos By Richard Holdener

Before even making the statement, we fully understand the ramifications, but more boost isn’t always the answer. Okay, turbo (and blower) guys, let the internet flaming commence, but before you rally the fellow keyboard villagers to get the proverbial torches and pitchforks, let us finish. More boost does indeed make more power. BUT, in the case of a 302 Ford stroker built with a production 5.0L block, more not only isn’t better, but it can be downright disastrous. The problem is less about how cool extra boost is, than how easy it is to exceed the strength limitations of a factory block.

Before illustrating how boost can be bad, we first need to understand the nature of boost pressure and the elements associated with pressurized performance. From there, we can illustrate an example of how to actually make MORE power with LESS boost, then finish up with how the stock block ultimately becomes the limiting factor.

Whether supplied by a blower or turbo, boost is additional airflow supplied to the motor that it might 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 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 (remember the word almost in the sentence). The problem is generic statements related to boost tell only a fraction of the truth.

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 Ford (or any stock motor), the results will be considerably less power than subjecting a stroker and/or heavily modified 302 to the same pressure. A related question is internal component strength, as the pistons in the modified motor making an extra 200 hp must withstand considerably more cylinder pressure than the stock 5.0, despite identical boost pressures.

Boost is often referred to as positive pressure, but the reality is boost is a measurement of back pressure. It can best be thought of as indication of a flow restriction in the motor. Basically, boost pressure indicated on the gauge is the amount of flow supplied by the turbo (or blower) 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.

We actually tried a couple of different turbo configurations on this 331 before disaster struck, but almost any (reasonably) size turbo has enough power potential to split a stock 5.0L block.

We actually tried a couple of different turbo configurations on this 331 before disaster struck, but almost any (reasonably) size turbo has enough power potential to split a stock 5.0L block.

The easiest way to illustrate this point is with a simple statement, followed by a few examples. As we have tried to stress time and time again, the best route to an exceptional forced induction motor is to start with a powerful normally aspirated combination. Building power in the normally aspirated combination is a simple matter of improving the airflow through the engine with improvements in cylinder head flow, intake design, and cam timing.

Improvements in power production before adding boost are important. As previously indicated, boost can best be thought of as a multiplier of power. The reason this works is that your normally aspirated motor is already subjected to what is known as atmospheric pressure. It is atmospheric pressure (14.5 psi at sea level and a given temperature) that provides the motivational force for the airflow to venture into the negative pressure created by the downward moving piston. A properly sized blower or turbo artificially increases this pressure differential.

Using the author’s power/boost formula (Boosted hp=NA hp x Pressure ratio +1), we can get a good idea of the power output of nearly any boosted combination with reasonable accuracy (note the term reasonable). 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 14.5 psi of boost (basically doubling the current atmospheric pressure), it is possible to double the power output of the 350-hp 5.0L to 700 hp. The formula works at lower and higher boost levels, as 7.25 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 350-hp 5.0L by 69 percent to 591 hp, while 20 psi will yield an increase of 138 percent to 832 hp.

From these examples, it should start to become apparent big boost is not the only route to big power, especially for street motors. If we apply 7.25 pounds of boost to a 300-hp 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 up to 400 hp using ported heads, a cam, and revised intake manifold, the same 7.25 psi will increase our 400-hp 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 .5 bar (7.25 psi) of boost.

Run with the blow-through turbo system, the power output of the 331 stroker increased from just over 400 hp to 761 hp, but it was the 766 lb-ft of torque that eventually pushed the stock block to its breaking point.

Run with the blow-through turbo system, the power output of the 331 stroker increased from just over 400 hp to 761 hp, but it was the 766 lb-ft of torque that eventually pushed the stock block to its breaking point.

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 is 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 always a much more effective combination (especially for street use).

The ease at which power gains come with boost (turbo or blower) can be both a blessing and a curse for 5.0L Ford owners. Even with a bone-stock 5.0L rated at 225 hp, it is possible to exceed the strength limits of the stock block by simply adding a turbo system (with no other changes). Making modifications to the normally aspirated motor only mean you will reach this point at a lower boost level. This was an important lesson learned when running a 331 stroker turbo combination.

The stroker came courtesy of a Scat crank, Speedmaster rods, and JE pistons. Topping the stocker was a set of Edelbrock Victor Jr. aluminum heads, a COMP XE266HR cam, and Performer RPM Air gap intake. Run with an MSD distributor, Hooker headers, and a BG 650 carburetor, the 331 stroker produced 405 hp at 6,000 rpm and 391 lb-ft of torque at 4,800 rpm. Running a compression ratio of just 9.2:1 meant the combination was ready for some boost.

15 (1200x900)

Less of a dedicated kit, the single turbo system employed on this 331 consisted primarily of components we had left over from HP Performance. Those leftovers included tubular exhaust manifolds, a three-inch cross-over tube, a Holset 66-mm turbo, and various hot and cold-side tubing sections. The kit originally included an intercooler, but we deleted this piece and ran boost directly to the blow-through carburetor. The HP system did include a T-4-flanged mount for the turbo, a dedicated waste-gate flange designed to accept a Turbonetics Delta gate, and three-inch down pipe. Using these components, we configured the outlet (discharge) section of the turbo to blow through a carb enclosure supplied by Vortech.

After dialing in the air/fuel and timing, we were rewarded with impressive power gains from the turbo system, but after running the boost up to a hair over 13 psi (and 761 hp), we noticed a drop in oil pressure. Sure enough, the excess cylinder pressure had split the block from the main bearing journal to the cam bearing journal. The motor was still running, but continued use would surely spell disaster.

This adventure showed us two things: one being how easy it is to add power to a typical 302 with displacement, bolt-ons, then boost, but more importantly how important it is to use a good block when making big power. In the battle of boost vs. the stock block, boost always wins!

331 Ford Stroker-NA vs Turbo (13.4 psi) Before the block split, the turbo motor posted some impressive power numbers.  Run in normally aspirated trim, the 331 stroker produced 405 hp and 391 lb-ft of torque. Credit the displacement, COMP cam, and Edelbrock induction for the plethora of power. After adding the single 66mm Holset turbo, things got serious. Running a peak of 13.4 psi, the turbo combination produced 761 hp and 766 lb-ft of torque. It is actually the massive swell of torque that ended the life of the stock block, and this thing exceeded 700 lb-ft from 3,600 rpm to 5,700 rpm.

331 Ford Stroker-NA vs Turbo (13.4 psi)
Before the block split, the turbo motor posted some impressive power numbers.
Run in normally aspirated trim, the 331 stroker produced 405 hp and 391 lb-ft of torque. Credit the displacement, COMP cam, and Edelbrock induction for the plethora of power. After adding the single 66mm Holset turbo, things got serious. Running a peak of 13.4 psi, the turbo combination produced 761 hp and 766 lb-ft of torque. It is actually the massive swell of torque that ended the life of the stock block, and this thing exceeded 700 lb-ft from 3,600 rpm to 5,700 rpm.

Sources: ARP, Arp-bolts.com; COMP Cams, compcams.com; Holley/Hooker/NOS, holley.com; JE Pistons, jepistons.com; MSD, Msdignition.com; Scat, scatcrankshafts.com; Speedmaster, Speedmaster79.com; Turbonetics, turboneticsinc.com; Vortech Superchargers, vortechsuperchargers.com

About the author

Elizabeth Puckett

Elizabeth is a seasoned writer and hardcore gearhead. She was born with motor oil in her blood and a passion for everything that goes fast.
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