Words And Photos: Richard Holdener

When it comes to making power, nothing beats boost. Don’t get us wrong, we love a nasty, all-motor build as much as the next guy, but when it comes to serious power levels, boost is the great multiplier. Even nitrous can’t compete with the ability of a turbo or blower to double, or even triple, the power output of a normally aspirated motor. Obviously there are limitations to what can be expected from a good turbo or blower, but it’s hard to argue with the extra power offered by just a few extra PSI of boost.

The only thing better than boost, is adding that positive pressure to an already powerful normally aspirated combination. We will get into the specifics later, but since boost is a simple multiplier of the original power output, starting with more power before you add boost will result in more after you add boost. This scenario holds true for any combination, including our 5.0L Ford test motor. Hardly a power player today, the original 5.0L fuelie motor was all the rage back in the day, but with the right parts it can still be made to produce serious power.

While it is true that making boosted power is much easier if you start with a bigger or more powerful normally aspirated combination, the 5.0L Ford is one example where the extra expense of a stroker assembly might not be warranted. Let’s start our discussion with some theory then follow up with some actual dyno results. Rather than make you wait until the end, the reason for the choosing the 5.0L Ford for this test was because of the inherent power limitations. Though plenty powerful in modified form, the thin-wall castings that helped produced a light-weight block ultimately limited power production.

The factory 5.0L hydraulic-roller block itself was actually the weak link in any performance build up, especially one that involved forced induction. When subjected to excessive cylinder pressure, the stock blocks have been known to split from the main webbings up to the cam journals. We have seen plenty of 500 horsepower 5.0Ls running around, but pushed to 600 horsepower (or worse 600 lb-ft), the stock blocks are living on borrowed time, especially when subjected to the shock loads and stresses of drag racing.

In almost every other situation, we would recommend starting with additional cubic inches to reach any given power goal. This allows the power level to be reached in a milder state of tune which in turn provides increased drivability. For powerful street motors, bigger is always better, but we don’t recommend that route with a turbocharged 5.0L (unless a block upgrade is part of the equation). If you plan on sticking with a stock 5.0L block, stick with the stock displacement, which will allow you to run the factory crank. The rods and pistons should be swapped out, but we have seen plenty of 5.0L performance builds that started with nothing more exotic than the entire stock short block.

Many factory 5.0L H.O. motors came with forged pistons, but the lack of adequate valve reliefs will limit cam choices. Why not run a 331 or 347 stroker with the stock 5.0L block on a turbo motor? The answer is quite simple, as the boosted 302 can provide more than enough power to split the block, so why go to the expense of the extra displacement? A little math here will help illustrate the situation.

Having recently run a stock, modified and turbocharged 5.0L Ford on the dyno, we had power numbers to help illustrate the potential. Run in stock trim with 1 ¾-inch Hooker headers, no accessories and no air intake, the otherwise stock, fuel-injected 302 produced 261 hp and 321 lb-ft of torque. We know from our power/boost equation that if we double the atmospheric pressure supplied to the motor by adding 14.7 psi of boost, we can (theoretically) double the power output.

Applying this formula to our stock 5.0L, we see that doubling the 261 hp and 321 lb-ft of torque at 14.7 psi should give us (261 x 2)=522 hp and (321 x 2)=642 lb-ft of torque. Now we wouldn’t recommend running that much boost on a stock motor, but it does show you that we have already exceeded the torque limits of the stock block. What we would recommend is to replace the factory heads, camshaft, and intake on the 302 to increase the power, a procedure we performed on our 5.0L. After adding RHS Elite heads, a COMP XE274HR cam, and Holley SysteMax intake, the power numbers jumped to 395 hp and 380 lb-ft of torque. Doubling these now would result in 790 hp and 760 lb-ft of torque, or well beyond the limitations of the stock block.

As luck would have it, the turbo can be run at any given boost level, allowing us to dial back the pressure ratio and attending power level. Running just 7.35 psi (50% of 14.7) would result in a 50% increase in power. If we multiply the 395 hp offered by our modified 302 times 1.5 we get a more realistic 593 hp. The added benefit is that our modified 302 now makes more power at 7.35 psi than the stock 5.0L did at a much more problematic 14.7 psi. Now let’s take a look at what happens when we add displacement. Using our 395-hp 302 as an example, we see that the 5.0L makes 1.30 hp per cubic inch (393/302). If we apply that same specific output to a larger 347, we get (347×1.30)=452 hp.

The larger 347 would reach the 593 hp level at just 4.6 psi. On paper it might seem that using the 347 would be beneficial, but remember that the stroker crank, rods, and pistons all cost money and the extra 2.75 psi of boost required to reach the same power level does not. It is very easy (and cost effective) to crank up the boost a few pounds (no one runs around with just 4.6 psi). Besides, the 593-hp level (and similar torque output) is already pushing the limits of the stock block. The moral here-If you plan on running a turbo 5.0L with a stock block, stick with the 302 displacement and add boost (then get a good block after you get greedy and turn up the boost).

The power output of a stock 5.0L seems pretty pathetic given the current level of performance, but the right modifications can really wake up the little fuelie. Tested on the dyno, the stock 302 (with headers, no accessories or air intake) offered 261 hp and 321 lb-ft of torque. Once modified with RHS Elite heads, a COMP XE274HR cam and Holley SysteMax intake, the power output of the 302 jumped to 395 hp and 380 lb-ft of torque. Even with the stock short block, heads, cam, and intake will always wake up a factory 5.0L.

The difference in power offered by the larger 347 really makes it look enticing, after all the 347 produced 455 hp and 418 lb-ft of torque. That the 347 offered impressive power gains through the entire power curve is why they are so popular, but the stroker kit might be a wasted expense if you plan on running a turbo on the stock block.

These curves illustrate why a 331 or 347 stroker kit might be unnecessary if you plan on running boost. The single turbo kit increased the power output from 395 hp and 380 lb-ft of torque to over 620 hp and 630 lb-ft of torque at just 10 psi. That put this 302 well into the red zone for block longevity, all without the aide of additional displacement. Sure, a stroker will allow you to reach any given power level at a lower boost pressure but since you can already reach the block limitations with the 302, why spend the extra bucks on the stroker kit when boost is free?

Sources

Aeromotive
aeromotiveinc.com

Accufab
accufabracing.com

COMP Cams
compcams.com

Holley/Hooker
holley.com

L&R Automotive
lnrengine.com

Lucas Oil
lucasoil.com

MSD
msdignition.com

RHS Cylinder Heads
racingheadservice.com

Turbo Smart
turbosmartusa.com