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Camshaft Testing - When More Is Less
Why More Boost Isn't Always Better.
The more the merrier-it's a common theme in the performance world. On the surface, it seems like a logical statement. After all, if some performance is good, then isn't more always better? The obvious answer to such a black-and-white question is, yes, but as always, there are subtle shades of gray.
Camshafts are a perfect example, as it's often possible to increase the peak power output of a combination by increasing the lift and duration specs on a cam. The problem is, the increase in power at the top of the rev range often comes with a drop in power elsewhere in the curve. Larger cams also require more valvespring pressure. In addition, putting a huge cam in a street engine can create a drop in torque production, idle quality, fuel economy, and overall driveability. The same thing happens with things such as oversized carburetors, static compression, and even cylinder-head flow, as all offer the potential for increased power, but the potential comes with trade-offs that must be addressed to enjoy said extra power. All of a sudden, more is not better.
One area where it seems more is better is with boost pressure. Doesn't more boost always result in more power? Naturally, this assumes things such as tuning are taken into account, but it's not out of line to state your engine will make more power at 10 psi of boost than it did running 7 psi. As with our previous examples, the more-boost-is-better theme comes with a few stipulations.
Increasing the boost pressure increases the cylinder pressure (especially true of positive-displacement supercharged and/or turbo motors), and with it the likelihood of detonation. Adjustments must be made in the form of timing, air/fuel, and even octane level to counteract this revised detonation threshold. Oftentimes, a substantial portion of the power gains offered by the increase in boost pressure are offset by the required drop in ignition timing. The same can be said for a required richer air/fuel mixture, though you can't cure timing-related detonation with increased fuel supply. The shortcomings of increased boost pressure can be combated to some extent with the introduction of intercooling. Obviously, a lower charge temperature decreases the likelihood of detonation, which is why most every OEM forced-induction system comes with some sort of intercooling. Essentially, there's no free lunch, unless the chef happens to be serving up a heaping side order of race fuel.
Another way to combat high boost is to reexamine your goals. It's our desire for more power that actually makes us crank up the boost, right? Understanding this statement is important, as our goal isn't usually more boost per se, but more power. If additional power is what we're after, there are other ways to achieve this goal-ways that will actually decrease the chance of detonation. How is that possible, you ask-more power with a reduced risk of detonation? The answer is to actually lower, rather than raise, the boost. Now everyone knows that less boost equals less power, right? Well, that's actually only half right. If we dropped the boost pressure by installing a larger blower pulley (spinning the blower slower relative to the engine speed) or reducing the wastegate setting on our turbo, then the answer would be, yes, we will make less power. However, if the drop in boost pressure comes from an increase in efficiency of the boost combination, then we achieve our goal of increased power with a decrease in boost pressure.
While it seems counterintuitive that we can increase power while decreasing boost pressure, a better understanding of what boost pressure actually represents is in order. I know it may come as a surprise to some, but boost pressure is actually nothing more than a measurement of backpressure. Whether it's from a blower or turbo, the boost pressure we see on the boost gauge is nothing more than excess air that is having difficulty making its way into the engine. If we increase the flow of air to the motor by increasing the blower or impeller speed relative to that of the motor, then we see an increase in backpressure. The increase in pressure provides more airflow to the chambers every time an intake valve opens, but it also increases the likelihood of harmful detonation.
What if instead of increasing the impeller or rotor speed, we increased the displacement of the motor? What if we installed better cylinder heads, a wilder cam profile, or both? As you might have guessed, the answer is that the same airflow provided by the blower or turbo supplied to a more efficient motor will result in a drop in boost pressure. But since the engine can process more air, this drop in boost pressure will result in a net increase in power. In a small twist of fate, the flow rate of a supercharger or turbo (or any pump really) is proportional to the pressure it supplies. That is to say, a blower, turbo, or even fuel pump will flow less at 40 psi than at 20 psi. This, of course, assumes we keep the impeller or rotor speed constant.
A few examples will work wonders here. The first is a test we ran years ago on a Kenne Bell supercharged 5.0L. It illustrated the gains offered by a wilder cam profile on a 302 equipped with a KB supercharger. The 302 was equipped with a set of AFR 185 heads, a GT-40 lower intake, and a stock 5.0 stick cam. The motor was run first with the stock 5.0 H.O. cam and then again with a Comp XE274HR grind. It's not surprising that the motor made more power with the Xtreme Energy cam, but the side benefit of making the motor more efficient with the aggressive cam timing was a drop in boost pressure of just over 1 psi. This came with no change in the blower pulleys because all we did was change the camshaft. The drop in boost pressure from 8.6 psi to 7.4 psi resulted in a jump in power from 440 hp to just over 480 hp. Imagine that-a jump in power that was accompanied by a decrease in boost pressure. You're probably thinking we should have cranked up the boost again to match the original 8.6 psi, but that wasn't part of the test. In our last example with the turbo motor, the turbo self compensates for the artificial drop in boost pressure.
The second test was run on another 302 motor equipped with a set of stock E7TE heads, a stock 5.0 H.O. cam, and a Holley/Weiand 174 supercharger. Just as with the test run on the KB-supercharged 302, this test with the Weiand blower combined a drop in boost with an increase in power. This test paired a cam and head swap together to offer some serious power gains on the supercharged 302. The carbureted motor was first run with a set of stock E7TE 5.0 iron heads and a stock 5.0 cam. After running the supercharged combination, off came the stock components, and on went a set of Canfield heads and an XE266HR cam. The Xtreme Energy grind was one step below the cam run with the Kenne Bell blower, but it offered a significant step up in performance over the factory grind. Improving the heads and cam increased the power output of the motor by nearly 50 hp while simultaneously dropping boost by 1.6 psi. The new cam and heads improved the motor's ability to process air (increased its efficiency), which decreased the amount of backpressure in the intake manifold. The net result was a drop in boost with an increase in power.
The final test illustrated was a recent one with a turbocharged 5.0. This time, we ran the 5.0 with stock components, from the intake and throttle body right through the heads and cam. The single HP turbo system was configured to supply 10 psi of boost to the stock motor. After running the car on the dyno and at the track, we replaced the stock 5.0 heads, cam, and intake with pieces from Lunati, Holley, and TFS. Upgrading these components offered a huge power gain, somewhere near 150 rwhp at the same boost level.
Unlike the superchargers, which have their rotor speed regulated by the relationship between the crank and blower drive pulleys, the turbo isn't regulated by impeller speed. It's regulated by boost, so running 10 psi on the stock motor produces a given impeller speed, while running the same 10 psi on the upgraded components results in an increase in impeller speed. Had we been able to keep the impeller speed the same, the gains would have been less significant, but since the blower supplies 10 psi (in our case) to both combinations, the result was a sizable jump in power with the new components. What we could do with the turbo is drop the boost pressure to 8 psi and still have a significant gain in power over the stock components running 10 psi-thus we would have more power with less boost. Due to the self-compensating nature of the turbo, what we got instead was a lot more power at the same boost level.