Muscle Mustangs & Fast Fords
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.