Straight Pipe vs Performance Muffler
For most vehicles, having a straight piped or open exhaust system is highly impractical as it typically results in a significant loss of low-end torque and has dismal effects on fuel economy. To better understand why opening up the exhaust system can have negative effects, consider the following:
Exhaust Scavenging - In a simplified and ideal world, an internal combustion engine is nothing more than an air pump, drawing air in and pushing air out. Under this model, reducing restrictions in either the intake or exhaust systems yields greater efficiency. To a degree, this is true. However, engine operation is much more complicated and dynamic when it comes to performance characteristics. By performance characteristics, we're referring to more than just how much torque an engine produces, but where in its usable engine speed range it produces it.
Exhaust gas scavenging is a result of the flow velocity of a volume of exhaust gas. Exhaust gas flow through each exhaust manifold runner is not continuous, it is pulsed. The duration between exhaust gas pulses decreases as engine speed (RPM) increases. As a "pulse" of exhaust gas flows from its isolated exhaust manifold runner into the collector and a common (shared) exhaust pipe, a vacuum trails the stream of gas. This vacuum then contributes to the evacuation of exhaust gases from an adjacent cylinder. Back to the air pump model, the exhaust scavenging effect reduces pumping losses during the period that the engine must evacuate the cylinder of exhaust gases before continuing onto its next intake stroke.
The exhaust scavenging effect is the reasoning behind tuned and/or equal length headers, because they ensure that exhaust pulses do not converge at the collector at the same time as this would reduce the velocity of each pulse and therefore the strength of the scavenging effect. It is impossible to develop an exhaust system that provides an ideal scavenging effect at every engine speed, and therefore engineers are forced to trade performance in one range in order to gain it in another. Truck engines, for example, are generally developed to exhibit greater low end performance at the cost of power at greater engine speeds. Likewise, most race car engines are designed to reach peak power at relatively high engine speeds, and thus low end performance is likely to be comparatively lower.
Exhaust Backpressure - Many friendships have been ended over the topic of exhaust backpressure. Some argue that an engine needs exhaust backpressure, others argue that, returning to the air pump model, an ideal situation would ascertain no backpressure whatsoever. Both arguments can be validated and both claims are correct, but misleading. Reducing exhaust backpressure can have positive effects on performance and fuel economy by reducing pumping losses, and these pumping losses are minimized with no exhaust backpressure whatsoever. However, there is a point of diminishing return where backpressure becomes low enough that the velocity of exhaust gas pulses and therefore the force of the scavenging effect dissipate. Without this effect, performance may actually suffer. Under such conditions, performance is usually impacted at low engine speeds where the duration between exhaust pulses is long. On the contrary, engines that suffer on the low end due to a minimized scavenging affect may experience improved performance in the mid to high range.
For a combination of these reasons most production vehicles, especially pickups, will favor a performance exhaust system/muffler over a straight/open exhaust.
Straight Piped F-150 Review
Straight piping your truck is going to result in severe sacrifices unless you intend to hit the rev limiter with every shift of the transmission. In our own experiences, we've found that passing power in the 4,000 to 5,500 RPM range is noticeably increased over a muffled exhaust system, but it comes at the cost of normal driveability and fuel economy. Our truck felt absolutely gutless below 3,000 RPM and had to drop gears on even modest grades. That's on top of losing roughly 4 mpg in city driving scenarios, although highway driving showed small to negligible effects on fuel mileage.
4.6L F-150 Straight Pipe Exhaust Clip
• OEM exhaust manifolds, catalytic converters, 2.5" exhaust tubing Y pipe
• 2.5" section of straight exhaust tubing in normal muffler location
• 2.5" to 3" transition at muffler outlet
• 3" tubing to 4" tip, side exit in front of rear passenger tire
Ready to give up performance and fuel economy to make a lot of noise? So were we, until we realized that you can get an equally pleasurable experience from a good chambered muffler. We still throw the straight pipe on from time-to-time just to get it out of our system. It earns an overall rating of 1 star simply because it is unrealistic, albeit invigorating. It's difficult to hide the sound, even under light throttle - a traffic ticket just waiting to happen.