Engine builder George Pils...
Engine builder George Pils tunes the carburetor on our Dirt Late Model project car's engine in an effort to evaluate our intake system. Photo by Rob Fisher
There was a time when a race engine's parts were selected based on the performance potential of each component. In fact, specialty parts manufacturers often designed toward the optimized potential of their parts with little regard for their impact on other components. Today, for engines from Sprint Cup to Saturday nighters, the functional integration of basic components is essential to reliable, appropriate, and repeatable power. Most of the responsibility for making component choices falls on the engine builder or racer assembling their own engines. So with that in mind, we offer this sidebar as part of the concluding steps in this Series of stories.
Shaping The Torque Curve We previously noted that an engine's volumetric efficiency (VE) characteristics are closely tied to the shape of its torque curve. Aside from the influence of pumping losses and based on its achieved level of combustion efficiency, VE and torque curves are fundamentally similar.
Let's restate that. If you want an engine to produce torque in its lower- and mid-rpm, it's necessary to select parts that enhance breathing characteristics in this span of engine speed. In fact, that approach applies to whatever rpm range of operation you've selected. And while an increase in mechanical compression ratio will also provide torque boosts in the selected range of rpm, increased cylinder pressure at higher rpm can be hindered if the engine fails to have sufficient air. So, VE is key to making torque where you want it produced.
In earlier CT tech material, we briefly discussed the concept of how airflow velocity relates to volumetric efficiency to the extent that you can design, modify, or build intake and exhaust systems on the basis of how they affect air rate. We further suggest that flow rates associated with torque vs. rpm (notably torque boosts at selected rpm) could be based on flow passage section area. Here's the linkage stream: Flow rate vs. torque boost vs. rpm vs. piston displacement is tied to flow passage section area. And while other engine parameters have an influence on the characteristics of a given torque curve, the design of intake and exhaust systems can have significant impact on VE performance.
Much has been written to describe the dynamics of intake and exhaust systems. To elaborate on the complexities of how contemporary design and analyses of these systems can be accomplished at the Sprint Cup level serves no immediate benefit to the Saturday night racer. However, those elements that can be reduced to simpler and more hands-on suggestions are helpful. It's in this context that the above information and what follows has been compiled.
Functionally Integrating Intake And Exhaust Systems These are powerfully influential components in a race engine, certainly in terms of where in the rpm range torque is produced. Regarding how they influence engine output, you can view them separately or in conjunction with each other. But with either approach, it's important to recognize and understand that both have significant impact on VE (torque) performance. To optimize their effectiveness, you need to decide on the range of rpm where the engine will be most often operated; minimum to maximum (we alluded to this in a previous Series segment.) Making such a determination isn't always a simple process, but the outcome can materially affect your parts selection.