Trends in Intake Port Design or Modification
With big valves and extremely high lifts, it is not surprising that the ports of today's Cup engines are quite large (in volume). These ports might be confused with drag racing-style ports from just a few years ago. In general, circle track rules tend to prevent ports from being raised to the extent that is legal in drag racing. However, one of the current trends has been to use a "line-of-sight" philosophy when creating the intake system. If you've been around the development of racing engines for very many years, this almost amounts to a "duh." Ed Winfield would be proud of this approach.

Line-of-sight, as it sounds, can be depicted by drawing the straightest line possible from the carburetor (or fuel injector) throttle plates to the intake valve. An intake manifold of this type is usually combined with large-volume ports that emphasize power at its peak and beyond. This is an excellent technique, when combined with a high rpm engine package that includes the appropriate camshaft. Typically, a line-of-sight porting technique produces shorter runner lengths and sacrifices a bit of torque at torque peak and below, unless the camshaft's valve events are adjusted to compensate.

Traditionally, circle track engine builders prefer smaller-volume ports to improve performance at lower engine speeds, enhancing throttle response. Today, a new technique is being used which has a slightly different philosophy. This technique is to make the intake valve large in area and the ports large in volume, to minimize any ducting losses, and uses a line-of-sight strategy for the intake track. In turn, this can improve low rpm performance when used with shorter and more aggressive cam profiles.

Such an approach achieves good low-speed torque, provided by the valve event timing and aggressive lobe profiles, and good power that results from the extremely efficient and large-volume ports that combine with the line-of-sight induction system. The traditional approach uses longer intake runners and smaller port volumes and longer valve events that are intended to build lower speed performance and improve peak power.

While both techniques achieve similar results, the new line-of-sight technique seems to generate a better torque curve and still make more peak power, while carrying the power past its peak. This type of intake system minimizes ducting losses in the intake system and minimizes air/fuel charge separation in the inlet track, when compared to the smaller-volume and bigger-cam approach. This seemingly small gain in efficiency makes a difference, because of the incredible flow rates required to support the power of today's Cup engines. It can work for your Saturday night applications, too.

Exhaust Ports
The majority of the mass flow through the exhaust port comes at high pressure ratios (cylinder pressure vs. atmospheric pressure) at or near sonic flow conditions. Under these conditions, the exhaust gases flow off the back of the valve in a line (as straight as possible) to the exhaust port roof and then out of the cylinder head.

Like the intake, the throat of an exhaust port is the key to improving exhaust port performance. Funda-mentally, the exhaust port throat must be as large as possible for a given valve size. Exhaust port development (and flow) must be kept in balance with port design/modifications on the intake side.

Intake Manifolds
Intake manifolds are the primary tuning component for four-stroke, spark-ignited engines. Inlet runners function as organ pipes. The optimized length of a runner will provide wave reinforcement (energy) at the correct moment of the engine cycle to increase the pressure in the cylinder, before the valve is shut. This "wave reinforcement" improves the filling of the cylinder (volumetric efficiency) and thus increases torque and power. Wave reinforcement occurs over a very narrow engine speed range. Longer runners tune at lower frequencies, like large organ pipes, and shorter runners tune at higher frequencies (or engine speed). The frequency of the tuning point directly relates to engine speed as follows:

* Longer runners "tune" at low engine speed
* Shorter runners "tune" at higher engine speed