Flow bench and CFD testing...
Flow bench and CFD testing can only take a design so far. Actual Pontiac engine testing is the final step in the development process. This engine is more radical than the 463 we tested with.
Intake-Manifold Design Considerations
An engine in a street-driven Pontiac possesses a unique number of contradictory requirements that make the task of a manifold designer very difficult. It needs to operate under transient throttle and varied speed conditions. Single speed or even race-engine manifold design is much easier; packaging considerations are allowed to follow function. The ideal manifold doesn't exist for a multispeed engine, so the goal of the engineer is to limit the compromises.
A manifold of good design will offer fast mixture delivery, low flow loss when compared to an open intake port on the cylinder head, uniform mixture distribution, low fuel film accumulation, and uniform temperature distribution. With a carburetor, temperature is an extremely critical issue. Heat is required when the engine is cold to help vaporize the fuel. Once warm, the same heat now becomes an enemy. For every 10 degrees Fahrenheit the temperature is changed, the power is altered by 1 percent. Heating also presents a problem by lowering the octane tolerance, making the engine more prone to abnormal combustion or detonation.
The volume of an intake manifold is defined by the length and cross section of its runners and plenum. The capacity of a transient intake system with single-point fuel metering has proven to significantly influence the engine response and emissions behavior. Runner cross section should be as small as possible for fast mixture delivery times, minimal wall wetting, reduced weight, and maximum rates of heating when cold and dissipation when hot. OE runners are usually designed for a balance of flow loss versus velocity. It's common for an intake designer in both the aftermarket and OE level to incorporate taper into the runner. The cross section of the runner would then be larger near the plenum and at a predetermined point start to close up to the size of the intake port in the head. Working under the venturi principal, this would accelerate the charge.
Physics states that if the pressure is kept constant and the size of the orifice is decreased, then the velocity will increase. Many cylinder-head porters, both amateur and professional, don't recognize this. When reworking an intake, they grind away all of the taper. This historically increases flow at very high piston speed but will diminish driveability and bottom-end torque.
The tapered spacer was designed...
The tapered spacer was designed by Keith Wilson a number of years back but were handmade. Advanced CNC-machining has allowed the prices to start at under $130.
Flow losses are higher in runners that are rectangular in shape than circular, therefore they need to have a larger cross-section.
Intake runner length on a wet-flow system is usually limited by packaging concerns and the need to keep the mixture from falling out and separating from the air column. By nature of these facts, engines using this style of induction will always be torque-limited. The length of the runners for a single-point mixture formation manifold is determined by factors completely different than used when calculating the length of a runner for a multi-point EFI system. EFI systems can enjoy the torque benefit of long runners for resonance tuning, while a wet-flow induction system needs to shorten the length for acceptable delivery times and to minimize fuel-film deposits.
On carburetor applications, the industry has always felt it was taboo to have a highly polished intake manifold runner; the logic being that a semirough surface should be retained to keep the fuel in suspension since it would add turbulence. This has been proven to not be true if the carburetor is tuned properly. Reputable engineering texts state that internal runner roughness plays only a minor role in keeping the fuel in suspension. If a problem of drop-out is present, it's usually a function of poor design in regard to cross section or velocity.