By David Vizard
A 4-cycle engine is far from being a simple air pump. The principle reason turning apparent simplicity into real world complexity is the dynamic ‘stop – start’ nature of the flow through the engine and the fact that air is very much heavier than is often supposed. Rapidly changing rates of pressure and suction bring strongly into play both the momentum and the pressure wave effects that can be used to boost cylinder filling. At the end of the day this means that for a given displacement and rpm band there is a set of ports that are right for the job. Anything more than a few percent bigger or smaller is not. The following tests will demonstrate the difference delivered by a range of port sizes.
The Test Engine
The test engine, a 383 Scat cranked stroker, is typical of the majority of small block Chevy’s built these days. Intended for use with 89 octane fuel this engine is well within the budget of most serious hot rodders.
For this test a Scat 3.75 inch stroker (budget cast steel series 9000 crank was teamed with Scat’s budget 6 inch I beam pro-comp rods. Pistons were forged Silvolite ICON items. These in conjunction with the “as-cast” 72 cc combustion chamber Dart heads, delivered 9.8/1 CR. Had the 64 cc Dart heads been used the CR would have been bumped to 10.7/1 for use with 93 octane.
For what it is worth the torque and horsepower jump by about 15 numbers with the CR increase. For this engine the cam used was a custom 276 hydraulic roller grind done to precisely cater for the cylinder head characteristics, CR and displacement involved. If the saving of about $250 is of interest then it’s worth noting that a hot street hydraulic flat tappet cam with a 280 duration will deliver very comparable results.
Headers used were a set of street Hookers with 1- 5/8˝ primaries and a 2-1/2˝ secondary (collector) 18 inches long. The carb was a entry level 750 cfm AED unit mounted on a Dart two plane intake. It is worth mentioning here that it is, for this test to be valid, important for the intake manifold is capable of servicing the needs of the engine at both low and high speed. This means a manifold that flow well by virtue of with efficient ports not big ones. The Dart two plane item met those needs. At each cylinder head change, which ran small to large, the intake manifold was re- matched to the bigger ports at the manifold face. Ignition was via a Pertronix contactless unit. These are low cost and get the job done well.
The Test Heads
The intent here is to run four pairs of the Dart Platinum Pro 1 heads having intake port volumes of 180, 200, 215, and 230 cc. Regardless of port volume all these heads flow about the same cfm until some 0.400 inches of valve lift have taken place (see Fig 1). Even at 0.600˝ were the test engines valve train tops out there are measurable flow differences. Because of these differences are the result of a port size increase they are a legitimate component in our bid to investigate the overall effect on the engines torque/power curve.
Check out the flow curves in Fig 1. and you will see that the majority of the flow increases with increasing port volume occurs at the higher lift value. So much so that any test that did not put enough lift into the valve to access the extra flow at high lift would be totally skewed in favor of the smaller port heads…
We talk port size in cc’c but the reality is that it is the port cross sectional area that is the factor we wish to control. So why is port cross sectional area important? If the area is bigger the flow surely goes up and that’s what we want is it not? Sure the engine wants as much airflow as possible but much of the flow through-put depends on port velocity and the generation of pressure pulses. This means an overly large port can hurt power even though it may, on the flow bench at least, flow better. The question is how does this work out on the dyno?