If, as has so often been said, an engine is nothing other than a simple air pump, then flow numbers alone should dictate the engines final output. From this it should follow that if it's just flow numbers we are after, then bigger must be better.
Unfortunately, reality is somewhat different. A four-cycle engine (or any engine, for that matter) is far from being a simple air pump. The principle reason-compacting simplicity into complexity is the dynamic "stop-start" nature of the flow through the engine. Rapidly changing rates of flow bring about effects caused by both the momentum and the pressure waves generated by the rapidly changing flow rates.
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, and anything more than a few percent bigger or smaller is not. What we are going to do here is test to see what the effect of changing port volume has on a cost-conscious hot street engine's power curve.
The Test Engine.
The test engine used here was our trusty T&L-built 383. Basically, this engine's role in life is to run such tests as we are doing here. The plan was to run four pairs of the latest Dart Platinum heads on this engine. These heads had intake port volumes of 180, 200, 215, and 230cc. Now it might seem that this is an easy test to do-just take a strong-performing engine and run four sets of heads across it. That would be nice, but to get meaningful results, things are not that simple, and a major problem arises from the way flow increases as valve lift goes up. Here's the situation. At low valve lift values, say around 0.050 to about 0.150, the flow has little to do with the port size because the limit is set by the still minimal through-flow area between the valve and the valve seat in the head. It is not until the valve lift exceeds about 20 percent of the valve's diameter that the port size/flow efficiency begins to influence matters. Check out the flow curves in Fig. 1 and you will see that the majority of the flow increases with increasing port volume occurring 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.
Another factor is that a cam with a relatively short duration would also be needed so any low speed attributes of the heads could be seen. If the cam was too long, it would prevent the engine from running decently at low speed so any head volume that favored low speed would look worse than it possibly was. For this reason, we had to pull the flat tappet cam from our test engine and install a roller as these have a greater lift capability for any given duration above about 270 degrees.
For this we had Comp grind a custom single-pattern Xtreme profile (part No. 3192) shaft on a 106 LCA. This hydraulic roller profile has 276 degrees of "off-the-seat" duration and 224 degrees at 0.050-inch tappet lift. This, coupled with a peak lift of 0.605-inch when paired with a set of 1.6:1 rockers, got the job done. If you are building a true street 383 small-block Chevy and you want a decent idle and street drivability along with stout torque and horsepower numbers, you might want to consider this grind spec-you won't be disappointed!
All the heads to be tested had 72cc combustion chambers (64cc ones are also available) which, with the combination of deck height, piston valve notches, and gasket thickness, gave our test engine a 9.8:1 CR. Had we opted to test the 64cc items, the CR would have been bumped to 10.7:1.