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.
Now for the intake side of things. Here, a carb and manifold decision had to be made. Whatever was chosen had to work well at both ends of the rpm range. Also, in terms of a match between heads and intake manifold, the manifold had to accommodate four different sizes of ports. The intake chosen because of very promising previous dyno tests was Dart's 180cc two-plane design. To accommodate the port size variance, the 180cc heads had a small chamfer applied to the intake ports of the heads, as the manifold was slightly larger. From here, on the manifold runners were opened up over about the first inch into a size just shy of the port size in each successively larger head.
Feeding the fuel to the system was a race-prepped 950 Holley. With this induction system, the engine had access to sufficient airflow for a good top end, while still catering to whatever low speed the smaller port heads might deliver. Undoubtedly, had we used a single-plane race intake such as a Victor Jr. or the like, the bigger port heads could well have shown a greater top end advantage over the smaller ones. Countering this, a race-style single plane could have compromised the smaller port heads' ability to deliver a stronger low speed output. In all, the induction system used proved to be a globally effective compromise.
Quoting port sizes by volume seems to have come about for two reasons. Way back when the only choice of heads was those produced by the factory, ports for street applications were about it. With less than ideal castings to start with, I can say from experience it took a lot of grinding to maximize the castings' capability. To give the customer an idea of how much work had been done, the head porters would quote port volumes as a measure of what had to be taken out to achieve the end product. Bigger equated to more work and thus more money.
When the castings were as limited to the extent those early ones, the more that came out, the faster the car went, and the sooner the customer would be back for more ported heads to replace those that cracked. And believe me, if you wanted to go fast, cracking had to be an accepted outcome.
So demonstrating the amount of work done on castings was one reason for quoting port volume. The other was to get some idea of the size of the cross-sectional area of the port. Since the port area changes substantially as the port progresses from the manifold face to the valve, quoting size in square inches is not practical
The other option, since the traditional small-block Chevy head has a five-inch long port, is to quote port size in terms of its volume. The bigger the volume, the bigger the mean cross-sectional area of the port is.
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 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.
As can be seen from the flow tests (Fig. 1), the bigger port does flow more up at the higher valve lift numbers. Part of this is due to a bigger intake valve, but about 70 percent of the additional flow in the 0.500 range up is because of the bigger port, not the bigger valve. So the big port/big valve combo flows the biggest numbers. The question is ,how does this work out on the dyno?
Now's time to look at a load of curves on our graphs. So you can better see what's going on here, the torque and hp graphs have been separated. The effect any particular head has on low speed output can be more clearly seen by considering the curves shown on the low end of the torque graph. To see what happens at the top end look at the high speed results on the hp graph.
As can be seen from the torque graph Fig. 2 on page 72, the 180cc ports (black curve) produced the best output up to 3,400 rpm, peaking at a stout 482 lbs-ft. The 200cc port (red curve), though was not lagging by much, and from 3,400 rpm up it ran up with or close to the bigger ports.
If we look at the torque curves and also consider the hp curves in Fig. 3, we can see that for our 383 incher, with the cam it had and the rpm range it would operate in, the 200cc ports delivered the best curve. The 215cc (green curves) heads made the highest hp by pumping out 478 horses, as opposed to 457 for the 180cc runners, 472 for the 200, and 475 for the 230s. The price the 215s pay over the 200 to achieve this seven hp advantage is that they give away up to 10 lbs-ft of torque in the rpm range from 2,300 to 3,200.
The 230cc port runner heads failed to deliver any worthwhile superiority anywhere in the rpm range on our engine. Indeed the smaller 215cc port heads beat the 230s everywhere! This, in case it was needed, is living proof that an engine is not a simple air pump, nor for that matter is bigger better. Had we targeted an engine capable of more rpm or one with bigger displacement, then the bigger port heads would have paid off. Experience with ports in the 230-245 range show that every bit of the port size is needed if you are building a 440-cube small-block. If we look at a comparison on a pro-rata basis, a 235cc port on a 440-inch small-block Chevy is only equivalent to a 186cc port on a 350.
So how do you decide what port volume your small-block Chevy should have for best results? As good a rule of thumb as any is to base the port volume, and we are only talking traditional 23 degree heads here, on the projected power output. A point to note here is that if you rate your engine's final output too optimistically, you will end up with a port that is too big and the target output will not be reached. In other words, you will have shot yourself in the foot.
Check out the chart (Fig. 4). This will give you a good starting point for port volume selection. With Dart's selection of port volumes, they pretty much have the range from a relatively mild 350 (180cc ports) to a rampant 440-incher (230cc ports) covered. One more point worth noting for those higher hp engines here is that these Dart heads are really easy to port. Doing so can get the port volume right where it needs to be along with more flow.
Just remember that a port a little too small will be a far better deal to drive than one that is a little too large. Just 20cc too big can easily cost 20 lbs-ft at a point in the rpm band that is most often used for a true street driver.
One final point, just in case you are wondering. With a bigger cam and 10.5:1 CR, our 200cc Platinum Darts, on this engine, allowed it to crank out 500 lbs-ft and a tad over 502 hp.
This chart gives a good working guideline as to the intake port volumes to target for a small-block Chevy. Remember, a little too small is much better than a little too big. If the port volume of the heads you have seems a little too big, go for all the compression ratio the fuel intended will stand, as this will, to some extent, compensate.