In order to understand the impact of intake port volume on performance, the folks at Dart Machinery tracked down engine guru, David Vizard, and asked him to write up the results of his testing on the topic.
Their aim was to clarify the relationship bewteen port volume and flow, as well as horsepower and torque. Vizard is an engine research consultant, university lecturer and world renowned performance tech author.
The results are interesting and for the full explanation, you should head on over to the Dart site and check it out. The short version is that bigger isn’t always better.
Part of this stems from the flawed perception that an engine is just a big air pump – it isn’t. What goes on in the intake and exhaust paths is far more complicated than, for example, an air compressor.
If you understand that exhaust tuning is possible with long tube headers, then it should not be a surprise to say that the intake system’s characteristics – of which intake port volume is one – can also have an impact on the final results. Sometimes we consider issues that, in the end, really don’t have much overall impact, so this is the first area that Vizard looked at.
Using Dart Platimun Pro cylinder heads with four different intake port volumes ranging from 180 to 230 cc, he compared the flow rates achieved at different valve lifts and found vurtually no difference amoing the four, until lift passed the 0.400-inch mark. The differences were just starting to become important with a 0.500-inch lift, and Vizard notes that “…to tap into the full potential of the bigger ports, a valve lift of 0.600 was necessary.”
Still, we’re all more interested in actual output, so it’s off to the dyno room to see what’s what. While the flow tests indicate that higher ports volumes can be beneficial, it all depends what you’re after. After running the dyno tests and separating the HP and torque results for easier viewing, several interesting things emerge.
On the torque side of things, Vizard observes that “The dyno results tell us that smaller, higher velocity ports, favor low speed output. These results also show that going too big on the ports produces worse results almost everywhere in the rpm range.”
Where horsepower is concerned, the story is different. “It is easier to see what is working best at the top end of the rpm range by looking at the hp curves. Here we see that the 215 cc port equaled or beat the 230 cc port everywhere, thus proving bigger is not always better,” he noted.
So, what port volume is best for your engine? Vizard showed some recommendations specific to a 23 degree headed small block Chevy, with power output ranging from 350 to 700HP. This essentially shows that you need to start small and increase the port size as power increases. However, Vizard offers some sound advice as well.
“Do not overestimate the power you are likely to see. All that will do is lead you into a bigger port than would be optimal. This leads to less power than you had hoped for. At the end of the day, a little too small is better than a little too big!” Other engines may have different starting points, but the same principles will apply.