With only 2 cc of metal removal...
With only 2 cc of metal removal from the exhaust port, both flow and velocity were improved. Increased velocity, especially during the overlap period, is instrumental toward improving low speed torque without any downside for top-end output.
Modified 170s on the Dyno
Maintaining (or even slightly improving) swirl and port velocity at low speed should mean we don't lose anything at these lower speeds. While the increase in flow won't really pay off until higher rpm, the increase in compression should return dividends everywhere, especially at low rpm. A check of the red output curves in the graph confirms just that.
If this had been a short cam, we would have seen only about an 8 lb-ft increase at the 2,200-rpm point, but because the cam was quite big (for a street cam), an extra 16 lb-ft was realized. This little exercise should bring home the importance of matching cam and compression. The bigger the cam, the more compression is needed to make it work.
Here's the finished ported...
Here's the finished ported Dart 170s. Note that the chamber shape is virtually unchanged from Dart's original form.
Moving up the rpm scale, we see that the ported 170 heads pushed the peak torque some 20 lb-ft up over the best of the unported heads. Again, that will be largely from the compression increase, although improved airflow and port velocity factors are definitely starting to contribute to the output. As for peak power, this went up by some 30 hp over the bigger-port 195 Darts and hung on longer, allowing shift points to be raised to near the 7,000-rpm mark. About 10 of those extra horses will be from the compression increase and the other 20 from a combination of airflow and port velocity improvements.
From the results, you can see that having good airflow along with good port velocity, plus swirl and compression, results in a vastly superior power curve. The numbers speak clearly. Not only did we improve the low-speed torque for better street use, but we also added a staggering total of 101 hp at the top end. Mission accomplished.