At our two key points of 0.250- and 0.550-inch lift, the 195s flowed 159 and 268 cfm. That's up by 8 and 13 over from the 170cc head. As for swirl, the bigger port was a little down until about 0.400 lift, then it picked up to numbers similar to the 170cc variant. As for port velocity, here is how it all plays out: By enlarging the port by 25 cc, the bigger port has a mean cross-sectional area some 13 percent greater. However, the flow increases we see are about 5 percent at the 0.250 and 0.550 checkpoints. The net result is a decrease in port velocity by a nominal 8.5 percent. In simple terms, what this means is that this head would be better on an engine that was either 8.5 percent bigger or one that turned 8.5 percent more rpm. This makes sense as we already know from past tests that these heads work great on a 331 stroker. As for the exhaust port, we found that the port, though about 3 cc bigger, was marginally better on flow between 0.300 and 0.500 lift. This makes them close to the same as the ports in the 170cc heads.
195s On the Dyno
On the dyno, the 195s produced the output curves shown in blue in our Power sidebar. Probably the first point you will notice is that the torque at low rpm is down compared to the 170cc heads. This is how the situation remained until about the 5,000-rpm mark. From here to 5,500 rpm, the bigger port heads matched the smaller port heads. It wasn't until the rpm exceeded 5,500 that the bigger ports produced any extra output. Even then, the extra power only amounted to about 4 hp. Extra output over stock at 2,200 rpm amounted to just 1 lb-ft of torque. Peak torque was up by 12 lb-ft and peak power by 72 hp.
What all this means is though this head is still an effective piece on a relatively big cammed 302, it appears much more suited to a 331 or 347-inch engine where it will be close to optimal in terms of port size. Just for the record, we tested this head in conjunction with a cam about 12 degrees shorter, and this considerably helps these 195 heads to outpace the stock heads' low-speed output.
If Some is Good, More must be Better
From the tests so far, it looks like port velocity is instrumental in delivering more power under the curve. If keeping port velocity up to some key value, such as we are doing here, can produce a fatter curve without sacrifice at the top end, then we effectively make a smaller engine run like it's got more inches. In other words, we're increasing efficiency.
Let's take the 170cc heads and pose a question: What can we do to increase flow to equal or better that produced by its bigger valve/port sibling while closely maintaining the same port volume? If we can do this, flow, velocity, and probably swirl will all be increased. If our theory is right, this should produce an engine with both more low-speed torque and high-speed output.
Since the basic form of the Dart ports flowed well, it can be assumed that the basic shape is effective. To go from here without embarking on a serious flow bench development program entails making only simple and more or less obvious porting moves. This meant limiting the metal removal to streamlining the guide bosses, blending in obvious irregularities (there were only a few of those), and making the most of a progressive radius on the short side turn. Apart from porting, the 1.94 intake valves would also be replaced by 2.02 inch units. All things being equal, this should improve the low lift flow, which has the effect of improving the top-end output and helping power hang on longer after the peak point has been passed.
Power PlayJust out of the...
Power PlayJust out of the box, the flow of either the 170 or 195 Dart heads was a huge increase over stock. This paid off by allowing our test engine to deliver a substantial increase in top-end output.The ported 170 head (green curve) took advantage of the Dart's inherently good as-cast form by allowing us to effectively port it by simply refining what Dart had already built into the port. Note how the ported head delivers more flow right off the seat. Also note how it tops out at about 0.550 lift. This is a good indication that the port is optimally sized for the valve and valve lift (0.560) involved. If a port continues to flow significantly more at valve lifts much above those to be used, it's a fair bet the port is too big for the job.
The seat design that Dart...
The seat design that Dart uses on the exhaust seat is an effective form and largely follows what is successfully used for the ultra-high-tech heads we see on Cup Cars and in NHRA and IHRA Pro Stock cars. This being the case, every effort was made to blend the seat form just downstream of the seat itself into the cast part of the port without altering the machined form on the seat insert. This, along with some streamlining of the guide boss and a short side cleanup, resulted in the green curve seen here. With only 2 cc of material out of the port, our detailing of the exhaust resulted in an increase in flow from about 0.075 lift up. Again, note how the curve flattens out by the time our intended 0.560 valve lift point is reached, thus indicating the port is no bigger than it need be for the job in hand.