All-out flow testing was done...
All-out flow testing was done on T&L's big Superflow bench to ensure consistency from one head type to another. Prior calibration established its accuracy.
Swirl was also measured during our tests. We won't show the curves because inferring whether or not a particular swirl characteristic is good, bad, or indifferent, is a science in itself. The swirl measurements made on our Dart heads all showed excellent swirl characteristics. In spite of having bigger ports (which, all other things being equal, normally reduces swirl) the heads tested had better-than-stock swirl performance.
As far as port velocity is concerned, we have a somewhat difficult situation on which to get a mental handle. The Dart head has a significantly bigger port than stock, so for a given flow, the velocity would be down. However, the valve is bigger, so for a given lift and flow bench depres-sion, more air is pulled in, thus boosting velocity. OK, that all looks simple enough, but the engine doesn't inhale air in the same manner as a flow bench. Let's consider the situation at low rpm. If the valve is open to the extent that it can more than satisfy the instantaneous need of the cylinder, then the depression drawing in air could be lower than we see on the bench. The net result is that the port velocity could actually be lower even though it was (as in our case here) higher on the flow bench. If we throw all the variables of port velocity, swirl, and flow into the melting pot, it becomes somewhat difficult to predict with any certainty what a head change as we are proposing here will do.
Advanced Induction's Phil...
Advanced Induction's Phil Odom used his CNC Newen seat and guide machine to cut a custom form on our 170cc heads to suit the bigger 2.02 intakes being installed.
Although swirl is out of the picture, the exhaust ports, flow, and flow velocity are also factors affecting the low-speed output as well as the top-end performance. One of the worst enemies of low-speed torque when a big cam is used is exhaust-flow reversion. Higher uniform port velocities on the exhaust side can have a considerable positive influence on low-speed output. For the exhaust side, our 170cc Dart Pro 1 flowed with its measured 63cc exhaust port and at the two key lift points, 123 and 185 cfm, respectively. The stock 52cc port, at those same key lift points, delivered only 92 and 121 cfm. This means even though the port was bigger, the Dart exhaust had more velocity because of the greater quantities it could flow.
170s On the Dyno
With the flow, swirl, and velocity character-istics we have in mind, let's see how the as-cast 170 Dart faces its true test of function on the dyno. Check out the curves in the Power sidebar graph. The green lines are the ones to look at compared to the stock head results shown in black. As can be seen, even though it had a much bigger-than-stock intake runner, the combination of swirl, flow, and runner velocity were such that output was improved right down to 2,200 rpm. At this point, the Dart 170 put out 14 lb-ft more than the stock heads. This combination also led to the peak torque going up by 16-17 lb-ft of torque and peak power by a satisfying 68 hp. In addition, the usable top-end rpm figure rose by about 700-800 rpm. By any standards, that's pretty good for just a cylinder-head change, but by no means have we finished the search for power from a set of heads.
Bigger Sibling
The 195cc Dart Pro 1 differs from the 170 in as much as the intake measured out at 25 cc more and the exhaust at 2 cc more. Along with this, the 195s have 2.02-inch intake valves instead of the smaller 1.94s of the 170 heads.
A Barry Grant 650 Road Demon...
A Barry Grant 650 Road Demon carb supplied the air and fuel mix to the variety of heads that were used on the test mule. Interestingly, the stock and the ported 170cc heads were, by three numbers, optimal on the smallest main jets.
Unless the two are side by...
Unless the two are side by side, it's hard to see the difference in port size between the 170cc port (left) and the 195 (right).
This drawing shows the seat...
This drawing shows the seat form programmed into Advanced Inductions CNC Newen seat and guide machine. Top cut was 37 degrees x 0.02, seat 45 x 0.06, followed by a 0.45 radius starting at 55 degrees and running out at 80.