Here's the Newen-generated...
Here's the Newen-generated intake seat as it came off the machine. From here it took only minor handwork to blend the seat insert into the previously ported bowl.
In addition to valve size, low lift flow is greatly influenced by valve-seat form. To make the most of the chance to recut the seats to a form we knew to normally work well, we took the heads to Advanced Induction. This company, which has its roots in NASCAR racing, is a family-oriented business run by Phil Odom. Its specialties are no-compromise CNC heads and induction systems for discerning high-end street and race clients. To support such endeavors, Advanced Induction uses one of the new, high-tech, Newen single-point, CNC seat and guide machines. The beauty of using Odom's Newen is that it allows the user to design the seat required right on the machine.
With the intake seats recut to suit the bigger valves, the seat inserts' lower part was blended into the rest of the port bowls. On the exhaust side, similar simple porting techniques were used to smooth out the short side turns and to better streamline the guide boss. All this simple work paid off as can be seen from the flow curves shown nearby.
The smooth-flowing contours...
The smooth-flowing contours of the reworked 170cc heads intake port can be seen here. To get to this form from Dart's as-cast form took little in the way of metal removal and resulted in a high-flow, high-velocity port well suited to our application.
Unfortunately, flow graphs do little to show the extent of improvement at low lift, so let's look at some numbers to bring the point home. First, the new valve at 2.02 inches in diameter is some 4 percent larger. This means if it's utilized at exactly the same efficiency as the valve it replaces, it should be 4 percent better. If the seat form is also of a more efficient design, that will also increase the flow. Countering this is the fact that as the valve size increases, so does the shrouding caused by the cylinder wall.
Let's see how the numbers shape out. At 0.025 inch lift, the flow with the bigger valve was up from 16 to 19 cfm for a 14-percent improvement-not bad for a starter. At 0.050-inch lift, the gain was from 34 to 36 cfm for almost 6 percent improvement. We see this trend all the way up to the peak valve lift that we're going to use. On average, the flow increase is about 8 percent, and this has been achieved with a port volume that is only up from the measured 165 cc to 170-just 3 percent. What this means is for any given flow rate, port velocity has also increased. Now we have heads that will deliver more flow and more velocity. In addition, if we have not altered the basic port shape, we should also see a little more swirl activity. Our swirl meter confirmed this was in fact the case.
Power These curves convincingly...
Power
These curves convincingly demonstrate what a set of well-designed and "spec'd for the purpose" heads can do for output. Although top-end figures are most often used to quantify the success or failure of heads to deliver, it is in fact the total area under the curve that decides the issue. Even though the 195 heads were better suited to a bigger-inch engine, the potential low-speed loss due to lower port velocity appears to have been offset by this head's strong swirl characteristics. When the smaller-port 170 heads were used, the increased port velocity provided an extra measure of low-speed cylinder filling that the bigger port heads did not. The result was a 16 lb-ft improvement in torque at 2,200 rpm. The smaller port paid off all the way to about 5,300 rpm before the bigger port head surpassed its results. By porting the smaller port heads to get the big port flow and then increasing the compression, some really spectacular results were achieved (red curves). In all, the ported heads delivered 101 hp more than the stock heads and considerably flattened out the torque curve everywhere in the rpm range.
Compression Comprehension
The stock heads and the as-cast Dart heads delivered a measured compression ratio of between 8.87 and 8.94:1, so for the sake of simplification, we will round this out to 8.9:1.
Our ported Dart heads were machined some 0.050 thousandths to reduce the chamber size to 52 cc.
This, on our short-block combination, gave a CR of 10.1:1. Using a basic equation for thermal efficiency, this increase in CR should deliver 2.9 percent more torque everywhere in the rpm range. However, the formula assumes that the intake valve opens at TDC and closes at BDC.
In reality, we have a pretty big cam in this engine, and the valves open and close way before and after both TDC and BDC. This means the effective compression ratio, one that applies using the intake valve closing point, is much lower than the theoretical or static CR. As a result of this, an increase in ratio of 1.2 is actually a bigger percentage increase of the CR the engine experiences than it initially seems. The result is that with a big cam, the low-speed torque can increase considerably more than might otherwise be expected. Also, increasing the CR causes the exhaust gas speed to increase at just the time when it's most important to do so. That is right around TDC in the overlap period. This cuts the tendency for the exhaust flow to go into reversion and delay the onset of intake flow into the cylinder.