David Vizard
September 1, 2007
Our tried and trusty 302 dyno mule made a good head test unit. Run on T&L's dyno, it consistently produces good results and fits the format of many street-built 302s.

When you finally decide that more power is needed for your precious Ford, you'll have to determine whether to add cubes, boost, nitrous, or to make the existing engine more efficient. Any of these routes can result in more power, and it doesn't even take much in the way of engine tech IQ. It's the more subtle things, such as what is a point in compression ratio worth, how big should the ports be, and how much will extra airflow be worth in terms of additional output, that most don't understand but would love to know.

Our plan is to start with a stock pair of airflow tested heads on a mule motor and develop a set of baseline power and torque curves. From here, we will install a set of as-cast Dart Pro 1 170cc heads. Then we'll progress to a set of as-cast 195cc heads and then on to a set of ported 170cc Dart Pro 1s with a reduced combustion chamber volume so the CR is raised by a ratio of 1.2:1.

Our 302 was equipped with a set of KB's low-cost forged pistons. Nothing exotic here, just a tough piece that will take the abuse of dyno testing even with a heavy dose of nitrous.

The Mule
A successful engine is one that has a parts combination that works together in an orchestrated fashion. In this case, the bottom end was anything but exotic. The stock crank and rods were used with a set of flat-top two-valve-relief, 0.020-inch-over KB forged pistons. We used the rods in full floating form rather than the stock press fit by honing the rod pin bores to give about 0.001 inch clearance. This means we could change pistons as required without destroying a perfectly good piston in the process.

For a cam, a 280-degree Comp Cams High Energy, single-pattern street roller was used (profile No. 1474). This, with 1.6 rockers, gave us 0.560-inch lift at the valves. This profile was chosen because of the smooth dynamics, while being aggressive enough to produce good output. The cam operated through a set of Comp's solid roller lifters and Magnum pushrods to operate the valves via their budget Magnum stainless rockers. Other than this, all the other bottom-end parts such as the timing chain, oil pump, water pump, and so on, were stock parts. We did, however, use a Moroso pan as its greater volume and surface area help dissipate the heat of repeated dyno runs.

The mule was equipped with a Comp 280 Magnum solid roller as this has good dynamics and produces consistent results. This was important as we were testing heads not cams.

For induction, an Edelbrock Performer RPM Air Gap intake along with a 650 Barry Grant Road Demon were used. Ignition was fired by means of a billet PerTronix distributor with a mechanical advance curve to suit the cam specs. So, as you can see, our dyno mule was far from exotic and, for that matter, had some considerable test time on the card anyway.

Before we get started, let's define exactly what it is we are trying to demonstrate. We are all aware that more airflow normally equates to more power, however, this is only the case where the higher-flowing heads allow the engine to breathe in a part of the rpm range where the original heads did not. At low rpm, almost any cylinder head will allow the engine to fill the cylinders as much as they will need. The problem comes when rpm goes up and the cylinder filling time is shortened to milliseconds. Under such circumstances, if the heads (and the induction system) don't breathe, the engine doesn't produce.

At this point, making power might look simple-just make the heads as big as possible to flow as much air as possible. Unfortunately, there's far more to it than just plain, old flow as measured on a flow bench. Air has considerable mass and is heavier than you might think, and in terms of cylinder filling, air is considered a fluid.

The valves were operated by Comp's Magnum 1.6:1-ratio cast stainless rockers. The rockers are cost effective and deadly reliable.

To give you an idea of what air weighs, figure a typical school gym contains between 20 and 30 tons of air. Surprised? Most people are. If that air moves fast, we can utilize the kinetic energy it contains to not only ram into the cylinders at high speed, but also reduce flow reversion at low speed. Getting the port area and shape just right for the engine combination being used means more power at the top end where a previously air-starved engine can now breathe as well as the low end. That's the end of the power curve we use 95 percent of the time on the street, so it should at least be a realistic priority.

Another item on the agenda is swirl. Without good mixture motion, the combustion process is compromised. If a head has good swirl, it helps improve combustion quality, especially at low engine speed. In a nutshell, good swirl often equates to good low-speed torque.

The last factor we'll consider is compression ratio. This is as important for a street-driven machine as it is for an all-out race car. The compression ratio has a considerable influence on the size of the cam that can be used before low-speed output becomes unacceptable. The higher the compression ratio, the more cam that can be used. Also, more compression equates directly to more mileage-something that can't be overlooked these days.

Getting Started
With the carb and ignition dialed in, our 302 mule engine produced the numbers (rounded to the nearest whole number) listed here.


Even with stock heads, it's a pretty stout unit. This can be largely attributed to the improved breathing capability imparted to the heads by the significantly greater valve opening area produced by the cam and supported by the 650 Demon carb and Performer intake combination. Because we are minimizing potential restrictions elsewhere in the induction system, any superior character-istics the new heads have should pay off big with significant results.

Test 1
The first head change was to a set of as-cast 170cc Dart Pro 1s. The intake port volume on these was measured at 165 cc and the exhaust 62 cc. So you have a reference point-the stock heads are typically in the low-to-mid-120s on the intake and low-to-mid-50s on the exhaust. The 170 Pro 1 heads are intended to be used as a direct replacement on an engine that has an otherwise stock bottom end. That means they must have valves that are not too big to be accommodated by the stock valve cutouts in the pistons. To do this, the intake valve is sized at 1.94 inches. That's up an appreciable amount from the stock 1.78 but significantly less than the typical 2.02-inch normally used when aftermarket pistons are in the engine.

Flow curves for the entire lift range are shown in the sidebar. So, we can make a comparison quickly and easily by looking at two reference points that will, for all practical purposes, define the heads' ability to get the job done.

These two points are the flow at 0.250 lift and the flow at peak valve lift as delivered by the cam and valvetrain used. In our case, that's 0.560, but to make life easier we'll use the 0.550 lift point because it's close enough. At these two key lift points, the intake on a stock head delivered 121 and 155 cfm, respectively. On our freshly calibrated flow bench, the 170cc Dart Pro 1 delivered 151 and 255 cfm. To put that into perspective, at as low a lift figure as 0.250, the Dart heads delivered flow numbers almost as good as a stock heads at maximum lift. At the peak valve lift point, the Dart Pro 1 head produced a full 100 cfm more than stock.

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 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.

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.

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 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.

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.

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 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.