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.