How heavy would you guess...
How heavy would you guess this block of air at standard temperature and pressure is? To give you an idea of the size, we have parked a 20ft long GM/Chevy extended cab truck on top.
Port Volume -- bigger is not always better!
For port volumes most racers think 'bigger is better.' This is due to not realizing how heavy air is or knowing what an idealized port should look like and how it works.First, let's deal with the question of 'how heavy' air is. Take a look at the 100ft cube of air in Figure 1 below and see how closely you can guess its weight. Having made your guess, we'll tell you it's 38 tons. Chances are, you completely underestimated how heavy air is and as a result underestimated its velocity-induced cylinder ramming potential.
In Figure 2, we see an idealized port form. On the right is the manifold plenum and at the other end is the bowl just prior to the valve seat. Bringing the air up to speed in the high-speed section and slowing it down in the expansion section achieves two important things. First, allowing the air to slow in the expansion section allows it, in a real port, to make the turn to the back of the valve easier. Second, the expansion turns some of the kinetic energy of the high-speed section into pressure energy. This in turn raises the air's pressure at the valve thus helping to push it into the cylinder. In a race engine, bringing this hig-speed air to rest at the end of the intake stroke can result in as much as 7lbs. of velocity-induced port pressure, which all goes toward supercharging the cylinder. Your street motor can benefit in exactly the same way, so don't assume that bigger is always better.
If we could straighten out...
If we could straighten out and idealize the form of a port, this is what it would look like. The key to its success would not only be the fact that it is a very efficient shape but also a form that maximizes the effects of energy contained in the velocity that is built up in it.
Port length is a factor toward...
Port length is a factor toward maximizing horsepower but the reality is that port velocity due to the port's area is even more important, especially if a wide power band and high specific torque is a goal.
Swirl = Torque
Swirl is very important for low- and mid-range torque and, with an automatic trans, it's torque that launches the car. Small-block Chevy heads have a port that has a natural tendency to generate good swirl and this can be enhanced by biasing most of your port reworking to the cylinder-wall side of the port. Many Ford heads are much poorer in terms of swirl and often have little or none. To help offset this, make a conscious effort to smooth out both the finish and the curves of the cylinder-wall side of the port. By helping the flow stick better to this wall some additional swirl can be generated. In a 5.0 this feels just like extra cubes.
A SB Chevy's intake port has...
A SB Chevy's intake port has a natural tendency to swirl because most of the flow takes place on the cylinder wall side of the port. As a result, most of the air enters the cylinder on the 'A' side of the valve and continues in the direction indicated by the arrows.
SB Ford heads typically lack...
SB Ford heads typically lack swirl because most of the air tends to flow on the cylinder center side of the port and dumps straight into the cylinder. To boost the ports swirl capability you need to get the air to stick to the outside wall better so more flow occurs along the path indicated by the arrows. Making as smooth a radius of the dog leg as shown here helps.
Compression -- The budget racer's friend
Increasing compression increases torque throughout the rev range. At low speed it has just the same effect as more cubes and at high rpm the same as more cam. It makes all your other mods--especially a bigger cam--work that much better. It improves drivability and gas mileage. If milling the heads and manifold is in your budget and you are sure piston to valve clearance is not an issue go for all the compression you can consistent with the fuel octane to be used.
Five Golden Rules for Successful Porting
1:Give the air room to move where it wants to go, not where you think it should go.
2:Identify the points of flow restriction and fix them (as far as they can be fixed) in the order of severity.
3:Develop charge motion (swirl) as this increases burn rate and reduces octane requirement.
4:High-speed ports with minimal redundant volume ram cylinders better and result in better mixture quality, higher torque and a wider power band.
5:Go for as much compression as the situation will allow.
The author has not ported every single head on the market, but of the many he has, here is his pick of the easiest to work on.
Iron Chevy SR Torquers. Cheap to buy and easy to do for excellent results for truck or real street application. Chevy Sportsman II and Roush 200 Ford. Reasonably priced and can be easily reworked for semi-serious race applications.Aluminum Ford Windsor Lites and Junior Lites. With a hot street cam (240-245 @ 50) and 12:1 CR can be easily ported for over 1.5hp-per-cube.
Iron Eagle. Slim guide bosses and emery roll the rest of the cast surfaces and you are done with a set of heads ready to make some semi-serious race power.
Chevy and Ford Track 1. These heads are good out of the box but with a basic port and chamber tidy -up become very effective entry-level race heads.
SB Ford and Chevy. All the Canfield heads from their smallest port model to the largest deliver good flow out of the box but can be significantly improved upon with little more than basic guide boss and port cleanup. The price is also good.
Airflow Research (AFR):
Have used a lot of AFR products over the last 25 years- especially their Chevy castings. Whether dyno'ed in 'out of the box' or ported form, they have always delivered the goods. Example: a race hydraulic flat tappet cammed 12:1 350 Chevy equipped with cleaned-up AFRs topped 550 hp. AFR's SB Ford heads also score big.
SB Ford Performer and Performer RPM. Edelbrock made some changes a year and a half or so ago without announcing it. These changes made a good head design even better. High intake swirl right off the seat gives these Ford heads Chevy low-speed torque. Along with good swirl are good flow numbers for power. Basic porting can be accomplished with little more than a few emery rolls.
SB Chevy Very good flow and swirl out of the box and can be quickly ported for pro looking/functioning results with almost zero cutter work.
SB Chevy Vortec. Blending out chamber plug boss with a cutter is the longest job you will need to do on these heads. After that the ports need only be smoothed out with 60 or 80 grit emery rolls to produce a top-notch pair of street heads that will deliver both ends of the rpm scale.