Cylinder head porting is often equated with time and expense, and we've all heard horror stories about porting through the casting and ruining the cylinder head. For these reasons, many enthusiasts are hesitant to port their own heads, due to a lack of experience, and reticent to have the engine builder or machine shop perform the work for fear of adding expense to the rebuild. But while extensive race-style porting jobs can take many hours of grinding, sanding, and testing to perform correctly, the fact is that most of the benefit of a port job can be seen in just a couple of hours per cylinder.
For most bracket, street, or dual purpose cars, spending a little time on the cylinder head ports can greatly improve flow, increasing the engine's power substantially without killing your budget. In our experience, most factory heads will gain a significant increase in flow simply by matching the ports to the intake and header, and cleaning up areas like where the pushrod “pinches” into the intake port and the bowl area under the valves. Performing a little port work is especially beneficial if your engine will be equipped with a high-flowing intake manifold and headers, since the ports (especially in factory heads) can become the most restrictive part of the engine.
1 Porting cylinder heads...
1 Porting cylinder heads isn't for the novice engine builder, so if you're not comfortable we suggest taking your heads to a professional machine shop. With a little practice, however, it's fairly easy to gain flow without hurting anything. Practicing on a head that is cracked or unusable is also a good way to gain experience.
2 In addition to practice...
2 In addition to practice or experience, you'll need a high-speed grinder and an assortment of carbide bits for removing heavy material, and sanding "tootsie rolls" for the finish work and polishing.
2b. In addition to practice...
2b. In addition to practice or experience, you’ll need a When porting aluminum heads spraying the carbide cutters with WD-40 will help keep the bits from loading up with material.
To prove our theory, we decided to flow the intake and exhaust runners of several popular factory cylinder heads and then port an intake and exhaust runner to compare the difference. We'll be performing our testing on the Superflow flow bench at Auto Performance Engines, so the numbers you see are real-world flow numbers for popular Mopar big-block cylinder heads. For comparison, we'll also flow two popular aftermarket heads, the 440 Source Stealth and Edelbrock Performer RPM, then port the intake and exhaust runners of the Edelbrock head and measure the difference on the flow bench.
Remember that we're not out to gain every last cfm of airflow through these heads, we simply want to show what spending a little time on the ports will produce in terms of flow numbers. To keep our testing legitimate, we will only spend one hour porting and polishing each runner, which will only allow us to hit the above mentioned beneficial areas and clean up the casting flashing of the runners, bowls, and areas under the valve seat. We also stayed with the factory valve sizes for both the factory and aftermarket cylinder heads, as increasing valve size and performing trick, multi-angle valve jobs is a whole other area in terms of cylinder head work.
3 We use the intake and exhaust...
3 We use the intake and exhaust gaskets as a reference for how big to make the port opening. Remember, we aren't taking a bunch of material out of the ports, just enlarging them to the size of the intake and headers and cleaning up some key areas.
4 When looking at the chamber...
4 When looking at the chamber of this unported cast iron head, you can see how the valve guide protrudes into the chamber. By simply blending this area, flow is increased dramatically.
5 Once blended, the valve...
5 Once blended, the valve guide mo longer inhibits airflow as much as it did. For a basic porting job, the benefits of cleaning up this little area is well worth the time spent. This in no way is a complete guide to porting, but you can visualize the intended results so you can do it yourself.
While it is tempting to get crazy with the grinder while porting heads, removing too much material from the ports not only risks damaging the heads, but significantly increases the volume of the runner which is not so good for low and mid-range torque. So rather than whittling away a considerable portion of the runners for maximum high-end flow, we're removing a reasonable amount of material and cleaning up key areas that should improve power throughout the normal rpm range of a street engine.
Follow along, and after showing you the flow numbers for three factory and two aftermarket cylinder heads, we'll show you why spending a little time porting key areas of your cylinder heads can really pay off.
6 We had a set of factory...
6 We had a set of factory 516, 902, and 906 casting heads at the shop, so we measured the flow of the intake and exhaust port of one cylinder on each head for a comparison. The untouched (except for hardened exhaust valve seats) 516 had a best flow of 228 cfm at .700 inch valve lift. These heads are a closed-chamber design and are a very similar casting to 915 heads. Our factory 902 casting big-block cylinder heads flowed very much like the 516s at low lift numbers, but didn't have the high lift flow capabilities of the closed chamber 516s. As expected, the 906 castings had the best flow numbers in stock form, nearly 238 cfm on the intake side, when compared to the other factory heads we tested. These numbers also apply to the later 452 casting cylinder heads, which flowed identically and have the added benefit of hardened exhaust valve seats. Realistically, most moderate street or bracket engines won't have .700 lift, so the flow at .500 and .600 lift is more important to most people.
7 For comparison, we tested...
7 For comparison, we tested a couple of aluminum aftermarket heads including the Stealth heads from 440 Source. The Stealth heads flow far better right out of the box than any of our stock cylinder heads, netting 258.5 cfm at .500 inch valve lift. The Edelbrock Performer RPM head is a favorite with many enthusiasts for the big-block Mopar, and these numbers show why. In stock form, the Edelbrock intake port flowed a best of 291 cfm at .600 lift, while the exhaust port flowed 232 cfm at .700 lift.
8 The 452 heads really responded...
8 The 452 heads really responded to our work, netting best flow numbers of 282 cfm at .600 lift and 248 cfm exhaust flow at .700 lift. Flow was up across the board, and average flow was way up as well. This definitely shows that there is flow (and power) to be had by giving the ports a little attention.