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Porting For Performance The Science of Horsepower

By Larry Carley

Air flow makes horsepower. The more cubic feet per minute (cfm) of air that flows through the intake ports and into the combustion chambers, the more horsepower the engine makes.

A larger carburetor or throttle body, bigger valves, a high-lift, long-duration camshaft, three-angle valve job and exhaust headers are all performance enhancements that increase airflow for more power. Add a turbocharger or supercharger, and even more air is forced through the engine. But, bolt-ons can accomplish only so much. To realize the utmost performance potential from a given combination of cubic inches and mechanical modifications, you also have to optimize the ports in the cylinder heads and match the ports to the runners in the intake manifold.

There are two ways to port and match cylinder heads: the right way and the wrong way. The right way is to refine the flow characteristics of the head and manifold so as much air as possible enters the cylinders at the engine’s peak power curve. Every engine is different so there’s no "standard" port configuration that is guaranteed to deliver maximum airflow on every application. The port profile that works best will be limited by the physical dimensions of the cylinder head.

Limiting factors include the size, position and angle of the stock ports; the size, configuration and angle of the valves; the thickness of the casting around the ports; and the location of the water jackets, head bolts and other valvetrain components. But, other factors must be taken into account, too, such as engine displacement (big block versus small block); the engine’s bore and stroke; the shape of the combustion chambers; compression ratio; the depth and angles on the valve seats; total valve lift; camshaft profile (duration, overlap, etc.); and type of intake manifold and induction system.

Porting can unleash hidden horsepower by increasing airflow — but it requires know-how, a flow bench and special tools to successfully pull it off.

Opening Up
One of the basic goals of head porting is to minimize obstructions so air can flow relatively unimpeded from the throttle plate to the valves. Two things that get in the way are the valve guides and valve guide bosses. Cutting down the length of the valve guide stem and narrowing the valve guide boss can improve airflow past these obstacles. So too can using smaller diameter valve stems or valves that are necked down just above the valve head.

Bolt bosses that protrude into ports also create bumps in the ports that disrupt airflow. Grinding these flush with the surrounding port surface can also smooth out the route, provided you don’t run out of metal and grind all the way through the boss or dangerously weaken the head.

Transition areas in the port also need to be reworked so air will flow more easily around corners with a sharp radius and into the seat throat just above the valves. Sharp edges and rough castings also need to be smoothed and blended to eliminate turbulence and improve airflow.

The cross-sectional area of most intake ports becomes gradually smaller as the air moves toward the valve. This causes the air to accelerate as it approaches the valve and actually helps ram more air past the valve into the cylinder when the valve opens. Any sudden changes in the cross-section of the port can disrupt this effect and restrict airflow. That’s why port modifications that are made in the area just above the valve must not upset the normal increase in air velocity. The same goes for the exhaust side, too, except where the cross-section of the ports gets larger as the exhaust gases flow away from the valves. Again, the secret to maximizing flow is to have a smooth transition and as few obstructions as possible.

The joint where the intake manifold and cylinder head meet is another critical area that needs attention. If the runners in the intake manifold are not perfectly aligned with the ports in the head, sharp edges can interrupt normal airflow and impair performance. Matching up the ports so there’s a smooth transition from manifold to head will ensure maximum airflow. The same goes for exhaust ports. The head ports must be aligned with the header openings so the exhaust gases can pass freely out of the engine without encountering any sharp edges or obstacles.

The right way to improve airflow is to locate the best places to remove metal (or in some cases, to even add metal). This takes experience (knowing what kind of changes work and what doesn’t work), using the right equipment and/or tools (properly shaped cutters for reworking the various portions of the ports, valve pockets and manifold), and a flow bench to measure the changes in air flow produced by the various changes that have been made.

The wrong way to go at it is to grab a die grinder and start hogging out the intake and exhaust ports with no idea of where you’re going or what you’re trying to accomplish other than to open up the ports.

Bigger is not always better. Grind away too much metal and you may end up ruining the casting if you cut into a water jacket. But even if you don’t grind all the way through, removing metal in the wrong places can actually end up hurting airflow more than it helps. Here’s why: The secret to maximizing air flow and engine performance is maximizing volumetric efficiency and airflow velocity.

Big ports with lots of volume will obviously flow more air than a smaller port with less volume — but only at higher rpm. A lot of people don’t know that. At lower rpm and mid-range, a smaller port actually flows more efficiently and delivers better torque and performance because the air moves through the port at higher speed. This helps push more air and fuel into the cylinder every time the valve opens. At higher rpm, the momentum of the air helps ram in more air, so a larger port can flow more air when the engine needs it.

The ultimate port would actually be one with a variable cross-section that’s small at low rpm for high airflow velocity and gradually opens up for more airflow as engine speed increases. That’s sort of the idea behind staged split-plenum intake manifolds that open up and feed more air into the engine at higher rpm.

The bottom line is this: To realize the most power and performance out of an engine, air flow has to match the breathing requirements of the engine within the engine’s rpm range where it is designed to make the most power.

A set of killer aftermarket heads with huge ports and valves that are engineered to flow more than 500 cfm with a .800˝ valve lift at 8,000 rpm may be the hot setup for a 585 c.i.d. Pro Stock engine, but would be overkill on a street-driven, big-block Chevy that has a less radical cam, gearing and a redline of only 5,500 rpm. That’s why big-block Chevy heads with smaller oval ports work better on the street, and big-block heads with larger rectangular ports perform better on the strip.

Therefore, when choosing either an OEM cylinder head or an aftermarket head, you should try to match the port size with the engine’s power curve and rpm range. Don’t waste your money bolting a set of high-flow heads onto an engine that can never realize the head’s full performance potential because of limitations in gearing, the valvetrain, cam specifications or carburetion. Likewise, if you’re going all out, then start with the highest flowing heads you can find and try to add even more cfm potential by massaging the ports and manifold.

Kevin Self of Self Racing Heads & Engines, Durant, OK, said CNC (computer numeric controlled) machined aftermarket heads are a great place to start because the port configurations are based on designs that have already been proven to deliver maximum air flow. Even so, there’s always some room for improvement. Self said he could usually find an additional 25 to 30 cfm improvement in airflow in many CNC heads by tweaking the head on a flow bench.

"One of the advantages of CNC heads is that they all have very consistent flow characteristics. But, because they’re all the same, you don’t necessarily gain any advantage over a competitor if he buys the exact same set of heads as you. If you can’t improve the head, you’ll have to find more horsepower someplace else, otherwise you’re not going to go any faster than the next guy."

Self said CNC machining is much faster than hand grinding, and produces ports that have uniform profiles. But somebody still has to develop the initial profile by hand and then prove it works on a dyno and on the track.

As for port alignment, Self said the runners in the intake manifold should be about .050˝ smaller than the port opening in the cylinder head to allow for any misalignment that occurs during engine assembly.

"We’ve also developed a special ‘Port Intruder’ kit that allows you to narrow the thickness of the wall between the intake ports on small-block Chevy heads without increasing the risk of cracking the manifold or deck area of the head. The kit allows the use of a shorter head bolt between the ports, which eliminates the need for a head bolt or bolt sleeve between the ports. The head must be machined to accept the Port Intruder plugs. This change alone can increase air flow up to 30 cfm," commented Self.

Bob Irvin of M2 Race Systems, Farmingdale, NJ, said his company specializes in CNC machined cylinder heads. "We use outsiders to develop the port configurations, then digitize the ports for reproduction on customers’ cylinder heads."

Irvin said he charges a one-time fee of $2,000 to digitize and program a port configuration, and $1,200 to $1,500 to CNC machine a pair of cylinder heads (which includes valve seat and guide work, too).

"CNC is foolproof and is much faster than trying to port a set of heads by hand from scratch. But there’s no reason why you can’t do additional finishing on the heads after they’ve been CNC machined to find some extra power," said Irvin.

Where It Counts
As a rule, the roof of an intake or exhaust port has much more influence on airflow than the floor or sides of the port. The greatest gains in airflow can often be realized by removing metal from the top of the port only and leaving the sides and floor relatively untouched. This can have the same effect as using a different head casting or an aftermarket head that has the ports relocated slightly higher to give a straighter shot at the valves.

Additional gains in airflow can often be found by carefully smoothing and blending the short-side radius in the port floor where the port bends toward the valve seat. This helps air round the corner more easily for improved airflow.

In the area where the intake manifold and head are bolted together, using a template to scribe alignment marks on the head and manifold can serve as a guide for hand grinding and smoothing this area.

Mr. Gasket Company’s plastic "UltraSeal Port Gauge" templates for small-block and big-block Chevys can be used for this purpose, and are much more accurate than using a gasket as a template.

Expert Advice
Joe Mondello, whose name has long been synonymous with high-performance cylinder heads, said a lot of people who don’t really know what they’re doing jump into head porting and make big mistakes.

"They take out metal where they shouldn’t be taking out metal and end up with ports that are too big and don’t flow as well as they should. The shape of the port is far more critical than the overall size of the port," stated Mondello.

Mondello, who teaches the secrets of building, porting and flow testing high-performance cylinder heads at his Mondello Technical School in Paso Robles, CA, said he also sells special porting tools that are designed for every part of the cylinder head.

"When you’re doing the short-side radius of a port, you don’t want to take out too much metal. You just want it to be nice and smooth," instructed Mondello. "Trying to get around the short-side radius bend is difficult unless you use a cutter that’s designed for that purpose.

"When cleaning up the bowl area, blending alone won’t improve flow unless you also remove some metal to increase volume. Many people don’t do valve bowls properly. You have to blend everything from the base of the valve guide to the base of the primary valve seat, and then do a 3-angle valve job. Otherwise you’re just scratching the valve bowl and ports, and aren’t really gaining anything."

As for matching ports, Mondello said not to use gaskets as a guide because there’s too much variation in gaskets and most aftermarket gaskets have openings that are up to 1/8˝ larger than the port runners. If the port is enlarged to match the gasket, it can reduce air velocity and hurt performance.

"We teach port matching, not gasket matching. I pick the largest port, match all the others to it, then do all the work inside the port to maximize air flow around the pushtube turn because that’s where the biggest restriction is in the port," said Mondello.

"The largest gains in horsepower are found on the intake side by raising the roof of the port (the side closest to the valve cover) by .100˝- to .175˝. The amount of metal in the top of the intake manifold runner will determine how high you can raise the roof.

"On late-model Chevy Vortec heads, you don’t want to change the shape of the port much. The best advice here is to clean up and equalize the ports so they have the same height and width. On small-block heads, there’s a large pocket right below the rocker arm stud in the roof of the port. This should be filled in with epoxy to improve airflow. Doing that will give you an extra 15 cfm.

"On exhaust ports, if you tried to match the port to a header gasket you’d probably destroy the port. The secret of exhaust porting today is not how big the port is, but the shape of the port and the velocity of the exhaust flowing through it. We don’t even flow test exhaust ports anymore because most heads have plenty of flow capacity as is. All we care about is velocity and pressure.

"Nearly every single exhaust port today, except for Ford 302, 5.0L and 351 heads, are big enough. The only thing we do to enhance airflow is raise the roof of the port about .100˝, depending on the headers used. We don’t touch the floor of the exhaust port or the sides unless we have to get rid of a hook, seam or rough area in the casting,” said Mondello. “Any time you start making the ports bigger on the exhaust side, you usually end up killing air flow in the head. I’m talking a reduction of 25 to 30 cfm. All you need to do is clean up the valve bowl, blend the short-side radius, and raise the roof slightly. Don’t touch the floor or walls."

Mondello explained that CNC machining and hand grinding are two different techniques for porting heads. "Everybody says CNC is the way to go. But you first need someone who can take a raw casting and rework it so it has good air velocity and flows well. Then you can digitize it and reproduce it with CNC tooling on other heads. There are a lot of CNC profiles being sold today, but I think most have some room for improvement. Additional hand grinding can usually pick up another 10 to 12 or more cfm."

As for polishing, Mondello said a smooth finish is great for exhaust ports, but a rougher finish flows better on the intake side. He recommends using 300- or 400-grit paper followed by a Cross Buff for polishing exhaust ports, and 50- or 60-grit paper for the intake ports. A slightly rough surface texture in the intake ports and intake manifold runners creates a boundary layer of air that keeps the rest of the air column flowing smoothly and quickly through the port.