The horsepower game is all about getting as much air into the cylinder as possible, adding the appropriate amount of fuel and lighting it off. Boom! The bigger the boom, the more power and torque. Outside theory, it’s not quite that simple. The “boom” is limited by how much the structure of the engine can handle, but it’s 100 percent true that the first part of the power equation is air.

Any naturally aspirated (NA) engine can only draw in as much air as atmospheric pressure and restrictions in the intake tract allow. Because most diesels are unthrottled, they have an airflow advantage over an NA gasser. Virtually any modern NA diesel can make 85 percent volumetric efficiency, or better, over a broad rpm range. Only really well-tuned NA gassers can do that and usually in much narrower rpm bands. Volumetric efficiency (VE) is the static percentage of air a cylinder of a particular size can hold at standard atmospheric pressure versus the actual amount it can pull in dynamically in the face of intake restrictions, cam timing and engine speed.

When it comes to making diesels breathe, forced induction is the great equalizer. With forced induction, turbocharging or mechanical supercharging, air is forced into the cylinder by a compressor and filled to more than its static volume of air. The cylinder is actually under pressure when it starts the compression stroke. Depending on the amount of air the compressor can deliver, that can be anywhere from 125 to 200 percent more than the static volume of the cylinder  at seal level atmospheric pressure (125 to 200 percent VE). With forced induction, a 360 cubic inch diesel with VE of 125 or 200 percent is taking in the a volume of air equivalent to 450 to 720 cubic inch NA diesels and with the proportionally correct amount of fuel added, the smaller engine can make approximately the same power and torque as the larger. Again, it all comes down to how much “boom” the engine can tolerate.

On an NA engine of any type, one of the major restrictions to volumetric efficiency is the intake tract. That starts with the air horn on the air filter assembly and ends at the intake valve. Air filter improvements are the obvious starting place, followed by the intake manifold, but very quickly the bottleneck will become the intake ports and intake valve. Each engine is different, so the exact cause of bottlenecks will have to be determined by experience and knowledge. To the handful of you out there still running old-school NA diesels, you are going to turn to cylinder head intake and exhaust port improvements and intake manifold improvements a lot sooner that people running forced induction. You have to! It’s really the only answer left before forced induction. The benefits will be only modestly cost effective in most cases versus going with forced induction.

To the majority of you out there with turbo diesels, improvements in the cylinder head ports and valve pockets will be a ways down the “Round-To-It” list. Any hotrodded turbo diesel can eventually get to the point where the cylinder head is an impediment to power output. One indication of that is excessively high boost pressure and power gains leveling off. Boost pressure is an indicator of a lack of flow and improvements in head flow can sometimes actually reduce boost pressure with no other changes while increasing power at the same time. How much? it’s highly variable, but according to various sources a 40-80 hp gain on a streetable B-Series Cummins is not uncommon. With extra tuning to make use of the extra airflow, even more is possible. On top of that, the other benefits include lower EGT, faster spool-up and less drive pressure (backpressure).

What is Head Porting?

Head porting is the art of reducing restrictions in the intake and exhaust tracts. We say “art” because it’s one of those things in which the “eye” and the sense of touch combine with hard science to get the job done. While porting involves a certain amount of “voodoo,” elaborate test equipment and trade secrets, it’s something a shadetree builder can play with too … as long as he understands some basic limitations.


To get some perspective on head porting, we turned to Gavin Knisely. Gavin is a well-known Southern Ohio puller that gained a lot of practical experience porting his own Cummins heads, discovered he had a knack and eventually start doing it for others. When the University of Northwestern Ohio decided to start building a second engine for their pulling truck, they turned to Gavin for a massaged 24-valve head. We were able to follow Gavin through the process and get some tips along the way. We also were able to do before and after flow tests on the UNOH SuperFlow flow benches to see the improvements.

The Importance of Valve Work

 Bear in mind that it’s the valve work that’s going to make or break your head flow numbers. Since most of us don’t have a Serdi valve cutting machine in the garage, the shop you pick to do the valve work could make or break your porting improvements. In many ways, the valve work is the most important part of the job because the valve is usually the most restrictive part of the intake tract.

Shelf or no shelf? It’s fairly big bucks to machine off the cast-in manifold on a Cummins 24-V and make/buy a new manifold. When you are grasping for every horsepower, it can reap enough rewards to make it worthwhile but it still ain’t cheap. The main gain is the ability to be able to improve the intake ports, particularly #1 and #6, which need the most help. Gavin Knisely gave us some informal numbers of a top quality job with the shelf on and one with the shelf off. With the flowbench results he presented, it shows around a 21 percent difference. The main difference is the ability to thoroughly work over the intake ports. The shelf itself isn’t the major flow impediment, though removing it and replacing it with a custom intake is worth some flow and power. The plate shown here in front is the mounting flange for a new manifold to be constructed. None of the mounting bolt holes have been drilled and tapped at this point. Knisely also used this as a template to match the ports.

Tools of the trade. The tool investment is not huge and many of these things you may have already. Hearing and eye protection tops the list. You will need lights to shine into ports. A high speed die grinder (air or electric) and an assortment of long-shaft carbide cutters. Sharp ones… they wear out. A bunch of 80-120 grit carbide rolls for the final smoothing, with the arbors to carry them. One of the more important tools is the digital internal calibers, a Fowler shown, used to make sure you get the ports dimensionally correct and even with each other.

Good valve work with pay the highest dividends at the low lift parts of the valve opening event. The valve is only fully open for a short period in that event and when you consider at high rpm the period of time the valve is open to feed the cylinder is measured in fractions of a second. At 3000 rpm, an average intake valve is only off its seat for 2 tenths of a second and the time shortens with increased rpm, so every millisecond the valve is off the seat is important to filling the cylinder.

In the process of testing the UNOH head, we had a graphic demonstration of this. We flow tested the head before and after without any special valve work done and then got numbers after a top quality, performance oriented valve and seat work. There was a 15 percent increase in flow after the valve work. Don’t forget, even without porting, good valve work can unlock a lot of airflow.

The Airflow Equation

 Keep in mind that head porting is only one part of a big equation. The valve work mentioned above is a very closely related factor in that equation but there are others. The cam profile is a major one. You may not be able to unlock all the airflow potential in a cylinder head without a change in cam profile. The turbo is probably of equal importance, as is the exhaust system and the air filter assembly. Guard against thinking of head porting as a single element and think about all the peripheral items before and after the cylinder head ports.

DIY or Don’t

So, is head porting a do-it-yourself job? Yes and no. It’s a matter of degree. With the right tools, patience and a little research, even a first timer can unlock some horsepower by porting his own head.

The first basic advice Gavin has for home head porters is don’t try to reinvent the wheel. Smoothing port walls, radiusing edges, port matching, etc., are all beneficial, but radically changing the shape of a port or runner should only be done with knowledge aforethought. What looks good may actually flow worse than stock and introduce adverse effects. Any shape changing of the ports should be limited to what has been tested to work and that’s pretty hard information to come by for a home porter, unless he has a stack of heads for trial and error and/or a flowbench and dyno handy. A verified “recipe” from someone who has done the R&D is another alternative but you will find few skilled head porters willing to share their top-secret recipes and it may be difficult for a first timer to match what a pro can do. Even Knisely was unwilling to show us all of his tricks.

Here’s what you start with on the intake side, assuming the shelf has been sliced off. Knisely points out that you will never be able to get the end ports to match the dimensions of the center ports due to the casting, but you can drastically improve the flow anyway.

In the bowls, you can see a lot of rough stuff and edges to take out. Note also the three angle valve seats. Three angles work OK for the exhausts but the intake seat need more to flow well.

To gauge how much can be taken out of any head port, pros “slice-n-dice” in the critical areas. On top you seen a 24-V intake port and on the bottom an exhaust. Knisely keeps a 24-V head that’s sliced into about 10 pieces for testing.

Get comfortable. You’re going to be there a while. Wear hearing protection. Especially with an air die grinder, the noise will be enough to damage your hearing long term. Needless to say, the grinding dust will ruin your eyes. A face mask isn’t a bad idea either and Knisely often uses them. Knisely keeps a shop vac handy and regularly sucks up the dust.

To match port on the exhaust side, use an exhaust manifold gasket as a template. Once done on the head, you can check the exhaust manifold and make sure it’s going to match the head. It’s OK for flow if the port running is smaller in diameter than the exhaust manifold but not vice versa. You can grind the flat spot on the floor of the exhaust port at the outer edge but take care as you move in or you will end up in the water jacket. This is one aspect of porting most DIY people can do.

The valve pockets are smoothed out and the sharp edges of the guide bosses are smoothed. Some of the “secret sauce” is in this work

Also, more than a few heads have been ruined by overzealous amateurs who got carried away. Unless you know the thickness of the port walls and runners in all locations, you should be very careful how much material you take out. Every head has a few areas where too much grinding gets you into a water jacket or so near one you later get a crack. The pros slice heads apart to both learn where these vulnerable areas are and to observe the shape and dimensions of the ports and runners. You need a safety factor in this too, because core shift during the casting process can lead to variances in what’s out there. The OE manufacturer accounts for this in production. A  1/16-inch variance may not mean much in a stock engine but it could for a guy taking 1/8-inch out of a port wall during a port job. Pro porters get bite marks in the ass over this too but they usually have a broader experience base to draw from and their own safety factors based on experience with particular heads.

The Real World

A DIY-guy  has to face the inevitable fact that it’s unlikely he can match something done by a pro with a proven recipe based on lots of objective testing on flow benches and dynos. Does it sound like we are trying to talk you out of DIY porting? Not really, but we want to give you some realistic expectations of what a novice can or should do, and the potential results.

Once you get the ports sized the way you want, you smooth them with the 80 or 100 grid rolls. You don’t need or want a mirror finish… even though that is “purty.” A slightly rough surface is actually beneficial, as it creates a thin boundary layer of turbulence that reduces air friction of the main flow running more towards the center of the port. The dimples on a golf ball do the same thing.

Plenty of secret sauce here too. The goal is to enlarge the ports, make them all equal in size and shape (except the two end ports and you just do the best you can), eliminate the choke points and a “iron boogers” (that’s the technical porter’s term) protruding into the runner.

The final step is to have the valves and seats ground. There is an art to this as well. Knowing where to put the angles on the seat to streamline the flow past the valve. On the intakes, five angle valve seats are common and exhaust four. The valve heads will also have several angles. The base seat angle is important too, with 30 degree seats flowing better on the intakes than 45 degree. The basic shape of the valve is also used to increase flow. All this is somewhat specific to the particular head.

There is a lot of bench racing banter between the advocates of 12 and 24 valve Cummins heads. Generally speaking, according to Knisely and other sources, a stock 24 valve head will deliver more overall flow than a stock 12-valve but once porting work starts, things a can equalize and it’s then down to the mojo of the porter. Airflow numbers are hotly debated but don’t always have a direct horsepower equivalency. More is better, of course, but how that extra air is utilized is key.

Slicing off the stock manifold and buying or fabbing another is beneficial but expensive. The UNOH diesel club built this manifold from scratch. This step radically changes the engine plumbing considerations.

So how much should you expect to pay for a porting job? That’s going to be highly variable according to your region, the shop and how much work you want done. Consider a thousand bucks the bare minimum on a six cylinder head, with the valve work extra. You could easily put $2500 in a head with all the work done, more if you want to fight over every CFM of flow and every horsepower. DW