Creating Horsepower:

The magic of cylinder head porting

To be up front with you I'll apologize now if this article is a little drawn out. I'm a technical guy and this is a technical article. It is intended to be easy to read in simple English terms, but it is still going to be the how-to and the what-is-going-on kind of stuff that surrounds the mystery of why porting a cylinder head is such an effective way of creating horsepower.

To be specific here, we are talking about 4 stroke engines of the normal reciprocating piston type (2 strokes and rotary engines are a little different). There is no real magic to what goes on in the workshop when a technician goes to work grinding on your cylinder head. Modern sport bike engines are designed with good shape and flow characteristics from the factory. The problem lies in the fact that they need to produce thousands of them quickly and inexpensively. It is those last few details that they can't deal with on a production basis. Now, to get a little more specific here, we'll talk about porting for a street engine. Race engines are ported a little differently, the ports are usually enlarged, and the focus is on creating more horsepower higher in the rev range for that passing opportunity. For a street engine, very little metal needs to be removed since we don't want to enlarge the port, lowering air velocity, and potentially losing some performance lower in the rev range. A good street port job will produce power gains everywhere (even low in the rev range) and usually improve drivability as well.

What we really want to do here is clean up all the flaws and mismatches that the factory couldn't work out. I'll point out what we will be looking for along the way. DANGER! ---Not all those little humps should be taken out! Before we start removing metal, I want to tell you what not to remove. Take a close look at your head and see where the coolant passages are, where the bolt holes go through, where the little drain hole for the spark plug goes, as well as things like air injection and any other passages you can see. A lot of the shape of the port may be decided by fitting it around all of these obstacles. If any of these passages look like they may be close to the port TAKE EXTREME CAUTION. It's not worth taking that little bit more metal off if you end up breaking through the port wall and creating some kind of leak that you can't fix.

Some basic guidelines. 1) Start working in the deepest, hardest to reach areas first, then work your way out from there. 2) Start with course tools, but don't try to get everything all at once, and then work with progressively finer tools. 3) Course tools can remove a lot of metal quickly and put gouges or groves that are hard to smooth out or even create ripples that will impede flow. Work with the finest tool possible to remove the necessary amount of metal. Too fine a tool may take a long time, but is unlikely to put any nasty gouges that will defeat your progress. The tools that I use are (in this order): carbide burr, rotary stone, course sanding roll and fine sanding flapper. I'll show you the tools as we go. 4) This one is important. Keep the tool moving. If it sits in one spot for too long it will leave a divot or hole, and we don't want that.

This head is off a GSXR750. The design is good enough that I didn't need to use a carbide burr anywhere on the head. The valve guides are recessed into the port ceiling so that they stick out very little. If you have a valve guide with a large boss cast around it, this would be a place to use a carbide burr. Use a tool long enough that you can get down into the port where you need to work, and can still see around your tool to what you are grinding on. Note: for a street head you don't want to remove the part of the guide that sticks out. By shortening the guide, you will make it wear faster, and flow bench testing shows that it doesn't greatly improve flow enough to warrant the extra wear. Shape the head to meet the guide smoothly. This is where it starts to get artistic. Imagine the air flowing through the port and what shape would be easy for it to flow around. Teardrop shapes and smooth gradual curves are good, square edges and ripples are bad. Also consider the direction of the flow, intake and exhaust ports are flowing opposite directions from the combustion chamber!

Let's get to work! Here I am using a course stone to clean up where the valve seat is pressed into the head. You can see on the untouched side where the factory machining ends that there isn't a smooth transition to the rough casting. Where I have started grinding you can see even more where the mismatch occurs. I am using a round tapered stone to gently work around this area until it smoothes out and the flaw disappears. Note: The closer to the valve seat the flaw is, the greater the improvement in flow will be when the flaw is removed. On the inside of the bend, the machining created a sharp edge that will detach the airflow from the port wall right before it gets to the valve. I use a round stone to work on the back side of the tool and make this a nice smooth curve. Tip: Don't just rely on what you can see. Get your fingers in the port and feel the shape, the curves, and the edges. When it all starts to feel smooth, you are getting closer!

Here is a good spot to stop and give you a few tips on grinding. When you are out buying porting tools, pick up a few cans of WD40 or something similar. I keep the stone wet all the time and use it to wash the grindings out of the port. It also stops the stones and burrs from getting clogged with aluminum. Use a tool the right size and shape to help create the contour you want. Bigger and smaller tools will be needed in different parts of the port, so adjust accordingly. Working with the grinder will take some practice, maybe start off porting your buddy's head before you start on your one!

On to the next step—Intake ports. The intake is more critical to get it right. The engine is relying on vacuum to pull air into the cylinder instead of exhaust pressure to force gas out. All the rough casting is being smoothed out and the bridge where the port splits is sharpened. Don't narrow the bridge too much, it will make the metal brittle and could crack or break. Shape the port so the edge gets sharpened but not narrowed, this will divide the airflow with the least amount of resistance. When everything starts to take shape and you start getting a nice finish on the port walls, resist the urge to polish it. For a drag race engine this is fine where full throttle is all that matters, but for a street engine this can cause starting and warm up problems by making the fuel mixture condense against the port walls (especially in cold or moist conditions) until the metal comes up to temperature. For a street engine, I leave a smooth sanded finish. This creates a small amount of turbulence on the port surface that stops the fuel from condensing. Shape is far more critical than surface finish here. The worst thing you can do is create ripples down the length of the port. After all my grinding and sanding is done, I'll finish up with a sandpaper flapper on a rod with a slit to get the best finish. Tip: Remember again to start working deep in the port and work your way out to the ends. Keep the tool moving at all times, this limits the possibility of putting a divot or gouge in the port.

The other side—Exhaust ports. On this head there were big divots where the air injection holes were ( California and emission control states only) even though this is not an emission control model. You can see where the hole has been filled to create a smooth surface. You can either have the head welded or use a metal based epoxy for this purpose. Here I have used an aluminum based epoxy filler. Remember the gas flows the other way here on the other side. I have removed the mismatch at the valve seat in the same way, but I shaped the flow around the valve guide a little differently, and it was not necessary to sharpen the bridge between the ports since the gas flow will not be bombarding the bridge from the front. Polishing the port on the exhaust has no ill effects at all so go ahead and make it shine if you like. Remember to get your fingers in the holes and feel around to all the parts you can't see.

Now that we're all matched and shaped we are ready to put it all back together, right? Nope, one more thing left to do. The combustion chamber itself needs a little help. Most of the time the valves are shrouded, to some extent, by the casting in the combustion chamber. The valve needs to open further before airflow picks up because the flow is being blocked by metal around the circumference of the valve. You don't want to remove a lot of metal here, it will reduce your compression, but do remove enough to allow airflow at small valve opening. You are essentially increasing the effective cam duration without the ill effects of a larger cam. Remember again that sharp edges are bad for airflow and you can see all the square edges around the valves. Also, the closer to the valve seat the larger the improvement will be. Here is a two for one! This only needs a little bit of work so start with a fine sanding roll and work gently. WARNING: don't grind the valve seat! It makes doing a valve job to get the valves to seal much more difficult.

Are we there yet? Our valves are cut and lapped, head reassembled and put back on the bike, valves all adjusted, now we've got 10 more horsepower right? Though a good porting job should be worth about a 10% gain in HP, you will only recognize it when your fueling is worked out as well. Now that we can draw in all this extra air, we need to let in a little more fuel as well. It isn't uncommon to need 15-20% more fuel to get the mixture right with all this extra air. This doesn't mean you mileage will drop; it just means your engine is more efficient and needs to get the right mixture into the engine. Spend the time to get the mixture right and you will be amazed at the performance that was hiding inside your engine waiting to get out, and it cost you less than that trick exhaust you were looking at. Another point to consider, porting works well with any other modifications you have on your bike, even if that means no modifications at all. With the police cracking down on loud pipes, more people are leaving the stock pipe alone, and looking for other ways to improve their performance. Either way, take it to the dyno and get your jetting straightened out, and if you were curious enough to run it on the dyno before you started, you can show all your friends your numbers before and after to tell them how fast you are.

For more information on having your cylinder head ported or to check our inventory of ported cylinder heads, contact 2WW (Two Wheel Workshop) at (562)980-2797 or visit our webpage