3/1/2001
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Hot Heads, Head Work On Harley Davidson Engines Can Yield Good Profits
By Joe Mondello
Porting these heads and increasing airflow and valve sizes does the trick
For decades, Harley Davidson (HD) owners have sought increased power and performance for their machines. We have found over the years that porting the cylinder head and increasing the valve sizes and airflow not only adds horsepower but also increases torque and driveability.
HD heads are easy to port but it’s very difficult to get high airflow numbers without welding the chambers to raise the compression. Welding the exhaust valve bowls and using epoxy on the valve bowls and floor of the intake ports helps air accelerate over the short side radius. The earlier style heads – Pan, Knuckle and Shovel – require most of this added work. Trying to straighten out the runners by welding really makes a big difference.
On all HD engines, the front head always flows better than the rear head because of the frame shape. The early heads work greal with bigger intake and exhaust valves and smaller valve stems installed; 5/16˝-intake stem size and 11/32˝-exhaust stems seem to work very well. Manley and Rowe Performance are good sources for guides for these valves. The aftermarket valves are available from most high performance valve companies including Manley, Wen-SI, Ferrea, Rev, PEP, Kibblewhite Precision Machining and others.
We have flow-tested many valves and found some remarkable differences from one manufacturer to another. The stock HD valves, in both material and shape, are really pretty good valves except for the excessive weight from the 3/8˝ valve stem. Aside from this weight, if you are staying with stock size valves, you’ll find the stock HD valves flow fairly well.
When porting the early style heads, do not remove any material from the intake and exhaust short side radiuses, as this will decrease airflow. You should only reshape, roll and contour the short side radiuses using a carbide cutter.
We offer the RT4-6-20-RC carbide cutter, a 7/16˝-diameter teardrop shape on a 6˝ x 1/4˝ shank. The RT5-6 is of the same type at 1/2˝ diameter. Both cutters are available on a 4˝ shank and can be ordered with 18 or 22 teeth. Our own Posi-flow 2010 aluminum porting lubricant, available from most major distributors, should be used while doing all porting and polishing on aluminum. This lubricant eliminates any and all clogging of your cutters, and it makes your abrasives last three to four times longer – cutters stay sharper longer, too.
The port matching of intake manifold and carburetors plus exhaust flange gaskets and pipes is very important. Raising the intake roof of the port is most effective at increasing intake airflow. On street performance machines, I shorten the intake valve guide boss to .600˝ while raising the roof at the same time. This measurement is taken from the spring seat to the top of the valve guide boss in the port. Of course, the valve guide is removed from the head.
If you or your customer race bikes on the street, shorten the valve guide boss to no less than .500˝. Any shorter than this will not leave enough guide boss to hold the valve guide in place.
Before reinstalling valve guides, we taper the valve guides with a 10° angle from the top of the valve guide boss to the top of the guide. This procedure should be done in a lathe. When you raise the roof this high, porosity will sometimes show up in the spring seat area. If this happens use "Gasoila" Hard Set and brush it on the head in that area to seal the intake port from oil and air leaks into the intake port. Usually the rear head needs to have the floor of the intake port raised; welding or epoxy works well in this area. Top parts suppliers distribute a good epoxy for this application.
We use a professional fuel-proof epoxy called Splash Zone A-788 on all aluminum heads and manifolds in our shop and at the tech school. It machines, ports and polishes well and it won’t crack, peel or fall out – but be aware, it cannot be used in exhaust ports. The intake valve bowl should be ported and polished. I don’t recommend any higher surface finish on the intake port than 80 grit; 50-grit handy rolls used with a flapper stick works effectively and flows really well on intake ports.
The exhaust ports need a little more help in the valve bowl area to increase airflow. The area between the valve guide and the back wall of the valve bowl needs to be filled in by welding nearly to the valve guide and just below the valve seat. I recommend installing a new exhaust valve seat after all welding has been done. Be careful with the short side radius and do not remove any metal; just reshape, roll and contour. Raise the roof the same way as the intake.
On the exhaust wall where the oil return drain hole is, enlarge the port wall until you nearly fall through the port wall into the oil return hole. If you do fall through, I have a sleeve available to seal up the mistake.
I want that exhaust port highly polished, so we start with 60-grit handy roll flapper paper on a rod and progressively use 80-, 100-, 120-, 150- and 180-grit paper. Then we finish with a very fine cross buff from Standard Abrasives. These cross buffs come in 1/2˝, 1˝, and 1-1/2˝ diameter sizes. A 4˝ mandrel for the cross buffs is available. I do little to the chambers except blend the chamber area into the top of the valve seat, eliminating all valve and valve seat shrouding.
Do a good valve job. The .050˝ to .060˝ wide intake seats and .060˝ to .080˝ wide exhaust seats work great. If you have a fixed cutter cylinder head station, I prefer a fully radiused exhaust seat below the 45° cut. Back cut the valves so the valve seating area is about .010˝ to .020˝ wider than the seats in the head. Try to maintain the original radius and angle under the head of the valves.
The new era Evolution twin cam, Sportster and Buell heads have 5/16˝ stem valves. The aftermarket billet CNC heads from Comp Cams Maximum Velocity, Alan Johnson, S and S and Edelbrock are all good. The porting and polishing techniques are very much the same as the early heads. I don’t recommend shortening the intake and exhaust guides on aftermarket heads unless needed for maximum flow as mentioned before. Less epoxy and welding is needed. The same finish will still apply, and always remove the CNC ridges in the heads and polish as outlined earlier.
On the new era HD heads, everything I have mentioned in this article – raising the roof, shortening valve guide bosses .500˝ and .600˝, welding, epoxy, putting the same porting finish on intake and exhaust, and cutting the same valve angles still applies. We do modify combustion chambers on the square wall model Evolution Sportsters by machining the exhaust wall with a radiused cutter between 5° to 7° with around 32° to 36° bottom radius so you don’t dig into the combustion chamber area on top of the valve seat. We move the wall over about .060˝ to .080˝. This alone creates airflow of between 7 to 10 cfm.
I like the shape of the new era valves with their deep tulip. If you are staying with stock size valves, follow all angles I mentioned, making sure to back cut the intake and exhaust valves. For a lighter valvetrain, I like the Manley Valve line; their angles, shapes and radiuses are good. I do not highly polish the chambers; I feel an 80- or 100-grit finish is all you need. Whether you are doing cast iron, old style or new era aluminum heads, all surface finishes I have mentioned will apply.
I have two styles of HD head porting vises with 10mm and 14mm, 6˝ long spark plug hole mounting mandrels. They can be mounted to a bench or grinder stand. The vises swivel 360°, rotate 180°, lock solidly into place and can hold up to 20 lbs. We have also designed a complete porting kit for HD heads that includes carbide cutters, cartridge rolls, flapper paper, various mandrels and porting and polishing lubricant.
The best cams we have tested with ported heads are from Andrews, S and S Cycle, Comp Cams, Sifton, Crane and Web. There are more cams out there, but we haven’t tested all of them yet.
I hope this will help you port your HD heads and get some good air flow power and torque out of them. Always remember: we make horsepower, not promises.
Joe Mondello has been involved in the high performance industry for 40 years. He owns and operates Mondello Technical School, Paso Robles, CA. jmondello@automotiverebuilder.com.