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NEW Tech. Article on Cylinder Head Modification
This is a technical write up on Headwork, it is based on my experience as an engine builder, race car driver, and import tuner, it should by no means be taken as fact, it's not the ten commandments, please use your brain and form an intelligent opinion of the information given in this article. You may find that some points apear to be contradictory, however that is my style... I like to present both sides of an argument in an equal manner and allow you to beleive what you think makes the most sense.

A cylinder head is the most important part of an engine. If it does not flow well, it will limit what the camshaft, induction, and exhaust systems can do. Correctly modifying a head relies on flowbench experience, there are many different designs and each one requires a different approach. What I'm going to do is lay out a few 'rules of thumb' which should give you a good starting point.

Often overlooked but MOST important, are the valve seats. The seat must perform certain functions:
Phsycally seal properly, so that there is no leakage of combustion gases
cooling. a large part of heat transfered is through the valve seat into the head, and the cooling system. hotter valves create less power.
FLOW WELL. obviously the seat profile is the factoring property of low and mid lift power.. the port only becomes restrictive at higher lift

Most newer cylinder heads already have a 3 angel valve seat. when you hear someone saying they have a 3 angle valve job... tell them so does your grandmothers Gti.. because it does.
Seat WIDTH is critical, and many racers tend to make this area to narrow, the seat fails to seal properly, and they don't flow well.

you cannot cut valves or seats properly with worn guids, major head modification should include replacing guides with harder brass units anyway.

overall, a good width for intake seats is between 4% and 4.5% of valve diameter, narrower seats improve high end flow, at the expense of low end and should be used in conjuction with HIGH LIFT cams. narrow seats tend to burn out faster, so if you plan on running your engine on the street often, consider leaning towards the wider end of the spectrum.
For exhaust, you can usually get away with the same seat width as the intake, or 5% of valve diameter.

most heads use a 45 degree angle for the valve seat. 30 degree and 70 degree top and bottom cuts are good general choices for the other cuts. this creates a nice smooth transition from port to chamber and visa versa. Width of these cuts depend on head design, but should be at least the same width as the seat. Many tuners may say that 60 degree bottom cut is better for geometry. that is not my opinion.

After the valve seat, the area most important is the area from about 1/2" above the valve seat to 1/2" belove the seat in the throat. Parts of the chamber that shroud the seat need to be opened up, and clear the valve head by 25% of valve diameter all around. A lot of heads have chamber walls closer and flow loss takes place. Hemi style heads have multiple valve heads where the stem is angles away from the vertcle and tend not to suffer from shrouding. "bathtub" heads are the worst, with their big vavles that show flow loss because of shrouding.
The valve throat should be bored out 85%-88% of valve diameter. this should leave enough material for a 70 degree bottom cut of decent width. any smaller and you'll start to experience flow restriction at high lift, larger and it creats a bend that is too extreme through the valve seat area. I see too many heads with no bottom cut and the throat bored out to nearly the size of the valve seat. This looks good, but hurts flow. I've found nearly 7HP gains above the average port job in just this area alone, these gains are possible just making the throats smaller rather than larger. The idea is VELOCITY, not so much volume. Velocity is needed at higher RPMs to get correct atomization of gases and swirl pattern inside the chamber.

Now the port bend. an area that affects mid and high lift flow. Although there aren't any 'rules of thumb' to shape, the idea is as large a bend radius as possible between throat and manifold end. cooling areas can restrict the amount of metal you are able to remove. it would be pointless for me to quote dementions, percentages or radii. At least run your finger around the port bend area to make sure no lumps are present before you do anything. also, there is no way to properly port a head without removing the valve guide, regardless of what others may have told you. Big shiny ports are more for show than go, and you'll immediately notice the difference both on the flowbench and on the track with a properly ported head.
Speaking of big shiny ports, we'll move to the Manifold end of the port. it's the easiest part of the port to get to, and too many hours have been spent on this area. This area only effects high lift flow and rarely needs anything bigger than 80% of the valve diameter for intake. exhaust is less critical if it's big, but should be no smaller than 80% of the exhaust valve. Obviously, with squared or rectangular ports you can't figure an exact diameter, but should be the same area.
When sizing the ports, take into consideration valve lift and intake manifold design. low lift cams don't require large ports. Also if you are using a poor manifold design, there is no point modifying the port size. the aim is velocity, without restricting flow. Mirror finishes are a COMPLETE waste of time on intake or exhaust ports. a smooth grind is all that is necessary, finished with 80 grit emery cloth on a round stick for high spots. Spend more time shaping valve seats and less time polishing but don't finish it with a hammer and chisel...
Thank you,
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