So after all the requests that Iíve had about porting cylinder heads and intake manifolds, I thought I would make a write up on the doí* and doníts of porting.
NOTICE, this is not a guaranteed method of improving horsepower and torque because the science of Cylinder Porting is ALSO a form of art, thus requiring skill. Use this guide at your own risk.
Typically cylinder heads will com from the factory with less then desirable ports. The most basic goal is in cylinder head porting is to smooth these ports to a finish of roughly 80 GRIT sand paper. Most people will typically tell you that a mirror finish is the best. However, it is not the case. A the air flow in the port will stick to the surface of the port causing a reduction in air flow. Thus in a 80 GRIT finish, the surface causes air pockets on which the air stream through the port rides on. This 80GRIT finish will typically yield 10-25 cfm at 28Ē on a SUPERFLOW 600 flow bench.
This is a stock port. Sorry, only picture I could find
This is a Finished Port of a NASCAR SB2.2 Cylinder Head used at Daytona 2006
Besides the finish of the port, there is the technical aspect that must also be taken into consideration. The main thing would be the CROSS-SECTIONAL AREA (CSA) or choke point. In the general, the CSA is a calculated size that is derived from the bore and stroke and the rpm of desired peak horsepower. The CSA determines where the cylinder head will flow the most with sufficient velocity.
3800 Series II/III com from the factory with a CSA of 1.54 Square Inches. As we start cranking up the power on our 3800í* and start turning them 6500+rpm, the need to open up the CSA grows. For the guys that have shift points of 6000-6500 rpm I would suggest 1.8 square inches. For those of you with 6500-7000rpm shifts, I suggest using 1.9 square inches.
For a really basic calculator on measuring what CSA you need refer here.
When porting, make a cut out of your desired CSA and port you head so that it fits snuggly thought the first inch of your head. Past that inch, the port should taper down into the valve seat, this is critical the edges be radius and smoothed out
This is an example of the transition from port to the seat.
The next step in porting would be to pay attention to the valve guide, typically we want to contoured the port that leads up the guide, BUT DO NOT TOUCH THE GUIDE ITSELF.
Here is an example of what I mean.
The Combustion chamber is another mess to deal with. On a 3800 head for the street, I would simply recommend smoothing out and blending any potential heat risers, these are the spots that lead to detonation and the Evil thing called KR. If you have a welder hand you can add some material around the exhaust side of the chamber to increase the quench pad. (this aids in cylinder pressure recovery). And then to keep the compression ratio the same, you can remove some material from the top of the intake side of the chamber
Here is a good example of what a well ported combustion chamber should look like.
When it comes to porting the intake manifold, everything previously mentioned is reapplied. The only difference is that when adjusting the CSA in the manifold, the CSA should remain constant throughout the port.
Questions, Comments, Concerns? Just post them or PM me.
USE AT YOUR OWN RISK.
Porting Cylinder heads is not on easy task, but can yeild great results, OR can wreck a cylinder head.