So after all the requests that I’ve had about porting cylinder heads and intake manifolds, I thought I would make a write up on the do’* and don’ts 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 28” 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 3800’* 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.
http://www.wallaceracing.com/chokepoint-rpm.php

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.

I agree that porting cylinder heads is better left for those with at least a solid understanding and experience in porting in general.

I obtained the manufacturer'* casting drawings of my cylinder heads before I ported mine.

You won't gain jack from just porting the intake and exhaust. The combustion chamber itself is really non-existent on 3800 heads, but you MUST pay ample attention to the valve bowls in order to gain from other porting (intake/exhaust). Modifying the exhaust port shape too extremely can also cause further problems.

Intake porting is one thing. Replacement parts are inexpensive if you goof. Heads are another story. One bad move and you wipe out a valve seat.

Thats why i always say if your goning to port your own heads, you do it at your own risk.

Just want to add it is quite difficult to add material to the combustion chamber of a cast iron head. In addition to really knowing what your doing (i.e. using a TIG, with correct filler rod, being VERY clean, and using the correct gas shielded mixture), the head will need to be preheated.

Also, you mention the port area should remain constant through the intake..
What about ramping up velocity?
The L36 LIM alone reduces CSA 30+% before it reaches the head.

I've read a good amount about intake design and even in high rpm setups, authors were suggesting at least 3 degrees or so of convergence.

And one other thing to consider while porting is exhaust to intake flow ratio.

I think this topic was intended to be a bit more 'general' in nature and not go into those specifics.

Those that have the porting skills and tools are probably already aware of some of the specifics, and if they're not, they'd ask.

I think his intent was probably to give some GENERAL guidance in order to help our members decide if it would be worth it for THEM to pursue this, rather than have it done by a professional.

I changed the title to incorperate General.

Yeah, i just wanted to cover the real basic that the novice should know before taking on the task. I figured that taper angles and bell-mouthing along with changing the short-side would have been best left out and let to the more advanced head porter.




Typically, on a NA MAXIMUM PERFORMANCE engine (2.7+hp per cubic inch) we focus most of our intention on the maximizing the intake side of the head. and then use the camshaft to bring up the exhaust. But this then also carries into wave tuning. Dont, get me wrong, we still port the exhaust side to maximize it.

Forced Induction, whether it be super or turbo, (we can include N20 in this also) is almost the opposite.

The old Intake to Exhaust Flow Ratio is the "Old School" approach to porting. It'* hard to work by that ratio due to engines being so vastly different from one to another.

I posted in response to this: