Established In 1981
Engine Rebuilding Vintage Marine & Performance Engines
Today there are literally hundreds of race tracks & sanctioning bodies that allow little or no visible cylinder head modifications in their restricted sportsman classes. This creates an opportunity for increased profits in an area of cylinder head modifications that performance and specialty shops may not be capitalizing on. With shrinking markets and crate engine programs we need to diversify our services. 
If you have any morality issues over cheating up a pair of cylinder heads, then perhaps you shouldn't read on. The first step in the process is to rough in our valve seats so that any undercutting from the chemical can be blended and show contrast from our bottom seat angle to the texture of the port. Now we can grind and modify the areas in the ports or chamber that will improve airflow. We will use a carbide cutter of the appropriate diameter and shape to suit the specific area we are working, typically a 1/2" diameter egg or flame shape with a 4" shank, the cutter style can be either a single or double cut. Let's say for example that we have a 492 small block casting and we want to straighten the divider wall of the intake port adjacent to the head bolt. This is a relatively small area of about 2 square inches. Stay in this area without walking off onto the floor of the port or anywhere else and gently blend. The radius of the short side turn wants to be increased and laid back slightly and we typically leave the valve guide boss alone. Narrowing and modifying the valve guide boss is easy to notice and provides a very small increase in cfm. 
One advantage you may have in doing this work is to know the methods and level of scrutiny that a particular track is practicing. This will help you determine the degree of creativity you will exercise in the process. 
At the other end of the intake port there are several considerations. The port opening at the flange is always untouched. It is obvious if altered and is not a major hindrance to flow. The floor of the port is also off limits. I typically will widen and straighten the port wall at the push rod aperture, again we are working a small 2" area and we want to focus on a concise area. Use a pair of inside dividers to keep your port width consistent. You may notice that we are only talking about the intake port and that's because this is where the majority of the effort is needed, also the exhaust port flow and balance will improve easily with much less effort. The exhaust port will get similar attention to the short turn radius, also we can widen the floor above the turn but don't get into the draft line. That's the line running down the port that is generated when the casting is poured at the foundry. The long turn of the exhaust port around the valve guide boss is another area of attention. This will pick up flow and is relatively unnoticed at tech time because you have a curtain of carbon over the entire port. 
Now it is time to move on to the second step, this is where the actual texturing begins. I am switching to a different carbide, it is a 1/2" egg double crosscut with a 4" shank. The shank is slightly bent which is a benefit here. Drop your grinder speed to approx 3-4000 rpm and now you can revisit all the areas you have previously worked. Remember to stay specifically in the areas you worked, no wandering willy nilly through the port. A light pressure is all that's needed along with the proper technique. This is a circular pattern either clockwise or counter clockwise. This technique is taught by Joe Mondello at his technical school and is referred to as wax on, wax off. It is this combination of carbide, speed and technique that will generate the foundation of the texture we are looking for. At this point your port modifications are complete. If you are a novice and you do not possess the hand eye coordination needed, your port could well look like it was assaulted by a runaway McCulloch. Remember, It is important that you are as neat and precise as possible. 
Now we can go to step 3. What we are going to do here is reinstall our draft line where it is missing on the divider wall at the pushrod aperture and at the window area adjacent to the head bolt. I use several tools here. The first is a long 90 degree scribe to reach deep into the window area. Using a flexible 6" rule or any other suitable tool as a straightedge align the old draft line and rescribe it. You have to go slow and steady until you have a line started and then just keep following it until you are happy with the depth. In the aperture area it is more easily accomplished with a needle file. 
On to step 4. This is where the catch phrase acid porting is probably better described as acid cover up. Basically we are preparing the cylinder head by installing a set of dummy valves with RTV sealant on the faces. Try not to use too much and spread it evenly on the valves, also remember to anticipate some etching, this is why at the beginning we only roughed our seats in. Next you will need to fabricate plates for the intake and exhaust flanges of the proper material that is impervious to the chemical. You will need two each so you can do both heads in one session. The plates need to have a chimney on the port to allow percolation of the chemical. Now set the heads with the port flange level and move outside. Remember to use mask, gloves and rubber apron. I always have a water hose handy in case of any spillage. You can now fill the ports exhaust or intake side first, doesn't matter. You've been wondering with what? Just good old fashioned battery acid. Do this over a suitable containment such as a plastic tote. The cylinder heads need to marinate for 24 hours, at this point the acid is spent anyway. Rinse, flip and do the other side, or if you want you can go for a second soak, it's optional. At this point your done with step 4 and it's time to clean up and finish. Remember to dispose of waste properly. Flush heads with water and you will need to bead blast the heads as they will look pretty nasty. The more careful and neat you were in the pouring process will save work now.
The final step is to finish the forms on your valve seats as well as any other machine operations. Make sure you save your other machining operations for last so you have good contrast. If you want you can go on the flow bench and document your improvements. It should be mentioned that the average tech man at most race tracks won't have a clue when he is looking at the cylinder heads that we just did and when you look at all the time and expense that goes into campaigning any race car for a full season it just seems to me that if you don't make all necessary preparations that it's like taking a knife to a gun fight. Let's keep making more horsepower.
Acid Porting: Uncovering the Cover Up 
Leo Croisetiere
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