SVO 4.6L SOHC Cylinder Head Porting
by: David Posea

WARNING: I am not a professional head porter/engine builder!!! In fact, this is my first attempt at head porting. Use this information at your own risk.

The SVO heads out of the box are pretty good. The castings are a little rough, but not horrible. Still, with some basic cleanup flow can be improved. I urge you to read Standard Abrasives article on head porting first, and order one of their head porting kits. It contains various straight and tapered sanding rolls in 40 and 80 grit, mandrels, stones, and SA's cross-buffs for polishing. You really need a die grinder to port heads. A dremel tool just doesn't have the power to remove lots of material. WD-40 will work as a grinding lube, but I finally bought some "Grinder's Grease" from Eastwood. This is a waxy substance that helps keep the abrasive from loading up with aluminum. It also speeds up the cutting action. I wore out a few grinding stones, so I picked up a few more at Home Depot. Also, you'll need a fairly good compressor to provide enough air for a grinder. I have a Campbell-Hausfield 36 gallon, 2 stage 5HP compressor. It runs on 110V and works great.

I went a little beyond the basic port and polish, but not too far as this is my first set of heads. I tried to avoid thinning out the port walls too much. The walls are nice and thick, but still some care needs to be used to avoid costly mistakes!!!! I talked to some people with ported SVO heads, and got a few pictures as well. This gave me some ideas on where to grind, and where not to grind. I later managed to get some detailed pictures of a professionally ported SVO, and found that mine were very close to those. Being an overhead cam engine, there are no pushrod holes to complicate the port design. Also, since these are "modular", all ports are the exact same shape.

Before we get to the actual head work I want to mention some grinding techniques that worked well for me. Keep the abrasive moving at all times. Either move it from side to side, or in and out of the port. If it is left in place it will quickly make a groove in the head surface which is hard to remove later. I found that angling the grinder so the end part of the stone or abrasive roll was the part touching the port worked well. I then moved the abrasive in and out of the port, varying the pressure depending on where I wanted to cut. You only need a light pressure to cut aluminum. When widening a port wall, I intentionally gouged out a groove with a pointed stone using just the tip. The groove was so I would know how deep I needed to go. Then the rest of the wall was brought down to the level of the groove bottom. Even my cheap mini grinder had a speed adjustment screw on it. I used about half of the maximum speed. When working in a tight area I used a lower speed and a smoother abrasive roll. It takes a long time to remove 1/16" of aluminum from a port wall, but don't try to hurry it too much. By the time you finish one set of heads you'll be a pro at using the grinder.

 Now lets' get to the porting stuff!!!

First we have the combustion chambers. Other than the basic polishing we can improve flow by reducing the size of the "swirl dam".



In the pictures above the red arrow points to the swirl dam. It was moved back away from the valve, and the ridge along the top of the dam was lowered, giving it a nice smooth transition. The picture on the right shows the stock swirl dam. It's hard to see, but the dam rises almost all the way to the top of the chamber wall. I used the small grinding stone supplied with the SA head porting kit along the bottom edge of the swirl dam to recess it. The I switched to a larger stone for the rest of the work. You can use a finger through the intake port to judge the thickness of the chamber floor. It's too thin to completely remove the swirl dam, but the size can be reduced a good bit. On the other side of the chamber from the swirl dam, between the valves, there was a raised casting bump, as shown in the picture on the right. It's gone now. Also you can see that the combustion chambers were polished. I used a tapered sanding roll for most of the chamber, and a small straight roll for the really tight spots. Some old valves stuck in the valve seats will prevent damage to the seats. The exhaust valve seat was cut wider than the combustion chamber, but I didn't try to smooth out the edges. I wanted to leave the chamber volume as close to stock as possible. The reshaped swirl dam will lower the compression ratio just a little. In the middle picture above you can see where the cut was made to unshroud the exhaust valve. I did smooth the edges of this cut where it meets the chamber walls. Also, the SA porting kit comes with two flap wheels (120 grit), and cross-buffs for polishing. I couldn't get to some areas with these, so I ordered some 240 and 320 grit tapered rolls from Eastwood to finish the chamber polishing. The 320 grit will leave an almost mirror like surface.

The righthand view above also shows the valve guides quite well. You'll notice these are not cast into the head, but are pressed in. Don't try to change the shape of these!

After all of the chambers were complete I moved on to the exhaust ports. I'll be using these heads on a supercharged engine, so I wanted lots of exhaust flow. The exhaust ports on the SVO heads are round, and smaller than the gaskets for the headers. I used an SVO header gasket to mark the maximum diameter of the exhaust port. In the right hand picture you can just barely see the outline scribed around the port. The red arrows point to one of the thinnest areas in the port walls. There is an overhang where the flat surface for the header flange goes. Be very careful here not to take off too much material. The blue arrow on the left points to the port floor (it's upside down), and the one on the right points to the exhaust valve guide. The port roof does have a cast in boss for the guide. Smooth it down, but don't remove more material than necessary. For my heads I wanted a little more port volume, so I removed material from the port roof, and widened the port on both sides. The wider port sides extend all the way down the port, blending smoothly with the exhaust bowl. You can see that the new port shape is an oval, wider than it is tall. The ports can be widened without affecting air flow over the port floor. The taller roof also increases port volume. The hardest part of the job here is getting all of the ports the same size and shape. After lots of debate I also decided to "square" up the area where the port floor meets the port side walls. After the walls were widened the port floor had a big radius where it met the walls. I used a narrow tapered grinding stone at the corners to reduce the radius (make a tighter turn). In the right hand picture you can see that the port floor is now eliptical rather than round. Other than a very light smoothing the port floor was otherwise left alone. The exhaust bowl was polished but it's shape wasn't changed. The valve seats were smaller then the bowl, which works out well because the flow moves from the smaller seat into the larger bowl. Working on the exhaust bowl is tricky. Great care must be taken not to hit the valve seat with the grinder!!! I used small straight sanding rolls spinning at a low rpm here. It takes longer, but is much safer.

NOTE: If you look closely you can see the scribe line made with the gasket. On the left side (red arrow) the port was not opened up all the way to the line because the wall would be too thin.




ADDED 06/12/99:

I couldn't resist the urge to grind some more. I checked the thickness of the port wall on the left hand side in the pics above. It was thicker than I originally thought. Out came the grinder again, and I opened the port up almost to the scribed gasket line. I still didn't remove any material from the port floor, which is at the top in the pictures above. Below is the polished, final exhaust port. If you look closely you can see the valve guide boss reflected off of the port floor at the top of the picture. The exhaust ports have a mirror-like shine. The valve guide bosses are still a little rough because it's almost impossible to get to them. The rough area is on the back side and shouldn't affect flow out of the port at all. From the pictures above, and those below, you can see that the exhaust ports have been enlarged quite a lot. Also the port roof (at the bottom of the pictures) has been enlarged all the way out to the gasket line. I'm using SVO headers, and they have also been opened up so the header flange opening is the same size as the header gasket used to mark the heads.



Now it's time for the intake ports. I'll be using the SVO intake manifold with these heads. I actually worked on the intake manifold first, so I matched the heads to the intake. An intake gasket was used for the port match. The bottom middle picture below shows the stock intake port. The lower right hand picture below shows the SVO manifold port. This was enlarged around the top of the port (opposite the injector hole) a good bit to straighten out the airflow into the head. The intake ports in the head will match the new manifold port. Like the exhaust ports, only the first inch or so of the port floor was touched, except for a light cleanup. The radius where the floor meets the port wall was squared up, and the walls were widened. With the intake port the valve bowl was slightly narrower than the port, so the area where the port meets the bowl were blended and smoothed. The port roof was raised, tapering down from the port entrance down to the bowl. Looking at the bottom left picture below you can see the shiny area where the port roof and port wall have been enlarged. The bowl itself was worked just enough to eliminate casting marks. The valve seats on the intake side are smaller than the bowls, leaving a "lip" where the seats overhang the bowl. Because the overhang was very small, I just widened the seat opening slightly with a small grinding stone. If more material had needed to be removed I would have had a machine shop cut the seats and bowl with a bowl cutter. The intake ports need to be slightly rough, so they were finished with 80 grit sanding rolls, but were not polished.



ADDITION 06/12/99:

I made some minor changes to the intake ports too. The port roof (bottom of the pic below) was very flat. I removed more material from the center of it to make it more round. I also moved the port wall on the right back some more. The area in the port near the valve guide was also enlarged. In the pictures above you can see that port is more tapered towards the bottom, while in the picture below the new port shape is more "square" and extends almost all the way down the port. It's hard to tell in the pictures, but the whole port was finished with an 80 grit abrasive roll, leaving it just slightly rough. The roughness is desired to prevent fuel from collecting on the walls. A rough surface provides enough turbulence to keep the fuel well "atomized". Comparing the picture below with the stock port (above, middle) shows that the port shape is now "fatter". The SVO heads have a very simple, straight port design. If you look at other cylinder heads you will see many more turns, obstructions, etc. Aren't OHC engines wonderful?


When trying to decide where to remove material from the port walls, visualize how a liquid (or air) would flow through the head. Which side will it run along? What would create turbulence? Don't remove a lot of material all at once. Grind a section, then check wall thickness, then ask does more material need to be removed, and what will create the straightest path to the valve? If you are working on heads for a naturally aspirated engine port volume becomes critical for best power. Too large and you won't have enough velocity to make torque. With forced induction you can cheat a little, but don't try to just hog out the ports and expect any improvement. I've probably spent 30-50 hours on these heads so far. I've gone back and made changes to all the ports because I found another way to improve flow working on the last ports. I also cut some templates to make sure all of the ports were the same size, and had the same curves. All in all it's been fun to port my own heads, but it does go slowly at first. The materials aren't that expensive, but you don't want to go cheap there. I bought a Campbell-Hausfield Standard Duty Die Grinder (about $30), and it's getting pretty well worn. A larger grinder would be better, as the small CH one bogs down if too much pressure is applied. Speaking of pressure, practice on some scrap aluminum to learn the technique before starting on the heads. If you are working in a critical area, use a finer abrasive roll, and low rpms on the grinder to avoid mistakes.

I want to thank several people who have helped me with this project. Sean Adkins for the pictures of his professionally ported heads, Marcus Patterson for his advice on the swirl dams, and Randy Stinchcom for all kinds of info on where to grind, port shape, etc.

I hope to have some flow numbers available soon.

David Posea