Do-It-Yourself Small-Block Cylinder Head Porting

What It Takes To Pull Off A Successful Home-Grown Port Job

Scott Crouse May 1, 2002 0 Comment(s)

Step By Step

McKenzie Racing flow-tested our World Products S/R Torquer heads before and after the porting procedure to measure our results.

The World Products S/R Torquer cylinder heads are a great performance piece for the enthusiast looking for an inexpensive head that will unleash a few extra ponies. We chose this particular head for its stock-like flow. A higher-flowing head like World’s Motown 220 offers much better out-of-the-box performance but would probably be less responsive to a backyard head-porting job.

Before starting our porting adventure, we picked up a Craftsman 1/4hp rotary grinder and Standard Abrasives supplied a cylinder-head porting kit.

McKenzie showed us how to roll the cutter with the direction of the port while removing material in a 180-degree area.

The ridge below the valve seat is the key area to remove in order to enhance airflow. Eliminating this ridge develops a smooth radius from the valve seat into the port.

McKenzie instructed us to remove material only within 1/2 inch of the bottom cut of the valve job. This is as close as you want to get with your carbide cutter. The area can later be blended in to form a smooth radius.

We kept our porting well above the valve guides, but if you’re feeling adventurous, it’s possible to remove material from this area to pick up a few extra cfm.

Be sure to leave a minimum of 0.125 inch of material around the valve guide.

Before attempting to unshroud the valves, it is important to line up the head gasket with the bolt holes and dowel pins in order to scribe a cylinder-bore line around the chambers.

With a line scribed around each chamber, it is easy to see the amount of material that needs to be removed in order to properly unshroud the valve. It is crucial to not remove material past the line or the head may be ruined.

McKenzie lent us some old valves to protect the seats before we started removing metal from the chambers. As you can see, we nicked the valves several times. Be sure to clean the guides with solvent and a pipe cleaner prior to sliding the valves through the guides.

Gasket-matching the intake ports is an easy task, but it’s only a small step in moving air through a head. Porting the walls of the runners is time consuming and offers almost no flow improvements.

It is a good thing we contacted Todd McKenzie of McKenzie Racing before we started porting. With his help, we discovered the right way to port our brand-new cylinder heads. Several common porting mistakes include removing too much material, creating concave pits, improper shrouding, polished intake bowls, and ridged contours throughout the ports.

The key to horsepower heaven is in the shape of the cylinder head. Inside these castings, ports direct airflow to and from each cylinder. Since horsepower enthusiasts often attempt to improve their own heads, Chevy High Performance has decided that the time has come to find out what it takes to pull off a successful home-grown port job.

Our first order of business called for selecting a set of heads that still resembled a stock casting. Choosing a pair of high-flow race heads would leave little room for improvement, so we decided on a set of affordable World Products S/R (stock replacement) Torquer iron cylinder heads. World Products offers the S/R Torquer heads with 2.02/1.60-inch intake and exhaust valves in either 64cc or 76cc combustion chambers. While these valves are larger than most 1.94/1.50-inch stock production pieces, we felt they would offer the best potential for our project.

Now we’ll show you how we ported the heads and the improvements registered on the flow bench. Next month we’ll bolt the heads on a new Agent 87-B 355ci engine to measure how much power we made with our porting. The box-stock S/R Torquer heads flowed a peak 219 cfm on the intake at 0.500-inch lift while the exhaust flowed a max of 134 cfm at 0.550-inch lift (28 in Hg). These are comparable numbers to a stock production iron head.

With the baseline numbers out of the way, we were ready to do some porting. We dove into our Craftsman tool catalog and found a Craftsman Professional ¼hp rotary grinder capable of delivering no-load speeds of 26,500 rpm. Standard Abrasives also helped by offering us the use of several polishing kits. These kits work wonderfully for polishing the chambers and exhaust ports as well as deburring a block or removing gaskets. To speed up the cutting procedure, we purchased an egg-shaped carbide cutter with a ½-inch head and a 2-inch shank. The plan was to have the author, with no prior porting experience, do the porting to illustrate the ease with which this could be accomplished. What you see here is exactly what the author created on his first attempt.

Before we went to work on our heads we decided it would be best to contact Todd McKenzie of McKenzie Racing in Oxnard, California, for some guidance. From the start, McKenzie made it clear that the goal is to create a smooth-flowing path for the air to travel across. Cylinder head porting is more about how well air flows through the port rather than port size. When we began to remove metal from the bowls it became obvious why it’s a good idea to first practice on a junk head. The cutter jumped all around and nearly ripped itself from our hands. McKenzie told us that our cutter would work best with a speed reducer, limiting operating speeds to 15,000 rpm. As we began porting, we realized that the new carbide cutter worked extremely fast, requiring us to constantly keep it moving under very light pressure.

Inside the intake and exhaust bowls there’s a ridge that forms after the valve seat is machined. McKenzie instructed us to remove this ridge and blend the area in with the contour of the port. With the head placed on its exhaust ports, we began removing material from the ridge in the middle and continued to work our way across the floor and up both walls covering a 180-degree sweep.

McKenzie instructed us to remove material only within ½ inch of the bottom cut of the valve job. After removing a little material, we used our finger to feel the contour of the port for high spots. A black marker is handy to have available to mark the high areas. According to McKenzie, the bowls should resemble a venturi where the top of the seat is open and necks down to meet the port where it begins to open back up. Once we removed 180 degrees of the ridge in all the bowls with the head lying on its exhaust ports, we flipped the head onto its intake ports to remove the other 180 degrees of the ridge. When we finished, all of the bowls featured a smooth-flowing 360-degree contour. The important point here is that when porting heads you want to remove the least amount of material possible. Creating concave areas within the port disturbs the air and hurts flow. The entire port should feature a continuous smooth radius. This holds true whether you are porting a small- or big-block cylinder head.

After porting the intake and exhaust bowls, we moved on to the chambers to unshroud the >> valves. This step involved matching the head gasket with the bolt and dowel-pin holes in order to scribe a cylinder-bore line around the chambers. With a line around each chamber, we could see just how much material could be removed. McKenzie informed us that removing too much material would create serious sealing problems and that it’s critical to stay within the line.

He also told us that the removed material from the chamber shouldn’t increase the combustion-chamber size by more than one or two cc’s. Before we went to work on the chambers, McKenzie gave us an old intake and exhaust valve to protect the seats. You can ask your local machine shop for used/damaged valves. Luckily we had the old valves inserted, because we nicked them several times. Having the valves in place won’t allow you to touch up the bottom of the chamber wall quite as far down, but it’s important to use them. We started our project hoping to avoid the cost of a valve job, but after several nicks during our bowl work it became necessary.

With the bowls ported and the chambers cleaned up, McKenzie instructed us to swap out our carbide cutter and use one of the cylinder-shaped polishing rolls from our Standard Abrasives porting kit. Polishing not only creates a smooth path to promote airflow, but also resists carbon buildup. The intake bowls are best left >> unpolished because it promotes atomization of the fuel. Minor cutter marks within the intake bowls are enough to keep the intake charge tumbling through the ports and into the cylinders.

The last thing we did to our cylinder heads involved port matching the intake runners. In McKenzie’s experience, this typically gains an insignificant amount of airflow. The most common type of port matching is done using the intake gasket as a template. Simply scribe a line following the gasket and open the port to match. Be careful in the corners as it’s easy to remove too much material with an egg-shaped cutter. It doesn’t hurt airflow to leave a little extra material in these areas.

With the porting work complete, McKenzie flowed the heads on his Superflow 600 bench and reported a gain of 11 cfm on the intake at 0.450-inch lift and 7 cfm at 0.500-inch lift. On the exhaust side we saw an outstanding gain of 43 cfm at 0.500-inch lift. >> The intake gains seemed low compared to the exhaust. McKenzie informed us that the ridge we removed on the intake side of the port was proportionately smaller than the exhaust-port restriction. Overall, we were very pleased with the flow results, but the only numbers that really matter are torque and horsepower. Next month we’ll find out how well we did. We can tell you that the gain was more than 20 hp.

What We Learned

We spent 10 porting hours to gain a maximum 13 cfm on the intake and 44 cfm on the exhaust. Had we attempted to port these heads without McKenzie’s guidance we most likely would have cut into the valve seats, hogged out too much material, and hurt the flow on both the intake and exhaust sides. Opening the chambers would have been a disaster without the old valves to protect the seats so we’re lucky we had McKenzie available to steer us clear of all these danger areas. The gains were certainly worth the effort.

Be Prepared

When porting heads it is nice to have an air grinder available for variable-speed cutting. Constant use, however, can really challenge even large compressors. During our porting adventure we used the Craftsman rotary grinder most of the time and McKenzie’s air grinder for minor touch-ups. The electric rotary grinding tool requires fewer resources but can be difficult to manage without a variable-speed controller. It’s also a requirement to wear earplugs, a dust mask, and eye protection. You should also work in a controlled area as metal shavings will go everywhere.

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