One key to increasing horsepower from your air-cooled engine is to maximize the air flow through the motor. The more air in the more power out. This is why turbo and superchargers, which force feed the motor a ton of additional air, help the motor produce more power. Several factors limit the amount of air an engine can breathe. On the induction side are the intake tract, including the carb, intake manifold and intake port of the cylinder head. The specs of the camshaft, which determine how much power the motor will produce. While it might seem that the obvious answer here is to make everything as big as possible, this is not the case.
One way to improve the airflow potential of these systems is to port them. Take out the old hand porting dremel and go at it. Unfortunately, it's just as easy to ruin the port as it is to improve it. The air flow is determined as much by the port shape as the size. Significantly altering the shape can actually be detrimental to air flow. While most VW fans are familiar with the phrase ported and polished, we have run across a rather unique form of porting that gives a whole new meaning to this term.
The process is call Extrude Hone Porting. What makes Extrude Hone porting different is that instead of hogging the ports out by hand using an array of rotational carbide and hard roll cutting tools, Extrude Hone forces a Silly Putty-type media through the passages under high pressure using hydraulic cylinders. The Silly Putty is impregnated with small abrasive silicon carbide chips that act as a cutting agent in the putty. By forcing the abrasive putty back and forth (sometimes in only one direction) through an intake manifold or cylinder head port, material is removed from the inner walls. This stock removal improves air flow through the passage.
The Extrude Hone process was invented by Ralph McCarthy back in the mid-60s. When the engineers at MSD were presented with a problem that defied current abrasive technology, they contacted Mr. McCarthy, who was then at South Shore Toll & Die. The company was working on deburring a particularly difficult radar guidance valve body for one of its aerospace customers. This valve body had no less than 84 intersecting holes, in a block approximately the size of a pack of cigarettes. The sizes of the holes varied from about 1/2-inch in diameter down to less than 1/8-inch. The aerospace customer set some fairly strict standards for the finished piece. The intersections of the holes had to remain burr free to meet these standards. The block had to pass a 20x microscopic inspection when finished. Deburring these intersections took the better part of eight hours, a time frame which actually had the company loosing money on each component. Not a desirable situation. While brainstorming one day, one of the engineers at Extrude Hone wishfully suggested that what they needed was a liquid file that would flow through the passages. From that statement, the Extrude Hone process was born.
The difference between wanting a liquid file and having a liquid file proved to be quite a challenge in itself. The engineers set up a small, hand-help hydraulic pump, similar to a grease gun, and ran a variety of different medias through it to test the cutting action. The group tried sand, butter, peanut butter, toothpaste and grease, all with varying degrees of success. It seemed that nothing would hold the abrasive particles uniformly, meaning that the particles would tend to collect in one area, limiting the effectiveness. While at a grocery store one day, one of the engineers came across Silly Putty and they gave it a try. The spongy substance proves to work much better than any of the others, and now Extrude Hone is the world's largest producer of the substance.
The Silly Putty-type media offers a number of unique properties that help the Extrude Hone process' effectiveness as both a deburring process and an airflow enhancement. The media exhibits properties of both a liquid and a solid. While at rest, the media exists in a fluid form. If you held the stuff in your hand, it would ooze onto the floor. How quickly this would happen would depend on the particular consistency of the media. But, like Silly Putty, roll it up in your hands and throw it at a hard surface, like the ground, and what happens? It bounces back like a rubber ball. Though the putty exists in a fluid form all the way to the ground, for that split second that it contacts the surface, it changes to a solid. This property can be demonstrated another way. The media can be stretched out like taffy if pulled slowly. Under sheer forces, such as those experienced by pulling it abruptly, the putty literally snaps. The same thing would happen if you struck the putty with a hammer. Even though it is soft and malleable, it would shatter like a rock.
It is this split personality that allows the Extrude Hone process to work so effectively. When the putty is put under pressure using the hydraulic Extrude Hone machines, the putty flows through the passages like a liquid, following the exact same path as the air flow. The hydraulic pressure from the machines is transferred through the putty to the abrasive carbide material. This allows the putty to remove material form the adjacent areas uniformly like a solid. When the puttys run into an air flow restriction, like the irregularities on the back side of the valves seats in the bowl area or the short turn radius in the exhaust port, it acts like a solid and removes the restriction. It's like having a tiny little hand porter go in, find where the restrictions are and remove them. This is very beneficial when porting long runner intake manifolds like those used on the newer water-cooled VINs.
As mentioned earlier, porting an intake manifold or cylinder head is more than just a matter of making it larger. The Extrude Hone process can be regulated to remove only that material necessary to improve air flow. A number of variable can be altered to achieve the desired results, one of which is the media. A number of different consistencies of media can be used, from HV to LV. The HV, or high viscosity, offers the consistency of concrete, while the LV, or low viscosity, takes the form of a gooey liquid. Like the media, the abrasive carbide particles can be altered to achieve a desired level of stock removal. The abrasive particles range from 8-grit, which is about the size and shape of gravel, to 1,000-grit, which feels like talcum powder or flour. When the process is used for deburring extremely hard substances such as carbide tooling dies, diamonds are actually used as the abrasive material. When deburring the tooling dies, as much as 6,000 carats of industrial grade diamonds are used in four pounds of media. This four pounds mixture costs between $8,000-$12,000. One good thing about diamonds is that they are very hard. Since they don't generally wear out like the other carbide abrasive material, they are reused. The media containing the diamonds is recycled to remove the metal contaminants that were eroded away from the part being deburred, and the diamonds are reused. Generally the media does not wear out, rather it becomes contaminated by the metallic particles from whatever part is being processed. Normal silicon carbide abrasive particles will wear out, so the mixture has a life span. With the diamond, the media is reused once it had been recycled.
The final component to the Extrude Hone equation is the hydraulic machine itself. These machines are designed and built by Extrude Hone and vary in sizes depending on the customers intended uses, as not only does Extrude Hone perform the process as a service, they also sell the machines to customers. The hydraulic machines are essentially two large cylinders, one hydraulic and one media cylinder. The machines are classified according to the different diameters of these cylinders, ranging from two inches, used in high-pressure applications, to a newly-developed unit that measures 30 inches in diameter. The larger cylinder size will be used for a turbine component from a jet aircraft.
Now that we have covered the history behind Extrude Hone and its application as a deburring process, it's time to take a look at how this fancy process helps an engine make more power. As mentioned, Extrude Hone did not design their process with air flow improvements in mind. The forcing of a silly putty-type media through passages had been used exclusively as a deburring agent until the company was approached by a pair of two-stroke chainsaw manufacturers. Like most of Extrude Hone's other customers, they were looking for a way to deburr a particular uct, and almost by accident, the air flow improvements were discovered.
The chainsaw companies were having problems with the cylinder bores (walls of the cylinders) of their two-stroke motors. Unlike typical four-stroke automotive engines, which utilize intake ports in the cylinder heads for air flow, two-stroke cylinders have small passages running directly into them called transfer ports. The problem was that while chroming the cylinder bores to achieve a nice smooth finish for the piston rings, the chrome would build up on the edge of the entry to the transfer port. This build up caused the rings to wear or even break, not something either company wanted in a consumer product. As such, Extrude Hone was contacted to help deburr the edge of the transfer port to stop this build up before chroming.
The transfer ports were treated to the Extrude Hone process, solving the chrome build up. An unexpected side benefit surfaced after the chainsaw manufacturers got the new motors up and running, more power. It seemed that Extrude Honing the transfer port increased the air flow potential of the motor dramatically, allowing a noticeable power improvement, and so the Power Flow portion of Extrude Hone was born. Although the air flow and horsepower improvements were substantial, the whole world wasn't quite as eager to jump on the Extrude Hone band wagon. It took some time before the process caught on in the tough performance market. While the Extrude Hone corporation in Pennsylvania continued to develop the deburring process, the Californian facility, located in Paramount, applied the process to two stroke motorcycle and Jet Skis motors, with excellent results. From there, the process has been applied to everything from rear end gears and automatic transmissions to cylinder head and intake manifolds, including the dual-port system we sent to them for the test.
Before we get into the results of the air flow tests, a word of caution might be in order. There seems to be a reoccurring theme among automotive enthusiasts, that of excess. To put it more plainly, bigger is better. If 40mm carbs are good, then 48s must be better. If a .450 in. lift cam is good, a .500 in. lift cam is better. If 150 cfm worth of air flow is good, then 200 must be even better. While we did say that the more air (and fuel) that a motor can ingest, the more power it would make, believe us, bigger here is not always better. Air-cooled VW engines are not different than any other performance machine in that it takes the proper combination of components to fully optimize the power potential. Having a pair of 48s on a stock 1600cc will do more harm than good, likewise a radical cam profile. The induction, valve train, short block and exhaust all must be built to operate in the same rpm range. What good is a 7,500 rpm cam if the heads sign off at 5,000 rpm? Conversely, having a low lift, short duration cam with a high compression short block will result in extreme detonation, not something to look for in a street motor. While Extrude Hone porting can be produce some mind boggling flow numbers, it is important to be honest about your motor's flow requirements.
For this reason, we opted to port a set of street heads rather than a full blown set of billet race units. Though the extra material in a set of race-only heads would allow the Extrude Hone porting to really shine, most of us don't drive race cars. Most of us drive stock or mildly modified VWs that have to get us to and from school and work. If our wheels don't work, we're hoofing to the beach. To that end, we selected a set of production dual-port heads that were destined to motivate a street-legal dune buggy. Since the motor would be using a camshaft with less than .500 lift, that would be the ceiling for our air flow tests. The heads were treated to the Extrude Hone procedure with all of these factors in mind. In fact, after testing the heads in stock form, Extrude Hone recommended that the heads only receive a minor clean up, as the relatively small valves would eventually be the limiting factor for air flow. A look at the air flow chart reveals that heads actually flowed pretty well right out of the box. (Actually the heads were treated to a 3-angle valve job just prior to porting.)
Rather than going for maximum air flow, Extrude Hone ported the heads to achieve the proper relationship between the intake and exhaust flow. Typically porter seek to achieve a 70-75 percent ratio between the intake and the exhaust. By this we mean that the exhaust should flow at least 75 of what the intake does. In this case, the dual port VW heads surpassed this by a good margin. Though the intake to exhaust flow relationship was very good in stock form, the intake flow was down a bit from where it should be. Once Extrude Hone ported, the both the intake and exhaust flow were raised, but the relationship was kept to 89 percent. Once more, the intake port in the cylinder head now flowed right up there with a good aftermarket dual-carb intake manifold. Looking to put a set of carbs on those stock heads? Extrude Hone porting will really help you make the most of that new induction system.
| Air Flow Data | ||||
|---|---|---|---|---|
| Valve Lift .100 .150 .200 .250 .300 .350 .400 .450 .500 | Intake Before 57 cfm 78 cfm 97 cfm 117 cfm 130 cfm 135 cfm 139 cfm 145 cfm 148 cfm | Intake After 55 cfm 78 cfm 99 cfm 123 cfm 141 cfm 150 cfm 158 cfm 164 cfm 167 cfm | Exhaust Before 49 cfm 69 cfm 90 cfm 104 cfm 113 cfm 123 cfm 131 cfm 137 cfm 141 cfm | Exhaust After 49 cfm 69 cfm 90 cfm 106 cfm 115 cfm 125 cfm 134 cfm 142 cfm 148 cfm |
| Intake Manifold- Before/After 164 cfm/191 cfm | ||||