Chevy Cylinder Heads - Insider - CHP

Cylinder Head Secrets With Porting Expert Judson Massingill

Stephen Kim Jan 19, 2007 0 Comment(s)
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Beneath that stern, intimidating glare and impressive stature is a man with few equals in the realm of cylinder head expertise. There are probably a few people who know more about cylinder heads than Judson Massingill, but since none of them are willing to talk, their existence is inconsequential. That's not exactly PC, but maybe it's just Judson's 100-percent B.S.-free demeanor rubbing off on us.

If you want answers, this straight-shooting Texan will give 'em to you in pure, unadulterated, and unfiltered form. Best of all, he's usually right. If you aren't familiar with the name Judson Massingill, there's no need to question his credentials. He and his wife Linda run the School of Automotive Machinists in Houston. The school's unique curriculum is strictly dedicated to the art of building high-end race engines. So successful is the program that some of the top race teams in the country, like Hendrick Motorsports, John Force Racing, Cosworth Engineering, Warren Johnson Enterprises, and DEI, all rely on Judson's graduates to put them in the winner's circle, and Nextel Cup teams enlist him to do development work on their heads.

Judson got his start like most hot-rodders, building engines in his garage and testing them on the street and at the track. Always pushing the envelope, he combined his street savvy with a university education in the quest to build more potent motors. In the heyday of the musclecar era, Judson prevailed in enough street skirmishes to pay for school with his illicit earnings. He was soon building engines professionally, but got sick of spending years training employees only to watch them quit and start their own shops. Realizing he was essentially training people for free who later became his competition, he turned that concept into a business by starting up a hard-core vocational school. Here are his words on cylinders heads, ranging from the most basic of principles to the most advanced of theories.

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Swirl
One of the more controversial areas of cylinder head theory is swirl, but it's a concept that packs more hype than substance. Swirl proponents say it contributes to a more homogenized air/fuel mixture, which eliminates lean pockets and reduces the potential of detonation. However, Judson isn't too convinced of its merits. "Experts can't even agree on how to accurately measure swirl, and every device that measures swirl affects it somehow," he says. "Promoting swirl adversely affects inertia and flow. While it may help with gas mileage, emissions, and to some degree power, optimizing quench is a much more effective method of homogenizing the air/fuel mixture without the adverse effects in airflow associated with swirl." In other words, quench trumps swirl every day of the week.

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Which Makes More Power?
"Unless rules require it, you don't want to run an iron head. The only advantage is the lower cost. All this hoopla that heat escapes too quickly out of the chambers with aluminum heads compared to iron heads is pure B.S. If you're that worried about heat dissipating too quickly, just move the water jacket farther out. How hard is that? Iron is more prone to cracking and much harder to repair; you have a whole lot better chance of salvaging an aluminum head if it's damaged. You can also port aluminum twice as fast. I remember the days of spitting up black stuff for two full days after porting a set of iron heads. Another major problem with iron is that you can't weld and add metal to it, which takes away from a skilled porter's creativity in reshaping ports and combustion chambers. The weight difference isn't too bad with typical small-block wedge heads, but the penalty is significant with big-block or canted-valve heads, or any time you raise the runners. Some circle-track guys say you need an extra 35 hp on a motor with iron heads just to make up for the additional mass, since the weight is so high up off the ground and so far forward in the chassis."

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Throat Diameter
A critical aspect of maximizing cylinder head flow is establishing the proper throat diameter. Going too big or too small can seriously impede airflow, but getting it right is pretty clear cut. "The rule of thumb is that the throat diameter should be roughly 90 percent of the valve diameter," says Judson. On race valve jobs, since the seats are moved farther down near the valve, the guidelines change slightly depending on the specific valve- seat angle. With a 45-degree valve seat, the throat diameter should be 0.200 inch smaller than the valve, and with a 50-degree seat, it should be 0.180 inch smaller. Stepping up to a 55-degree seat requires a throat diameter 0.160 inch smaller than the valve.

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Angle-Milling Heads For Power
Angle-milling cylinder heads is always beneficial to airflow. "Removing more deck material off of the exhaust side than the intake side reduces the com-bustion chambers, raises the runners, and helps deshroud the valves by moving them closer to the center of the combustion chamber," says Judson. It also allows more material to be milled before hitting the valve seats. However, there is a practical limit to how much a head can be angle-milled. Rolling over a 23-degree head to an 18-degree valve angle just isn't practical or feasible. On a small-block, "cutting 0.017 inch off of the exhaust side per inch of cylinder head width reduces the valve angle by one degree, and 1.5 to 2 degrees is the absolute max. If you get the chambers and seats optimized to go along with an angle-mill, the procedure can net a solid 15-cfm increase in a head that flows 300 cfm." The only downside is that the minute change in geometry that results from angle-milling may require some creative installation when fitting the intake manifold and headers.

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Working Runners Vs. Working Bowls
On production cylinder heads, the bowls require much more attention than the runners. "The average backyard hack needs to stay away from the port, gasket-matching it at most, and work on the bowl," says Judson. "It's hard to mess up the bowls, and using common sense will improve airflow. The goal is to create a nice transition from the bowl into the valve job." However, just the opposite is true with a quality aftermarket casting. "If you buy a good aftermarket head, the bowls are already 90-percent there, and the average person is more likely to mess it up than improve it."

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Valve-Seat Angle
"Generally, a valve-seat angle greater than 45 degrees will make more power. There are some trade-offs, though. As the seat angle increases, durability decreases. That's why lower angles are common in many production motors, and just about all diesels have 30-degree seats. With 50-55 degrees you'll lose 10-15 percent of flow from 0.200- to 0.400-inch lift that you can't get back. However, you can't sacrifice high-lift flow for low- and mid-lift flow because that's not where you make power. Some of the top engine-builders in the country don't even turn the flow bench on until 0.300- to 0.400-inch lift. And the improved high-lift flow of bigger angles lets you open up the venturi-at that point the venturi becomes the restriction. You lose a ton of energy when air exits from the port into the cylinder, so a bigger venturi helps maintain that energy. Porting is all about area relationships, and you want to maintain the valve area as the restriction, not the port. In other words, you don't want a weak port with 50- to 55-degree seats. A weak port with a valve seat area that flows well creates lots of turbulence, which hurts flow. The more you know what you're doing, the less you lose down low by going with a higher seat angle."

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Reshaping Combustion Chambers
Porters typically don't pay enough attention to the combustion chambers. The basic idea is for the chambers to be an extension of the valve job all the way into the cylinder. Following this principle, with wedge heads, a heart-shaped combustion chamber is ideal. Judson tells us, "If you perform a valve job, you have to work on chambers. The goal is to keep velocity even all around the valve." If the chambers aren't optimized, the penalties can be severe. "On one of our race heads, we cut 1-2 cc of material out of the chambers to get some extra piston clearance for the aluminum rods we were running. Our flow dropped from 410 to 385 cfm. It just goes to show you every little thing on today's heads is so much more critical than on the junk heads we had 15 years ago."

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Port Velocity & Flow
"I tell my students they'll spend the rest of their careers trying to find the right balance between flow and velocity. A simple way to look at it is if you increase the cross-sectional area of a port and pick up flow, then you haven't hurt velocity. On the other hand, if you open up a port and don't pick up flow, you've hurt velocity. It's a delicate balancing act, and air velocity is not uniform throughout a port. There's the average velocity and localized velocities, and air moves faster toward the center of the port, where friction from port walls doesn't affect it as much. The trick is mini-mizing the differences between localized velocities. If air moves too fast, it won't want to make the turn at the short-side radius, which makes a big difference between localized velocities in that part of the port and hurts flow. Although there are people who swear by high-velocity ports, it isn't nearly as important in a high-winding motor. The lower the motor's rpm range, the more velocity you need, but you can't make runners big enough if you want to turn high rpm in a race motor."

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Porting ToolsThe easiest way to begin is by picking up a porting kit from Standard Abrasives that includes most of the abrasive materials necessary for head porting. Either an air or electric grinder capable of at least 10,000 rpm is required, and the abrasives can safely handle 18,000-20,000. Judson prefers using a Milwaukee 5196 electric unit. Additional necessities are a selection of carbide cutters, radius and telescoping gauges, a protractor, dial calipers, scribes, different-length mandrels, cartridge rolls for finishing, and lights with magnetic bases to help illuminate the ports.

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Port Volume
A common method of determining properly sized cylinder heads for a given displacement and rpm range is port volume, but it's not necessarily the most precise method. "Port volume doesn't mean anything," says Judson. "All that matters is cross section, because you have to compare like ports. You can't compare 23-degree heads to 18-degree heads since the longer runners in an 18-degree head means it has more port volume for any given cross section." Generally, port volume is just a substitute for measuring cross section, and the larger the cross section, the higher the rpm the motor must turn. Here's the industry standard formula for determining the proper average cross section of a port:port speed = piston speed x (bore area port average cross section)

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Short-Side Radius & Power
As air moves through the intake port, its naturally tendency is to continue moving forward rather than transition downward toward the valve, which causes turbulence and impedes flow. "What you're trying to do as a porter is stick as much air as possible to the short side to help it make the turn toward the valve," says Judson. "Many times the short side matches the angle of the port roof, and laying it back farther away from perpendicular to the valve guide generally improves flow. The trick is laying the short-side radius back as far as possible without going so far that you hurt flow."

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CFM Vs. Horsepower
For most street/strip and mild race motors that generate up to 1.7 hp per cubic inch, improvements in airflow translate directly into increases in horsepower. However, at anything beyond that level of specific output, the flow bench is no longer the Holy Grail of predicting power. "Once you hit 2 hp per cubic inch, things start getting crazy," says Judson. "Take a Pro Stock engine, for instance, that puts out right around 2.7 hp per cube. On a motor like that, it's a given that you have a serious cylinder head that can fill a cylinder quite well. So when the exhaust valve opens, residual pressure reaching hundreds of psi escapes out of the cylinder. The velocity of the exhaust coming out the head is a bazillion times higher than what can be replicated on a flow bench." To put it succinctly, experts can find no correlation between airflow and power on the exhaust side beyond 2 hp per cubic inch. "There are things going on at that power level in a motor we just don't understand. You can have one head that's down 50 cfm to another head on the exhaust side that makes just as much power. Don't get me wrong, the flow bench has contributed more to horsepower than any other part or tool in the last 30-40 years, but after a certain point flow numbers don't mean squat."

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Shaping A Port
Small- and big-block Chevys feature a variety of rectangle, oval, and even cathedral ports, but not all are equal when it comes to airflow. "In terms of cross-sectional area to flow, an oval port is the most efficient because it has no sharp edges," explains Judson. "Air travels in the path of least resistance, and eliminating sharp edges minimizes resistance. If there's enough meat on the head, ports should be oval." Some rectangle ports can be reshaped into ovals by filling in the corners with epoxy. Extending this concept into the intake manifold, runners that taper from the plenum into the port makes incoming air less sensitive to the changes in the contours of the port. This effect is more pronounced with longer intake runners, where taper helps out even more. "Air likes consistency, not a lot of changes."

Sources

School of Automotive Machinists
Houston, TX 77055
713-683-3817
samracing
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