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Jump To The Flow Data Page To Compare Heads
Small-block Chevy cylinder heads can be categorized in several different ways. One of the easiest is to group them by intake-port volume. For the purposes of this story, we will organize these heads into three categories: Category 1, 179 cc and smaller; Category 2, 180 cc to 199 cc; and Category 3, 200 cc to 220 cc. Unmolested production heads generally fall in Category 1, but not always. For this test, we've evaluated most of the popular heads. For the earlier heads, we'll refer to them by the last three digits of their casting number, as in 462 heads, which were early castings that had a 64cc combustion-chamber volume and were used on 327 and 302 engines. The later heads will be referred to by their application, such as the aluminum L98 TPI castings, the LT1 and LT4 5.7L engines, as well as the cast-iron Vortec head and the new LS1.
While we have separated these heads by port volume, greater size doesn't guarantee increased airflow. The advantage to midsize heads is that they combine increased airflow without sacrificing velocity, which tends to promote strong torque. One way to look at cylinder-head flow is to include velocity when evaluating heads. In other words, an intake port the size of a tennis ball will probably flow tons of air but at a snail-like velocity. Conversely, smaller port volumes will suffer from flow restrictions but offer fantastic velocity. Velocity, then, can be equated with good torque and excellent throttle response.
There's much more to cylinder-head selection than just choosing the head with the best flow numbers. In fact, as we've explained, large intake-port volumes with high peak-flow numbers may not be the right choice at all. There are actually several criteria for selecting the best overall cylinder heads for your application. The best way to look at cylinder-head flow is to closely examine and compare the entire flow curve from low lift to max lift.
If you must look at only one point, most airflow experts prefer to look at mid-lift flow. For street engines, airflow at 0.400 inch is a good place to evaluate all cylinder heads. The main reason for this is that maximum lift flow at 0.550- or 0.600-inch valve lift is at the ragged edge of most streetable valvetrains. More importantly, the valve is only at max lift once in the entire valve-lift curve, while the valve achieves mid- and low-lift numbers on both the opening and closing sides of the valve-lift curve.
Cylinder-head selection also means evaluating exhaust-flow numbers as well. A cylinder head with killer intake numbers but poor exhaust flow is not going to perform as well as a head with a slightly weaker intake but an excellent exhaust port. It is possible to prop up a weak exhaust port with longer exhaust duration and more lift, but generally that engine will not perform as well as an engine with a strong exhaust port and a more ideal camshaft. One way to evaluate the relative strength of an exhaust port is by comparing the exhaust flow to the intake flow as a percentage at the same valve lift. This is generally referred to as the exhaust-to-intake (E/I) relationship. For example, let's say the intake port flows 250 cfm at 0.400-inch valve lift and the exhaust port flows 187.5 cfm. Exhaust-port flow divided by intake-port flow will express this relationship as a percentage. In this case, the exhaust flows 75 percent of the intake at the same valve lift.
Conventional wisdom holds that anything more than a 75 percent E/I relationship is considered good. Eighty percent is even better. But anything more than this might be suspect. In other words, a great E/I may point to a weak intake port rather than to a strong exhaust port.
Once you're armed with all the information in the test, the key is proper selection of the right cylinder head for your combination. As is the case with most things high-performance, bigger is not necessarily better. For mild street engines of 350 ci or less that rarely see the high side of 5,500 rpm, there's little reason to select a cylinder head with a port volume of more than 200 cc. Engines in this category respond better to a smaller intake-port volume that will contribute to excellent low- and midrange torque. This also contributes to excellent throttle response and even decent fuel economy when teamed with a mild camshaft, a dual-plane intake manifold, and a carburetor with small primary throttle bores, such as a Quadrajet- or Carter-style carburetor.
Engines of larger displacement, in the range of 383 to 406 ci or heftier, can take advantage of a larger intake port, depending upon how the engine is intended to be used. For example, a 406 small-block that will end up in a four-wheel-drive Chevy truck would be better served with an intake port of between 180 and 190 cc, since this smaller port volume will again promote excellent torque from this larger-displacement engine. For an engine intended for higher-rpm horsepower, a larger port volume will tend to promote greater horsepower when combined with a longer duration camshaft. For example, a 210cc intake-port cylinder head is rather large for a 383ci small-block, but if you plan to spin this motor to 6,500 or even 7,000 rpm, the large intake-port head might contribute to a significant power increase.
As you can see from this rather brief overview, the business of selecting a good cylinder head can be downright confusing. Port volume, port flow, E/I percentages, cost, weight, chamber size, valve size, and probably at least another half-dozen or so variables contribute to making a cylinder-head choice rather difficult. On the other hand, spending a little more money for an aftermarket cylinder head is usually a very good idea. Airflow is the key to making horsepower, and all of the aftermarket heads we tested offer improved airflow over stock, 25- to 30-year-old castings. To paraphrase engine builder John Lingenfelter, "The world's best cam combined with a poor set of heads will produce an engine that's a dog. But bolt on a set of great heads even with a poor cam, and that engine will still make great power."
If there is a magical combination, it would have to be a port with great airflow and outstanding velocity. This is especially true for intake ports. A good example of an extra-large intake port that didn't work would be the '69-'70 Ford Boss 302 engine. The ports on that engine are huge. Those engines made decent horsepower but were infamous for their lack of torque. This is also due to cam timing but mainly to the expansive intake ports. That's why pure airflow numbers can be misleading. You must also consider the application for which the cylinder will be used. For example, it would be folly to bolt a huge 220cc intake port head on a daily-driven 283ci street car.
The idea behind this effort was to use one flow bench to test as many small-block and big-block Chevy cylinder heads as possible. By using one flow bench and the same test procedure, we eliminated many of the variables that exist between different benches and operators. All the tests were performed at Westech Performance Group by John Baechtel and Steve Brule using a SuperFlow 600 computerized flow bench. We tested each head not only for intake and exhaust flow at 28 inches of water, but also measured each combustion chamber and intake port for actual volume.
There are a couple of important notes worth remembering when using this data. The cylinder heads are broken down into two main areas :small-block and big-block heads. Within each of these areas are three categories. We found the easiest way to categorize these heads was by intake port volume. For the small-blocks, Category 1 encompasses all heads with intake ports smaller than 180cc’s. Category 2 includes heads from 180cc to 199cc while Category 3 lists all small-block heads larger than 200cc. For the Rat motors, the port volumes are larger but also split up into three categories. Category 1 for Rat motors lists all heads under 220cc while Category 2 includes 220cc to 299cc intake port heads. Category 3 lists all Rat heads with ports from 300cc to 370cc.
Most of the heads were tested both with an open exhaust port and with an exhaust pipe. This way, if you are more comfortable with one type of test, we have data in either form. Another line on each chart calls out the E/I, the exhaust-to-intake relationship at 0.400-inch valve lift. This gives you an idea of how well the exhaust flows in relation to the intake. Most cylinder head experts will tell you that a number above 70 to 75 percent is considered good, especially if the intake port flow is also a good number.
While the temptation is great to concentrate on the peak lift flow numbers, we suggest concentrating on the mid-lift flow numbers and how they compare. For street engines with valve lifts that do not exceed 0.500-inch, it makes little sense to choose a head with great flow numbers at 0.600-inch valve lift since the valves never open that far. The valve lift curve hits the mid-lift numbers such as 0.300- to 0.400-inch twice in each valve lift curve while peak lift is only achieved once per cycle. Judging a cylinder head by its mid-lift flow potential can be beneficial in choosing the right cylinder head for a street engine.
This will be an on-going project. As new cylinder heads are created and produced, we will continue to test these heads and report on what we find. So if you want to know the latest information on the state of art in street-oriented cylinder heads, it would be wise to check our site on a semi-regular basis.
We designed this web site so that you can call up each cylinder head individually and look at the flow numbers. We did this so that it would be easy to create your own personal cylinder head reference manual by merely calling up each category and printing out the information. Then those pages can be placed in a notebook where you can use them for instant reference whenever you need it.