Street Performance Cylinder Heads - Engine Builder Magazine

Street Performance Cylinder Heads

Where I grew up in Connecticut wasn’t the hotbed of street racing like Woodward was in Detroit, but we had the Berlin Turnpike, River Road in Middletown, plus a few local small-town hotspots. It was during this period that I got my first education about the importance of cylinder heads.

A local rich kid got a brand new ’66 Chevelle SS 396, 325 hp four-speed car. It was beautiful but ran out of steam at about 5,000 rpm. We figured that the hydraulic cam and oval port cylinder heads just couldn’t keep up. A couple of my buddies volunteered to install 425 hp solid lifter cam for the guy, which they did.

I wouldn’t have anything to do with the project because I thought that it was a waste of time. Without installing the larger rectangular port cylinder heads I knew they were just spinning their wheels. Man, was I wrong. This oval port-headed Chevelle ran as good or better than most of the factory 375 hp rectangular port cars we raced against. It would simply tear the tires off in the first three gears.

It took the aftermarket cylinder head industry about 20 years to figure the same thing out, and to start offering high performance oval port heads for the big-block Chevy enthusiast. In the final analysis street performance is not just about huge flow numbers and port volumes. It is the proper sizing and velocity that determines how well a cylinder head drives on the street.

As time has gone on, the lines between street and race have become so blurred that it depends on whom you talk to whether an engine or component is race or street. So when I got the assignment to research street performance heads for Engine Builder I thought long and hard about what really separates the two.

Most of the differences are the obvious things like mounting bosses for accessories and intake and exhaust manifold flanges that match up with OE manifolds. Some folks (especially the musclecar purists) even demand that aftermarket heads actually replicate a factory head’s appearance on the outside. Many of the heads we will discuss here have minor changes such as raised exhaust ports that greatly enhance their performance. If that is an issue for you or your customer, keep shopping because there are plenty of alternatives.

Probably the most important thing an engine builder can do is interview his customer to determine his concept of street performance. If the customer’s approach is to build a racecar with license plates, then a full-on race head is probably the best choice. On the other hand, if it is a 4,000 lb. daily driver with a stock torque converter and a 3.00 rear gear, a more moderate approach is called for. A cylinder head that is too big acts very much like a camshaft that is too large.

The vehicle feels lazy until it hits its happy operating rpm. You can “Band-Aid” the situation with a shorter camshaft, but that is not the right approach according to most experts. The relationship between the camshaft and cylinder heads is extremely important. In fact, most of the experts agree that a smaller, high velocity port with a bigger camshaft is a better combination.

The good news is that there have never been more choices for street performance cylinder heads. The bad news is that there have never been more choices for street performance cylinder heads. With more than 10 major domestic cylinder head manufacturers the choices are endless. Whether you choose cast iron or aluminum, as-cast or CNC ported – there is a cylinder head to fit your budget and application.

Many cylinder head manufacturers mentioned that engine builders should beware of budget cylinder heads available on eBay and through other sources. In some cases, the materials and workmanship have been found to be slipshod to the point that by the time you straighten the problems out, you could have purchased a top-quality head to begin with. Also, you could be giving up 30-70 hp due to inherent design deficiencies not found in the highly developed cylinder heads offered by the more advanced companies.

We started our research with noted street and race engine builder Joe Sherman. Joe has always based his engine combinations around his cylinder heads. He makes an interesting correlation between port volumes and engine displacement. He said that he looks at what size intake port the engine came with – for example a 350 small-block Chevy came with a 160 cc intake runner. He recommends increasing that by about 15-20 percent or to 185-190 cc.

For a big-block Chevy that came with 260 cc intake ports he upgrades to a 310 cc intake cylinder head. Joe also made the point that there is virtually nothing you can do to improve on factory performance below 4,500 rpm so the OEs obviously got the port-sizing pretty close to optimum for the street. Any power gains you will experience from larger port volumes will most likely occur above 4,500 rpm.

So what are desirable characteristics of a good street performance head?

Good low- and mid-lift flow are extremely important in any performance head, but especially in a street head where cam lifts are usually limited to .600? or less. The total flow under the lift curve is what you should shoot for. Good low-lift flow is the product of unshrouded valves in the combustion chamber, the correct bowl shape, and of course the valve job is very important. Many head manufacturers have flow charts on their Web sites. Comparing these flow areas “under the curve” for various heads is fairly easy to do with these charts. Just make sure that they were tested at the same pressure differential (28? of water is typically the standard measurement).

We spoke to a few cylinder head experts to get some guidelines about selecting a good street performance cylinder head. Certainly, these are not the only suppliers of quality products and you can find additional sources in the High Performance Buyers Guide in this magazine.

AirFlow Research’s Tony Mamo says that when discussing a potent street application AFR’s mantra is to “go for the most flow with the smallest hole.” A good example of that is AFR’s latest big-block Chevy oval port heads. GM’s larger oval port head has an intake port volume of around 255 cc and flows about 270 cfm of air. (These heads are becoming scarce as they were replaced early in production with “peanut port” versions.) AFR’s smallest oval port head has a 265 cc intake port volume and flows up to 350 cfm – a whopping 30 percent more airflow through basically the same size port. This would make a very impressive street cylinder head on a 396-468 cid engine. A word of caution and AFR spells it out right up front that it has raised the exhaust ports .375? so it may take some modifications to the vehicle’s exhaust system.

They also plainly state that this is not an emissions-legal head, so if that is important to you there may be better choices.

AFR does offer some emissions-legal street heads such as its LSX Mongoose Street heads that feature a near-stock 205 cc intake port (stock LS1s are 201-202 cc), however it flows 300 cfm versus a stock head’s 230 cfm – again a 30 percent increase in flow from a stock sized port.  It utilizes the latest CNC porting technology that allows AFR to produce these sophisticated street heads at affordable prices.

Since AFR only offers aluminum cylinder heads we had to talk to Dart’s Jack McGinnis about the virtues of aluminum versus iron heads. Dart has built its reputation on the racetrack, but Jack says that about 30 percent of Darts business is the street cylinder head market. The tradeoffs between iron and aluminum are obvious and quantifiable. Dart’s Iron Eagle line of iron heads is approximately $300 cheaper than its aluminum counterparts and about 50 lbs. heavier per pair. So if your customer is looking to improve the handling of his vehicle, getting the 100 lbs. off of the front end of the vehicle is a huge advantage.

It looks like you might pay a slight penalty in performance with the iron heads too, because the flow numbers look a little softer than aluminum. Dart offers two iron street heads for small-block Chevys; the Iron Eagle that comes in various port sizes and the Iron Eagle S/S that is smaller, but still outperforms factory replacement heads. Dart sells a full line of aluminum heads in various sizes and applications.

Another advantage of aluminum heads over their iron counterparts is tolerance to increased compression. Alloy heads can easily handle 10.5:1 compression ratios on pump gas where iron heads are more comfortable at 9.5:1. So if you have an existing shortblock that works out to 9.5:1 with a standard size combustion chamber, then an iron head might make more power and perform better. World Products has you covered in those situations – they offer their same Motown style in both iron and aluminum. World Products also offers several styles of iron heads so you have many affordable alternatives.

A few companies like Edelbrock are offering aluminum heads designed specifically for the street with powerbands from idle to 5,000 rpm for its E-tech and Performer line of cylinder heads. Need a little more performance? Edelbrock’s Performer RPM is the next step up the ladder. For street heads that also see the racetrack on on regular basis Edelbrock’s RPM XT series might be the right choice.

They take the Performer RPM casting and CNC port the combustion chambers, bowl areas and the port entry and port exits. You get 90 percent of the benefit of CNC porting without the expense of a fully ported CNC cylinder head. Edelbrock also sells a line of dyno-tested camshafts tailored to match the flow characteristics of each head.

Racing Head Service is one of those companies that has been around for years in various forms, but is now solidly addressing both the street and race market. Like World Products, Dart and many others they offer an improved cast iron replacement line of heads, plus a Pro Action line available both in aluminum and cast iron in several runner sizes, many of which are targeted for the street performance market. Like Edelbrock, RHS has a sister division (Comp Cams) that has worked out a bunch of cam profiles to match the flow characteristics of the RHS heads. So if you are a little uncertain about cam/head combinations, this could be a good way to go.

The final company in this category that we will mention is Trick Flow Specialties or TFS as they are more widely known. TFS not only offers several versions of its heads that will work well in a street application, they have held their prices down where they are extremely price competitive with a set of iron performance cylinder heads. A quick pricing search at Summit Racing shows that a complete assembled pair of heads ready to go is approximately $1,000, which is a good value.

Tri-State Cylinder has developed a line of fully assembled, ready to bolt on, import and domestic racing heads for both street/strip and hard core racing engines. The company says its Flo-Tek Chevy 350 heads is comparable to Summit’s SMU-162111 cylinder heads and comes with 180cc intake runners, 64 cc combustion chambers, angled or straight plug, 2.02? and 1.60? stainless  valves, a 5-angle valve job, valve springs good to .550? lift on a hydraulic roller cam, bronze guides, and 3/8? screw-in studs and guide plates are included.

Patriot’s Freedom Series heads meet the demands of the BBC world, while keeping the quality high and the price reasonable. Patriot’s BBC heads come as cast with a 320 cc intake runner, US made bronze guides, and a 5-angle valve job. Chevy High Performance used these to build their 675 hp 496. These heads come standard with a 119 cc chamber, assembled in Patriot’s own facility with 21-4N stainless 2.250?/1.880? valves, .600? hydraulic roller springs, steel retainers, 7/16? studs, guide plates, and Viton seals.

EngineQuest offers a new replacement cylinder head for the Chevy 327/350 made from the high grade cast iron. Designed to fit 1955 to 1986 model Chevy vehicles, the new head is available in either a 67 cc or 76 cc combustion chamber with either a 2.02? or 1.94? intake valve and uses a straight positioned spark plug.

Utilizing flow cast technology, EQ’s high performance cylinder head improves on the original design with a thicker deck for added strength, hardened exhaust seats for greater resistance to cracking, and a redesigned water flow system for improved thermal conductivity. With CNC machined guides and seat, a superior port finish for increased flow and a three angle valve finish, these cylinder heads offer both OE fit and improved performance.

There are many performance aftermarket head choices for small block, big-block and LS Chevys, and small-block Fords and Chryslers. Unfortunately, other engines have fewer options. When you get to that oddball engine that requires you to actually modify an OE cast head, remember, your biggest gains will result from unshrouding the valves in the combustion chamber, porting the bowl area under the valve and doing a good three-angle valve job. A little work here can result in huge gains at the rear wheels. Some manufacturers like Racing Head Service offer identical cylinder heads such as its Pro Action series in both cast iron and aluminum. Save some money with cast iron, or save some weight with aluminum 
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