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Building Drag Racing Cylinder Heads
Depending on the class, tuning for the quarter-mile requires different approaches. We offer expert advice from successful racing professionals.
By Larry Carley
One thing every drag racer wants is to go faster - even bracket racers. There are lots of ways to shave tenths of seconds and even whole seconds off an elapsed time. It all comes down to how much money the racer is willing to spend in his quest for speed.
One way to go faster is to improve the breathing efficiency of the engine. Installing a hotter camshaft with more valve lift and duration will add more power, but to maximize the engine's breathing potential the cylinder heads also have to be reworked to take full advantage of the increased valve opening and timing. Stock cylinder heads can be ported and massaged to a certain extent but eventually you run out of metal. So when a racer gets serious about wanting to make more power, the stock heads are usually replaced with a good set of aftermarket performance heads.
All of the engine builders we interviewed for this article said that drag racers today have a great selection of aftermarket performance heads from which to choose. Most of these heads are affordably priced and deliver excellent performance gains right out of the box. Of course, there's always room for improvement. But with heads that are fully CNC machined with a state-of-the-art profile, there's not much more to do except to finish the seats and assemble the heads. Most of the builders we spoke to said they prefer to work with bare heads rather than fully assembled heads so they can have more control over the finished product.
The amount of work and money that goes into a set of heads will obviously vary a great deal depending on the drag racing class. Budget-conscious index racers who are competing in a Stock, Super Stock or even Competition class don't have the deep pockets or the sponsorship money that racers do who compete in Super Competition, Pro Stock, Funny Car, Alcohol Fuel and Top Fuel dragster classes. On the other hand, racers in the slower classes don't need the huge flow numbers of the more competitive and expensive classes. All they need is a car that runs consistently and is competitive.
In addition to money (or the lack thereof), another issue engine builders have to cope with is rules. Every sanctioning body for drag racing has their own rule book that defines what can and can't be done to a set of cylinder heads. The rules vary from one class to another and depend on the sanctioning organization. Generally speaking, the lower classes usually restrict combustion chamber volumes, valve size and location, valve seat and guide locations, spark plug locations, even how much porting is allowed.
For example, one new rule change for 2004 by the National Hot Rod Association (NHRA) for cars racing in the Competition class says engines must use heads that are "commercially available over the counter from the OEM source and must conform to specifications provided to NHRA by the OEM." In other words, no custom aftermarket heads. The new rule also says any size valves are permitted, but the guides and spark plug holes must remain in their stock locations. The same goes for the angle of the valve guides.
For those who are unrestrained by such rules, the only limiting factor as to how far you can go are the physical dimensions of the casting itself and how much money your customer is willing to pour into a set of all-out racing heads.
Many top competitors spend tens of thousands of dollars on their cylinder heads. Darin Morgan, who does all the head work at Reher-Morrison Racing Engines in Arlington, TX, says his shop runs several Pro Stock engines with heads that cost $35,000 to $40,000 per set!
"We start with raw unmachined castings that are almost a solid chunk of aluminum. We have two CNC machines and develop the port configurations and combustion chambers ourselves in house. It's a very labor intensive process that takes about a month of fulltime effort.
"After the profiles have been developed, the castings are rough CNC machined. We then redo the heat treatment and freeze the heads to stabilize the metal. This probably doubles the life of the heads, which is about four years on average. The heads are then final machined and assembled."
Morgan says he only does about three new sets of Pro Stock heads a year, which are run on Reher-Morrison's own engines or used on engines that are leased to several race teams (for $15,000 to $20,000 per race!). They also build about 250 engines a year for bracket racers.
"We currently have 13 Pro Stock engines in rotation. Each one takes about four to five months to build. By the time we're finished we probably have $120,000 to $130,000 in man-hours and materials invested in each engine. They are 500 cubic inch big block engines that develop 2.175 horsepower per cubic inch at 9,000 rpm, and 1.7 ft.lbs. of torque at 7,000 rpm. They are very efficient and require only 27-1/2° of spark advance at 9,100 rpm to produce peak power."
Morgan says the most important aspect of building competitive cylinder heads is achieving maximum efficiency in air flow and chamber burning. "The trick is to find the right balance between port area, air velocity and the shape of the runners. You also want the chambers to give you a quick, complete burn.
"In our Pro Stock engines," he continues, "we use beryllium/copper valve seats because they are softer than cast iron seats and work well with titanium valves. We run triple springs that are alternately wound with 2.25?- to 2.30? installed heights. Seat pressures are 375 to 400 lbs. with the valve closed, and 1,200 lbs. at maximum lift - which is over one inch on these engines. We usually replace the valve springs after four runs. The heads themselves are very stable and are good for about four years of racing. Even so, we check the heads every 50 runs and freshen them up as needed. Usually all we have to do is take a very light cut on the valve seats, maybe only .001? or so unless there's been a problem."
"Cylinder heads today are not like heads 30 years ago," explains Joe Mondello of Mondello Technical School in Paso Robles, CA. "Today, you can buy a good set of aftermarket heads from many manufacturers and suppliers that are CNC-machined and ready to go. But for those who are racing in stock classes or don't have the money, porting is still an option for OEM cylinder heads. If you can find a good porter who knows what he's doing, for $900 or so he can clean up the bowls, raise the roofs of the intake and exhaust ports and give you an extra 40 to 50 hp."
Mondello says it doesn't make any difference whose cylinder head you use, head stability is an issue with green castings. This is because they all move around as the metal heats and cools, which can shift the location of guides, seats and decks, possibly affecting valve sealing and valve life.
One of the most important things that should be done to prep a head, says the Living Legend Mondello, is to stabilize a green casting before it is machined. He recommends stress relieving the head by shaking it (a frequency of 100 Hz works well, he says) until it is "normalized," then cryogenically freezing it to further settle the metal. If you don't have access to a shaker or freezer, he says another trick that helps stabilize a green casting is to simply cycle it through a hot water parts washer several times. Once this has been done, the guides can be installed, the seats machined and the head surfaced as needed to achieve the desired clearances.
Mondello recommends using high-quality bronze guide liners in cast iron heads, and diamond honing the guides after they have been installed. But he also recommends slightly greater valve stem-to-guide clearances than stock for drag racing. He recommends .0015? to .002? for intake guides and .002? to .0025? for exhaust guides. Why? If the guides are too tight, they may gall and grab the stem when the engine is hot. This may cause the valve to stick momentarily causing a piston to hit a valve or cam failure.
The angle of the valve seats is critical for achieving maximum performance, says Mondello. "To get good flow numbers, you need to use a four to seven angle valve seat cutter - not the usual 3-angle cutter that most shops have. We cut the intake and exhaust seats to 50° to 55° on high performance heads with 215 to 230 cc intake runners. Increasing the seat angle really improves the wet and dry flow characteristics of the head. By the same token, you can also screw up the flow characteristics of a good head by giving it a standard 3-angle valve job. We've seen heads lose as much as 30 cfm after somebody gave it a 3-angle valve job."
Mondello says any engine builder who wants to use a higher than normal valve angle to improve the flow characteristics of a performance head can now buy his special Joe Mondello Signature Series Cutters, available from Goodson Shop Supplies.
When Mondello builds a head, he catalogs the exact angles that are used so they can be duplicated when the head is freshened up after a season of racing. "We log all the cutter angle information so we can go in and recut a seven angle seat exactly the same as it was originally cut. If somebody goes in with a stone and tries to touch up the seat, he'll probably ruin it."
Prepping the valve springs is also critical when it comes to reliability and performance. Mondello also recommends shaking and freezing valve springs to extend their life. He says this can increase spring life five-fold, from an average of six runs to 30 runs in a high competitive class. For engines that sit for more than a couple of weeks between races, he also recommends loosening up the rocker arms and relaxing the springs. If several springs are held at or near full compression for a long period of time, it can weaken the spring and increase the risk of spring failure.
Another trick Mondello recommends is coating the valve springs with a product called "Heat Seek," a special assembly lube that can also be used on other valvetrain components to help protect them from heat. He says springs that are installed dry can lose as much as 10 to 20 lbs. of pressure when the engine is first fired up.
As for valves, Mondello says stainless steel valves work great for most drag racers - except the top teams who run titanium to save weight.
Champion Racing Engines
Dick Fox of Champion Racing Engines, located northeast of Indianapolis, IN, said his shop builds engines for SuperComp, SuperQuick, Top Sportsman and Index class drag racers. "We use CNC-machined heads. We buy bare castings and do some hand touch up work before machining the seats and installing the guides. We use top quality valves, cams and springs, locks and retainers, as well as the best hardware."
Fox said they pressure test every new head before it is machined and assembled to make sure there are no porosity leaks or cracks. The combustion chambers are then cc'd and the heads resurfaced.
After a season of racing, Fox said they will disassemble, clean and pressure check the heads. They will also replace all the valve springs, retainers and locks, and install new intake and exhaust valves. "We're hell on preventative maintenance," he said. "We recheck every dimension to make sure nothing has moved or changed. We don't find many cracks but we do see some seat walk in the heads so we redo the seats as needed."
Fox said most of the heads he builds are good for four or five years of racing - or until something better comes along that the racer wants on his engine. "Some of these guys want to change heads every six months to keep an edge over the competition."
"Our heads cost anywhere from $3,000 to $6,000 per pair depending on what the customer wants. If they want titanium valves, it will cost them," he said.
One of the first things Eddie Browder of Ed's Automotive in Abilene, TX, does when a customer wants him to do a set of heads for them is to look at the camshaft so he can set up the heads and valve springs to match the cam.
"We buy the bare head castings and custom build them to suit the application," says Browder. "We'll use different valve lengths depending on the cam and type of lifters."
Browder says the average drag racer should avoid using too much spring pressure or a cam with an overly aggressive cam profile if he wants his engine to last. "If you use reasonable spring pressure, keep the lift to around .650? and engine speed under 7,000 rpm, you should be able to go 300 runs easily. Of course, if you want to rev to 8,000 rpm or higher, you're going to need more spring pressure and you won't get as many runs out of the valve springs.
"When we freshen up a set of heads for a customer, we always replace the valve springs, and the valves, too, if the engine has more than 300 runs on it," Browder continues. "We also change the valve locks. If a customer is changing a cam, we always check the retainer to valve seal clearance and make sure there's no coil bind at maximum lift. We also check to make sure that all the spring pressures are equal."
For sealing heads, Browder says he uses mostly performance head gaskets. "I don't recommend copper head gaskets unless the engine has a blower or turbocharger on it and is running a lot of boost pressure."
R&R Engine & Machine
Al Roth of R&R Engine & Machine in Akron, OH, races a late model Mustang in the National Mustang Racing Association. "The aftermarket heads we're running on a 5.0L engine have been CNC-ported, reworked by hand and flow tested by Fox Lake Power Products. We're also using titanium valves and running a 15-1/2:1 compression ratio."
"We don't use that radical of a cam profile so the springs last us the entire season. We used to go through a set of springs every three or four races, but since we changed cams the springs are no longer a problem."
Roth agrees that having the right valve seat angles is critical to making lots of power. "We use multi-angle seats to get good flow numbers."
Match Heads To Budgets
We'll wrap up this article with a few thoughts about choosing aftermarket cylinder heads.
To get a winning combination, the breathing characteristics of the head has to match the cam, the induction system, the rpm range of the engine and the gearing of the race car. The size, length and shape of the intake and exhaust ports, the size and angle of the valves, the restrictions in the ports created by the valve stems and guides, and even the surface finish of the ports all influence how much air the heads flow and how well the heads perform in a given application.
In theory, the more air a set of heads can flow in cubic feet per minute, the more power they will make. But in the real world, things aren't so simple. Air velocity is just as important as air volume, especially for street-driven dual purpose vehicles and heavier vehicles with automatic transmissions.
At low rpm, an engine actually breathes better if air velocity in the ports is kept high. Air entering at high speed fills the cylinders more quickly and efficiently. That's why stock engines have relatively small intake ports, runners and valves. It boosts low rpm air velocity and torque. Installing aftermarket heads with huge intake ports on a relatively stock engine or a small displacement engine that's going to be driven on the street can be counterproductive and hurt low end performance.
On the other hand, if you're building a race-only engine that will rev to 7,500 rpm or higher, you need a lot of flow to achieve top end power. That means big ports and big valves.
When looking at the flow numbers for various aftermarket heads, keep in mind that airflow depends on the profile and volume of the runners, the shape of the bowls and combustion chambers, the size of the valves, the rate at which the valves open and maximum valve lift (which depend on the lobe profile of the cam and ratio of the rocker arms). Watch out for inflated flow numbers that are measured at valve openings that exceed the maximum valve opening the engine can realistically achieve (for example, measuring airflow at .800? valve lift when the cam and valvetrain can't achieve more than .700? of lift).
As a rule, the bigger the engine, the more intake runner volume you need in the heads. The intake port volume on a stock Chevy 350 small block head is about 170 cc's. So if you're building a 400 cubic inch small block, you need 12 percent larger runners, or about 190 cc's just to maintain the same airflow. If you want to improve performance over stock, then you have to add 20 to 25 percent more intake runner volume to increase the airflow. For drag racing, you'd want heads with 200 cc or larger runners on a small block Chevy.
For big block engines, even larger runners are needed. A typical aftermarket street performance head for a big block Chevy might have 265 cc oval ports. A big block racing head, by comparison, might have port sizes ranging from 300 to 420 cc's or greater depending on the rev limit of the engine.