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Drag Racing Cylinder Head Selection
When you talk about performance heads for drag racing – or any other performance application for that matter – the best heads aren't necessarily the ones with the biggest cubic feet per minute (CFM) numbers.
By Brendan Baker
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Experts say that the key ingredient is high velocity matched with good flow. But the high flow numbers may blind your customers from seeing the whole picture, so it is up to you to explain.
Some cylinder head experts compare flow numbers to horsepower numbers on a dyno - but guess what? They're not all equal. So if you see one head with extremely high CFM numbers there are a couple of guesses what may be going on. One cylinder head expert says that the general enthusiast/racer doesn't know if the numbers are bogus, all he sees is a big number and that's what he wants.
Larger engines need larger volume ports. And today there are many aftermarket cylinder heads to choose from with larger ports. But before these heads were available, drag racers didn't have many options as to what size heads to use. Most racers would look for the biggest stock head available and adapt it to their application. Yet one of the biggest problems with using stock heads is that you're stuck with the port locations and the thickness of the casting, so you can't get too radical.
Some aftermarket heads have features such as raised runners and relocated ports to improve airflow. Today's aftermarket "as cast" cylinder heads with unmachined ports often flow better than stock heads that have been ported. And "bare" aftermarket heads are available to allow CNC porting to create almost any shape port you want.
Cylinder head specialist Darin Morgan says that with all the aftermarket heads available choosing a cylinder head today is a difficult task. Unfortunately, a bad choice can cost thousands of dollars in wasted time, says Morgan, and a bad head choice may go unnoticed without ever showcasing how good your engine could have been.
So with all the heads on the market, how do you make the right choice? Morgan says it's a complex issue with no simple answer.
"I wish I could lay out some quick and easy mathematical equations or some simple guidelines to help, but there simply aren't any," says Morgan. "It's a complex issue, which is why so many people have trouble. The best way to grasp what's most important is to use what I consider the five most important variables used to tune the induction system."
- Average velocity;
- Individual instantaneous velocities;
- Shape/design (maximize a homogeneous velocity profile over the entire port and at the same time promote efficient flow);
- Rate of velocity change; and
Morgan says that if you follow his five variables you'll soon find the most important rules of designing an induction system are: Velocity, Velocity, Shape, Velocity and, finally, Airflow.
We then talked to Curtis Boggs at Race Flow Development (RFD), who says his company takes a bare casting and comes up with its own port designs. He says the design being made determines which head casting will end up being used. "It could be a Dart, Edelbrock or something else - each casting has its own design issues," he says.
Boggs says that most of his customers are professional engine builders who call him to make custom cylinder heads. "Customers who call me tend to call three different shops on a regular basis - we all do high end head work. I can tell who they've called by the CFM number they quote," jokes Boggs. "If it's 20 CFM higher than the laws of physics, then I know who they've talked to."
Boggs reiterates what other cylinder head specialists have said about comparing flow numbers to a dyno rating. "It's become a popular way to sell cylinder heads," says Boggs. "Publications have promoted the CFM number too, probably from a little ignorance (we plead the fifth! - Ed). "While it makes sense that a larger CFM number indicates more air flow, and therefore more power, that's partially true, but it's not the most important aspect in selecting a good cylinder head."
Boggs describes how he designs a cylinder head for an application: "Darin Morgan is a friend of mine and we tend to have a similar philosophy on head design. I tend to be a little more generous on valve size than he is, and I also use the percentage of a bore size as criteria for choosing an intake valve. That is also tied to the size of the throat under the valve, which, as far as I'm concerned, is the most critical dimension. That sets the air speed at which the air exits the valve into the chamber.
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