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Drag Racing Cylinder Head Selection
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"It's all air speed," says Boggs. "If you understand basic fluid dynamics (how things flow and what kind of shapes they like), you'll have well-shaped ports. And if you've designed your port to hit the proper target air speed (it's going to give you a certain size port to hit that speed if you design around the cross-sectional areas), in general, the CFM number should follow that.
"But the CFM number is the last thing I pay attention to," continues Boggs. "If everything else has lined up properly in designing the port, the CFM number should be there. But it's not the number you design around."
So if someone claims 20 more CFM, it's not a good thing? Not necessarily, according to cylinder head experts.
A very good example would be a 500 cid NHRA Pro Stock engine, explains Boggs. He says you can make that cylinder head flow 600 CFM, but for the most part, the front running engines don't typically flow that much.
"I know of one set of Pro Stock heads that won a race and were top qualifier at several other races last year that flowed only 560 CFM peak. That's 40 CFM less than what other people claim their heads produce."
So, explain the experts, while you can technically have the same air speed you may not get the same CFM number.
"That is more common than most people realize," says Boggs. "Many Comp Eliminator style heads have 2.5 or 2.6 hp per cubic inch, or even more, and are very efficient engine combinations. But most people would be surprised at the CFM number for these engines. An example would be the current trend for 300-inch motors with SB2 heads in Comp Eliminator. That particular cylinder head only flows 360 CFM. And at that number most people would want to throw that head in the trash, but it's a record-holding car."
In general, you set air speed in a couple of different places in the cylinder head. The simplest example would the 18-degree SBC. Generally, the smallest part of the intake port through the entire tract will be at the pushrod pinch. That's why everyone keeps using offset rockers, trying to make the port bigger. The peak airspeed is from the manifold plenum to the back of the valve.
In addition, the throat behind the valve is the second place you can set airspeed. At the pushrod you have a choke point called the primary choke, so that's the fastest part. And that's the first part when the air goes too fast that it'll choke. So you set your air speed at the pushrod and your exit air speed into the chamber at the throat.
Once you know the bore and stroke of an engine, you have a hole with a piston going up and down, you have to fill it with air. So you know what the volume of air you have to move. Now you can calculate back up to the intake tract and figure you have a certain amount of time to move a specified amount of air, so the port has to be, for instance, 3.2 sq.in of area to move that amount of air at 300 ft. per second.
Boggs says you make adjustments in .100" increments of the cross-sectional area, which are small adjustments. If you adjust .100" you could be affecting the air speed by 20 or 30 ft. per second. They're very precise adjustments.
Boggs and other cylinder head experts say they need to get as much detail as possible from a customer before they can properly design the port. Everything comes under consideration at that point. If it's a road racecar, for example, you have to consider what track it is. If it's a drag race engine it's a little easier because you only have straight-line acceleration.
Subsequently, with drag racing you're solely worried about acceleration rate say experts. In drag racing applications you can get away with a bigger cylinder head and use more gear, a big converter and so on. But there's always a balance. If you make it too big, the car won't ET well. A bigger head will give you more top end power but you can't launch as well.
Here's where guys buying cylinder heads from CFM numbers alone get into trouble, says Boggs. If the cylinder head really does have a bigger number, and it's designed by someone with an uncalibrated flow bench, there's only one way to do that: The hole has to be bigger! You just can't change physics.
"So people who are buying the biggest number they can find also have the biggest holes, and the biggest holes aren't the ones going down the race track the fastest," explains Boggs. "You make most of your ET in a drag race car in the first 60 to 100 feet. You've got to get the car going - that's where the best ET is. Top speed doesn't mean anything. You're not accelerating at the top of the track. If you buy a head based on CFM and the air speed is too slow because of the big hole, the car won't accelerate."
Matt Driskell, Driskell Enterprises, says he looks at how heavy the car is and what's going to be done with it. "On anything up to a 555 cid to 565 cid engine, I'll use a head that's anywhere from 350-360 cc intake runner volume," says Driskell. "If there's an application where the car is a lot heavier, I might use a 320 or 330 cc range, if it needs a lot of torque and doesn't need to peak really high in engine rpm."
Driskell says most of what he's learned about cylinder head work is from his experience at the track and in talking to other experts. "From what I've found, you can look at flow numbers all day long in this range (320-360 cc). But it's just something to look at, in my opinion. You can dry flow the heads all day long and pick the best one but it'll perform differently out on the track."
If you have a Top Dragster that weighs 1,800 lbs. and you're trying to go 8.20 to qualify for your Quick 8 Class at your local track, experts say you'd better pay attention to what you're doing. And that's where you need a cylinder head built for the application.
Street-Strip guys can buy an as cast aftermarket head and put a valve job in it and go. Nothing really comes out of the box ready to go, which is where engine builders come in. A custom head that is built for a specific drag race application will require a lot more work and effort than a mass produced head that you clean up and ship out.
The criteria changes for what you need to do the higher up you go on the drag racing scale. At the mid-level to high-end drag racing classes, air speed is the real key to pay attention to. Remember: a CFM number is a just a number that someone decided to advertise.
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