The first 60 to 100' are critical to ET and the head choices and what was done to them is a win or lost.This article speaks to that and the advertized intake runner CC might not be what you need to look for.Do read this link because it is important to your project:
http://www.enginebuildermag.com/Item...selection.aspx
http://www.enginebuildermag.com/Item...selection.aspx
AlkyIROC
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Apr 29, 2026
A couple of things not mentioned in that story. Air flow is important but it also depends on the entire engine combination. Using ProStock for example. They are still using carburetors (soon to be upgrading to EFI) and a carb needs a good vacuum signal to pull fuel from the float bowls. Too big a head and you don't get the right velocity to get the proper amount of fuel from the carb(s). Once you get into those really big heads to feed big engines, large ports do start to cause problems and on carbureted engines like in ProStock, they've gone back to an oval port design. They're large CFM heads but use a smaller oval port compared to the large rectangular ports to increase the air velocity.
Now that fuel needs to mix with the intake air. Too slow a velocity and the fuel starts to solidify before it gets pulled into the intake ports creating a bad mixture.
Now on the other end of the scale is injection. Mechanical or EFI it doesn't matter. Fuel is injected directly into the intake port of the head and doesn't have to mix with the air that's already in the intake manifold. The air right at the intake valve is then sucked into the cylinder as the piston travels down the cylinder and pulls in the air with the fuel that's been injected right at the port. By that point, head CFM doesn't really become an issue because the piston will pull in as much fuel and air as possible.
In your typical bracket car or street car at the track, they're built for lower rpms where intake air speed is more of a requirement. I launch my car at 5000 rpm and shift around 7000 so I have no low end power to get me off the line. I'm launching at full torque and shifting around max HP. When my throttle snaps open just before the light turns green, the 2 big butterflies on the toilets open and 3600 CFM of air is sucked into the intake. Instant pressure drop but at 5000 rpm, the engine doesn't care. Fuel is being injected directly into the cylinders with whatever air is being pulled in from in the intake.
Getting into Comp Eliminator as mentioned in that story is even more different. A 300 cubic inch engine with head flow large enough to feed a big block but they're also screaming those engines to 11,000 rpm where they need that kind of head flow.
Head flow is all about volumetric efficiency. The average street car is lucky to get 80% efficiency. Theoretically, my engine gets at most, 105-110% at WOT going down the track. A Comp Eliminator car at WOT should be getting 120-130%. This means the engine is spinning so fast that when the valve starts to open, the air is already pushing itself into the cylinder before the piston has a chance to pull it in. Blowers and turbos do the same thing to increase VE but at lower rpms.
Because blowers and turbos are artificially forcing the air into the cylinders, head flow isn't a big issue. You can run basic unported aftermarket heads and still make good power.
For most of us, a standard head design is all we can afford. It's unlikely you'll find anyone bracket racing a SBC with SB2 heads. Same goes for me. I'm to the point where I'm just about maxed out with a conventional head and if I really want to go faster, I need to step up to something like Big Chief heads or some other spread port design. The valves are at different angles and in different positions to change the air flow through the heads. This can make huge differences in HP limits.
It's how much budget you have that really limits your head choices. If you're in cubic dollars racing like Comp Eliminator, a $10,000 set of fully prepped race heads is cheap if it gets you that extra .1 ET against another racer. It's scary to think what a Comp Eliminator racer has done to the engine when he runs 1.500 seconds under his index. It's cheaper to run in ProStock.
Now that fuel needs to mix with the intake air. Too slow a velocity and the fuel starts to solidify before it gets pulled into the intake ports creating a bad mixture.
Now on the other end of the scale is injection. Mechanical or EFI it doesn't matter. Fuel is injected directly into the intake port of the head and doesn't have to mix with the air that's already in the intake manifold. The air right at the intake valve is then sucked into the cylinder as the piston travels down the cylinder and pulls in the air with the fuel that's been injected right at the port. By that point, head CFM doesn't really become an issue because the piston will pull in as much fuel and air as possible.
In your typical bracket car or street car at the track, they're built for lower rpms where intake air speed is more of a requirement. I launch my car at 5000 rpm and shift around 7000 so I have no low end power to get me off the line. I'm launching at full torque and shifting around max HP. When my throttle snaps open just before the light turns green, the 2 big butterflies on the toilets open and 3600 CFM of air is sucked into the intake. Instant pressure drop but at 5000 rpm, the engine doesn't care. Fuel is being injected directly into the cylinders with whatever air is being pulled in from in the intake.
Getting into Comp Eliminator as mentioned in that story is even more different. A 300 cubic inch engine with head flow large enough to feed a big block but they're also screaming those engines to 11,000 rpm where they need that kind of head flow.
Head flow is all about volumetric efficiency. The average street car is lucky to get 80% efficiency. Theoretically, my engine gets at most, 105-110% at WOT going down the track. A Comp Eliminator car at WOT should be getting 120-130%. This means the engine is spinning so fast that when the valve starts to open, the air is already pushing itself into the cylinder before the piston has a chance to pull it in. Blowers and turbos do the same thing to increase VE but at lower rpms.
Because blowers and turbos are artificially forcing the air into the cylinders, head flow isn't a big issue. You can run basic unported aftermarket heads and still make good power.
For most of us, a standard head design is all we can afford. It's unlikely you'll find anyone bracket racing a SBC with SB2 heads. Same goes for me. I'm to the point where I'm just about maxed out with a conventional head and if I really want to go faster, I need to step up to something like Big Chief heads or some other spread port design. The valves are at different angles and in different positions to change the air flow through the heads. This can make huge differences in HP limits.
It's how much budget you have that really limits your head choices. If you're in cubic dollars racing like Comp Eliminator, a $10,000 set of fully prepped race heads is cheap if it gets you that extra .1 ET against another racer. It's scary to think what a Comp Eliminator racer has done to the engine when he runs 1.500 seconds under his index. It's cheaper to run in ProStock.
I know it is a long article,but if you read it all it does speak to the low dollar racer and how a cross section of a given port effects him bring about a awareness that CC runner values are not all there is to head tech.
Senior Member
Quote:
Because blowers and turbos are artificially forcing the air into the cylinders, head flow isn't a big issue. You can run basic unported aftermarket heads and still make good power.
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This a misconception about forced induction that it increases VE of an engine.Originally Posted by AlkyIROC
Head flow is all about volumetric efficiency. The average street car is lucky to get 80% efficiency. Theoretically, my engine gets at most, 105-110% at WOT going down the track. A Comp Eliminator car at WOT should be getting 120-130%. This means the engine is spinning so fast that when the valve starts to open, the air is already pushing itself into the cylinder before the piston has a chance to pull it in. Blowers and turbos do the same thing to increase VE but at lower rpms. Because blowers and turbos are artificially forcing the air into the cylinders, head flow isn't a big issue. You can run basic unported aftermarket heads and still make good power.
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The VE of the engine depends on the engine itself. Once you build an engine its VE is set and cannot be changed without changing engine components.
For example say you had a positive displacement supercharger like a roots or screw supercharger installed on your engine. you set the supercharger to force 2x the cfm air your engine could pump byitself. If your engine VE increases you would not see boost pressure in the intake.