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Performance Chevy Cylinder Head Comparison - Go With The Flow

When It Comes To Cylinder Heads, How Much Is Enough? Glad You Asked ...

By Mike Petralia, Photography by Mike Petralia

This is a perfect example of why you should clean up your intake ports. These stock ports wouldn't really be the best for performance, but cleaned up like the ports in the next photo, they'll make more power. Note how the cleaned ports are not really much taller or wider, except at the top. This is an easy way to make power, and fortunately, most aftermarket cylinder heads already come this way so you don't have to mess with them.

Shown at the top of the next page is what I'd call a fully ported intake port- almost too much for the application. Note how the top of the port has been opened up to the point where the gasket doesn't have much to seal against. What this guy really needed was a raised-port cylinder head instead of trying to make one himself. By raising the intake port's entrance in the head, you get a straighter shot at the intake valve, and the air/fuel column gets to make a less steep turn into the chamber. But raised port heads also require a properly matched raised port intake.

Average Flow Is The ReaL Deal
When shopping for heads, one of the best indicators for performance potential is average flow data. Take this example:

  HEAD A HEAD B
LIFT INT EX INT EX
.200 122 110 138 97
.300 175 153 194 129
.400 223 182 234 155
.500 252 196 243 177
.600 259 204 245 179
AVG 206 169 211 147

If you were reading an ad for both of these heads, Head A would probably proclaim "260 cfm @ .600-inch." And Head B could only meekly shout "245 cfm @ .600- inch!" But which one do you think might be the better head? Looking at intake flow numbers alone, I'd be pressed to choose Head B over Head A. That's because its average, not peak intake, flow is better. In fact, it flows much better down low in the scale, which is a very important thing to look at. But then I look over at exhaust and see that Head A flows better. Tough decision to make, isn't it?

Well, these flow figures aren't made up, and here are the test results from both heads run on the same engine using the exact same parts, except for the heads, of course. This engine was a stout 427ci Mouse, and the difference is slight but notable. Head B made more peak torque than Head A, but it started out making less. Average horsepower and torque went to Head A, so I'd call Head A the winner.

But Head B features an 180cc intake port, while Head A was more than 200cc, so which one do you think might work best on a smaller engine? It's a tough call, and because this engine had a large displacement, it could make use of the bigger intake ports in Head A. Had it been a good ol' 350-inch engine, Head B's 180cc intake port would respond very well on the street.

How Everything Else Affects Flow
Now we're going back to that thing I like to preach about: building the entire combination correctly, not just randomly bolting on parts when you can afford them. When choosing a camshaft for any engine, the first thing I look at is the airflow from the engine's cylinder heads. I pay particular attention at what valve lift peak airflow occurs, because it tells me how much lift needs to be put on the cam.

By Mike Petralia
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