Performance Chevy Cylinder Head Comparison - Go With The Flow


Mike Petralia Mar 1, 2008 0 Comment(s)
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This example can be translated directly to cylinder heads. A head that's too small will not let enough air into the cylinders to make power. On the other hand, a head that has too much port volume will cause the intake air/fuel velocity to drop and not "ram" into the cylinders fast enough for the most efficient filling, which will also cost power. A very famous Pro Stock engine builder once told me, "Give me a set of heads with the smallest intake port cross-section that'll flow all the air I need." What he means by that is he already knows how much airflow he'll need based on his entire engine package. Now all he needs are the right heads to make the number.

Port Size Makes the Difference
I can think of a great example that I saw in action once. It was on a bigblock, but regardless of engine size, the principle still applies. A guy takes his heads to a local inexperienced head porter and lets him go to town on them. They were some older castings, and he really took the grinder to them. He produced a set of heads that he was very proud of and handed them back to his customer. Now, $1,500 later, the guy clamped his freshly ported heads onto a flow bench in another shop to show off the awesome porting job. The numbers read something like this:

Sucp_0803_08_z Performance_cylinder_head_comparison Duel_intake_manifold 6/17

Not too bad on paper. But then the shop pulls out a flow sheet from similar, new, off-the-shelf CNC'd cylinder head made by one of the bigname manufacturers, and our buddy just about croaked:

After his heart had been thoroughly shattered, he looked deep into the issue and just about passed out. You see, not only did the out-of-the-box CNC heads beat his ported castings, but they also did so with a 45ccbarometric smaller intake port. This is a prime example of how the right port size and shape can affect airflow. Needless to say, the guy who owned the heads wasn't too thrilled about this, but regardless, he'd spent a lot of money on them, so he bolted them on, added a big nitrous kit, and drove off a happy customer anyway.

Sucp_0803_12_z Performance_cylinder_head_comparison Installation_complete 7/17

But all this is straying away from where we need to go. Unless your engine is big, like over 500ci, street heads won't need more than about 300 cfm max. And even that's too much for most engines. One of the best combinations I've ever tested used Edelbrock's E-Tec 170cc heads on the infamous Super Chevy 355-cid "Danger Mouse." That little pump-gas Mouse made almost 460 lb-ft of torque and more than 485 hp. And it idled decently, too, but I bet it would be one helluva kick to drive.

As a follow-up test, I bolted on Edelbrock's bigger E-Tec 200cc heads and got my butt handed back to me along with all the torque I'd just lost. Overall, torque fell off by 24 lb-ft. But interestingly, peak horsepower stayed right at 485, although it moved 200 rpm further up in the powerband. That'd be nice if I were planning on racing in a tirelimited class and didn't care so much about torque, but I wanted this to exemplify the best combinations for the street.

That day's testing really taught me a lesson about proper head management. I learned that the E-Tec heads are a great design overall, capable of making good power. But the smaller of the two, the 170cc, was better suited to overall running around, at least on that little 355-inch Mouse.

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Chamber Shape & Flow
Air and fuel flow through an intake port at a pretty good clip. I don't know exactly how fast, probably slower than the speed of light but faster than grandma getting off her rocker. I'm sure there's a study on intake air velocity somewhere, but that's not important for you to consider when choosing a set of heads.

In a test I did a while ago, I dyno'd an engine with a pair of mildly CNC-ported heads right out of the box and then sent them for all-out aggressive CNC porting after the day's tests were done. When I got them back, the port volume had increased by 12cc and dry airflow dropped off at the bottom of the lift curve, but it picked back up at the top, surpassing the original set. More importantly, the shop that did the CNC work hit the chambers with their mill and reshaped them into what they believed to be a better design. The differences were subtle, but you could see it. Anyway, I bolted the heads back onto the same engine, went back to the same dyno, and retested it with no other changes at all. And I found more power. Not a huge amount, about 2.5 percent average overall, but it was there. After talking with the CNC shop, they pointed out that airflow measured on a dry flow bench does not indicate where the air is going as it enters the chamber.

Fuelish Ways
My longtime friend and cylinder head expert, Joe Mondello, has developed a "wet flow" bench to find out what the air and fuel does as it enters the chamber. Many bigtime engine builders and cylinder head companies are using it to find things they'd never thought would occur. That's because air and fuel mixed together become a fluid, and fluids can move in strange ways.

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Much of this effect comes from the chamber's shape and how it relates to the intake port. When the air/fuel column is rushing past the intake valve, especially at higher rpm, it's important for the cylinder head's combustion chamber to give it some direction. Note in the photograph how far back the chamber has been milled away from the intake valve. The arrows indicate the direction of air and fuel travel as it enters the chamber. You can see that the chamber's shape will at least give the mixture some direction of travel. This typically creates "swirl," which has been determined to be a good thing in combustion chambers.

Now, check out this stock chamber from an older iron head. See how it's got no shape to induce swirl? Nothing in this chamber is designed to direct the column of air and fuel anywhere. Its only purpose seems to be keeping the combustion pressures in the block.

For the longest time, it's been said that matching your intake manifold's ports to your cylinder head's ports is the only way to make real power. And that's not far from the truth, but it has to be taken in context. If you're building an engine to make max power at high rpm, then you'd better have your ports aligned perfectly. But on a street engine where you're trying to get the best power on a budget, it's kinda tough to perfectly match the ports. You can get close, but that's usually the best you can do.

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Port-matching a set of cylinder heads is really not matching the ports completely. Instead, the ports are typically matched to an intake gasket size, and then the intake manifold can be opened up to match that same gasket, or left just a bit smaller. But by doing so, you could effectively put your head's performance out of the range you need.

What I like to do for stock or ported heads is to take the existing port shape in the cylinder head and straighten its walls and smooth in the entry just a little bit. Don't worry about opening it up to match a gasket exactly unless you're going all out. Then I do the same thing on the intake manifold and bolt them together. It seems to work pretty well and is a lot less hassle.

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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:

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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.

Sucp_0803_27_z Performance_cylinder_head_comparison Chevy_engine_block 13/17

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.

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Ways To Improve Performance Without Porting


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Factory vs. Aftermarket heads

Valve Angles

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The valve angle changes I'm referring to here are the angle of the valve's face in relation to the piston's flat surface. Way back when, in order to fit the wide 90-degree V-8 inside a stock engine compartment, OEM engineers angled the top of the valve stem in toward the intake manifold, compromising flow. Race engine builders then started angle-milling cylinder heads to move the valve stem back out toward the exhaust side, which puts the valve in a better relation to the piston. This produced a better-breathing cylinder head, and as such more power came along with it. But angle-milling the heads meant that you also had to move everything else to match.

Today, cylinder head companies are simply moving the valve angles within the heads, but the intake manifold and rocker arm companies have a tough time keeping up with the varying angles. And so far I've only talked about rolling the valves toward the motor's exhaust side.

Canted valve heads' intake and exhaust valves are at different angles, and splayed valve heads' valve centerlines point out from each other as viewed from the top of the heads. This allows large valves room to open in small cylinders. Actually, canted/splayed valve technology is nothing new. Big-block Chevys have had it since their birth, which might explain why a stock oval-port big-block head can flow just about as much as many race small-block heads. The most important thing to keep in mind here is that whenever you move the valve angles around, it's going to cost more money to build the engine.

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This illustration highlights how complicated the end of a normal V-8 valve can become. Study it carefully. All these angles, widths, and dimensions can be modified for increased performance. It takes days, even years to develop the perfect setup, and only the guys with the biggest budgets have the resources to find minute differences in power here.

The three-angle valve job you've heard so much about refers to the angle of the seat cuts on the valve and in the head. Typically, the valve actually has only two of the angles (highlighted in the circles), and the head has the third, a transition cut commonly called a "throat cut," used to blend the valve seat into the cast area of the port's bowl. It does not have a corresponding cut on the valve face, and the valve will never touch this area.


Trick Flow Specialties
Tallmadge, OH 44278
World Products
Ronkonkoma, NY 11779
Dart Machinery
Troy, MI 48084
Holley Performance Products
Bowling Green, KY 42101
BMF Racing Heads
Mike Petralia's Hardcore Horsepower
High Velocity Heads (HVH)
Knoxville, TN
Brodix, Inc.
Mena, AR 71953
Racing Head Service (RHS)
Chapman Racing Heads
Woods Cross, UT
(801) 292-3909
Canfield Cylinder Heads
Canfield, OH 44406
All Pro Aluminum Cylinder Heads


Engines Drivetrain
Check out our findings as we compare a variety of performance cylinder heads against GM's iron Vortec castings!
Mike Petralia Mar 1, 2008


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