Mailbag: 10 Important Considerations About Small Chevy Cylinder Heads (Part 2)

Got questions?

We’ve got the answers—the Summit Racing tech department tackles your automotive-related conundrums. This week, we finish helping a reader pick out the right cylinder heads for his small block Chevy engine. We answered the first half of his questions last week; we answer his last five questions now.

B.A. Smyrna, GA

Q: I am thinking about getting a new set of of cylinder heads for the 400-cubic-inch small block in my 1965 Impala SS. The engine has been bored .030 inches over, has dished hypereutectic pistons, a Summit hydrualic camshaft (224/254 degrees advertised duration, .510/.533 inches of lift), and Edelbrock Performer intake with a Holley 750 cfm double pumper, Mallory Unilite distributor, HyFIRE ignition and a Promaster coil, and Hedman headers dumping into a 2.5-inch dual exhaust. The engine is connected to a Powerglide transmission with a B&M shift kit and Holeshot converter. The 12-bolt rear axle has 3.30 gears.

My research so far has left me with more questions than answers. I am trying to balance performance gain, driveability, gas mileage, and cost. Here are my questions:

1. What size should the combustion chambers be in relation to engine compression ratio?
2. How do I determine proper intake runner size for my application?
3. What size intake valves should I use—2.02-, 2.05-, or 2.08-inch?
4. Does valve angle make any difference for my application? Should I go with 18-degree heads or stay with the standard 23-degree valve angle?
5. Should I use straight or angle plug heads? Is there a performance difference?
6. I have seen references to the term CNC concerning heads. What does the mean?
7. Some head literature mentions limitations for valve lift. Is this due to the head design or valve spring limitations?
8. Are bronze valve guides important?
9. What kind of longevity can I expect compared to factory heads? Are aftermarket heads easy to rebuild?
10. Should I go with aluminum or cast iron heads?

A: Picking up where we left off last week, when we answered your first five questions in Part 1 of this post, here are the answers to questions 6-10:

• 6. CNC stands for Computer Numerical Control, which is a computer controlled machining process that allows exact reproduction of a cylinder head design on a mass production basis. CNC machining is one big reason why there are so many affordable heads on the market today.
• 7. Limitations on valve lift are usually due to valve spring limitations, but you also have to keep in mind port size and runner length. A lot of lift won’t do you any good if your ports and runners can’t flow enough to support that lift figure.
• 8. Bronze valve guides are good because they wear longer than cast iron guides, thanks to bronze’s better lubrication qualities.
• 9. Longevity is not really an issue with today’s head designs. One thing to keep in mind is pushrod length and rocker arm geometry. If the pushrod length is not correct (see our recent post on How to Determine Proper Pushrod Length), it will throw off rocker arm geometry and cause the valve guides to wear prematurely. Make sure you measure pushrod length with a good pushrod length checker. As far as rebuilding heads, the only difficult part is repairing a crack in cast iron heads. It isn’t much of a problem with aluminum, but it requires someone with experience to repair a cracked cast iron heads.
• 10. Aluminum has two advantages over cast iron. It is lighter and transfers heat out of the combustion chamber quickly. That means you can run a little more compression with aluminum heads over cast iron. Don’t go over 10.25:1 compression for your street engine, though. Also, make sure to drill the heads for the 400’s steam holes if your new heads do not have them. Use the head gasket as a template.