Got questions?

We’ve got the answers—the Summit Racing tech department tackles your automotive-related conundrums. This week, we help a reader pick out the right cylinder heads for his small block Chevy engine.

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 use2.02-, 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: We will answer your questions in the order you asked them, starting with the first five today. Next week, we’ll go over the second half of your questions. We begin here:

  1. Combustion chamber size affects compression ratio in a simple waythe smaller the chamber, the less area you have to compress the air/fuel mixture, and the higher the compression ratio. In other words, a small chamber will give you a bigger “bang.”
  2. Intake runner size (along with your cam profile) determines the power range of your engine. The larger the runner, the higher up the rpm range your powerband.
  3. Intake valve size does the same thing as the intake runner size. If you have a street car, we would stick with a 2.02-inch intake valve
  4. Valve angle refers to the relationship of the intake valve to the piston or cylinder bore. In an 18-degree head, the valve stands up straighter, which in turn moves the valve away from the cylinder wall at high valve lift. This is called unshrouding the valve. This allows more airflow, the use of higher-lift cams, higher rpm capability, and more top-end horsepower. However, 18-degree heads are expensive, require special valvetrain components and intake manifolds, and aren’t noted for low-rpm power. For your application, stick with 23-degree heads.
  5. Spark plug angle is important because it determines where the air/fuel mixture starts burning. If the burn starts closer to the exhaust valve (which occurs with angle plug heads), you get a better scavenging effect when the exhaust valve opens. That in turn provides a cleaner intake charge the next time the cylinder fills, creating more power.
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