Low- & Midlift Flow
Peak flow numbers sell heads, but low-/ midlift flow is far more important for a street motor. Much of the secret to finding good low- and midlift flow lies in combustion chamber design, valve-job profile, and the actual shape of the valve itself. "Back-cut valves are a must, and time must be invested in trying different angles as well as different widths of those angles," Tony says. The actual width of a 45-degree seat needs to be considered as well, while the short-turn height and shape also play a smaller role. "It's a give and take, and really strong peak numbers can cost you a lot of low- and midlift flow while really strong low- and midlift numbers may knock more off the peak than you may be willing to sacrifice. In most of the products we manufacture, our design goal is to produce industry-leading low- to midlift results yet retain solid, respectable peak numbers. Part of this reasoning is the fact that a lazy port with big peak numbers may ultimately be handicapped by the intake manifold choice anyway, leaving you with weak low- and midlift performance and only so-so peak flow where you were expecting that combination to shine."
Bore Size
You should always flow the cylinder head in question with as close to the exact bore size you plan on running in your engine combination. "While it won't make a huge difference, too small a bore can hurt numbers by 5-10 cfm, and too large a bore can artificially inflate them by approximately the same amount," Tony explains. "It's always best to be within 0.030 of your actual bore size. That's only 0.015 per side, so being a little bigger or a little smaller in that range won't matter very much. Once you have over a 0.100 mismatch, the numbers are going to start getting skewed and won't be very representative of real-world performance with your particular combination." CHP