Originally Posted by Caballerokid
How would someone go about selecting an intake runner size rather than someone just saying I should be ok with such and such? How would I know what to pick, say a 165cc, 170cc, 180cc, 195cc, 200cc, 210cc, 215cc, 220cc, 235cc etc... The 235cc will flow the most air because it's bigger. Why would 180cc be ok for my setup? Help me understand why. Thanks
My ex-wife used to ask me about the size of her *** and I'd always tell her, "It's not the size that matters, it's the shape."
Same with intake and exhaust runners. The tiny little 170cc intake runners in the L31 Vortec 5700 heads will do a better job of filling the cylinder than a lot of aftermarket heads with larger volume runners. A rule of thumb for a small block Chevy street driver is cubic inches times 1/2......283/140, 305/150, 327/160, 355/175, 383/190, 406/200. This assumes STREET use, idle to 5500 rpm's, not racing. The runner must be sized so that the "slug" of air/fuel mixture has sufficient VELOCITY to continue to pack the cylinder even after the piston has reversed course and is headed back to the top of the cylinder on its compression stroke. Remember, compression cannot begin until the intake valve closes and that is quite a number of degrees after bottom dead center with the piston coming up the bore. Of course, you also need VOLUME. If the runners were the size of a drinking straw, you'd get some hellacious velocity, but you wouldn't get much volume because the other factor involved is TIME. Your goal is to fill the cylinder with as much mixture as possible during the time that the intake valve is open.
Most of you fellows who are building street motors will use too much head and too much cam in an effort to make high horsepower. Horsepower is not what moves the car from point A to point B in a street use setting. Torque is what does that. If you really want to build a pleasant driver that will pull like Jack the Bear, bolt a set of L31 heads onto a 383 and enjoy the torque created from 1500 to 5500. Build it 9.5:1 with a matching cam, long-tube, equal-length headers with 1 5/8" primaries, dual-plane high-rise intake manifold such as an Edelbrock RPM #7116 or Professional Products Vortec Crosswind #52028, 650 vacuum-secondaries carb and you'll have a really pleasant, tractable daily driver motor.