Calculations may exist but the "common gist" in racing is to basically ignore the exhaust port & crutch it along with the cam. Maybe not all of the time but some of the time engines will respond to a combination of a bigger/better exhaust port and less exhaust duration. Sometimes this kind of thing happens on engines that are notorious for big duration splits.Newold1 wrote:Let me try to put forth a certain scenario in a normally aspirated engine using 93 octane gasoline as fuel and at a certain V-8 pushrod OHV engine with a 53 cubic inch cylinder with a bore of 4.125" and a stroke of 4.0" a static compression ratio of 11.0 to 1 and a total combustion space of 120cc . This engine has an intake tract that is 14" in length that provides 46 cubic inches of total area from throttle plate to intake valve seat. The curtain area for the intake valve is 3.45 sq.in. This intake tract will flow 330cfm at the camshafts maximum valve lift of .650" and will open at 20 degrees BTDC and close at 50 degrees ABDC. With an air fuel ratio of 12.5 at WOTand ignition starting at 29 degrees BTDC, at 6000rpms after fairly decent combustion, how much volume of exhaust will be created when the exhaust valve begins to open at 60 degrees BBDC and at that volume and pressure how much cfm of exhaust gas is going to flow until the exhaust valve closes at 18 degrees after TDC on the exhaust stroke @ sea level of course. It seems as though this is said to be a mathematically produced number that can in essence determine the required size of the exhaust valve, its curtain area, port length and volume of the port to its exit? I realize this might be a stretch for a calculation but is it possible to calculate this requirement based on givens, or as some here are insisting that this is just a guess at best, or not really important at all? I guess what I am also wondering is do cylinder head designers or builders use formulas and math or do they just kind of guess at a valve size, port size and cfm they feel they need to outfit their cylinder head with based on previous designs and testing?
And sometimes flow measured on a bench at 28" H20 depression isn't any better or even as good in some places with a bigger/better exhaust port but the engine makes mucho-more power after peak, especially when events are narrowed up a little.