Can't remember entirely. But something to do with the exhaust gases being much less dense than the incoming mixture. Thus to maintain a good air speed whilst exhausting you need smaller exhaust valve area than you need inlet valve area....
Bit of text (which doesn't explain things), but it's from Vizards porting book:
Here follows the list of data as I copied it from a 60s tuning book. I think it was called "Tuning Four Stroke Engines". Unfortunately, I didn't copy the title or the writer's name.
port to valve ratio (inlet) inlet to exhaust valve ratio
oval = 0.81 - 0.83 to 1
= 0.72 - 0.73 to 1
= 0.75 - 0.76 to 1 1 to 0.65 - 0.68 for road - rally
1 to 0.70 - 0.73 for race
port to valve ratio (exhaust) = 0.95 - 1 to 1
equation: RPM = Gs x 5600 x Va / Cv Gs = gasspeed at inlet valve in F/s
Va = valve area in Square inch
Cv = cylinder volume in cc
All the ratios are area and not diameter.
The gasspeed table is in the spread sheet and so is the valve size table. I therefore didn't copy it here. The valve sizes are optimal for an engine producing maximum bhp at 7000 - 8000 rpm.
There are two problems with the equation : the definition of valve area and the number 5600. I have taken valve area to mean the circumference of the valve x the lift. This may of course be wrong. Some of the rpm that come out of the equation are clearly of by a big margin. I'm trying to determine where things go wrong.
Working the equation so it is in metric units :
RPS = Gs x 0.475 x Va / Cv Gs in m/s
Va in m²
Cv in m³
This can also be written as : 2.107 x Cv x RPS = Gs x Va = pumped volume per second.
Why the number should be 2.107 instead of the expected 2 I cannot say. After all it suggests a volumetric efficiency of 105 % at maximum revs. It may be that in combination with his list of estimated gasspeeds this gets the best results. For now I've left it as it is but I do realize that results are only approximate and I may have to adjust later.