Big runner&head VS small runner&head in a highly dummied down version & pretty short.
A 4 stroke engine has 4 strokes.
The charge stroke - the intake valves open & the mixture is sucked into the combustion chamber by the vacuum created from the piston moving down
The compression stroke - where the intake valves close & the piston moves up to compresses the mixture
The combustion stroke - spark plug fires, combustion the compressed mixture, which expands & pushes the piston down
the exhaust stroke - the exhaust valves open, the piston shoves what's left over of the combustion out of the combustion chamber.
That's how it works. What you get into with head & intake work is that this is a horrible way to tune an engine.
What needs to happen, is the different strokes need to be broken down.
3 intake phases
i1) The intake & exhaust valves overlap (both open at the same time) as the piston is still traveling upwards. -> The vacuum created from the exhaust leaving the combustion chamber sucks mixture into the chamber EVEN THO the piston is moving upwards.
i2) The "normal" suction phase -> This is what people are familar with. The downward motion of the piston creates a vacuum that suckes mixture into the combustion chamber.
What people don't realize is that the mixture being sucked into the cylinder doesn't immediately follow the piston. As RPM's climb, this becomes more & more pronounced - lowering effeciency greatly.
i3) After intake charging -> Most people assume when the piston reaches the bottom of the stroke & moves upwards (The compression stroke) that the intake stroke has stopped. THIS IS NOT TRUE! The intake valves are left open AFTER the piston is traveling upwards.
The mixture that lags behind the piston in the i2 phase has momentum behind it. Even tho the piston is traveling upwards, there is pleanty of room to pack in more mixture.
This phase is extremely important for high power at high RPM's. The longer the intake valves are left open, the more mixture will pack into the cylinder. Idle quality is reduced if the intake valve is left open too long, and if left open much too long, obviously it will push the mixture right back past the intake valves into the head.
That's the intake stroke in a nutshell.
A large port can flow more that a small port can with less effort, but the mixture moves slowly.
Smaller porting will not flow as much as a large port without added effort, but the mixture will travel at a high rate of speed.
Large porting will give a good amount of flow during the i1 phase, a large amount during the i2 phase, and nothing during i3 as there isn't enough momentium to pack mixture into the cylinder.
Small porting will give less during the i1 phase, slightly less during the i2 phase, and exponentially better during the i3 phase when enouhg momentium is built up to increase the amount of charge packing 0 the affect also increases as RPM increases! (At high RPM's, it's not only just exponentially better, but the flow lead increases as rpms increase!)
On a stock, fairly low RPM engine, small ports will generally produce more power at low & medium RPM, while less during peak power.
Large ports will make less power at low rpm, about the same thing at mid rpm's and more at high rpms.
If you're running the engine at high RPM's, oversized (large) ports loose power to stock! Small ports not only make more power through the low-mid rpm ranges, but they wax the floor with them at high rpm.
That's intake & head theory 332.(more advanced than basic engine 204 woot!) Here's how to apply it!
On a 4 valve head, it's good to enlarge the intake ports & runners to around 120-135% of the intake valve diameter.
If you're trying small ports, the intake ports & runners need to be filled back down to 85% of the intake valve diameter. Go any smaller, and you simply gain huge amounts of low end power, that severly chokes off as rpm climbs.
2 valve heads, you should enlarge to 130-135%, or fill down to 70%. (You can actually still get gains around 60%, but that's very drastic. It's only gonna help if you race north in every gear over say 6500rpm).
