Originally Posted by
juffer
Don't know if this helps you in any way but my power engineering head has large valves. Looking at it in the photos there is more material that has been taken away compared to the NMS heads. Maybe having the bigger valves is something to do with the larger port size for a faster flowing volume of air? I could be barking up the wrong tree of course lol
Ahhh cheers mate, think the other two heads have got larger valves too,
Bigger valves and ports will actually slow the air speed at low rpm/boost, but allows for a greater flow at high rpm/boost.
Some Jap motors (200sx S13 for example) have only half the inlet ports open at low rpm/boost and then open them up fully at higher rpm/boost, this maintains good response low down but still allowing good flow up top.
You also loose a lot of swirl with larger ports even good ones apparently, and bad ones are plain ridiculous, bad swirl and bad fuel atomisation, which doesn't always show up on a flow bench. The st200 sort of does the same with the switching inlet, long thin runners for low end grunt and short fat tubes for high end, works really well.
I'll post up my book marks later for the head porting. Some interesting articles in there
Rob