"Interesting" Flow Comparison...
12-02-2004, 10:45 AM
#21
Volume versus "Cross Section"
Originally Posted by Grant B
Tony,
Why do people always talk in terms of port volume, and not cross-sectional area? Obviously the later is responsible for velocity. I know the cross section is mostly fixed by the factory intake manifolds, but I'm still confused as to how port volume is so important. When porting heads, do people remove most of the material from the bowl area or do they increase cross-sectional area after the port inlet? It seems to me like removing material from the bowl could increase port volume without really effecting velocity at all.
Why do people always talk in terms of port volume, and not cross-sectional area? Obviously the later is responsible for velocity. I know the cross section is mostly fixed by the factory intake manifolds, but I'm still confused as to how port volume is so important. When porting heads, do people remove most of the material from the bowl area or do they increase cross-sectional area after the port inlet? It seems to me like removing material from the bowl could increase port volume without really effecting velocity at all.
I also speak more generally in regards to "port volume" mainly due to the fact it is much easier to get an accurate comparison on. The reality is that if your comparing same "style" cylinder heads with a known intake port shape, assuming there wont by any major changes in the size of that entrance, port volume will certainly give you a pretty good idea of cross-sectional area and it is certainly a good "barometer" of how efficient a cylinder head is based on its valve size, total flow "curve", and the volume (cc's) it displaces. Cross sectional area is much more difficult to measure due to the varying curves and inconsistant shapes typically present in the intake/exhaust port. (Obviously it would be a lot easier to measure if you were dealing with rectangular or square surfaces). But you are correct that cross-sectional area will give you a better idea of airspeed based on the total flow and the smallest cross-section. The actual "cc's" or pour volume of the AFR 205 is about 5-6 cc's smaller than a stock LS6 casting which is only about 2-3% smaller in total volume, but if any of you guys had the chance to peer down a stock LS6 intake port and an AFR 205 intake port, check out the cross section between the highest point of the short turn and the roof of the port....it is significantly smaller than even an UNTOUCHED LS6 casting (and outflows it around 50 CFM!)
Good stuff....
Regards,
Tony
