Posted by ptfarmer on February 27, 2020 at 22:05:04 from (70.114.32.211):
In Reply to: Porting Cast Iron head posted by Jwilhelm770 on February 26, 2020 at 10:17:00:
Air flow is one thing, but velocity is just as important. I have filled bottom of some ports up to 1/4" with no loss of flow, but the engine made more power because of the increase in port velocity. On the intake ports the increase of velocity helps keep the air/fuel mixed better thus you get better combustion. On an exhaust port the increase in velocity help draw out more spent exhaust gases out of the combustion chamber, and it also helps the intake draw in more air/fuel.
Another thing is how much valve lift you have, most also look at the airflow with the valve at it's maximum opening, and not at flow at half open (remember a valve reaches full open once, but it reaches half open twice).
I mentioned filling the bottom of ports can increase port velocity without reducing flow, but also if you can fill a port floor you are reducing the radius of the turn that air has to flow. The straighter you can make a port the better it works.
Before good aftermarket heads were available for the Ford 351 Cleveland the extreme bend in the exhaust port was a real power killer so what was done back in the day was modifying the exhaust ports with port plates. A section of the exhaust ports were milled away, and new exhaust port plates were put on to straighten out the ports, and it increased flow, and velocity dramatically.
Stock 351C 4V exhaust ports.
351 C 4V with port plates installed (note the ports are now smaller than the stock ports).
Hopefully this shows that it's just not about making ports bigger. A lot goes into cylinder head port work!
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