| Re: Interesring article on fluid dynamics and airflow |
Subject: Re: Interesring article on fluid dynamics and airflow by jsup on 2008/11/23 0:15:28 From the archives, for you Dan: Smaller port size, the better for same flow Looking above we see that there is a great deal of effort put into not only how much air could be pulled through the port, but mostly, how it will get through the port, thats what is really important, getting air into the engine during actual operation effectively and efficiently, its not all about VOLUME, its about QUALITY. You must always remember that a flow bench is only measuring how easily air can be pulled into the cylinder, not how much fuel is in that stream or the QUALITY of the flow when operation is actually taking place. This means that when comparing cylinder head sizes the most important aspect as the SHAPE of the port and how that shape relates when viewing flow all the way to the combustion chamber. Airflow must be controlled, and hence the shape and measurements of the port are far more important that just looking at flow versus runner size. In other words DRY FLOW BENCHES ARE USELESS IN HEAD DESIGN AND FLOW VS. RUNNER SIZE IS A MYTH. Again; a flow bench does not measure how much air will be pulled through the port during operation or the quality of delivery of that air, it measures the most simple of measurements, how easily it can be pulled though in a vacuum. Which tells you nothing about head performance. When looking to design a port design a big consideration is to see what the airflow demands will be; they also must look to what type and how much fuel will be induced into the mix, what RPM the engine will be operating at and lastly, must look past the port into the combustion chamber and into the intake manifold. Again, a head with lower flow bench numbers which produces better charge delivery (through cleaner airflow and physics) and does so throughout the operating range of the engine, is a better, more efficient head that creates more power. Despite the mantra we hear here about efficiency being measured by some made up formula of port size vs. flow numbers, that is not the case in true engineering, or the laws of physics. It is simplistic marketing hype for the simpleton masses. Significant hp can be gained or lost by port design regardless of the airflow number changes. GASP! A cutting edge cylinder head not only has low restriction and a proper shape for tuning purposes it also has to control velocity along with cutting back on irregular flow (turbulence). Now, a combustion chamber can promote flow or hinder it, it can also promote proper filling and flame travel (often trade offs); how a cylinder is filled is as important as how much of it is filled; all of the above have to balanced throughout the design process as there is no end all be all design that contains the best of every aspect. Flame travel, ever hear flame travel mentioned here? I didnt think so. No one here wants to think real hard about stuff, its easier to spew meaningless numbers. Another very complicated aspect of how the engine makes its power, valve and spark plug placement and combustion chamber shape have to consider this as well as how much air it can pass! To sum up, without the benefit of a wet flow bench, you would never be certain as to how the design works (and even the wet flow bench leaves something to be desired amongst designers but hey its a step in the right direction that measures additional variables). Now, thats not to say you cant get lucky by coping someone elses design, or through trial and error like many speed shops, however, there is no science behind guessing on a dry flow bench (just what worked well in the past ). Its brute force vs. elegant design. |
