We bought and sold 2 SF 110 flow benches for $800.00. The SF 600 bench is a 28" @ 1 inch valve lift and are not cheap. We made our current flow bench and it measures flow @ 28" with the engine assembled without the pistons , rods and crank in it.
As much as we would like to replicate real engine conditions, the truth of the matter is 28" of water does not even represent flow at an idle, but it's all we can practically measure.
The port ratio can be influenced by cam timing and velocity of the port , but you have to start with a basic ratio of 70% exhaust to intake flow. The positive flow characteristic of the exhaust is the reason you only need 70% of intake .
All this being said, the flow bench is only a guideline tool. The dyno tells the story and ultimately the road test or the track test is true denominator.
I wish I could have a dollar for every idea that showed promise on a flow bench and showed nothing on the dyno. The one and only thing that has proven itself over and over again is the port ratio.
For instance, the Duggan head has a 45% exhaust port. In our early test days we got it to 60% and 80 HP showed up. We built 5 different intake systems during that same test period and we only hurt power. The best intake system was made clear by 40 HP , once a cylinder head with the correct balance was bolted on.
None of this is an exacting science, but for the best bang for your buck, get or build a flow measuring device and follow these gudelines. You will wake that engine up.
