RS Motorsport :: Classic Speed Restored

I need some help here with some logic. I must be missing something but maybe not...

When flowbenches are used it is usual to make measurements at either 10" or 28" of H2O. I understand that using 28" rather than 10" provides more suck by the flowbench motors and results in more air being drawn through the head.

My question is, from some quick calculations I have done, at 8000rpm a 2046cc Pinto will require only 72CFM assuming 100% volumetric efficiency. Why then would we use 28" of H20 or even 10" of H20 as these result in flowbench figures of around 120 and 220+CFM at full valve lift. Doesn't this mean that we are testing the head at far greater flow than we will ever actually have whilst the engine is being used???

My calculation of the 72CFM may be wrong...

Engine CFM.jpg (104.42 KiB) Viewed 17193 times

I have read the various books by Vizard and others (including Vizard's latest porting book) and he explains why 28" of H2O is a suitable figure but I still don't get it. Anybody care to explain in lay person's terms?????

Stop thinking so much!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
The flow bench is only there to measure the changes you make when you're porting
Trying to work it out like you are when send you to the nearest mental health unit!

I agree about the mental bit but getting the right flow is important. Air will flow much differently at speed than it will whilst going slow eg making it around the short side turn. The flowbench I am making (loooong term project) will probably struggle to make much over 10" (3 vac motors).

I like thinking. I like understanding rather than accepting and just doing things 'because'.

If you wanted to be truly technically correct, the flow would actually need to be pulsating like it is in a real engine.

You have not factored in the number of engine degrees out of 720, that a valve is open and the port flowing.

Iniatially the port delivers less than cylinder demand, then equalises, and, if the combo is right, then delivers more than demand by the time the valve closes.

Yeah, 10" or 28" or any other nominal figure is just that. Originally, most likely determined by what a box full of vac motors would pull on some chev head at max lift.

I dont have a link to that research. It is just mentioned in his book but not referenced.

As far as I can gather you want the smallest hole that allows a given airflow.

There are a few flowbench specific forums on the internob so I may ask a few technical questions on there. I would still like to know the answer to my original question....I am sure somebody out there has answers! As I said before, I like to understand not just replicate.

Ah I think I see what you are trying to say...in order for the cylinder to be filled, the flow at any point of the valve opening can be significantly higher (smaller opening = faster flow for a given volume). I suppose even without a valve in place, due to the valve throat and port size being significantly smaller than the cylinder cross section the air flows significantly higher than if a 92mm bore cylinder was sucking air through a 92mm port.

Will look up those books. Cheers!