This thread makes me wonder...
When heads are commonly flowed on a bench, what is motivating the air through the ports? For instance, do they usually place a vacuum on the port outlet, or pressurize the inlet? And either way, to what extent? Perfect vacuum, or something less? And if pressure on the incoming side, how much?
Seems to me, the only thing that determines flow for any given orifice is the pressure differential across that orifice.
What are the standards for flowing a port I guess?
To start a flow bench measures a drop in pressure or vacuum across a given orifice. On a scale calibrated to that orifice.
The flow bench has several different orifices on a disc that you rotate depending on the scale you want to measure
I’ve aspent lot of time using one . The one thing that a diesel engine can benefit most from is cylinder head flow. There is nothing more important the air flow, remember with out air , all the fue l in the world will do you no good
The Cummins 24 valve head is at best, very poor in its ability to move air, a stock head at peak flow is around 144 cfm @ 28 inches
The best I have seen is around 270, this is still not enough to feed a Cummins what if would really like
I have seen boost drop by 25 to 30 % at the same or just a little greater power level , with a good ported head and Camshaft program.
This drop in boost means a corresponding drop in drive pressure and retained heat.
I have many flow charts of most every head out there , and its amazing the variance , in a 24 valve if you not removing the intake , your getting a small gain, . The best I have seen is around 185 to 190 cfm. . The restriction is in areas that can not be reached with out removing the intake.
A few misconception and myths
First Myth
First people say that they want to keep velocity up and porting the head hurts this , , this is horse dung , the Cummins head with the best port job possible has unreal velocity thru the runners , if you could make the runners even larger then the water jacked the port is still small.. There is no need to worry about velocity , in that there is no fuel . we are trying to keep in suspension . .
Second Myth I can just crank up the boost and make the same air flow gain, Yes you can to some extent , but the higher boost , has a lot of baggage that comes with it , and most damaging is .Retained Heat in the combustion chamber , second is spool up time and lag. .
You can reduce compression to compensate for this heat , but this is a crutch at best.
Third myth
Resonance tuning in a diesel doesn’t make a difference because your using boost to push air in to the engine . The truth is that even a unboosted engine has pressure pushing air in to the combustion chamber. Its called atmospheric pressure , the combustion chamber is at a vacuum as the piston goes down the bore and 14.7 psi at sea level and 29.29 barometric pressure .
Resonance tuning is using the inertia of a moving column in the runner and port to pack air behind the valve when closed. .
This happens in any engine , and there is a lot of power in this area
Here are a few flow numbers , of stuff several years old .
