I was wondering if anyone has done any testing of different valve throat diameters. I read where they should not exceed 90% of the valve size, however my AFR 190's measured at about 92-93%. Just wondering where the breaking point is before you actually loose flow.

 

Generally, a port throat diameter of between 90-95% is optimum for most port configurations. Now, how you arrive at the "optimum" is based on many variables, as are so many things in IC engine design. The trajectory angle plays a significant part in this, as does the combustion chamber design, shape of the short side radius and the valve seat geometry.

Understand that there is no "constant" that always fits the rule, things just aren't quite that simple. That's why race teams spend so much money paying cylinder head experts to develop port shapes better than the competitions. One way to arrive at an optimum design is to try every possible combination. At least, that's how it use to be done. Computers make the job quite a bit easier these days but it's still more black art than science for some.

I developed a formula here a few years ago (actually borrowed pieces from here and there) for calculating what I like to call a "Port Flow Coefficient". Using many variables, including absolute viscosity of upstream air (required in calculation of a Reynolds number) and approximated with a linear relation... which was sufficiently accurate in the range of 500R < T < 550R. The thermal expansion factor, taken as unity (negligable) and compressible flow expansion factor (based upon ASME Power Test Code). From there we arrive at mass flow rate (Wh)and in that formula, you will find the valve seat minor diameter (d).
Kv = C F Fa Y = Wh/359d^2 sq rt(hw Y)

Point being, it's not overly difficult arriving at a "target" when you know what you're trying to achieve. The throat minor diameter does play a part in Port Flow Coefficient and ultimately, the coefficient of discharge (Cd) or how efficient the port is with the valve in place, as opposed to the "perfect" situation.

Maybe more than you wanted to know but it's too late to turn back now.
A deep subject for sure.

Take care,
Chuck Riddeck
Progressive Race Engine Development

 

Thanks for the reply. The main reason I was asking is that I just had the seats cut for 2.055 valves with a radius valve job. The throat is still the same size though. I think Ill open it up a little and flow them after every change just to make sure Im going in the right direction.

 

Thanks for the info, Chuck. I enjoyed the read. Not everyone(?) understands Reynolds numbers, etc. but for those who do, we need more posts like this.

Jon

 

Ok so for applied science sake, what isthe optimum throat diameter for a 1.94, 2.00 and 2.02 valve in a LT1 head of average sized ported intake runners

 

To put it simply

1.94=1.785
2.00=1.840
2.02=1.858
2.055=1.891

that's just 92% of each valve size.

Which is basically what Chuck said. Anything deeper would need you to have the requirements of the engine on hand.

I think he said in laymans terms to 91tpi-zf "don't $%^ with it!"

Bret

 

I spent quite a bit of time today on the flowbench, trying some things out. I did not gain flow by enlarging the throat from 90.5 to 91.5 %. Flow did not drop off either. I guess this just proves that the 90% rule is a pretty good start.