Don Terrill’s Speed-Talk

Just out of curiosity, whenever referencing valve throat percentage in a head/port, how do most of you make the calculation? For instance, the intake throat is 90% of the valve. Do you take the actual 0.D. of the valve head and use that in the calculation, or do you account for the fact that the actual seat interface is generally about 0.040" less than the valve O.D., so when making the calculation, you use the valve O.D. minus the width of the seat interface (the valve O.D. x .98)?

I was recently discussing some measurements with someone and we kept getting slightly varying numbers. Eventually I realized he was calculating one way, and I, the other. A small difference, I know, but just got me thinking how most reference that value, as it is used often in discussions on here.

Port to valve ratio should represent a comparison of flow areas. As such, port to valve curtain area would be more realistic as an indicator of choked port flow.

For a quick and dirty comparison, mean valve seat diameter to port diameter can be used.
But others will work just as well.

I use valve OD and throat ID.

Erland

I also use valve OD to throat ID.

The articles/post that show numbers are usually valve OD and throat ID as far as I've noticed.

I'am on the Valve OD , Throat ID Boat also!

I noticed in some articles, they refer to the valve OD minus the .040 taken up by the seat as the "flow diameter". Is that correct? Do you guys call it something else? Thanks.

You can make the seat width whatever you want , anywhere from .040" to .110" if you care to dependant on what low lift flow numbers your looking for, but the valve OD of what valve your using is it!

You may also be interested to know that steeper valve seat angles will allow for a larger Throat diameter which can allow greater high lift flow potential, as in going from a 30 seat up to a 45 , or 55.
The down side to this is that on most heads you will give up some 15 cfm between .100" lift and .250" lift even with for example a 30 degree back cut added to blend the seat into the rest of the back side of the valve.
On most motors if your running lifts above .550" this drop off can be made up for with added high lift flow.

Other factors in the seat angle game are that seat angles of 30 degrees ware less then steeper angles , but they need more spring seat pressure to control any given Cam lobe.
The bad side of the coin for seats of 45 or greater are that as the guides get worn they loose there seat seal to a greater degree.

Actual valve diameter, multiplied by whatever throat percentage. For example, 2.02x.9=1.818. Also, don't forget the valve diameter in there. That takes up space too. 11/32 valve, .341/2=.1705x.1705x3.1415=.0913. So, add .0913 to 1.818, for a "true" 90% area of that valve size. So, to have a true 90% throat percentage of a 2.02 valve you need it to measure 1.91. That accounts for the stem of the valve being in the way.

Also don't forget some throats aren't totally round....... Measure up and down and side to side.

Frankshaft wrote:Actual valve diameter, multiplied by whatever throat percentage. For example, 2.02x.9=1.818. Also, don't forget the valve diameter in there. That takes up space too. 11/32 valve, .341/2=.1705x.1705x3.1415=.0913. So, add .0913 to 1.818, for a "true" 90% area of that valve size. So, to have a true 90% throat percentage of a 2.02 valve you need it to measure 1.91. That accounts for the stem of the valve being in the way.

you subtract the stem area, not add it to the csa = 2.5 csa
Tom

How do you figure? If you have the stem in the way, taking up area, you need to make the "throat" larger by the area taken up. So, I guess, what your saying, is, you subtract the stem area from the throat %, and that's what your actual throat % is, that's why you add that area to the measured throat.

Frankshaft wrote:How do you figure? If you have the stem in the way, taking up area, you need to make the "throat" larger by the area taken up. So, I guess, what your saying, is, you subtract the stem area from the throat %, and that's what your actual throat % is, that's why you add that area to the measured throat.

I guess it depends on weather the OP wants to just bench race talk about a throat percentage or have a useful csa number to logically work a port
Tom

You need the throat % number to calculate the actual cross section at the throat do you not? 2.02 valve as the example, 90% throat=1.818. 1.818/2=.909

.909x.909x3.1415=2.59-.09=2.5 So, I get what your saying. But, to get a true 90%, 89% whatever your targeting, as the op asked about throat %, and how we calculate it, this is a bit more than he asked. The valve stem is in the way, and takes up space, so, the area needs to be adjusted, or, you won't have that much area. Its pretty simple. Last time I checked, the valve stem is always in the way. So, now we subtract our stem area to get the actual cross section, which I think is what you were saying zums. I was saying above, to actually be 90%, you have to add or make the area larger by the valve stem area. Follow me?

Here is the easy way to do it!