Formula for Curtain Area @ Valve Lift ?

[SchmidtMotorWorks]

I think the seat angle does matter; mostly at low lifts.
The way I think about it is, imagine an exagurated valve seat angle of 1 degree. The valve would lift 0.010 inch and the curtain area would be about 0.002.

[ozrace]

Yep, the seat matters, and after doing a physical check like Bill did, it becomes apparent that this can't really be solved by a formula (I don't think so anyway).
The actual exposed area at lifts above .100" is affected by top angles and valve backcuts.
My solution at .100" lift is lift * cosine seat angle.
i.e. - .100 x cosine 45 degrees = .07071
Fairly close to Bill's measured .073". But after that it's not so easy.
I think the best way to solve this would be to draw it in CAD.

[SchmidtMotorWorks]

it becomes apparent that this can't really be solved by a formula (I don't think so anyway).

It can be, but it would be a program about 100 lines.

The concept I would try first is to make several formulas that calculate the offset of each face on the valve and seat compared to opening distance.

Then make another set of formula that does the same thing except that it would measure the distance from the edges of each face on the valve job.

Then make a set of conditional expressions that sort out the irrelevant measurments because at some low lifts, some measurements might not be useful.

Then make a function that finds the smallest value from all of the expressions listed above. I think that would be the curtain height, from that it is easy to calculate the area.

IN UG NX you can make a dynamic minimum distance measurment between two bodies, as you move the bodies the measument points move to what ever the current minimum points are. With a lilttle thought and effort you could probably output the results to a graph.

If someone wll post the dimensions of a good valve job I can do it when I get back from Japan.

[gas]

Somebody is making this out to be far more complicated than need be. Afterall, isn't this thread referring to the CURTAIN area?

[Ape]

Actually
i also think it could be put into somewhat real numbers but do not think it would be worth the hassles. Being somewhat off school for quite a while i vaguely remember formulas for different vaneangles on radial turboshovels(airpump, gasturbine) similar to a peltonwheel. i would much rather stick with the sin / cos formula to take into consid. the different curtain circumference windows caused by different seatangles. Everything else might be calculable but with enormous effort.

christian

[ozrace]

Somebody is making this out to be far more complicated than need be. Afterall, isn't this thread referring to the CURTAIN area?

Yes Gas, it's about curtain area... but actual curtain area.
To calculate that properly, you need to know the effective opening at the valve seat. If you look at Bill Jones' figures near the start of the thread, you will see that the numbers are nowhere near valve lift and they change considerably with valve seat angle.
If you want to calculate seat efficiency and velocity, you need this figure.

Schmidmotorworks - thanks for the offer. I will post some numbers later today, based on angles that Larry Meaux and Darin Morgan have recommended.

[ozrace]

SchmidtMotorWorks -
Here are some recommended angles for valves and seats. I think the only angles that affect this are the seat angle, top angle and valve backcut.
45 degree seats - 35 to 38 degree top angle - 30 degree backcut.
50 degree seats - 40 to 45 degree top angle - 35 degree backcut.
55 degree seats - 50 degree top angle - 40 degree backcut ?
The hardest figure to define would be the size of the backcut on the valve and I guess you would have to make an assumption here. Maybe to within .020" of the seat i.d. ?

I hope you get a chance to play with it.
I think the results will be interesting.

[bill jones]

-OK,,,I did a purely scientific study of this subject in my shop today.
-I took a large diameter 45" tapered counter sink tool and stuck it in my mill so I could move it downward in .100" increments and measure that with my quills digital readout.
-Then I set up a 1" travel dial indicator so the stem is exactly on a 45 degree angle upward from the bed and this has the indicator stem at a 90 degree angle to the face of that 45 of the countersink tool.
-When I move the quill down the dial indicator shows right at .072" movement for each .100" of the quill travel.
-So quill travel is the same as valve lift and the indicator would represent the corresponding gap at the 45 seat angles.
-This means that with a 45 degree angle you gain .072" open gap at the valve seat interface for each .100" of valve lift.
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-.100" = .072" (gap to be used for scientific curtain area measurement)
-.200" = .144"
-.300" = .216"
-.400" = .288"
-.500" = .360"
-.600" = .432"
-.700" = .504"
-.800" = .576"
-.900" = .648"
-1.00" = .720"
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-This is about as scientific as I ever get but I still don't believe these numbers here are as accurate for what happens in REAL LIFE as using pin gages or dial calipers to measure like I mentioned earlier.

[gas]

I see what you are up to. It is a basic trig equation. Initially (at .100" lift) the .072" 'window' is accurate. However, and with due respect to you Bill, the figures you have amassed, (realistically) are not logical. At 1.0", it be more like .972". Using the math you used to determine the .072", the .720" dimension would be put out in 'space' somewhere. As I question how well I have explained it, I doubt you'll agree with me, though. I didn't expect my post to be taken so negatively, though.

Gary

[bill jones]

-But you know, all of us aren't educated to where we know what we (and you educated people) are doing, so we have to try some idea that we can understand and believe.
-That's what's neat about these websites is there is all sorts of good thinking going on and most of us make do with what we have available regardless of what someone else has or does.

[Rick360]

You are very much correct Bill and I had to measure it myself to believe it. My measurements come out within a couple thou of yours. I formulated and calculated trying to come up with your answers and was always ~ .030" wider. The valve back-cut can create the smallest window area and sometimes the chamber side seat cut also does.

My earlier formula does as I explained, but it does not give the actual window area, but the vertical area at the inside of a valve seat to the valve face. This I now see is far from the actual area the air & fuel has to pass thru. Thank you Bill and Oz for enlightening me. Both of your posts helped me visualize the correct problem. It is a trig problem that can be solved.

I am working on a new spreadsheet that I hope will calculate the area correctly. It already properly calculates the true area between the seat and valve face and I am working toward including the valve backcut and seat chamber angle for lifts where they come into play(mid to upper). When I get it done and it calculates what I measure I will post it.

This has already made a few things apparent about different seat angles and widths that I didn't realize before.

This is a great forum. What I first thought to be a fairly basic question with an easy answer, with a few others help along the way, has made me say hmmm.

Rick

[ozrace]

Bill - the actual movement apart of the 2 faces will be about .072", but as they move further apart, the minimum area is not directly between the 2 45 degree faces but more between the top angle on the head and the back of the valve. The way you did it the first time is a good way to do it. i.e. actually check with drills and calipers.

To give an idea of the difference the seat angle can make - I believe that at .100" lift, the gap is just the Cosine of the angle.... so:-
30 degrees gives .087"
45 degrees gives .071"
50 degrees gives .064"
55 degrees gives .057"
You can see it makes a big difference.

Gary (gas) - I don't know who you thought was negative about your comments - I can't see anything offensive, but let me say that if it was anything I said - that wasn't my intention. I was just explaining what we were trying to achieve and why.
Likewise for SchmidtMotorWorks - I believe he was just stating the facts as he saw them.

[bill jones]

-To guys like me a cosine is greek or some foreign language I don't understand, but I actually went and looked up and found that .07071 number and thought maybe that was what I was seeing on the dial indicator I had reading the 45 angle.
-Then I went back and looked around to see what was really called the curtain area and it appears that nobody cares about the angles of the seats, but those engineers who do can't explain it so that I could ever understand it.
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-We're having fun with this tho and somebodys going to get their light lit up with this discussion.

[ozrace]

-To guys like me a cosine is greek or some foreign language I don't understand,

Bill - whatever you may lack in education - you more than make up for in experience. Your input is invaluable.
I am also not big on high end maths and Trig, luckily for me the trusty "Zeus" toolroom reference book has a table of Cosines in it.

Go Rick.

[shawn]

This is a really interesting post. It will be neat to see what the different valve angles produce for area at the different lifts, how a 55* seat will differ from a 45* at .800+ lift.
shawn