Air flow vs valve size

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travis
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Air flow vs valve size

Post by travis »

If you had a decently ported head with a 1.78” intake valve, how much gain in airflow would you expect to see going to bigger valves and throat sized accordingly? Say to a 1.84” valve, and then again to a 1.94” valve? I know this is going to be highly dependent on the rest of the port, but would/could the airflow gains be directly proportional to the increase in valve area?

On a head with a small or undersized port, would a bigger valve show more gains at low lift, with the difference in flow decreasing at higher lifts (basically running out of port to feed the bigger valve)? This is of course assuming that the port/bowl/seat shape is decent.

How do you determine the correct mcsa or average CSA for a given valve size? Is there a formula for calculating this?

Last question...can you buy a cutter that will unshroud the valves, that will leave a small radius between the chamber roof and walls? It seems like I have seen this before, but I may not be using the right words as my search is coming up with everything except what I am picturing in my head.
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Re: Air flow vs valve size

Post by PRH »

What I usually see by installing a bigger valve into a stock head with minimal porting is better low lift flow, and often the peak flow is nearly the same as with the small valve...... but occurs at a lower lift.
Often the c/d is lower than it was with the smaller valve.

Of course, only one way to really know for the particular application.
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Re: Air flow vs valve size

Post by mag2555 »

I guess we are talking a factory iron head here?

If so forget stepping up to a 1.94" valve as even the flow at low lift will be too shrouded off to be of value , no less not being able to make use of any high lift flow potential do to having not enough port wall meat to even get the valve to Throat percentage up to the minimum needed 85% more then likely.

If you assume that you have safely maxed out the 1.780" valve seat OD to 1.770" then your maxed out Throat potential of 91% leaves you with a needed 1.610" Throat .

If I recall right the stock Throat of Ford 1.780" valve heads is like 85% or like 1.50".

If you assume a minimum port wall thickness of .160" then it's plenty safe to take the Throat out to 91%, not that that will get you the fattest flow numbers at .400" lift which is where these heads do there best work if your sticking with the stock 1.780" valve size!

Going up to a 1.840" valve can make things Dicey with what will be left of the port wall thickness if your shotting for max high lift flow by means of grinding out a 91% Throat if you ask me.

With a 1.674" Throat for use with a 1.840" valve swap and then assuming doing most of the port enlargement work on the port wall that give the most flow increase means hogging out that wall wider to the tune of .130" which leaves you with the big likelihood of a blow out!
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Re: Air flow vs valve size

Post by denisj »

I use valve sizes based on bore size and set the CSA specific to my target peak HP/RPM spot. I’ve never heard of average csa. It’s pretty exact. Depending on what the engine is supposed to do I shoot for 600-640fps limiting port velocity. Check out Wallace racing calculators.

Of the engines I build for street motorcycles (2 valve push rod twins) I have never come across a valve that was too small. They are always too big. Maybe iron car heads are different

A bigger valve on a small port will kill your torque and airspeed...unless your engine is drastically under-valved

Hope this helps
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Re: Air flow vs valve size

Post by PackardV8 »

This reads like the area where Mummert made his gains on the Ford iron 289" head. Maybe he can share some generalities of what he'd do in your circumstance.

FWIW, we have the same tradeoffs with the Studebaker V8 heads. The OEM intake valve is 1.65625” with 140 CFM. When we get the intake ports up to 200 CFM with a 1.875" intake valve, the horsepower gains are not proportionate to the flow gains. Obviously, we need to have Mummert work a Studebaker 289" head.
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Re: Air flow vs valve size

Post by mag2555 »

It's no where near a Apples to Apples deal to compare the end result power made with a 200 or whatever cfm flow level between a Ford combustion chamber and one in a Studebaker!
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Re: Air flow vs valve size

Post by Mummert »

I don't see much wrong with a Stude combustion chamber, but I saw some heads on the Jalopy Journal and the exhaust valves are huge! What size are they in the cnc ported heads.
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Re: Air flow vs valve size

Post by modok »

Comparing how much a thing flows to how big the hole is.... generally called "flow coefficient".
In fact that's many flow benches work. You compare what the thing flows to a calibrated hole of known size. Find out how much it flows as compared to the size. You could think of it as efficiency. Either that or the first thing you'll learn on a flowbench is bigger things flow mare than smaller things, most times, because.....in fact it's true :P
Sharp edge hole 60something%
A poppet valve usually 65-75%
Of course, you have to decide what to compare it too. I don't remember if 100% is defined a venturi or a straight tube with raduis entry, right off the top of my head can't recall which.....perhaps depends what book your reading. I'd start with David V's oldest book on it, it's discussed thoroughly.
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Re: Air flow vs valve size

Post by Mummert »

Is the valve the only area in a port that you should look at when considering flow coefficient?
Is it smart to use the valve as the determining factor for you flow coefficient if you feel that it is too big for operating range?
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Re: Air flow vs valve size

Post by modok »

You can compare the flow coefficient of the Valve size, throat size, and intake runner size. And compare them and see what part is the restriction. Hate to say just read the book but...... I'm sure I can't explain it better, tho personally I probably do think of it a little different.

Since I'm more into street/endurace stuff, and a manifold tuning nut, I think of the runner and port velocity as being the main area of concern, that's the part I WANT to be the restriction. So the valve and throat has to flow enough to let the port and runner reach high velocity. Since a rather good valve/chamber is only 75%....that means I want the port and lower runner size 75% of the valve area, or, (about 86% by diameter), and often that is about right, or maybe kind of a minimum valve size(65-75%), the worse the valve flows the larger it'll need to be of course.
The the logical question is can a larger valve than NEEDED have any advantages? and...IMO, very few.
Unless lift is restricted, or the throat is very large and you have bad wet flow.

The again if you have a case where the intake valve sizes is fixed, or already maxxed out, might look at it a different way.
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Re: Air flow vs valve size

Post by modok »

And to blow a hole in the idea...
Check this out.
lets say you have a valve seat ground with 55, 45 and 37 degree angles, each .065 wide. not far from right...right?
You could make matching valves for 55, 45 and 37 degrees. :P
the 55 degree valve would be the smallest size therefore have the most impressive flow coefficient for it's size, tho it's not necessarily the right choice.
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