Valve exhaust port location--distance from valve seat and effect on valve flow?

[mackeral5]

I recently modified a valve by removing the OEM composite valve seat and creating a valve seat directly on the aluminum valve body under where the composite seat would have rested.  This relocated the seat towards the exhaust port, virtually right next to it.  Previously, with the composite seat, the valve seat was located more  forward, away from the exhaust port by approximately .45"

I'm curious what effect, if any this has on valve performance.  In other words, if I hurt performance by moving the seat up close to the exhaust, I may go through the effort of making a new seat that places it further forward, away from the valve's exhaust port.  I may be tempted to go even farther forward, if there is a potential for gains in doing so.

Anyone have any real experience/data around this?

[rsterne]

When I have a valve where the valve seat is a long way ahead of the exhaust port, I try and angle the port towards the seat, making sure to leave enough material between the two that I can round the upper front corner where the two meet without damaging the seat.... I don't like a long throat between the seat and a vertical exhaust port, as it forces a complete 90 deg. turn in the flow.... It also adds unnecessary volume between the seat and pellet, which means a greater pressure drop before the air gets to the pellet....

On the other hand, if the seat is too close to the exhaust port (like on an MRod), you cannot enlarge the exhaust port very much without cutting through the seat and ruining the valve.... The advantage to having the exhaust port near the seat is that you can see through the port, past the seat into the valve body, which to me indicates that the flow can also see that straight line, and would be easier than a long throat meeting a vertical port well aft at 90 deg. (like a stock Disco)….

On a valve like a Disco, I mill the exhaust port angled forward on a 20 deg. angle towards the seat, then round the corner where they meet.... This allows me to look down the angled exhaust port and see the lower half of the seat where it enters the throat.... To me, it is a simple matter of straightening the flow path as much as possible, and keeping it as short as practical.... When I am designing a valve from scratch, I angle the exhaust port 20-30 deg. towards the throat.... like this....



It is not shown on that drawing, but I also radius where the port and throat meet, to the left of the letter "P" in that drawing....

Bob

[Motorhead]

Shallow throat with seat near transfers vertical exit ... No negative i can think of EXCEPT, as stated by BOB increasing the transfer exit diameter is limited as well if the bare aluminum seat gets wear damage valve has a greater potential to become a paper weight.

As an old Cliche from my previous profession .... THERE IS RESISTANCE IN DISTANCE and getting the shortest delivery path to intended delivery point saves energy, delivers more due to less losses.

[rsterne]

Generally speaking, intake and exhaust ports in engines tend to be angled somewhat to the valve stem.... they are seldom either at 90 deg., or inline with it.... Having said that, shorter is better, not only for flow length, but also because the smaller volume reduces the pressure drop....

Bob

[mackeral5]

Scott and Bob, thank you very much for that information.  I think in terms of "signal" and making the air tract longer typically damps signal-- ie intake tract on a 2 stroke, extended transfer port system on a PCP.  This damping effect isn't always a bad thing.....  in the 2 stroke the signal is telling the carburetor to draw air/fuel and adjust to load in a quick, dynamic fashion.  In the PCP that signal is telling the pellet to GO, and to do so at a certain rate of acceleration, depending on how strong of a signal.  In both of these instances unnecessary length, which translates to volume, translates to a weaker signal....the carburetor isn't going to respond to changes in load as quickly, as dynamically.
The pellet isn't going to accelerate as quickly, as sharply.   that may not be the precise, 100%correct use of the concept but it is a way my pea brain can try to understand the impact of adding unnecessary volume to a gas path. 


I had never thought in terms of moving the seat away from the transfer.  I do recognize all of the concepts you described and understand the logic behind them. 

In this particular instance, the Extreme I am currently playing with, the transfer system design doesn't allow for  angling, nor does it lend itself to any significant radiusing on the inside of the turn from throat to exhaust.  The transfer sleeve extends too low in the valve.....

The sad thing is there was no reason to do this, there is plenty of "meat",above to have stopped higher above the throat.

I was not involved nor am I aware of this guns design history/evolution.  But simply based on valve design it would appear to me that someone built an AWESOME performing .308, the later decided to go up in caliber  to .457, but didn't invest in proportionately sizing the valve system, other than increasing exhaust to .375.....

A short rant....

I am amazed that it had such an cult like following, considering the dismal state of tune.  It was loud, it was violent.   It made a ton of power...for one shot...perhaps that appealed to some....it is well designed and well made in all other areas though...

After 2 simple changes it puts out more power and is more docile in doing it.  Only uses about 1200-1300psi  whereas before it used in excess of 2500psi....

[rsterne]

When Brent was around, I tried convincing him to tune his guns for 2 equal shots, or even a bell-curve of 3 shots within a 1-2% ES, both of which I know can be achieved.... I had a DAQ Exile that was an air hog, 3 shots of 836, 786, 742 and dropped from 3000 psi to 1800, which works out to 0.38 FPE/CI (and a 6% drop on shot #2).... Like you said, it was loud, and violent.... By shortening the hammer spring by setting it, and no other changes, I got 4 shots within 3% ES, 833, 832, 824, 811, 792.... 5 shots within 5% from 3000 psi down to 2000, which is 0.88 FPE/CI.... My 5th shot was faster than the 2nd shot the way it came.... and 5 shots used less air than 3 originally.... 

The cost on the first shot was a staggering.... 3 fps....       

Bob