Unshrouding valves - how much is it worth?
I know unshrouding valves will increase airflow, but it is just a miniscule increase or is it significant enough to justify a loss in compression? This especially comes into play when installing larger valves and raises the question, is it better to have larger valves and a little more shrouding, or slightly smaller valves and more room on the tight side?
It depends on how bad they are shrouded. My Sportsman IIs were pretty bad. The combustion chamber wall came really close to the intake valve. I cut it back to 3/16" clearance.
unshrouding the valve will "somtimes" increase air flow, it depends on the head,
Some heads dont even flow much air on the wall or shrouded side
I remeber spending hours reworking the chambers on a set of 492s, i made a speisel tool for cutting the side of the chamber as well as having it angle out so as the valve opened it becance more de-shorded, in the end i only picked up a few more feet per minute.
In a racing enviroment where every little bit adds up to an edge, it mite make more sence.
Its easyer to pick up more air flow by shaping the bowl aera so
air tends to flow more to the centre.
I look at it like how can i grind this so works like a helix
Just look at the chamber of a vortec head to see how smaller valves flow more air.
Thes baord has an exellent spel shecker to da ryght of the Submiit
Et mayks us bad spelers seam a littl mor edjumacated!
Here is a picture of the Sportsman IIs, (look at the picture of the combustion chamber). You can see how bad the intake valve is shrouded. In this case I think unshrouding the valve makes a big difference. I have a 4.145" bore, so I was able to open it up a little more than if it was for a 350.
It all depends on the combustion chamber design. As a general rule, unshrouding valves will improve flow figures, but then you have to compensate for the increase in combustion chamber size in some way, IE different piston design or cutting the heads.
Unless you have a lot of experience working with modifying heads, I wouldnt try doing it myself.
Pause to think, the guys making the aftermarket heads often have what could qualify for severely shrouded valves, yet they out flow stock heads, to great extent. Could be they are on to something.
While I have always done my own porting, I rely on someone with the proper equipment to do the valves etc. One thing that has been shown to work on most stock heads, despite of the design is to have a 15 degree back cut on the chamber side of the valve seat. This has been known to pick up some serious flow numbers without any real effects on increasing the size of the combustion chamber. As I said, its been shown to work well on most heads. If you are working with an open chamber head like a 2 bbl cleveland, the effect isnt worth the effort.
[QUOTE]Originally posted by lluciano77
[B]Thes baord has an exellent spel shecker to da ryght of the Submiit
Et mayks us bad spelers seam a littl mor edjumacated!
15 years ago i built my flow bench, you could say i've flowed a few thousand cubes of air.
Iv'e learned a thing or to, biuldt some real, not imagened horse power and set fast time more often than not.
you want to look at the content and not how it's spelt you mite find something you can use.
Ask me a question, tap my expereance. Ask me were the air flows in a chevy port.
I dont care if i seem edjumacated!
As for the 15* top cut it's true you will pick up some flow numbers but theres another trick that picks up more power, and that is to
cut it flat and leave a little steap or edge, that way feul that has fallen out of suspention and is crawling along the walls/floor and over the seat is torn back into the air stream when it hits this edge, its reatomized with air and not as dependent on the compression stroke to be forced over to vapor. The more vaporus
the envroment the better the burn the more power.
If you look at some factory GM heads you'll see it there
its done for emissions!
Most Chevrolet ports are designed with a bias towards the center of the cylinder. Airflow has a tendency to want to move in that direction, and generally, the higher flow areas of the intake aperature are located on toward the cylinder's center.
Increasing valve size from 1.94" to 2.02" on the early fuelie type heads is a common practice. Airflow will actually suffer if some sort of deshrouding operation is not performed on these types of heads. A situation where the intake valve is directly against the port wall creates fuel wash and poor swirl and disrupted flow.
Going to larger valves without doing the deshrounding operation can actually hinder flow over the stock valve size.
The amount of deshrouding doesn't have to be a huge amount, bearing in mind that airflow has a tendency to move towards the cylinder's centerline.
Most aftermarket performance heads are adequate in this area. Most factory heads, except for maybe the newer fast burn or Vortec heads are not. The factory Vortec head is a good example of what is necessary for optimum flow numbers.
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