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Old 10-04-2010, 10:50 AM
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thoughts on altering seat angle

On a hot street engine do you think it would be worth running 50 valve seats on a 1.84" valve with a cam that has a max lift of about .550.

My reasons to do it would be that the valve reaches .25 L/D much sooner than with a common 2.02" valve AND the seat would kill low lift flow making a cam seem less radical. This would be on the intake valve only. I can move up to a 1.94" valve but at that point shrouding becomes an issue.

heads are 059 305 Vortec castings, 305 bore size.

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Old 10-04-2010, 11:37 AM
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Quote:
Originally Posted by turbolover
On a hot street engine do you think it would be worth running 50? valve seats on a 1.84" valve with a cam that has a max lift of about .550.

My reasons to do it would be that the valve reaches .25 L/D much sooner than with a common 2.02" valve AND the seat would kill low lift flow making a cam seem less radical. This would be on the intake valve only. I can move up to a 1.94" valve but at that point shrouding becomes an issue.

heads are 059 305 Vortec castings, 305 bore size.
No, decreasing the seat angle provides more curtain area per unit of lift than does increasing the angle. For high output engines 30 degree seats had a spate of popularity back in the 1960s and early 70s and continues to be used but not with the enthusiasm of the those days. The gain is quite small while maintaining a tight seal becomes a problem as these seat angle are more sensitive to crud becoming trapped and holding the valve from completely closing.

One or all of these books by David Vizard has a long dissertation on seat angles, if memory serves which it doesn't so well any more, it's the first one in the list.

How to Build & Modify Chevrolet Small-Block V-8 Cylinder Heads
by David Vizard

How to Build Horsepower
by David Vizard

How to Build Max Performance Chevy Small Blocks on a Budget
By David Vizard

Time has established that seat angles in the range of 43 to 47 degrees provide the best compromise between seat life, valve to seat heat transfer, and flow.

Odd angles of seats is something like grooved crankshafts which were also popular in the 1960s, these proved to cause more problems than they solved, looked good on paper being able to move more oil thru the journal and insert, but proved to destabilize the hydrodynamic wedge of oil in that separated the two resulting in some spectacular bottom end explosions.

Bogie
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Old 10-04-2010, 12:08 PM
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I own all those books and it was actually a video of Vizards that got me thinking on this. The steeper angle does shroud at lower valve lifts, but it increases flow at higher valve lifts, namely where curtain area is no longer the restriction but the throat diameter is (over ~.25 L/D). This happens at only ~.460" valve lift.

I really can't see a losing aspect of a 50 seat in the application- but that's why I'm leary of running it- there must be something I'm missing. I was hoping a restricted circle track racer could chime in- I imagine this situation is something they see fairly often.
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Old 10-04-2010, 01:12 PM
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My thoughts align with Bogie on this, you would see more from a 30 seat as the .550" lift cam is considered low lift by modern racing standards. Your giving up low lift flow will actually make the engine seem cammier.

If you were talking about .750" lift, you would be in the area that 50 would help, but not at .550", the amount of time spent over .460" lift in your example is a lot less than the time spent below .460"

Only place I've seen 50 seats used are high rpm(5000-9500 rpm) high stall (5500+)drag engines, and vary large cubic inch engines where low rpm response is not needed due to starting line rpm, or the amount of cubes making more than enough low end.
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Old 10-04-2010, 01:18 PM
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Quote:
Originally Posted by ericnova72
Your giving up low lift flow will actually make the engine seem cammier.
I've always held the opposite to be true.
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Old 10-04-2010, 01:30 PM
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Poor cylinder fill will make an engine seem cammier, reduce low lift flow and you reduce cylinder filling at rpms below midrange. At least thats how I see it
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Old 10-04-2010, 01:42 PM
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Quote:
Originally Posted by ericnova72
Poor cylinder fill will make an engine seem cammier, reduce low lift flow and you reduce cylinder filling at rpms below midrange. At least thats how I see it

Oh boy... Its not exactly that simple and its kind of hard for me to explain. What a 50 would do is cut low lift flow and increase high lift flow (where exactly this switch over occurs is another discussion). What this means is that you're valve will look like its closed tighter than it is at low lifts as compared to a traditional 45 or 30 degree seat. It also means that it will look like its open farther above mid lift- essentially giving a flat tappet cam in a types B head (one with 50 degree seat) a flow curve similar to a roller cam with a type a head (45 degree seat).

The opening and closing ramps will appear more aggressive as the flow is cut off faster, which will lessen reversion and make an engine appear less "cammy."

If I get time I will try to generate a rough excel plot of this and post it.
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Old 10-04-2010, 02:52 PM
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I've ran some basic figures and running a 1.94" valve is more beneficial than running a 1.84 with a 50 degree cut, but shrouding is still a concern of mine and I would like to kill low lift flow, now I'm kicking around the idea of a 50 degree seat on a 1.94" valve...

I have a feeling this is one of those times where it may be smarter to keep the standard seat angle but I would really like to discuss this further.
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Old 10-04-2010, 03:38 PM
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Quote:
Originally Posted by turbolover
I've ran some basic figures and running a 1.94" valve is more beneficial than running a 1.84 with a 50 degree cut, but shrouding is still a concern of mine and I would like to kill low lift flow, now I'm kicking around the idea of a 50 degree seat on a 1.94" valve...

I have a feeling this is one of those times where it may be smarter to keep the standard seat angle but I would really like to discuss this further.
Low lift flow is what make the high speed internal combustion engine possible. For many years early engines used a suction intake where the descending piston created sufficient vacuum to overcome the closure spring and open the valve. These engines just could not be made to rev very high. The research efforts of Sir Harry Ricardo showed that the key to high RPMs and high power levels was to cam control the intake valve, opening it early and closing it late. This makes it apparent that low to moderate lift periods are most important on the road to power.

Shrouding the valve in a typical wedge chamber is a minor problem, the greatest flow bot intake and exhaust takes place to and from the center of the cylinder. In the case of the intake too much relief on the wall side sets up a flow counter to the main flow which diminishes swirl making the engine more ping prone, so this is something that requires care in execution.

Additionally a port will only flow to about .5 or .6 Mach. Usually the pacing dimension that establishes when the most critical part hits Mach shut off is the curtain area of the valve. Reducing the valve's diameter, exacerbates this problem by reducing the curtain area. The stock Chevy 350 with 1.94 inch valve is already pushing this limit by 6000 RPM so making the valve smaller would work against increasing flow regardless of seat angle used by increasing the Mach number across the seat. Needless to say the peak Mach is hit around full lift which again redirects us to the importance of getting the flow numbers up at the lower lifts. So performance of low and mid lift flow isn't something that can be ignored.

Bogie
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Old 10-04-2010, 03:52 PM
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http://speedtalk.com/forum/viewtopic.php?f=1&t=23486 Interesting that this topic is discussed on Speed talk forum and the conclusions are very similar too, Im of the opinion that the theory does not always match the results, unless someone has back to back testing done ,
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Old 10-04-2010, 04:33 PM
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Quote:
Originally Posted by technicaltom
http://speedtalk.com/forum/viewtopic.php?f=1&t=23486 Interesting that this topic is discussed on Speed talk forum and the conclusions are very similar too, Im of the opinion that the theory does not always match the results, unless someone has back to back testing done ,
The theory is well supported by results. engine performance is much more a pile of pragmatic test results from which some math models were created rather than starting from theory and searching for matching results.

Bogie
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