Thanks again. I'll play around with modeling some lower lift values and I'll try a 30 vs 50 comparisons to exaggerate the differences.Rick360 wrote: ↑Tue Jun 12, 2018 1:49 pmYes, I think you've got a pretty good understanding. One other thing about the area is that the steeper seats starts gaining back some area deficit to the 45 at about .100"-.150" lift depending on angles and widths. The rate of change in area per lift or "slope" of the area vs lift gets much better around that lift. Slower opening area at real low lifts where not much can happen yet then faster area opening at about the lift when the valve can really start to move out of the way. It acts like a faster lobe. Or thats how I see it.NewbVetteGuy wrote: ↑Tue Jun 12, 2018 1:15 pmRick, I really appreciate you posting a link over to that thread, making the XLS, and making and posting the pictures.Rick360 wrote: ↑Mon Jun 11, 2018 9:52 pmHere's a link to an old Speedtalk thread from 2005 that might be useful for some. This post/thread was my initial inspiration as to a new reason other than flow "why" steeper seats might be better, although I already had a set of heads with 50º seats by then. This certainly got me thinking along new lines.
I just used the XLS to look at the minimum window area of a 2.02" valve with 50/43/35 valve angles vs. 45/35/30 valve angles at 0.100" increments up to 0.700" and then also added one more data point at 1.000". I did not see what I expected and I think I'm more confused, but possibly on the cusp of understanding something new...
2.02" Valve "typical" 50 vs 45 degree valve job min window area at various lifts:
Valve Lift 45 50
0.100" 0.459 0.413
0.200" 0.975 0.855
0.300" 1.504 1.417
0.400" 2.075 1.984
0.500" 2.661 2.568
0.600" 3.254 3.16
0.700" 3.85 3.756
1.000" 5.649 5.555
I fully expected to see the 50 degree seat start out with less window area at lower lifts and then at some point transition to having a larger window area (much like we see the CFM flow #'s cross over in the HotRod article), but we don't see that actually ever happen. The 50 degree angle starts out with less window area and stays that way at all lifts... Did I do this right?
Is this correct that the 50 degree angle valve will have less actual curtain area at all lifts but still somehow flows more? (simply because the angle of the valve and seat let the air flow FASTER?) More air flow with the same area == more velocity, but this seems to be more airflow with less area -so potentially significantly higher velocities through the valve curtain area??? (At least at the lifts where the valve curtain area is still the bottleneck /choke point.)
On a related note, unless I totally screwed up something above (WAY more than possible): When calculating at which valve lift point the choke moves from the valve curtain area to the intake port's choke / minCSA, should we actually be using this more complicated Window Area Calculation? (If so it seems like it will take more lift with a 50 degree seat to reach the point when the throat becomes the choke, right?)
Your description of "it acts like a faster lobe" -is exactly the mental concept that I've been using internally to explain to myself how I think it works. Makes it SEEM like the seat-to-seat duration and lower lift durations are shorter while the higher lift durations are roughly unchanged AND you get higher flow high in the lift curve with more air velocity.
I haven't seen anything approaching a "rule of thumb" in terms of how much switching to a 50 degree seat reduces your peak HP RPM (I'm not saying I expect to see one, I'm using this as a general concept to see if I understand or not), but it certainly seems like that's the kind of thing you should expect to see/the changes that are seen when moving to a 5 degree steeper valve angle seem like the change that would simply move the point at which a port would fail to make more power to a lower RPM. Let's say a particular 195cc head with a min CSA of 2.15" peaks on a 383 @ 6,500 RPM OOB with a 45 degree valve seat angle because velocity gets too high and the port goes super turbulent / unstable, if the move to a 50 degree valve seat angle increases the velocity of the air through the port and past the valve, then I'd expect it to reach that super turbulent / unstable point at which it fails to make more power sooner.
My thinking right now is that moving to a 50 degree seat on a hot street build would be a good "tweak"/"hack" to improve average airspeed/torque if your heads are sized slightly too large for your desired HP RPM peak and cubic inches. (Example that highlights my thinking: My particular engine build but as a 383: SBC 383 6" rod, long-runner intake (19.25"), 195cc Profiler heads, Stans 1 5/8" tri-y headers; Mike Jones 227/228 @0.050" 272/280 0.006", 0.600" lift with 1.6RRs on the intaek cam: neither the long runner intake, nor the cam, nor the 1 5/8" tri-Ys are going to want to run past 6,000 RPM, for 6,000 RPM on a 383 the heads' min CSA is slightly oversized - moving to a 50 degree seat will increase peak air flow, reduce reversion, and increase the average speed of the air across the used RPM band. -The airspeed at 6,000 RPM is now faster and closer to what it would have been at a higher RPM with the 45 degree seat.)
-Again seems like a good trade-off in a hot street build to me. -Rather than further pushing to a faster and faster lobe or higher and higher ratio rockers and having to control it with stiffer springs and impacting the lifetime of the valve train, get that last bit of "effective lobe speed" via a 50 degree seat -and also get some reduced longevity from the 50 degree seat angle. An asymmetric lobed cam can get those valves opened quickly to get the valve "into the good air" and you can still have the closing side decelerate it so it doesn't bang shut as intensely. Beehive / Conical springs and lightweight retainers to keep the pressures at a minimum to help with longevity. (And WIDER 50 degree "street seats" to also help with longevity.)