Bigchief632 wrote: ↑Mon Mar 13, 2023 1:11 pm
I use diameter of od of valve also. And add valve stem area. This is another old wives tale that's held on for decades. 85-90% MAX!! It's all dependant on application, valve diameter, shrouding, and profile. By default, if you cut a 55 degree seat on a 2.5+ diameter valve, for example, you'll end up at close to 93%, with all the angles on the seat, 50-55-65-75. Just an FYI, some engines use seat angles even steeper, 60 or even 65. With the correspondingly steeper "bottom" angles, a 60 degree seats uses a 70 bottom angle, or more, by default the throat % ends up even larger without removing anything with a carbide. It just tips everything up, and it ends up wider. That's the reason for it too. Literally makes the hole larger under the valve. On a 800 inch deal trying to make 1600+hp at 8000+ rpm, that's just what it needs. The extra angles, vs just blowing the hole out with a carbide and destroying all the angles that help turn the air or in this case, turn it less, and helps wet flow.
I revise bottom angles all the time, no "rule" that says a 45 degree seat HAS to use a 60 bottom angle. I've changed it to 65 more times than I can count, I have a Sunnen VSC cutter I use, and I had another one made. For the serdi style blades. I'll also use a 65 single angle blade to change the bottom angle. This works well when you're valve size limited do do bore size and the engine "thinks" it has a bigger valve without increasing shrouding.
My favourite 45 deg intake cutter that I use on most things with over .600" lift is 35/45/65. Usually with 75/80 or 75/85 throat cuts.
I have no idea why everyone defaults to 60deg bottom cut.
The only issue is getting the cutters, I hang on to my old Mira ball drive cutter bodies coz I can get that cutter profile in a mira cutter. The rest of my tooling is goodson fast cut.
But to the question - it's valve diameter, not area.
And it's a rule of thumb, there are many exceptions, it's just a basic starting point.
Flat tappet lower lift stuff - 85-88%
Aggressive flat tappet or street/hyd roller - 87-89%
Solid roller decent lift - 88-91%
Aggressive roller - 90-92%
Again just a starting point.
I've used up to 91% on SFT stuff when the port design makes that feasible.