Throat % is it area or Diameter?

[HQM383]

It is easy to get confused if it’s diameter or area as there would be many instances where authors are careless with terminology. As in this instance;

“According to Darin Morgan at Reher-Morrison Racing Engines the sweet spot is where the throat area is between 91 and 92% of the valve diameter. He further notes that various applications are best served within this range and often differ by only a few tenths. An optimum general figure seems to be 91.5 percent.”
Hot Rod Engine Tech

91.5% throat area for a 2.02” valve is a 1.93” diameter throat. That would be getting awfully close to the inner diameter of the seat.

91.5% of diameter for 2.02 is a throat diameter of 1.84”.

Head porters, what size would you make the throat for a 2.02” valve @ 91.5%?

[brad_m]

OP not sure why you can't get the one word answer for your question. All the porters I know use diameter.

Reading all the answers in this thread shows why I spent so much time looking and not finding the simple answer in other threads.

I've asked a question that effectively requires an answer of A or B.
But this thread like all the others, quickly derails into a bunch a replies that don't actually answer the question.

[HQM383]

OP not sure why you can't get the one word answer for your question. All the porters I know use diameter.

Reading all the answers in this thread shows why I spent so much time looking and not finding the simple answer in other threads.

I've asked a question that effectively requires an answer of A or B.
But this thread like all the others, quickly derails into a bunch a replies that don't actually answer the question.

[modok]

Around here when you hear 'throat percentage', it means the diameter of the "valve seat throat" compared to valve diameter,

What is the valve seat throat?.....it is the location right before the TURN to go all directions when flow reaches the valve and valve seat profile.
The size of this area controls the speed at which the flow enters the turn.
Most of the time, 86-90% is right range for a 45 degree valve.

If the throat is too small, the air is entering the turn too fast,
Too large and the turn ends up too sharp.

[Walter R. Malik]

Reading all the answers in this thread shows why I spent so much time looking and not finding the simple answer in other threads.

I've asked a question that effectively requires an answer of A or B.
But this thread like all the others, quickly derails into a bunch a replies that don't actually answer the question.

[/quote]

Correct or not ... I have always used "diameter" of the outside edge of the valve seat to figure the throat size.

[Bigchief632]

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.

[SpeierRacingHeads]

Most bracket, street type heads are all 91.5% of the valve without stem deducted.

I do some at 92-93% of valve, depending on what the application is.

I'm not a steep seat guy. Own one 55º cutter.

[David Redszus]

The proper throat flow ratio should be area. All flows are determined by area.
As shown below, there is a clear relationship between diameter and area.
The usual area ratio target is 80% which is obtained by a diameter ratio of 90%.
The use of diameter has become an easy and quick shorthand used by builders but
not by engineers who will use flow areas.

Valve........Throat
Diameter.. Area
0.95,....... 0.90
0.90,....... 0.81
0.85,....... 0.72
0.80,....... 0.64

So which throat ratio is correct? Well, that depends.
As Panic clearly observed, valve curtain area must be considered, which is a function
of valve diameter and lift.

But, in addition, piston air demand must be considered, which is a function of piston
area and piston velocity.

When valve curtain area (diameter & lift), piston velocity, and bore are increased,
the throat flow velocity must increase and can only be accomodated by increased
throat area.

Bottom line.
For low rpm, low lift engines, a smaller throat area ratio is beneficial. For high rpm,
high lift race engines, increased throat area ratio is required. How much is too much?

Since flow benches cannot replicate the air demand required, they cannot provide
acceptable test data. Either dyno testing or engine simulation is necessary.
But dyno testing requires working prototype engines (for each variable parameter)
which is quite expensive. So the best path forward is to use engine simulations
to test alternative designs and operating conditions.

Increased power should be a secondary objective. The primary objective should
be the optimiztion of component parts as indicated by torque curves vs rpm.

[SoCalDave]

I do this as a beginner and as a side gig, so I go conservative on throat % @ 88% of intake valve diameter and 86% of exhaust valve diameter. If you're just learning that should work; however for SBC I had to use 89.6% since the intake was already ~89% which will still work as SBC heads use 11/32 valve stems.

[digger]

It is based on diameter. People can use whatever makes them happy but when people talk 92% or whatever, then common sense says it’s diameter as you are not going to fit proper angles and sizes to turn air if using area especially with 45degree seat

[juuhanaa]

It is based on diameter. People can use whatever makes them happy but when people talk 92% or whatever, then common sense says it’s diameter as you are not going to fit proper angles and sizes to turn air if using area especially with 45degree seat

Yes and no. It depends the coefficient of discharge. Even a static flow testing can show this. I could call it a day if a see a high CD value + the flows are not backing up on a higher lifts i.e. indicating a unbalanced, unsteady port operation (perhaps choking by trying to accelerate through a given area) the trick might be to slow it down instead, but obviously it depends what were trying to achieve.

Perhaps a good easy-to-understand example could be putting a non back cut valve to gain higher lift flows.



-juhana

[digger]

It is based on diameter. People can use whatever makes them happy but when people talk 92% or whatever, then common sense says it’s diameter as you are not going to fit proper angles and sizes to turn air if using area especially with 45degree seat

Yes and no. It depends the coefficient of discharge. Even a static flow testing can show this. I could call it a day if a see a high CD value + the flows are not backing up on a higher lifts i.e. indicating a unbalanced, unsteady port operation (perhaps choking by trying to accelerate through a given area) the trick might be to slow it down instead, but obviously it depends what were trying to achieve.

Perhaps a good easy-to-understand example could be putting a non back cut valve to gain higher lift flows.

-juhana

i dont think you understood what i was saying, it is geometry and maths problem.

if you use 92% of area ratio the diameter ratio is 96%. if you take say a 1.5" valve and have a 45 degree 0.060" seat the ID of the seat is just about 96% already meaning there is the 45 seat and almost no room for any other angles which is not good

[juuhanaa]

It is based on diameter. People can use whatever makes them happy but when people talk 92% or whatever, then common sense says it’s diameter as you are not going to fit proper angles and sizes to turn air if using area especially with 45degree seat

Yes and no. It depends the coefficient of discharge. Even a static flow testing can show this. I could call it a day if a see a high CD value + the flows are not backing up on a higher lifts i.e. indicating a unbalanced, unsteady port operation (perhaps choking by trying to accelerate through a given area) the trick might be to slow it down instead, but obviously it depends what were trying to achieve.

Perhaps a good easy-to-understand example could be putting a non back cut valve to gain higher lift flows.

-juhana

i dont think you understood what i was saying, it is geometry and maths problem.

if you use 92% of area ratio the diameter ratio is 96%. if you take say a 1.5" valve and have a 45 degree 0.060" seat the ID of the seat is just about 96% already meaning there is the 45 seat and almost no room for any other angles which is not good

The question is what are the speeds through the system at different locations and how they serve us in building the engine. It depends the velocities for example at seat, throat and port on average.



-juhana

[HQM383]

Yes and no. It depends the coefficient of discharge. Even a static flow testing can show this. I could call it a day if a see a high CD value + the flows are not backing up on a higher lifts i.e. indicating a unbalanced, unsteady port operation (perhaps choking by trying to accelerate through a given area) the trick might be to slow it down instead, but obviously it depends what were trying to achieve.

Perhaps a good easy-to-understand example could be putting a non back cut valve to gain higher lift flows.

-juhana

i dont think you understood what i was saying, it is geometry and maths problem.

if you use 92% of area ratio the diameter ratio is 96%. if you take say a 1.5" valve and have a 45 degree 0.060" seat the ID of the seat is just about 96% already meaning there is the 45 seat and almost no room for any other angles which is not good

The question is what are the speeds through the system at different locations and how they serve us in building the engine. It depends the velocities for example at seat, throat and port on average.



-juhana

Thread title: “Throat % is it area or Diameter?”

Merely a question of terminology. Not definition or modus operandi.

Its diameter.

Just.........diameter.

[juuhanaa]

i dont think you understood what i was saying, it is geometry and maths problem.

if you use 92% of area ratio the diameter ratio is 96%. if you take say a 1.5" valve and have a 45 degree 0.060" seat the ID of the seat is just about 96% already meaning there is the 45 seat and almost no room for any other angles which is not good

The question is what are the speeds through the system at different locations and how they serve us in building the engine. It depends the velocities for example at seat, throat and port on average.



-juhana

Thread title: “Throat % is it area or Diameter?”

Merely a question of terminology. Not definition or modus operandi.

Its diameter.

Just.........diameter.

I think you are right. The throat has no references to gender and you cant hurt it feelings, so it can be whatever we want.

I have this sticker on my flowbe.. sorry shop vacuum cleaner as a reminder about the same thing. I havent been much on a garage so thank you for correcting me.

Screenshot_2023-03-14-13-26-45-36_92460851df6f172a4592fca41cc2d2e6.jpg

-juhana