Don Terrill’s Speed-Talk

I've spent time searching and reading many thread looking for the answer but they always quickly derail into a discussion about what throat % is needed and never answer the question of: When you're talking throat size as a % of the valve size, are you talking about Diameter or Area?

brad_m wrote: ↑Sun Mar 12, 2023 1:16 am I've spent time searching and reading many thread looking for the answer but they always quickly derail into a discussion about what throat % is needed and never answer the question of: When you're talking throat size as a % of the valve size, are you talking about Diameter or Area?

It is usually talked about as 90% of valve diameter, BUT
when you are measuring the area of the entire port as in cross sections you need to convert to throat area minus valve stem area as well.

steve cowan wrote: ↑Sun Mar 12, 2023 1:40 am

brad_m wrote: ↑Sun Mar 12, 2023 1:16 am I've spent time searching and reading many thread looking for the answer but they always quickly derail into a discussion about what throat % is needed and never answer the question of: When you're talking throat size as a % of the valve size, are you talking about Diameter or Area?

It is usually talked about as 90% of valve diameter, BUT
when you are measuring the area of the entire port as in cross sections you need to convert to throat area minus valve stem area as well.

All I'm asking about throat %
So to be clear, You are saying, when someone says for example 90% throat size, they are talking about the throat diameter being 90% of the valve diameter. NOT the throat area being 90% of the valve area.?

I don't know why but when I started reading up on it I must have assumed we were all talking Diameter. But looking at a head I have in front of me had me questioning that because the size as machined by the OEM is rather on the large size if we are talking strictly in Diameter.

brad_m wrote: But looking at a head I have in front of me had me questioning that because the size as machined by the OEM is rather on the large size if we are talking strictly in Diameter

I've some LS rect port heads in at the moment, not touched since they came from the factory, and the 2.16" inlet is showing a throat percentage of 89% as is referenced off the diameter

With the older tech stuff OEs seem to be putting out what would have been almost race spec machining twenty years ago to wring out every last pony

Power wise it’s always about area since that’s the ultimate factor in local port velocity which always controls the width of the motors power band and at a certain level causes the hp to drop off like a rock.

At least to me diameter gives me a quick idea of what I have to work with valve job wise and also in terms of the overall short turn height since on many heads with shallow chambers the short turn runs right into the deck, so on heads like this the overall length / depth of the valve job starting at the seat OD really needs to be considered part of the make up of the overall short turn height .

Example.
A 2.05” intake valve with a dead nuts max of a 91% throat is 1.86”.
This leaves a total of .185” left for the valve job which then needs to divided by 2, which then leaves you with only .092” to encompass the main seat which should have a minimum width of .045” and then however many bottom cuts you choose to stuff in.

And I have not even factored in the -.005” to -.010” of the valve diameter that you should have to promote good sealing and deal with the expansion of the valve and chamber floor.

If you have a head already with 91% throat and a idealized valve job for that throat size and you have a mishap which calls for cutting a good amount of meat off of the seat ti clean things up, then that ideal flowing valve job is lost!

Unless you have very deep pockets to just install new seats to never go over that 91% I much prefer to when max porting a head to not going over a 89% valve to throat ratio.

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

I dont know if its standardized. We also have metric and imperial systems. I have yet seen a metric wrench, but in my daily work almost everything is something divided by something.

The IOP program actually asks for a measure and not a percentage



-juhana

This is very disappointing.
Those of us who remember plane geometry know the answer.
The others choose diameter.

LotusElite wrote this...

That's an interesting point. Pulsed flow has a much lower dependency to wall friction as to inertia. A simple experience of that is, if you test headers on mostly straight piping on engine test bench level and go with the optimized dimensions into the chassis, but you need 10 bend to fit it, the results will be almost the same on the rollers. Just because wall friction is way lower affecting losses as the inertia of the gas. The fluid is undergoing a highly dynamic change of its thermodynamic status, that is common knowhow in fluid dynamics for immediate vented high pressure tanks or gas pipes like Nordstream 1 and 2 after the explosion. Same is valid for alternation of load parts. Of course there is a dependency of pressure amplitute height and time scale, but even IM system follow that recognition.

The effect of wall roughness is way lower as the effect of cross section changes. Every change there creates a loss, but if designed right some of the kinetic energy can be converted back in static pressure, which is the one accounting for density in the chamber. Kinetic pressure or impluse is important for the combustion process, but should be not driven to far as density losses will follow that. But as the inhale process is highly dynamic each phase has it own rules: low lift opening, low lift acceleration, low lift to high lift and so on till low lift closing. The last phase need an high impulse which can be reflected at the valve back as a high static pressure to crump as much as air as possible.

So finally the inflow process is connected to all processes before and after. Therefore bench flow port development follows requirements of hole process not just flow numbers. The cylinder head market unfortunately doesn't work like that, if follows straight flow numbers, which is an indicator but says not much about the power output.

At the engine forum I am Admin of we have a nice example regarding port velocity. A head ported by the top dog in drag racing in the K20-scene compared to a stock near head, which get material added in the port (increasing the short turn radius and increasing the length of the high speed section). That top dog head had cuts in the bowl and inflow area as well as on the divider (typical knife edge cut). Of course the top dog head lost everywhere as the flow velocity was too low for a 2-Liter engine, we talk about 7-15 ftlb and peak power was about a handful ponies offset'd. Later on that top dog CNC company released a much smaller port, seems they learned their lesson . BTW, that guy adding material to ports was from Finland. I am sure you will know him, as the scene here and there is too small not to know some essentials.

It has nothing to do with geometry, it is just a quick method to see where you are at before you begin the work, and where you need to be by using the diameter of the valve and throat. Been that way for 51 years for me. Diameter vs diameter. 88-89% exhaust, 89-90% intake. Works just fine for 99.9% of the industry. New cutters tend to give you 92% for new heads and valve jobs if you care to check. The latest TFS valve jobs are at 92% out of the box. Joe-71

The valve and the port have completely different shapes. The valve head is a circle, but its function is the curtain area (the surface area of a cylinder).
The curtain area (how valve seat flow presents to the combustion chamber) of a 2.00" valve with .500" lift is a function of only D, L, and π. The actual curtain area is 3.14 In².
Adding 10% to the diameter to 2.20" is a linear increase: the new curtain is 3.46 In² (+10%).

However, the throat and port flow are proportionate to the square of the diameter. A 2.00" port area is 3.14 in². Adding 10% to the port to 2.20" is not linear, the factors are D² and π. The new area is 3.80 in² (+21%).
See the difference?

brad_m wrote: ↑Sun Mar 12, 2023 1:16 am I've spent time searching and reading many thread looking for the answer but they always quickly derail into a discussion about what throat % is needed and never answer the question of: When you're talking throat size as a % of the valve size, are you talking about Diameter or Area?

Diameter.

panic wrote: ↑Sun Mar 12, 2023 1:57 pm The valve and the port have completely different shapes. The valve head is a circle, but its function is the curtain area (the surface area of a cylinder).
The curtain area (how valve seat flow presents to the combustion chamber) of a 2.00" valve with .500" lift is a function of only D, L, and π. The actual curtain area is 3.14 In².
Adding 10% to the diameter to 2.20" is a linear increase: the new curtain is 3.46 In² (+10%).

However, the throat and port flow are proportionate to the square of the diameter. A 2.00" port area is 3.14 in². Adding 10% to the port to 2.20" is not linear, the factors are D² and π. The new area is 3.80 in² (+21%).
See the difference?

Thank you for detailed explanation. I prefer percentages more now. 91 is easy to remember, but it really depends.



-juhana

Diameter works good enough, stay on the small side unless you wanna get into fancy math.

From what i have seen from nice competition heads it would be area not diameter. If it was strictly diameter the throat would be perfectly centered on the valve. I have noticed the throats are offset and sometimes not actually round. Cheated one way or another. So i guess i would say %area