Valve throat.

[kokkolanpoika]

Hi.

Like to ask why there is a rule that valve throat size is usually 85-87% of valve diameter?
Is that a reason, because all engine builder always do this? I make an converstation of our head porter, and he say that you wont get all valve area to use if use throat like 85%. He usually go for bigger over 90% to get best results. Throat will resist flow if it is smaller.? Also depending on port volume.

He look at my Rover head´s and say that you get best results to use 41.5mm inlet valve and use throat approx 90-92%. This size is near 38mm witch will against 44mm valve and 87% rule..

He build toyota 1.6l engine, it is pushrod engine and it will made 204hp with 41mm inlet valve. Engine make vacuum also. So there might be some resistor on throttle bodies witch are 42mm.. Might need 45mm bodies? He say that Rover port size is to small to use over 42mm inlet valves. You can get over 400hp with 42mm inlet. He estimated 440hp at lest if all parts will match.
And also lifting inlet port floor is waste of time. Original port desing is way to make power. On the flow bench floor lifting is better, but not in real life..

So can someone explain why there is a throat size rule?

[JP.]

Had to google it........ but do you mean throat.... ( I am not even English )

Basically it all depends on valve seat. Smaller seat = more throat room but less durability. Cut you valves and valve seat in a steeper angle and you create more space, but less durability.

Most valve cuts come in basic angle so there's your rule of thumb.

[kokkolanpoika]

Yes i mean throat.. My mistage..

But will this angle command throat size? And i do not see why durability is worse?

[Darkspeed]

With a 41.5 mm valve you would normally use a throat diameter of around 37.5mm you could go a little larger but that's a small seat and not much of a lead into it which would harm low lift flow The larger the valve the larger the % of the throat to the valve dia. Any transition into the throat would typically remain the same dimensions whatever the valve diameter.

Typically the dimension across the seat is about 2mm, 1mm for the 45 and then another 1 mm for the 60 etc. Thats 4mm off the valve diameter

25mm valve - 4mm = 21mm - 14%

42mm - 4mm = 38mm - 9.5%

However the bigger the valve and the smaller the seat the more critical the guide wear becomes.

Typically the 82-84 % figure is not for the throat but for the port behind the throat - the throat is always opened up due to the valve boss and stem taking up port volume at that point.

That's why its usual to velocity probe the port as you want to keep velocities stable and the velocity will show where there are restrictions or expansions in the port - Expansions being the worse in many cases.