Throat % is it area or Diameter?

[Wetflow]

I don’t see how you can determine a throat size without considering the size of the port. The throat percentage rule is an estimated maximum, not necessarily a target.

Shouldn't port size be a result of throat size? Shouldn't you start at the valve to determine throat size which ultimately determines port size?

[juuhanaa]

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.

I was under the impression that air does not like turning over sharp angles more than 15 degrees?
That is a statement from Darin Morgan

Think about it logically, and visualize it. Air turns easier and less, with say a 38 or 40 degrees top cut vs say the "classic" 30 degree. Draw the angles. 38 top, 45 seat, 65 bottom, 75 transition angle. Now draw 30 top, 45 seat, 60 bottom and 70 transition angle and tell me which one do you think turns easier around the given angles.

An extra 5 degrees of undercut sets or leaves the turning loose, so that a top cut can make a touchdown?



-juhana

[SpeierRacingHeads]

I use nothing less than a 40 cut top these days..

And yes I agree the rest of the port should be sized off the valve size, starting with the throat...

Kind of blows a hole in the biggest valve possible though. Ends up a huge port.

Nothing wrong with having the secondary choke up stream. Sometimes it's the only way.

[BobbyB]

I use nothing less than a 40 cut top these days..

And yes I agree the rest of the port should be sized off the valve size, starting with the throat...

Kind of blows a hole in the biggest valve possible though. Ends up a huge port.

Nothing wrong with having the secondary choke up stream. Sometimes it's the only way.

Do you ever use more than a 40 degree top cut with a 45 degree seat?

[SpeierRacingHeads]

I use nothing less than a 40 cut top these days..

And yes I agree the rest of the port should be sized off the valve size, starting with the throat...

Kind of blows a hole in the biggest valve possible though. Ends up a huge port.

Nothing wrong with having the secondary choke up stream. Sometimes it's the only way.

Do you ever use more than a 40 degree top cut with a 45 degree seat?

No I don't.. I use a 40, 45, or 48 top with my 50 degree.

I put the same 40/45 on LS, SBC, BBC, and even my SS heads. It's a custom cutter that I found through using stones.

I use a 45/52 on some shallow valve heads as well.

[6.50camaro]

Is there a correlation between throat area and valve curtain area being to large. As an example I have a set of heads that has a 1.84 throat with a 2.02 valve 91% [that how they came OTB] that gives me a throat area of 2.58 subtracting the valve stem area. The curtain area equals the throat area at .406" lift . this put the L/D at about .20 does it not. Is this correct, is this OK. I am no expert just a guy that does most of his own work trying to learn more.

[Tom Walker]

As far as the terminology, as has been said, I believe percentage of area is more an all encompassing engineering terminology, while valve diameter percentage communicates a quick rule of thumb for a starting point.
Both can be useful, but It does help to accurately communicate which method we are using at the moment.

[David Redszus]

Is there a correlation between throat area and valve curtain area being to large. As an example I have a set of heads that has a 1.84 throat with a 2.02 valve 91% [that how they came OTB] that gives me a throat area of 2.58 subtracting the valve stem area. The curtain area equals the throat area at .406" lift . this put the L/D at about .20 does it not. Is this correct, is this OK. I am no expert just a guy that does most of his own work trying to learn more.

During the process of evaluating hardware, there are a few points to observe and consider.
Port and throat diameter/area are fixed dimensions. The valve size is also fixed but lift is not.

As lift is increased, so is valve curtain area. Our real consideration should be the ratio of
valve curtain area to port area. Even this is subject to adjustment due to the effects of Cd.

How much is too much? How much is too little?
That would depend on engine displacement and rpm which determines maximum flow.
Everything else merely reduces flow.

[steve cowan]

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.

I was under the impression that air does not like turning over sharp angles more than 15 degrees?
That is a statement from Darin Morgan

Think about it logically, and visualize it. Air turns easier and less, with say a 38 or 40 degrees top cut vs say the "classic" 30 degree. Draw the angles. 38 top, 45 seat, 65 bottom, 75 transition angle. Now draw 30 top, 45 seat, 60 bottom and 70 transition angle and tell me which one do you think turns easier around the given angles.

I get what you are saying, it was just a reference I heard a few years ago,my opinion is if a person like Darin Morgan says a statement I try to listen.
On my sbc gen 1 stuff I do at home using stones,as I don't own a valve and guide machine I use angles of -
40 top
50 seat
60,70,82 undercuts to throat,most as cast sbc heads already have a throat of at least 88 % - 89 % so it doesn't leave any room to offset throat without making it bigger .
One set of SHP heads I had to add 2 top cut 35 - 40 and then 50 seat .

[SoCalDave]

Thorat percentage is used to set the opening. You use the throat as your venturi to promote flow of air fuel mixture/exhaust gas flow so it's one important part of making heads flow efficiently to make power.

[Wetflow]

No I don't.. I use a 40, 45, or 48 top with my 50 degree.

What is the minimum change in angle that will still shear fuel? From a 50 degree seat to a 48 top doesn't seem like much.

[steve cowan]

I don’t see how you can determine a throat size without considering the size of the port. The throat percentage rule is an estimated maximum, not necessarily a target.

The problem I see is you can buy as cast cylinder heads from known brand with a 2.02 intake valve as an example
The intake runner lengths are very similar let's say 5.40" - 5.45" for a 23 deg sbc head,they will be advertised as a 180cc , 190cc,200cc 210cc with a 2.02" - 1.6" valve combination.
The difference will be the MCSA at the pushrod pinch, and average CSA through the port to the seat ring.
The valve,seat ring,throat will be close to the same in most of these heads.
For the person not porting they go the bigger as cast size head and the person who wants ported heads they start with a smaller casting.
I am not in disagreement with you but when you buy heads bare or assembled the valve job/ throat is set by the factory whether we like it or not.

[SpeierRacingHeads]

No I don't.. I use a 40, 45, or 48 top with my 50 degree.

What is the minimum change in angle that will still shear fuel? From a 50 degree seat to a 48 top doesn't seem like much.

The truth is, the top cut of any cutter should be what the chamber allows.

I also like long top cuts. All my cutters are a min of .100...

[KnightEngines]

No I don't.. I use a 40, 45, or 48 top with my 50 degree.

What is the minimum change in angle that will still shear fuel? From a 50 degree seat to a 48 top doesn't seem like much.

The truth is, the top cut of any cutter should be what the chamber allows.

I also like long top cuts. All my cutters are a min of .100...

100%, I'll sink the valves to get a long top cut, the only time I won't is when the short turn is already very low.

[Mummert]

The top angle seems to be most critical to fuel momentum, i.e. fighting seperation, I think the first 2 angles the fuel encounters from the port are most critical to to fuel shear.