How to determine best throat size?

[travis]

Is there any reasonably easy or rule of thumb for determining the right throat size to use in an application? What variables would you use?

I know this is a vague question but after reading through so many threads on the subject, I’m more confused than ever

[mag2555]

Since your choosing to be elusive by not even posting up what head you want to talk about here all I/ we can do in my opinion is post up minimums and maximums for you, and as you might surmise there are a ton of conciderations that go into what might work best for a given combo/ motor!

1) 85% of the valve size is where you will just start to get into the range of making good use of flow capabilitys of that valve size.

For Intake ports with 45 degree seats I like no more then 90% as a maximum.
For those Intake ports with steeper then 45 degree seats then 91% is the number.

2) Exh ports 92% is the dead nuts maximum, and even for you better be talking about a all out race motor with over 13 to 1 compression!

[travis]

I’m just speaking in generalities…nothing elusive about it. But, if you want specifics, here’s 2 different examples I’m working on.

1) Ford E5AE smog heads on a 351w, it has 1.78/1.45 valves, and doesn’t flow very well (177/134@.450” no pipe). I’ve done a bit more porting and am upgrading to a bigger intake valve. It’s currently at a 87% intake/89% exhaust throat. Heavy truck application, small 208@.050 cam, 9.0 compression, idle to 4500 rpm range.

2) Dart 180 iron eagle heads, 1.94/1.60 valves, on a 351w in a ‘85 mustang. 351w, 9.9 compression, 268 voodoo hydraulic cam. The owner hurt the engine after yet another trans failed. It ran constant 12.50’s…he wants to get it in the mid 11’s. Looking at going with a 2.02 intake valve, some porting, a bit more compression, going from 3.55/5 speed to 4.10’s/C-4 combo, and a solid roller cam.

I guess I really don’t understand how the throat size affects everything. Is it as simple as a low rpm truck application may work better on the lower end of the scale (say 86-87%), while something expected to perform at higher rpm’s wants a bigger throat? What happens if you do nothing other than increase the throat size for either of the combo’s listed above? Is the difference even significant?

I recently threw away a set of heavily bowl hogged iron heads that flowed pretty damn good for what they was, but the intake throats was around 94-95%…both my machine shop and members here said these things would be a dog in the car. Why would they suck so bad?

[travis]

Found this on another post from 2008…

Int throat to small, would equate to an obviouse lack of breathing and limit top end HP.
Int throat to big, would no doubt cause a slow lazy port that would have a very mild Tq curve, lack of throttle responce and local velocities all over the place.

Too large exh throat, would equate to severe loss in top end power/TQ.
Too small exh, would create pumping losses, therefore rob HP/TQ being a major restriction. I must say though.... i'd rather have a smaller super fast exh port than one to large.

[PRH]

I recently threw away a set of heavily bowl hogged iron heads that flowed pretty damn good for what they was, but the intake throats was around 94-95%…both my machine shop and members here said these things would be a dog in the car.

The best way to learn what works and doesn’t is to do it first hand.

You had heads with flow numbers you were happy with, but had some areas of the port outside the accepted parameters for good performance.
That was a missed opportunity for a teaching moment.

If you would have used the heads on something, you would have experienced the results of that configuration first hand.
They might have been fine.

Imo, Its going to be pretty hard to be taught what shapes work and what shapes don’t by gathering info from the net. Especially when you’re at the novice level.

One of those shop vac flow bench kits(or a used flow bench) is probably the best money you could spend at this point to further the effectiveness of your porting efforts.

[mag2555]

You also have to concider the simple machanics of things too!

Once you get above 92% and especially once your up at 94% or more you not getting a freshen up valve job into the head if something like a valve head comes off or a piston comes apart without installing a seat or yanking and then installing a replacement seat.

This also means you may not get a multi angle valve job put back in place the same which then throws off the flow balance from the other cylinders.

[PRH]

To put another perspective on it.......

Travis....... didn’t you build something fairly recently with some aftermarket heads on it?
Went into a pick up truck I think, and I seem to recall you saying it was worse at doing “truck type stuff” than the stock motor was.

Did you measure the throat diameter on those new heads?
Where did it come in at?

It’s hard for me to imagine those new heads came ootb with a throat diameter that was totally out to lunch(although it’s not impossible).

[travis]

I threw those bowl hogged D0OE heads away because there was nothing left for a proper valve job. There would have been no bottom cut at all, and the machine shop said that what was left of the seat angle wouldn’t have lasted any length of time because they was just too narrow. Otherwise, I would have ran them even with the too big throats.

I don’t remember what the throats measured on the aluminum heads, but it was nothing out of the ordinary…seems like they was 89-90% but I didn’t write it down apparently. I will try to remember to measure them when I pull them back off here in the next month or 2. The lack of torque is being addressed…with about 50 more cubes.

[steve cowan]

I recently threw away a set of heavily bowl hogged iron heads that flowed pretty damn good for what they was, but the intake throats was around 94-95%…both my machine shop and members here said these things would be a dog in the car.

The best way to learn what works and doesn’t is to do it first hand.

You had heads with flow numbers you were happy with, but had some areas of the port outside the accepted parameters for good performance.
That was a missed opportunity for a teaching moment.

If you would have used the heads on something, you would have experienced the results of that configuration first hand.
They might have been fine.

Imo, Its going to be pretty hard to be taught what shapes work and what shapes don’t by gathering info from the net. Especially when you’re at the novice level.

One of those shop vac flow bench kits(or a used flow bench) is probably the best money you could spend at this point to further the effectiveness of your porting efforts.

I like how PRH says it's the shape and not a percentage,I personally think percentage of anything can be misleading and is only a good starting reference.
I know a couple of guys on this forum who use steeper than 45deg seats and under 90% throat and there engines run really good.
I start off with a exhaust flange CSA / % of throat depending on application/ rpm.
I have found on most SBC heads I own have the intake throat a bit bigger than I want as cast which does not leave a lot of room to work.
One thing to consider as well is your bottom cuts on your valve job and where the throat size could end up.
I look at good cfm/in2 at the throat but you have to be mindful of chasing cfm.
Making a fixture on the flow bench to mount port side down and put bench on Exhaust mode and pitot around the valve/ throat area is one area I want to look at,just need to make the fixture.

[PRH]

I don’t remember what the throats measured on the aluminum heads, but it was nothing out of the ordinary…seems like they was 89-90%

So, even with the throat diameter well within “normal parameters”, the engine performance is still not up to snuff.
Pointing out that the sum of the parts(the combination) is more important than getting a few aspects correct, and a few others maybe not so correct.

In my view “correct” is whatever is appropriate for the task at hand.

When it comes to factory heads, especially when used in the milder applications (say, .85-1.05 hp/ci), I don’t try and make the head into something it’s not.
I’m a big fan of trying to get the most gain for the least work....... and I’m not expecting miracles.
When using non-HP heads that come with small valves........ it’s usually beneficial to go to a slightly bigger valve. In many cases I’ll do this just so I can end up with a longer bottom angle on the valve job....... especially on those heads that have no, or almost no bottom angle between the seat and the factory bowl cut.
When the cam will be pretty low lift(under .450-.500), I often wont increase the bowl diameter at all(after machining for larger valves)...... but it really depends on the particular heads you’re working with.
The larger diameter valve gives you a larger curtain area, and that along with the longer bottom angle can make a nice improvement in the .200-.400 flow range, with basically zero extra port volume.
I keep the SSR fairly steep, and put just enough of a radius on it to let the flow hang on to as close to peak lift as possible.
For this type of mild rework, laying the SSR back excessively will often result in a flow loss in the .100 to .300-.350.
For this type of build it’s rarely worth it to chase the high lift flow.

[David Redszus]

Is there any reasonably easy or rule of thumb for determining the right throat size to use in an application? What variables would you use?

I know this is a vague question but after reading through so many threads on the subject, I’m more confused than ever

Throat size is dependent upon several variables: valve head diameter, cam lift, rpm, and displacement.

If the valve curtain area is smaller than the throat, it will remain as the choke point, not the throat.
As long as the valve curtain area is greater than the throat area, the throat section will become the choke point.

This analysis is based only on comparative area ratios, i.e. static ratios.

In a running engine, piston air demand becomes a factor. As the rpm increase the valve curtain becomes the
choke. If we increase the valve curtain area (more valve lift) to accommodate increased air flow, at some rpm
the throat will become the choke point.

After a few hundred calculations, a trend becomes apparent.
At low engine rpm (under 4000 rpm), a throat diameter that is 70% (.490 area) of the valve diameter is close to optimum.
But at higher engine rpm (8000 rpm), the throat diameter must be increased to 80-85% (.640-.723 area) to avoid
port choke. At higher rpm, an even larger throat would be beneficial.

A throat that is either too large or too small will cause a reduction in air flow.
But, those values are based on valve diameter; they should include valve lift in order to obtain the valve curtain area.

Another point that needs clarification is whether we refer to diameter ratios or area ratios.

Valve = 2.0, Port = 1.8, diam ratio = 90%, area ratio = 81%.

Certainly, some important variables are absent, such as flow CD as a function of lift and pressure ratio.
It is not easy to write a single equation that will consider the applicable variables although a spreadsheet can be
constructed that will provide usable insight for a specific application.

[travis]

So which would be the preferable choke point…the throat, or the curtain area?

Would it be preferable to have the throat area and the curtain area about equal at a certain lift point, and what would that lift point be? Max valve lift? Lift at 70-75 degrees ATDC? I haven’t done the math yet to see if this is even realistic.

I understand that this is just a small part of the bigger picture. And on a sub 300hp 351” engine, it may not matter enough to even think about.

I haven’t tested it on a flow bench, but I would assume that as long as the throat is the choke point, increasing the throat size from say 87% to 89% to 91% would show increased flow numbers…at least until some other part of the port becomes the restriction. I guess this is why finding the MCSA is important, but then that makes me think…where is the preferred point to have your port choke???

The more you learn, the more questions it raises…

[steve cowan]

Throat as the minimum area is desired but depends on application.
Some say the pushrod pinch is just for packaging, some say it's for better low speed driveabilty as in keeping airspeed up at lower rpm,but there is a trade off.
Going Back to CSA.
work out your intake valve area's - 1/2 DIAMETER^2 xPI = AREA - valve stem area 11/32" = 0.09" area as an example.
2.02" valve -
2.02 ÷2 =1.01
1.01 × 1.01 = 1.02 x 3.1416 = 3.20"
3.20" - 0.09" = 3.11" area of 2.02" valve.
Now 90% throat using a 2.02" valve
2.02" x 90% = 1.81"
1.81 ÷ 2 = 0.909"
0.909 x 0.909 = 0.826
0.826 x 3.1416 = 2.59"
2.59" - 0.09" = 2.50"
So 2.50 " is the throat area @ 90%
If we use a 23 deg SBC head as an example
They normally around 5.40" long give or take.
Using 2.50" as the minimum makes for a large port if you do the maths.
Port vol in CC
MIN CSA X PORT LENGTH X 16.387
2.50" x 5.45" x 16.387 = 223 cc
If you had a 383 Sbc that you want to peak around 6500rpm. 223cc is probably to big .
This is just tip of the ice berg and a starting point with a lot to consider.

[Momus]

I work on small single cylinder 2 valve Honda bike engines.

One highly developed inlet port on a Japanese aftermarket head has an 83% or so throat to valve ratio.

This is an engine that is making over 150 hp per litre with good driveability.

[travis]

Steve, I’ll ponder on that more once I’ve had some sleep. Using this calculator http://www.wallaceracing.com/ca-calc.php it says for a 4.03”x3.5” bore and stroke, for 5000 rpms needs a MCSA of 1.63”. Using an average port length of 5.25”, it comes up with a 140cc port…which sounds pretty realistic. But…the heads I’m working on right now have a 1.78 intake valve, and a throat of 1.63” diameter, which is about 2.08^2”. I’ll circle back to this later.

But in the meantime, another question. These heads I’m working on have a 1.78” intake valve, and a 1.63” throat. This comes out to about 91.5%. Now if I move up to a 1.84” valve, and leave the throat at 1.63”, this comes out to 88.5%…theoretically a good place to be. What difference would this make in the way the engine runs? Would it make any difference, since the choke size is still the same?