Throat/PR pinch sizing

[travis]

Sorry for the noob questions...just trying to learn here.

This is a quote from a well respected head porter in the Mustang world. Your thoughts?

“You do not want the throat too big in relation to the rest of the bowl. Bowl hogs usually do this. You want the same or slightly larger cross sectional area at the pushrod restriction as the throat area. Over the short side will be even larger. Low lift cams (.550 and below) will not want the runner ground with equal cross sections at the runner throat whereas cams with high lift will. Smaller lift cams will want to be smaller in section to keep velocity up since the lift is short and the valve is not moving as much air. Basically, with high valve lift, the pushrod area can become a choke point whereas with low lift it usually won’t, unless it is extremely small.”

I’ll throw some measurements and more questions out there once I got some opinions on this statement.

If it matters, I’m working on several different under 6500 rpm street applications...not referring to any race type builds.

[RevTheory]

I've bumped into the same conundrum so I'll be following along. Great topic.

[David Vizard]

Sorry for the noob questions...just trying to learn here.

This is a quote from a well respected head porter in the Mustang world. Your thoughts?

“You do not want the throat too big in relation to the rest of the bowl. Bowl hogs usually do this. You want the same or slightly larger cross sectional area at the pushrod restriction as the throat area. Over the short side will be even larger. Low lift cams (.550 and below) will not want the runner ground with equal cross sections at the runner throat whereas cams with high lift will. Smaller lift cams will want to be smaller in section to keep velocity up since the lift is short and the valve is not moving as much air. Basically, with high valve lift, the push-rod area can become a choke point whereas with low lift it usually won’t, unless it is extremely small.”

I’ll throw some measurements and more questions out there once I got some opinions on this statement.

If it matters, I’m working on several different under 6500 rpm street applications...not referring to any race type builds.

for me to comment on this I would want to see some comprehensive flow data plus some dimensions and a port mold. If I am understanding this correctly this is a shade out of step with what i do.That does not mean it does not work (the dyno will make have decision) but it is not quite what I do.

DV

[travis]

Sorry for the noob questions...just trying to learn here.

This is a quote from a well respected head porter in the Mustang world. Your thoughts?

“You do not want the throat too big in relation to the rest of the bowl. Bowl hogs usually do this. You want the same or slightly larger cross sectional area at the pushrod restriction as the throat area. Over the short side will be even larger. Low lift cams (.550 and below) will not want the runner ground with equal cross sections at the runner throat whereas cams with high lift will. Smaller lift cams will want to be smaller in section to keep velocity up since the lift is short and the valve is not moving as much air. Basically, with high valve lift, the push-rod area can become a choke point whereas with low lift it usually won’t, unless it is extremely small.”

I’ll throw some measurements and more questions out there once I got some opinions on this statement.

If it matters, I’m working on several different under 6500 rpm street applications...not referring to any race type builds.

for me to comment on this I would want to see some comprehensive flow data plus some dimensions and a port mold. If I am understanding this correctly this is a shade out of step with what i do.That does not mean it does not work (the dyno will make have decision) but it is not quite what I do.

DV

Unfortunately I don’t have access to a flow bench...I sure wish I did before I dive head first into this.

1 set of heads I have are ‘70 351w castings that someone has done some work on at some point. They have 1.94”/1.50” valves, and probably the worst looking bowls and short turns I’ve ever seen (nice radius over the short turn and then drop straight off, like someone did a plunge cut deep into the bowls). I haven’t measured the throats yet, but the PR pinch on the intake ports is 1.92”x1.16”.

I guess part of what I’m trying to figure out is where to start. The PR pinch certainly isn’t the mcsa on these. Do I want to open the throat area up to close to what the pushrod pinch is?

[GARY C]

Good luck getting anyone to give you a real concrete answer, I discovered that on a thread I did not long ago... I have been told everything from "It's the most critical" to "It doen't really matter" but 0 actual test data of one vs the other.

I think this is another online discussion where everyone repeats what they have been told but no one has actually tested it to any extent.

Here is my thread if you want to read through. https://www.speed-talk.com/forum/viewtopic.ph ... 2&start=15

[modok]

The "throat" as I understand it, is basically the bottom edge of the valve job.
If you have a 35/45/60/70 valve job, the throat is the diameter at the bottom of the 70 cut...or, um, somewhere near there anyway.
So the throat size basically means how wide your valve angles are, and that's it.

what it does just upstream of that, is not necessarily concentric to the valve, so it's not easy to measure and describe.

If you have bowl hogged out FAR thus actually have a 90 degree angle (straight bore0 under the 70, then that's easy to measure,......but rarely that's what you want.
Although, sometimes you do want that, BUT, farther upstream what is the diameter there? that's the key, is the diameters at what distance.
how exactly do we describe that? not well.

When lift is in the range of .25 valve size, and you look at that in cross-section, you see the air hits the back of the valve and makes a 50-60 degree turn.
when air goes through a turn.... the outside path is long, the inside path is short.

One might think going into the turn slow is the best way.....no. Even greater efficiency can be had if you have the center of the flow fast, because it takes the outside path of the turn, and slow down the air by the seat angles, because they take the short path.
So some area just up stream of the valve can be small to speed up the flow, then from there to the back of the valve the diameter expands to slow down the air around the outside as much as possible by the time it hits the valve seat angles.
What that ends up looking like can take several different forms, and is hard to describe. Since the chamber shrouding is not equal all the way around the valve, the bowl does not want to be equal either. Different sides of the valve flow different amounts so you take advantage of that.
so, I can see how using a bowl hog to do the whole job... is not smart. But you could have the bowl hog do part of the job, and it would be fine. i don't know what they were doing.

[travis]

I honestly can’t make heads or tails of what someone did or tried to accomplish with these heads in their past life. The guides, valves, and seats all look/feel pretty good in my amateur opinion, but the bowl/throat area looks like some combination of rough factory cuts and some ham fisted machine work. The spring/retainer combo was clearly SBC parts...1.265” diameter springs and retainers that was walking around in the 1.46” spring pockets with no locators and trashed the spring shims. The valves are SBC sized as well...1.94/1.50 undercut stem stainless pieces. The guides are cut for .500” seals and have been trimmed down considerably. With a comp 740 retainer and 611 lock, there is easily 1” of retainer to guide clearance.

I’ll try to get some measurements and post some pics at some point today.

I need a flow bench and a lot more free time...neither of which is happening any time soon.

[mag2555]

Depending on where you want the motor to make peak TQ and HP there are a few places where you might want the minimum port area to to be for best performance in terms of a Intake port of overall runner lenght.

For example the Ford FE motor has a very short intake port runner in the head as compared to a Windsor motor.

As for me I have never seen a need in terms of average flow wise for a valve to Throat percentage of greater then 90%, especially when a head has minimal short turn height to begin with!

In a build as you state ( non race) I would want the velocity choke / pinch point ( velocity not flow!) to be one stroke lenght back up the runner from the center of the back side of the valve

Note also that to make the most out of any complete Intake pathway there should only be one choke point and this can be hard to do with all the curves in a duel plane type Intake.
Man I use to love the true street type signal plane intakes that use to be made that would work on 327 cid motors in that regard!
Also not that to turn high velocity air any more then 15 degrees you will in short order find that with a flow bench you will need to slow it down some , as in expanding the area that good chunk of the air passing thru/ over.
This is also another reason to have a valve to Throat percentage that give the air a ever expanding area to pass thru as the valve opens.

Also note many times as David Visard has shown to us many times and I have proved to myself once I got my flow bench the fact that in any cylinder head the flow rate up to some .350" lift in some heads is totally controlled by the shape of the valve bowl and the valve job , and not so much by the thrust size!

This line of thinking will lead you to the conclusion that if your running a Cam of only .480" to .500" lift which is a max of 30% more then .350" lift some other changes will need to be made to make the best use of that added 30% in lift.

For example if you where running a 1.78" valve with a stock Throat percentage of let's say 80% , then to make the best use of a added 30% gain in Throat size you would need to step up to a bigger size valve .

Many times in street / limited strip use motor it's advantageous to step up to a bigger valve and then just go 10 to 15 percent greater in Throat size over stock, and yes this assumes that any added shrouding can be dealt with.

This decision will also hinge on how much meat the rest of the port has to be reworked into the needed port area from the pinch point in the Throat on back to the flange.

In general it's easy to confirm the port circumference need to have close to the needed port area.

For example a 1.65" Throat when times by 3.14 gives a ball park circumference of 5.181" minus the corner radiuses which in a Ford iron head like yours are like a 1/4" radius so that's is .050" more in circumference you would need to add in to that 5.181" dimension to be in the ball park for the needed port area.

Many times when reworking these Ford iron heads to max out there flow with the usage of bigger valve even considering there good valve inclination angle you will find the need to make the floor over the short turn a little wider then the roof path as in a trapezoid shape so that you do not loose control of the flow coming over the short turn .

This can start to be seen when the high lift flow numbers start to regress instead of just reach a peak.

[travis]

From what I have been reading it looks like you want the throat on the intake side to be 90% max, and about 85-86% on the exhaust.

So what happens as the throat size moves up, from say 85% up to 90%? Do you lose low lift flow or velocity as the throat gets bigger? Does the higher lift flow get better?

It *looks* like low lift flow (say under .25” lift) would be more affected by shrouding in the chamber than anything. That and the valve job/seat angles.

With a small port/valve, I know DV preaches tighter lsa’s. Would it also be more advantageous to put the cam in more advanced as well, to help get the most possible from the small heads? It’s obviously not going to make big top end power anyway but it seems like you would have a better street power curve this way.

[mag2555]

It's very simple !
1) there is no guarantee that opening up the Throat will not effect the low lift flow, as most times it will produce at some lift point a drop off in flow, and without a flow bench there's no way to determine that.

2) if the added Throat size does not pick up flow by the peak lift range you will be running, or if the amount of high lift flow gain is not enough to off set a lower lift flow loss then you will have inturn dropped port velocity by the same percentage as your change in Throat size / port area.

To maintain the stock amount or better throttle responce you want your gain in air flow seen in your Cams lift range to atleast equal the percentage of increase in your Throat size.
So a 20% increase in Throat size should be met with a 20% overall gain in air flow in your Cams lift range.

Of course if your rebuild will kick up the motors CID for example by 5% then you can tolerate a port velocity reduction of that same 5% and still maintain a stock level of throttle responce.

On the other side of this velocity coin just let be known that many of us have made more peak Hp out of a full race motor by cutting peak air flow levels which in turn brings the peak port velocity down to a level more suited to the rpm range the heads on the motor should be working in terms of port area.

[RevTheory]

I've got some notes here I jotted down on a sticky-pad using an online area calculator a while back that I'll offer up. They all use an 8mm valve stem and 89% throat diameter to arrive at actual throat area.

2.08 valve = 3.397 area
8mm stem = .078 area
89% = 1.85 diameter = 2.688 area
2.688 - .078 = 2.6 actual throat area

That's how I got there. This is layed out so I can be corrected if need be. So using the same stem and throat percentage diameter to get total area:

2.08 valve = 2.6 throat area
2.055 valve = 2.55 throat area
2.02 valve = 2.46 throat area
1.94 valve = 2.26 throat area

If you were doing a hot-street 350, the required mcsa would probably be somewhere around 1.95 which isn't even close to the throat area of a 1.94 valve, 89% throat diameter with an 8mm valve stem, unless I'm completely fubared in the way I did it.

How does this jive with Darin Morgan saying that you never want the pinch smaller than the throat? Granted, I don't think Reher-Morrison messes around too much in our world.

[SchmidtMotorWorks]

With regards to intentionally reducing CSA at a pinch point upstream from the valve...

Can anyone think of a well developed, designed for high performance, overhead cam engine that has a pinch designed into the port?

[RevTheory]

We have to play from the hand we're dealt. Most of us have a pushrod pinch point and need to find some sort of balance between that pinch, local and average velocities and the throat area for a given application. We also have to get around a turn that really wants to separate unless velocity before it is slowed and is reluctant to give an improvement here without taking it from there.

[SchmidtMotorWorks]

We have to play from the hand we're dealt. Most of us have a pushrod pinch point and need to find some sort of balance between that pinch, local and average velocities and the throat area for a given application. We also have to get around a turn that really wants to separate unless velocity before it is slowed and is reluctant to give an improvement here without taking it from there.

My question is in response to anyone that thinks a pinch point is desirable.

[Stan Weiss]

I have seen both sleeved push rod holes and welding to move push rods to increase / eliminate the pinch. The only problem I have seen is in a few case where this could not be done due to what the new push rod angularity would be or a crazy offset rocker arm that would be needed.

Stan

PS Also a roller lifter can help push rod angularity over a flt tappet since the push rod does not need to be centered.