Porting School #10 - Pushrod Pinch Point Power Issues

[DavidVizard-GFN]

Here it is –

- the subject every beginner needs to know before they start porting and every Pro needs to know for the frosting on the cake.

Pushrod Pinch Point Power Issues

Problems and Solutions

By

David Vizard

We will start by defining the pinch point of a typical small block Chevy intake port. First I am using the Chevy as a prime example of a pinch point simply because it is by far the worlds most modified engine and therefore represents a majority case study. But what we learn here will also apply to a greater or lesser degree, to those heads that closely resemble the small block Chevy heads. The Chrysler 318 – 340 - 360 range of engines comes to mind here but these are not the only ones. If the engine you are working on has a port that is Chevy like in appearance then much of what we are going to deal with here applies – but first let’s define the pushrod pinch point. Below we see an illustration that does just that.

The top and to the right arrow above shows how the port wall is pushed inward by the need to give space to the pushrod slot adjacent. The initial response to this squeezing in of the port is that it must be a prime restriction to flow. The reality here is that although it ultimately can become a restriction is not the power robbing element that it may first seem. Also it’s elimination can actually cause the power curve to suffer even though the flow may have increased. All this points toward the fact that when it comes to cylinder heads for power all is not what it may at first seem!

Pinch Point Efficiency.

The prime reason the pinch point is attacked in the first place is to free up what appears to be dormant flow potential at what seems to be a restrictive part of the port. The logic here is that this move will free up flow so that the rest of the port downstream has access to more air. Sounds like an obvious positive move but when we investigate further it proves to be far less positive than might be initially expected. Here’s why. If we look at the pinch point in isolation (as shown below) it looks a lot like a venturi.

From this illustration it can be seen that the ‘Pinch Point’ at the pushrod location closely mimics a venturi. In isolation it will also flow like a venturi – i.e. really well. In practice it’s flow characteristic would also closely follow what you would expect of a venturi if were not for the complex port shape down stream of it.

Unfortunately as much as it simplifies how we visualize it the pinch point’s is strongly influenced by the rest of the port down stream of it. This means that it is not quite a simple an issue to deal with as might be initially supposed. In other words there is more to it than just enlarging it or even eliminating it.


An important aspect to appreciate here is that the flow is not uniform across the port at the pinch point. The illustration below shows a somewhat simplified velocity map of a typical ‘as cast’ production small block Chevy head.

Note how most of the air is flowing in the top right had corner – that, on this handed port is thew cylinder wall side of the port. The adjacent port would be the mirror image of this. The difference in velocity between the red area and the yellow is about a 3 to 1 ratio. This means if the fastest is moving at 300 ft/sec the slowest is moving at 100 ft/sec. By the same token it also means that the kinetic energy per unit area of the red section is, since it follows a square law, a whole 9 times higher than the yellow section. The form of this velocity map brings about some porting aspects that we need to give plenty of thought to. An overriding one is that if the port is enlarged in an area where activity is low then the effect toward increasing airflow will also be low. This means we need to devote attention to high velocity area’s and establish why they are flowing at such a high velocity and ask ourselves if this high velocity makes the area within which they occur a prime point to enlarge. Let’s look at our options here.

On the left is our basic pinch point velocity map. To the right of that are some of the options we can apply to the pinch point and the area’s immediately up and down stream of the pinch point.

In the above diagram port #1 is about average for what we see in a stock casting. Whether intentional or not (probably not) the ports are, in most instances, a little wider at the top than at the bottom. This is OK as the top of the port is far busier than the bottom. What this means is that although section #2 may show a benefit in flow any extras flow achieved was mostly from the widening of the top half of the port.

If we know that the airflow is predominantly toward the top of the port we should ask ourselves if port #3 might not have been a better way to go. Why? Because a greater part of the port would be flowing at high speed so not only would the flow have gone up but also the average velocity. On top of that the kinetic energy the air has would have gone up. This would have done so in proportion to the flow increase and as a square function of any velocity increase.

There is also another factor here. Fuel drop out always congregates on the floor of the port. Generally, as the velocity slows so it becomes easier for the fuel to drop out onto the floor of the port. Here we see a prime example of applying porting rule #2 (from porting School #9) That is: ‘Try, as far as possible, to let the air move where it wants to go, not the way you think it ought to! If we work the area where the port is most active we are more likely by far to get better results.

Let’s go one step further with efforts to rework the busy part of the port to the fullest extent allowed by the casting. Usually the thickness of the dividing wall between the two adjacent ports is 3/16th to as much as ¼ inch. There is no reason why this needs to be as wide as this so why not enlarge the top of the port along the lines of #4 in the illustration above. And guess what – it works!

I mentioned fuel drop-out a moment ago and that brings me to a wet flow situation that is worth addressing. Since the airflow along the floor of the port is so much slower than the top the wet flow along the floor is difficult to reintroduce into the air stream. Take a look at port #5 in the above illustration and note the red port ‘fill-in’ area. In this instance the floor and the sides have been filled to cut the less functional area of the port down smaller so as to increase the velocity here. Now this move normally makes a marginal reduction id flow from the mid point up but it works wonders for the wet flow situation. The red section is not streamlined on the approach but is a vertical wall. In other words it looks like a perfect manifold miss-match in as much as it is a step. What this does is act as a dam preventing any floor running fuel stream from entering the head port without any impediment. When the fuel reaches this ‘dam’ it builds up to the lip and is sheared off back into the airstream.

So does the floor located fuel dam work? Yes -- but you need to understand the circumstances under which it works best and this is partly tied in with the size and effectiveness of the short side turn. The higher the short side turn is the better the approach there is to the seat. This in turn tends to make the bottom of the port at the pinch point a more active area. This leads to less of a problem with wet fuel drop out. If we add to this a good carb that has consistently even and suitably fine fuel atomization and an intake manifold that has good fuel support and distribution the need for the port fuel dam is diminished. But that said I have sometimes seen power gains with highly functional CNC heads when an intake manifold mismatch that forms a step at the bottom of the port (about 0.060 inches and the manifold bigger than the port in the head) actually delivers better results everywhere.

What’s Most Important – Approach or Departure?

One aspect that is worth highlighting is the form of the approach too the pinch point and the form of the departure. In terms of flow getting the form on the port wall down stream of the pinch point is more important than the approach. This is sort of bad news if you are porting your own heads as it is relatively easy to put a nice streamlined form on the approach side but far harder, due to access limitations, to do so on the down stream (departure) side of the pinch point. Just because you cannot see it, don’t short change it. Apply as large a radius as possible, consistent with retention of casting integrity, to blend the pinch point into the port wall down stream. If you have a swirl meter you can expect to see diligence here rewarded with more flow but slightly less swirl, This will be more than made up for by appropriate work on the cylinder wall side of the port from the pinch point right down to the bowl and around the guide boss. Just how all that is done will be covered in a PS feature in the near future.

Eliminating the Pinch Point

Way back I decided that it looked like a good idea to eliminate the pinch point. I had a set of Brodix head castings that I intended to rework to the max so here was my chance. Some offset rockers would be needed to do this as would a set of lifters with the pushrod pickup point offset. The heads were duly ported and the absence of the pinch point bumped top end flow – but not by that much. Still every bit counts. On the dyno the results were pretty indifferent to say the least. Power at about 7000 was up and at 7800 there was an advantage of about 7 hp over the best I had done to date. But below about 6000 rpm the engine was off by some 5-7 lbs-ft, In all a little disappointing. So what was different here? When I cut the pinch point out entirely I noticed a measurable drop in swirl but had hoped that it’s possible negative effect would be below the rpm the engine would run at while racing. Obviously that was not the case. The pinch point is seems, is an effective form to push the incoming air onto the cylinder wall side of the port. That’s the area that needs to be busiest to generate the best swirl. This taught me one thing for sure – eliminating the pinch point in an effort to maximize high lift flow is not actually a good way to go. If you are hopping up a true street motor that really should have a wide and strong torque curve the presence of a reasonable pinch point is actually an advantage.

Although the pinch point might just be a good source of swirl we still would like to maximize flow so is there a way to do this without eliminating the pinch point. The answer here is there is not an easy 100% fix but there is a simple half way solution toward generating a little more functional port area at the pinch point. Take a look at the illustration below.

What you see here is the cylinder side port wall depressed opposite the bulge of the pinch point. As drawn here I have much exaggerated the amount of the depression on the port wall at this spot. In reality you can only depress this wall about 0.030 inches and then only in the top half or so of the port. Any more than that and it seems the flow gains reverse to losses. The good part about this is that the swirl usually goes up a little as well as the flow. A one time dyno test on an extensively ported set of aftermarket iron heads showed, with a before and after test, gains from just before peak torque on up. The gains were not huge but 519 hp is better than 514 – especially when it comes with no downside.

Physically Optimizing the Pinch Point

What you see here is a cross section through a highly functional aftermarket iron performance casting. There are two factors to note. First the position of the pinch point and secondly the large approach radius on the short side turn. That big radius is instrumental toward making the lower half of the pinch point a more flow active area. On a head casting like this making the pinch point as effective as possible becomes more of an important issue – especially if the intake valve sizes are maximized. The question here is how to do this quickly and effectively.

Let me tell you how I used to rework the pushrod side port wall at the pinch point. I would painstakingly mark out the limiting width on the manifold face and using two small squares continually check to see what material was left. At best it was slow especially as the limit was approached. I have to admit even with years of porting experience there were more than the odd few occasions when I went through into the pushrod void and had to weld up the mistake. Not good – especially for the less experienced porter. The answer here is the ‘E’ Tool from Dr J's Performance, 436 Montgomery Street, Orange CA 92868 Tel: 714-943-3404 Dr J's Performance - Dr. J's Performance - Home.

Here is what the ‘E’ Tool looks like and here is -----------------------

------------ how you use it. When located as shown here the gap at the arrow is twice the width of the thickness remaining between the vertical bar in the pushrod void and the horizontal bar in the port. It’s fast and simple. In most instances you don’t even have to completely remove the ‘E’ Tool from the head to continue working. One you have used this tool you will be reluctant to part with it.

I have been using this tool for the best part of 18 years now and the first time I used it I came to the conclusion that for the job is was the best deal going. Designed by (it’s that guy again) Roger ‘Dr. Air’ Helgesen this tool offers more than convenience and speed. It’s use means the ability to better optimize the form at the pinch point. Not only can the amount of remaining material be super quickly determined but also by moving the tool up and down the port to mark the surface it also shows where the point of minimum width needs to be. This in turn give a clear picture of the point at which the port needs to start to flare out on the downstream side so the biggest radius can be applied here for best flow results. In all, this tool finds CFM fast and saves the embarrassment of welding up miss-ground castings.

What you see above is a typical shape I target for a hi-perf aftermarket head casting when I am also going to port the intake manifold to match. Points to note are as follows. First the width is only maximized at the top of the ports as indicated by the arrows. The maximized width drops down to about the half way mark . At the bottom of the port the pinch point is only given a clean up and blend into the rest of the port up and down stream of it. On the other side of the port – that’s the cylinder wall side – the port is widened right out to the manifold face (the gasket sometimes needs to be trimmed to suit here). As can be seen the form will mean that the port is wider at the top than the bottom. A port modified with these moves is just a part of an overall mod program that results in CFM numbers bigger than normal for the port size involved. The payback is the production of better than average top end power without the often seen loss of low end output.

Here is the pinch point on one of AFR’s entry level budget CNC heads. The light color around the port is from the clay I used to make the port entry radius in case you wondered why it was not shinny and picture perfect. The subject here is the pinch point. If you look you will see that this is a well executed but none-the-less conventional looking parallel wall top to bottom pinch point. Part of the reason that AFR can get this to function the way they want is the large short side turn radius used on the approach to the seat. This allows the bottom of the port to function better so calling for a more nearly parallel pinch point opening. Also take note of the port size. This is a 195 cc port which is often considered by the average hot rodder to be too small for their need to satisfy there power lust. But sound port design has allowed AFR to make a port that makes the best use of the space/volume available. We have seen very good results with these heads for high output, pump fuel, street motors.

David Vizard

Other parts in this series are at:-

#1 Porting School #1 - Why engines need airflow

#2 Porting School #2 - Super Cheap Flow Bench

#3 Porting School #3 Budget Bench Calibration

#4 Porting School #4 - Budget Bench Electronics

#5 Porting School #5 Identifying Primary Restrictions

#6 Porting School #6 - Secrets to reduce valve shrouding

#7 Porting School #7 - Power & Port Volumes

#8 Porting School #8 Optimal Port area's

#9 Porting School #9 - 5 Rules to Goof-Proof Porting!

#10 Porting School #10 - Pushrod Pinch Point Power Issues

In addition to the Porting School articles there are directly related cylinder head development subjects at the following sites:

Wet Flow :-

Six Wet Flow Mistakes

Combustion Dynamics:-

#1:- Turbulence and Combustion Dynamics

#2:- In cylinder Turbulance and Combustion Dynamics

#3:- Turbulance and Combustion Dynamics - Part 3

#4:- Coming soon

#5:- Turbulance and Combustion Dynamics - Crevice Volumes - Stealth Power Thief

[99R/T]

Another great article, thank you David.

A few questions though.

Assuming you raised the port as high as possible, and opened the pinch and opposing wall as far as you could (as in shape #4), but the minimum CSA was still too small, would it be better to continue opening the pinch as in shape #2 to achieve the required CSA, or would it be better to keep the CSA on the small side and keep the velocity higher along the floor?

Is there a rule of thumb regarding the angle/distance/shape of the reverse taper on the pushrod side between the pushrod pinch and the bowl?

And tying these together, if there is a rather steep taper between the pushrod pinch and the bowl, and the minimum CSA required you to open the full length of the pushrod pinch, wouldn't this be the way to go in order to reduce the amount of taper angle in the CSA between the pushrod pinch and the bowl?

Thanks,

John

[DavidVizard-GFN]

99R/T

John,

I am going to make a bet here that this question is one that many other readers have also framed in their mind. It is not an easy one to answer as it means making a judgment call or dyno testing a few configurations to establish. However let's go through this and build on a couple of scenario’s that will illustrate the situation as it relates to the CSA.

Here is your post:

Q. - Assuming you raised the port as high as possible, and opened the pinch and opposing wall as far as you could (as in shape #4), but the minimum CSA was still too small, would it be better to continue opening the pinch as in shape #2 to achieve the required CSA, or would it be better to keep the CSA on the small side and keep the velocity higher along the floor?

A. - To best answer this I need to pose another question. Let us say that there is no (as in zero) air flowing at the bottom of the port. If this was the case then opening up the port to give the CSA you think it wants would achieve what? Answer - pretty close to nothing as nothing much other than the CSA will have changed. But when you calculate the port velocity at which peak power occurs the answer you get will be a lower number than before it was opened up. This little mind experiment will tell us that the CSA is not the be-all and end-all of port sizing.

But that example is an extreme case. If we back away from that situation a little we find that we are in a situation we looked at in PS#7 - namely the effect of port volume on the power curve.

Because most of the V8's we are dealing with will be of a displacement that is way too much for the heads flow capability we find that there will be a time when the engine becomes starved of air to the point that any extra flow, even if it is low grade from a slow moving part of the port, helps the engine make more power. The real issue here is a how effective the CSA is. If there is a big velocity gradient across the port then the peak power intake velocity based solely on the CSA will be much lower than the peak power velocity based on a port with near zero velocity gradient. An example here off the top of my head. The peak power Mach # for a typical pushrod engine with a typical velocity gradient will be about 0.55. However an all out, purpose designed, four valve unit along the lines of an F1 engine will have the peak power Mach # up around 0.65.
The point I am attempting to make here is that the CSA can only be used as an absolute yardstick for determining the peak power rpm (or visa-versa) if the velocity gradient across the port is near zero.

If it is not near zero than the more we try to extract from the engine in terms of maximum output the more the power becomes dependant on total flow even if it's achieved from an area of the port that has a poor return in relation to the area increase. But in following this route we will usually see a substantial degradation of output below peak power. The old ‘Tunnel Port’ Fords of the 70’s were a prime example of poor area utilization and as a result the power curve was best suited to a Bonneville engine.

To sum up the rule here should be to make the most of any high flow area first and then see where you might need to go for any additional flow that you think may be beneficial to output - even if it is low grade. This is where a dyno comes in handy.

Q.- Is there a rule of thumb regarding the angle/distance/shape of the reverse taper on the pushrod side between the pushrod pinch and the bowl?
A.- Best I can offer here is that the departing radius needs to be as large as possible and the departing angle needs to be 12 degrees or less.

Q. - And tying these together, if there is a rather steep taper between the pushrod pinch and the bowl, and the minimum CSA required you to open the full length of the pushrod pinch, wouldn't this be the way to go in order to reduce the amount of taper angle in the CSA between the pushrod pinch and the bowl?

A. - Exactly right - but this is a judgment call one again. If we are talking of maxed out inches we will see that any flow increase from almost whatever source is good but if there is a decent match between CID - RPM and head flow (lets say 300 cfm intake flow at 0.600, a 350 inch engine and peak power at 6000) then we could make the power curve worse by indiscriminately grabbing any flow from any low grade source.


Granted the CSA answer may seem a little convoluted but I am sure as we progress with our PS series it will become clearer.

DV

[old blue 75]

David a couple of questions.

1. Can you draw up a little sketch to show where you are measuring this angle from.

Quote(Q.- Is there a rule of thumb regarding the angle/distance/shape of the reverse taper on the push rod side between the push rod pinch and the bowl?
A.- Best I can offer here is that the departing radius needs to be as large as possible and the departing angle needs to be 12 degrees or less.)

2. Can you at some point do a write up on how thin certain ares of a head
can be before they get to week. Things like port dividing walls, bowl to water jacket
port roof thickness, combustion chamber to water jacket and things like that.

Thanks old blue 75