[steve25]

Hi all.
Stan Weiss a member on this board a number of months ago asked me to do a flow test of the stock port area at the push rod pinch point.

Since I did not want to cut up a head to do such I had to 1) make a mold of the port, 2) mask a casting of that mold , 3) cut that casting up and make it mountable and sealable to the flow bench.

I used a stock 1978 6X casting to make the port mold from.


Here are some photos of the whole deal.

The short section comprising the push rod pinch is what I made a casting of and flow tested.

In the last of my photos I had to make a very gentle curved entry radius to get the highest possible flow number out of it.

The flow results where this.

At 28” that stock amount of port area at .974” wide can pass 239 cfm.

[Stan Weiss]

Steve,
Thank you very much for all of the work you did here.

The port you made the mold from. What is the peak flow that port shows?

Thanks again,
Stan

[steve25]

Stan, peak flow was 208.

[johnta1]

Do any pitot testing on it?

[steve25]

I can do such later in the week John.

[Half-Inch Stud]

So the bowl represents the 1st choke point?

[steve25]

Correct.

[Steve C.]

Conversation only and not any recommendation....

Related here ? Probably not, but it might be of interest. And some conjecture on my part about removing and sleeving the pushrod bulge. The sleeve becomes a 'speed bump' ?

In regards to studying airflow characteristics during flow directional changes, particularly in inlet paths where fuel and air tend to separate due to the compressibility and mass (weight by volume) differences between fuel and air Jim McFarland uses the following analogy:

To illustrate how this weight difference can affect mixture homogeneity, consider this example- A train comprised of loaded cattle cars is moving down its tracks at a constant speed. In this example, the train is analogous to air and the cattle to fuel. At this point, the combination is well "mixed". Now, visualize what happens if the train suddenly changes in speed, direction or accelerates or decelerates. It doesn't require much imagination to realize how the quality of this "mixture" will be effected, accordingly. Such is the case between air and fuel in an inlet path, during changes in air direction or speed.

Now comes the question of how much of the air/fuel mixture in the intake port runs along the wall with the pushrod bulge verses the outside wall and roof. The intake wall "pinch" at the pushrod may not be that much of a factor.

Here is something that was written regarding flow improvements on a Chevy cylinder head that may or may not apply here (?). It touches base on the subject of the pushrod pinch:

Although many rookie porters start here (in the belief it must be a restriction), this rectangular venturi form is very flow-efficient. When the rest of the port flows well, work at the pushrod pinch point will pay off. The previous mods produced good results, so now is the time to widen the port at the pushrod pinch. As expected, this move only produced results above .550-inch lift. Here, the flow jumped by almost 7 cfm, but the swirl took a dive. This was probably due to the flow attaching itself more firmly to the inner wall, and the increased amount of air exiting the valve (toward the cylinder center) killed the swirl. Many engine builders have found that enlarging the pinch point past a normal amount may marginally increase flow, but does little (or nothing) for power output until very high rpm. The reduced swirl is probably why.


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[johnta1]

Here's a quote from Larry Meaux:

if the FPS thru a Pushrod Pinch is way too fast ( too small CSA )
then it reflects back a positive pressure Wave against the incoming positive pressure wave disrupting and somewhat blocking incoming Mixture Flow to the Cylinder, and even worse bad effect is now the Total Induction path seems shorter , and changes the best acoustic wave pressure timing at the Intake Valve closing point ABDC

if you have a too fast Floor Apex area at the beginning of the Short Turn Curve it will cause Fuel separation out of the incoming flowing Mixture and also can cause Flow Stall
... that is , Dry(Flowbench) or Wet( Live Engine) Flow separation from the Shape of the Floor's Apex area thru the Short Turn Curve Shape and some Flow will shoot across the Curtain area opening towards the Long Turn ... blocking or hindering any further Flow CFM gains ( Flow CFM Stall or Flow CFM reversal effects )
........you want to use the entire Valve circumference Curtain area for maximum Flow CFM , and not Flow separation causing a portion of Flow to shoot across the Curtain Area blocking Flow CFM

The worst situation is :
you can loose a lot of HP around Peak HP RPM point and above
if you have a "combination" of both a too fast Pushrod Pinch area and a too fast Floor Apex area at beginning of the Short Turn Curve

on NHRA SuperStock SS/IA 362cid at 7500 RPM and above RPM , it lost over 105 HP in a #041X casting 164.0 CC Port Volume Port at 250 CFM at 0.700 Lift and ran 10.80's at 3330 Lbs

a likewise pair of the same #041X castings, again with 250.0 CFM at 0.700 Lift and 164.0 CC Port Volume but this time reshaping Ports with grinding + epoxy , it set NHRA SS/IA Record at 10.132 ET

10.132 ET from 10.80's

The Pushrod Pinch is typically located 4" or inches before the Intake Valve(which is basically the end of the Induction Path) , so a too fast Pushrod Pinch can hurt you more than a too fast Floor Apex/Short Turn Curve area by changing Tuned Length.... but a combination of both, by far, can cause a severe HP lost !

He also says that the Pontiac heads are really bad with their pushrod pinch. (and long port length probably)

[Stan Weiss]

Originally Posted by steve25

Stan, peak flow was 208.

Steve,
Thanks. So the pinch flows 31 cfm more by itself than when part of the whole port. Is it possible to do 2 more sections? For me it would be very interesting to see if it is the throat / bowl area or the short side which is the real bottle neck.

Stan

[steve25]

Yes Stan I can get those two test done next week sometime.
I think I have enough mounding liquid left to do that.

Like Larry has said the Pontiac pinch off area takes place in a very short lenght of port which makes it worse then a SBC iron head pinch point.
The absolute worst I can recall in a common American iron head is the push rod pinch in the 318 Mopar heads.

Anyone recall the super stock Pontiac days when the Intake runner approaching the flange was purposely necked down to match the push rod pinch?

Such was done to midigate that sudden area reduction and narrowing of the motors power band.
It helped out even though it did not increase flow levels of one bit.

Port lenght wise the 67 and up heads are only like 1/4" longer then a SBC iron head, but the pre 67 heads due to there valve inclination angle have a port lenght 1/2" longer then the later heads, and that's a lot!

[Skip Fix]

Steve what did you "pour" the port with. Thought about getting some to see what those Speedmasters look like.

[steve25]

Skip I used a product called FlexFoam-iT! X
I got it from Micro-mark since they are somewhat local to me and as such I did not shop around for price .

This is mixed at a 1 to 1 ratio and has a expansion ratio of 6 to 1 so do a test pour in a paper cup to get a feel for it.

It’s a dense foam type also when fully cured in 2 hours.

Also note that I found out the hard way that there room temp suggestion of 74 degrees or higher should be adhered to.

I did not use any fancy mould release agent, I just used a very liberal amount of dish washing liquid to coat the port and valve with.

[steve25]

John I had a chance to velocity probe the mold and here’s what I found.

The highest velocity was in the center of the runner in a vertical manor.
Interestingly even with the slight amount of floor rise that this short length of port has it was effecting where the air wanted to go.

As such the peak velocity in the center of the port as seen vertically tapered off as you dropped vertically from the center of the port and from 3/4s the way down the velocity dropped off a lot.
This is a trend that would likely still take place with a intake manifold bolted on.

[johnta1]

Thanks, Steve!