MCSA location (?) in intake runner

[BradH]

This question is pretty much off the top of my head as I'm waiting for the first cup of coffee to kick in...

If my ASSumption that the intake runner MCSA between the carb and the valve seat is typically the pushrod pinch as a result of space/architecture/packaging constraints, where "should" the MCSA be?

Also, is the reason why removing the pushrod pinch in many heads is actually detrimental because the rest of the runner downstream of that point might not allow for sizing and/or positioning the MCSA in a more optimal location?

Thanks - Brad

[steve cowan]

Brad,
I think you know
I will give my opinion on 23 deg chev heads as that's all I have done, and I am just a beginner.
Ideally the seat ring as the minimum would be best but usually not possible.
When you work out the area of throat (1/2 dia x 3.1416 minus valve stem) depending on cubic inches,rpm,valve size etc the average CSA will be to big for application.
EG- 1.94" valve.89% throat
1.94 -11% = 1.72
1.72 /2=0.86
0.86 × 0.86×3.1416=2.32
2.32-0.09 (stem dia)
2.23" area of throat
If you look at making the port AVG CSA 2.23" that will feed a 383ci to peak approx 6500 rpm.this is just a starting point but do area calculations on bigger valves and you will see where I am going.
I have run smaller pushrod pinch areas and proven to myself that is not the best option.
It has also been said before that in a street car application a faster airspeed at the pinch may help with low rpm drive ability??
Its seems difficult to get a really consistent CSA through a 23 deg head but when people really oversize the window over the SSR and blow the bowl out to get those big cfm numbers that kills your cfm/in2 potential at the valve/ seat ring.

[modok]

IMO pinch point is fine if
-it is a straight line downstream of it
-it is in the last 1/4 of the total intake runner length.

[Bob Hollinshead]

I've heard some people use the stroke length to determine the point of minimum cross sectional area

[skinny z]

I'll resurrect this old thread as it's time I get into these heads on the work bench.
When discussing the MSCA, it seems it's generally but not always about the pushrod pinch.
However...

Ideally the seat ring as the minimum would be best but usually not possible.

Steve's reply above referencing the seat as the restriction is one of the few outliers I've come across (not that my research to this point is exhaustive by any stretch).
While I haven't completed my measurements, and quick look at the pinch shows 2.0"x 1.0". And from the looks of it, it satisfies the criteria as posted below:

IMO pinch point is fine if
-it is a straight line downstream of it
-it is in the last 1/4 of the total intake runner length.

By last 1/4 of the total intake runner length I'm assuming that's the portion closest to the head/intake flange.

In the case of these heads, with a pinch or MCSA of 2 sq./in., various calculators suggest that this would satisfy 357 CID (4.04 "x 3.48") to just north of 6000 RPM.
Oversimplified I'm sure. Reasonably accurate? That's my question.

FTR, I'd like to see this engine make peak HP at 6k and carry to a 6500 shift (3 speed auto).

[mag2555]

I have heard that in a perfect world that it should be one stroke length back up the runner from the back side of the valve.

[6.50camaro]

I have heard that in a perfect world that it should be one stroke length back up the runner from the back side of the valve.

Just for clarification, Would that be at the port centerline? as there is around 1.5" difference between floor and long side dimensions.

[skinny z]

I have heard that in a perfect world that it should be one stroke length back up the runner from the back side of the valve.

Hmm. Well within the capabilities of my measurement skills. I'll look into that.
Thanks.

Although...

I wonder if there is any science behind it as I haven't heard that before?

[skinny z]

I have heard that in a perfect world that it should be one stroke length back up the runner from the back side of the valve.

Just for clarification, Would that be at the port centerline? as there is around 1.5" difference between floor and long side dimensions.

Good question.

[juuhanaa]

I have heard that in a perfect world that it should be one stroke length back up the runner from the back side of the valve.

I wonder if there is any science behind it as I haven't heard that before?



-juhana

[juuhanaa]

IMG_20221113_184609.jpg

Not sure about the wires, but stroke is 93mm, port centerline lenght is 97mm and it start to expand soon after the manifold flange.. Errr

Its an undersquare 4 banger that ill try to run with a later timing events and see what it does. I have another version in my rally car with MCSAs at the valve pockets.



-juhana

[Erland Cox]

On most 2 valve heads ideally the MCSA should be at the throat.
And this is because air hates to turn and expand so don´t do both at the same time.
Area should go down from the turn to the throat to accelerate the air.
Because this lowers the static pressure and thins the boundary layer.
If you slow the air the pressure goes up, the boundary layer thickens and can get blown backwards.
Then we get separation and awake and that causes port stall.
In a straighter port like a big valved 4 bike 4 valve the MSCA can be at the valve guide to get better diffusion into the chamber.
But taper from the guide must stay below 7 degrees included.
Most 4 valve heads benefit from filling the bowl to lessen the expansion.

Erland

[juuhanaa]

7 degrees 14 included is often cited for the port angles, but the valve angles that work in conjunction with the port are not so to most people.

Like why a non back cut valve support higher lift flows/increased curtain area is not that simple...


-juhana

[Erland Cox]

Flow over valve seat areas is controlled by the valve.
For short expansions, less than 2 diameters it is possible to go to 16 degrees included.
But the faster you expand the harder it is to avoid separation.

Erland

[juuhanaa]

Speed ​​corrects mistakes, but we may need to brake earlier



-juhana