Don Terrill’s Speed-Talk

On a 23* head where say your combo calls for a 1.95 MCS and this is at the pinch.
Using the AFR 195 as an example you have a gasket opening of 2.76 minus the radius. (2.185" H x 1.265" W 5/16 Radius) that then goes to a (1.920 sq. in.) https://www.airflowresearch.com/195cc-s ... nder-head/

Wouldn't it be better to open the MCS up some and maintain it from the gasket face to the pinch? Just curious I have never seen any info on MCS Length before and it seems like such a drastic reduction in a short span to pinch off a 1/2" of port.

How can it be mcsa if it's not the minimum ?

digger wrote: ↑Thu Mar 14, 2019 3:18 am How can it be mcsa if it's not the minimum ?

If it's smaller than the throat and the rest of the port before and after wouldn't that be the minimum?

Something like this (see my squiggly line) to keep the port size more consistent.

Or this vs this... I am just trying to get an idea of how much my needed MCS would change if I filled the port to make it more consistent.

GARY C wrote: ↑Thu Mar 14, 2019 3:47 am

digger wrote: ↑Thu Mar 14, 2019 3:18 am How can it be mcsa if it's not the minimum ?

If it's smaller than the throat and the rest of the port before and after wouldn't that be the minimum?

Didn't you say it was 1.95 st the mcsa which was the pinch but then say the entry was 1.92? Might have misinterpreted

digger wrote: ↑Thu Mar 14, 2019 4:22 am

GARY C wrote: ↑Thu Mar 14, 2019 3:47 am

digger wrote: ↑Thu Mar 14, 2019 3:18 am How can it be mcsa if it's not the minimum ?

If it's smaller than the throat and the rest of the port before and after wouldn't that be the minimum?

Didn't you say it was 1.95 st the mcsa which was the pinch but then say the entry was 1.92? Might have misinterpreted

I am sorry, the combo I am thinking about calls for around a 1.95, I was just using the AFR head as an example of the size change on a similar size mass production head.

I am just wondering if I fill that in where it is more consistent would I need to increase it to say a 2" due to the added length of the cross section.

GARY C wrote: ↑Thu Mar 14, 2019 4:04 am Something like this (see my squiggly line) to keep the port size more consistent.

My take on this, i think i know where you're coming from,, as in blending that mcsa area in smoothly??
In the squiggly line case, each 'section' of the port needs to take into account the section leading into it and the section it leads into, the throat/bowl end is not complementing the shape of the area it leads into so while the area thru the PRP may be a smoother transition if the area behind that is a poor transition you are putting a poor area of the port right at the transition and turn into the bowl.
The PRP is not necessarily also the MCSA, a tall narrow port at the prp can have more area than a low wide area infront or behind it.
Within the constraints of a given architecture the position of the measured mcsa may not be easy to move but if i read you right, yes the way flow is managed prior to and after this point can make a very large difference in the performance of that port.
There will be many more here with vastly more knowledge on this than me, so I will glady be told if my thinking is off.

Are you monitoring the port velocity in several points ?

mag2555 wrote: ↑Thu Mar 14, 2019 6:28 am Are you monitoring the port velocity in several points ?

I have not had the unfilled port on the bench, I did not port them, Competition Grinding out of Tucson did those back in 2004 and I never have used them.
The opening is about 2.7, prp about 1.9 and 1/2" past the prp about 2.04 and about 2.66 right before the short turn...Theses numbers are not accounting for the corner radius. I will probably try probing them although I will have no idea what I am looking at.
The change just seem to abrupt in that short of a distance but I know everyone use to gasket match heads instead of port matching.

Using a gasket to match a head to or base a port job on is not the correct way to do things. Fair enough if the gasket opening just happens to be what the engine and port need but really the port needs to be correct for the engine then the gasket just needs to seal a gap.

cjperformance wrote: ↑Thu Mar 14, 2019 7:03 am Using a gasket to match a head to or base a port job on is not the correct way to do things. Fair enough if the gasket opening just happens to be what the engine and port need but really the port needs to be correct for the engine then the gasket just needs to seal a gap.

Yes when I get to what I want I will use trim to fit gaskets.

I did what you are talking about (more or less) by taking nearly all of the length to get from the prp out to the width heading up towards the apex rather than coming hard off the pinch with the angle pointed straight at the apex/wall corner radius and I didn't really touch the wall leading into prp.

I don't know if that was right or not but it seemed to make sense.

Rev, thats what I am looking at, I think I just need to set my pinch size and record the air speed and the fill in the area before the pinch and see how it effects it, the runner after the pinch seems ok.

I've often thought about this myself . Is that part of the port an extension of the manifold or is the manifold an extension of the port? Either way wouldn't you want to keep the taper close to the close to same ? Going from a manifold with a 7° taper into a port that has a say 11° taper for the first inch, isnt the optimal situation IMHO. But what do I know.
Justt my thoughts Gary . Dan