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Al Eckstadt
Al Eckstadt
Jordan, NY
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DMadigan #854136 07/20/21 12:19 am
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Tried another variation taking into account the area of the guide and valve stem on the width. To make the area follow the quadratic curve the width has to make a sharp jump essentially the opposite of the area curve. I put a fourth order curve fit through the points and made a port with ellipses of height and width at each station (blue line). The width has to come back to the valve seat diameter somewhat quickly. This port measured 95 CFM as printed.
Accounting for the guide and stem obstruction does not appear to make a big difference over the quadratic area port.
To eliminate the diverging walls of the quadratic port where it meets the seat I calculated a cubic area function with zero slope at the carb and seat.
[Linked Image from live.staticflickr.com]
This results in a wider port at the start of the guide (~3") with a shallower convergence angle of the width at the seat. This will be the next print.

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DMadigan #856146 08/14/21 11:52 pm
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I started another big valve head the other day. And made a new curved manifold to get the angle better. I tried a little different shape but both cfm and speed are quite down so far.

So I put the new manifold on the previous 44.5mm valve head. These two heads had big valves and ports from years ago. These had less than 160cfm trough 38mm ports. Because the ports needed filling I could move the port down and line it up better.

This compares gaskets, The port with the 30mm AMAL fitted flows a little less than 109. And the big valve port is probably giving more than a 650cc could use without turning very high rpm.

[Linked Image from i.postimg.cc]

What I did to try another way to measure was run the 4 vacs flat out. On the plate with a 162.4cfm hole and on the ports. The 162.4cfm plate pulled 34"w the good head through the carb 30.5"w and with no carb and radius 29"w.

Last edited by Mark Parker; 08/15/21 12:04 am.

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Finally got the print done for the cubic area transition intake and attached it to the head. Same settings as the others read 97.5 CFM as printed, a slight improvement. The sides appear to angle more inward toward the stem than the previous port. I will put the quadratic port inside the cubic and difference them to check the transition is correct.
From the top the port looks like a cobra head seen from behind.
[Linked Image from live.staticflickr.com]

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That's really good flow with that valve size Dave. The question would be port size and speed in the port, but the carb size is limiting it down. There is so much suppeltey to the bowl shape, I do not know how you can print them. If it isn't smooth you also loose flow. I looked up some port molds and they often have approaches to and from the guide, though they can disturb stuff depending on where the air goes.

The difference between the good and bad port is width at the guide, good is 39.5mm other is 37mm. The port height is also less 23.25mm compared to 25mm. 39mm width at the guide is getting dicey on the std casting, going through between the cooling fins is patched easy but into the pushrod tunnel more severe and complicated.

The 34mm carb takes around 10cfm 191 drops to 180 and looking at this beautiful short stroke Norton 750 running against V4 Hondas and stuff from the '80s you can see how top end on a fast track would be important. Doug uses 36mm carbs and 8500rpm. It's great to watch as he makes top videos.


Last edited by Mark Parker; 08/16/21 4:17 am.

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I'm on the second big valve port and waiting for JB to cure. I fixed the first one. Width is the thing, it may be different if it's higher but there isn't much point when there is nowhere to go and it isn't necessary. It's dicey getting them to 39mm width or more but it gets the flow.

The ports with 42mm valves may work better wider, but maybe when I do another one. These with 42mm valves are 37mm wide approximately, at the guide, compared to 28mm with a std 1970 head. I think improvements on the 42mm valve heads might come from width. If they put a little more metal in the pushrod tunnel and between the fins on the other side it may be better, but compared to Triumphs and Nortons and triples it's miles ahead.

The 38mm XR port is 49mm wide, my 883 port is 43mm, but it goes through both sides. The same width ratio as the Harley, the 34mm port would need 43.84mm width. But going through means a less than smooth surface.


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The last port has a maximum width of 41.54mm at 77.6mm from the front of the carb spigot, 31.4mm from the upstream side of the seat insert (along the port centre line). 34mm carb I.D., 36.5mm I.D. seat insert. The valve head is 42.16mm diameter.
A trend line through the cubic port area including guide peaks at 46.5mm width at 82.4mm from the carb spigot, 26.5mm from the upstream side of the valve seat.

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I have the r/h port done ready to test, it's 39mm wide, if it's not measuring 178 through the carb I'll go wider on the outside as it's easy to patch between the fins. I'll have to watch just where that stud is though. This is another old head that had 44.5valves and 38mm ports. Ben's 750 has that size but is better shaped and flows much more. I can swap his head with the 34pwks and it should pull better, and I don't think maximum flow is much different.

I'd like to know what the Westlake/Rickman 8valve heads flow, the modern Triumph 4 valves are 181cfm.

On a triple, though I no longer have one, I'm pretty sure those studs are very close. The triple heads are definitely not that cheap or easy to get.

[Linked Image from i.postimg.cc]

This is that 38mm carb head, it would be interesting if I can get it better as it's going to be near the valve that is the limitation. If this flows more the speed will go up.

[Linked Image from i.postimg.cc]


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