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There is a hump in the roof of the exhaust port just behind the valve guide. Can this be taken down some and narrowed? Or is it best to leave it alone? Curious how much material is there to work with.
Probably best to leave it alone. Most likely it won't be The Restriction, or even close to it, in your finished product.
The "low hanging fruit" of home head porting is in the bowl, right behind the valves. Clean up all the weird angles, steps, ledges, and other artifacts where the factory just jammed a cutter down into the intersection of all those casting sections; removing the parting lines and steps between the casting sections themselves; smoothing (NOT changing the shape, just smoothing) the short side radius esp the exhaust; gently making the guides into a teardrop or airfoil shape; and of course a good quality multi-angle valve job; those are the things that pay off. Messing with the overall port shape and volume (*cough*"gasket matching"*cough*) is mostly a waste of abrasive and provides nothing beyond some eye candy for the uneducated. In a REALLY maxed out set, when done by an experienced person with access to a flow bench or templates, and multiple castings to work on when one gets scrapped from going too far, there's a bit to be gained by narrowing the push rod pinch in the intake.
Think of the purpose of porting as, improving flow streamlining (reducing turbulence), and making the velocity constantly increasing (intake) or decreasing (exhaust) as the port approaches and leaves the valves. "Hogging out" is usually a mistake.
There is a hump in the roof of the exhaust port just behind the valve guide. Can this be taken down some and narrowed? Or is it best to leave it alone? Curious how much material is there to work with.
Thank you
Post a photo of the hump you're writing about. As RB04Av wrote it's the throat and bowl blend that will gain you the most power.
This area was a flat hump. I brought it down just a touch in the center and tapered the sides. I did concentrate mainly just under the seats/bowl area smoothing and blending. Very little material removal. I did raise the roof of the port at the exit .050 and blended into the port. I removed zero from the floor. Port is just a tad wider than stock due to blending in where I removed from roof. You can see the casting lines are still visible on the sides.
Anyway, AFTER I did this, I read where one person said not to mess with this as you would hit coolant passage. I didn’t break through, just not sure how much meat is left there. All was done with just 80-120 cartridge rolls. No cutters.
Cool glad you found an answer. If you did that with sanding rolls you are probably ok. Either way if you have a lot of work into the heads you could always have it welded if you're work was too aggressive.
Slimmed the guide boss just a little and slightly widened area between boss and wall. Smoothed out transition from seat to bowl. Again, very little material removal. As you all mentioned, probably should not have even messed with anything past the bowl as it’s not that beneficial or what I am doing, which is just a low duration .550 lift truck cam in a 5.3 that will never see north of 6000 RPM.
Found this cutaway of a 241 head(on this site BTW). This head should be very similar to a 706/862 head. Looks like a good bit of material there. This may be helpful for future readers.
There's a few other things you can do, not directly related to "porting":
Get rid of any/all sharp edges in the chamber; around the valve seats, and where the deck meets the chamber especially. And, back-cutting the valves, and rounding over the margins, helps a bit with low-lift flow; as well as, the margins being a sharp edge that can glow red-hot, especially the exhausts.
Also maybe worth mentioning, there's what's considered to be an "optimum" range of bowl diameter in relation to the valve size. Usually somewhere in the 85 - 90% range. Most of these heads, seems like by the time you get the throat all smoothed out, it's not to far from that anyway.
Looks to me like on yours, you've got most of the "bleeders" fixed; should produce a noticeable bump in power.
Found this cutaway of a 241 head(on this site BTW). This head should be very similar to a 706/862 head. Looks like a good bit of material there. This may be helpful for future readers.
I remember this photo. I believe it came from a GM article on developing the C5 Z06 243 head.
Did you by chance see a 243 cut away?
I have a dead 317 head here. I think I’ll chop it up with my chop saw and look see.
Was thinking, I have some 823’s here. Slightly bigger exh valve, much larger exh port...and they do flow better. Hmmm, duplicate that on a cathedral head? I see the answer now.
Water injected exhaust port.....
Here is the throat and bowl of 799 intake with just removing the casting surface texture. Maybe everyone can see that the 799 and 243 have that extra area to make the short side radius easier for the fuel to move past. I have an old book from David Vizard and one of the many diagrams/illustrations in the book shows the evolution of the sbc intake port and the 243/799 port looks very similar to those illustrations of the latest ports.
01-29-2021, 07:45 AM
Bump in exhaust port 706/862 heads 
