Porting and polishing question

[sww914]

I enlarged my intake and exhaust ports to 36mm in a mill with a boring bar. It went very well, the machining left a perfectly round port exactly 36mm in diameter with only a small step to grind out from where we stopped with the boring bar before the port was larger than 36mm to avoid milling the guide boss away. I'm preparing to finish off the first head. I've polished the exhaust port as much as I'm going to polish it and I've finished the intake port with 80 grit in accordance with the booklet that came with the tool kit. 80 grit seems awfully coarse. I think that this book was written mostly for iron head SBC guys, I'm wondering i it wouldn't be better to finish my aluminum ports with 120 grit instead because I know that 80 grit leaves much finer scratches in steel than in aluminum. I've also enlarged my intake manifold ports to 36mm.
What say you experts?

[dion914-6]

Looks like nice work so far. But as my old race engine builder used to tell me, you can port and polish all you want but unless you have a flow bench, it's just an exercise in grinding that could turn out making the heads flow worse. The intake manifolds look great.

Dion

[jpnovak]

A rough surface will help disturb the surface boundary layer that can cause fuel droplets to stick to the sidewalls of the manifold and head port. This can cause temporary changes in AFR due to the fuel "wetting" the surface. Usually this happens during an acceleration event where extra fuel is squirted into the intake tract. Some heads are worse than others in this regard. Do a web search for wall wetting or X Tau parameters for more information. In my opinion if you are running larger ports than normal for the size engine and given cam configuration then a rougher surface is beneficial. This is due to the slower intake gas speed and resulting slow cylinder filling. The slower gas speed will give more time for the fuel droplets to fall out of suspension. The converse would require a slightly smoother intake. I do not think that polished ports do anything but keep carbon from sticking. This should not be an issue on the intake side.

btw, nice machine work.

[aws]

I have never used anything coarser than 320 grit for the final finish on the intake port. My opinion has been that 911 intake port is so straight and short a coarser wall finish only makes the boundary layer thicker and effectively makes the port smaller in diameter. It is nice to have a flow bench, but my experience over the years is that the 911 port is so short it responses to enlarging very well without any downside. I use a 944 guide on the intake which is shorter and have removed the ledge behind the guide on the intake for good results ( 250 rwhp from 2.7 with 11/1, RSR sprints, 46 IDA/42mm vent. ) I polish the exhaust and enlarge it to match the cam/header combination but do not "pocket" port behind the valve except to smooth out the rough casting. I do polish the valves The only caveat here is to show judgement to port size. 41mm intake ports on a 2.7 CIS 8.5 to 1 engine with a stock cam and muffler will NOT get good results. A trip to an abrasive store will show you all sorts of sanding rolls and "flapper" sanders to make the job easier and look better. A lot of available air pressure helps. Most importantly, turn the air pressure down and TAKE YOUR TIME!!. You can remove material easier than you can put it back.

aws

[sww914]

From what I understand, the first rule of porting and polishing is the same as the first rule of medicine- Do No Harm. My goal is not to produce some radical new port shape, but rather to slightly enlarge the ports from 34mm to 36mm and simply blend and smooth them.
I'm building a 2.7 with RS pistons, DC60 cams, Weber 40's and open exhaust for track use with a few test drives on the street here & there. I'm using the Webers because that's what came with the 2.7 that I bought for less than the Webers are worth alone. I'm planning to twin plug the heads while they're off because I can do it for free at this point and if I ever find a deal on some 2.8 RSR P's & C's the heads will be ready. I don't want to afford to race a high compression engine yet anyway, I'm looking for reliability for right now and I'm building within the confines of what I can get for cheap and do for free myself. I know, what part of "Porsche Racing" made me think it was cheap? Was it the Porsche part or the racing part? The only cheap part that I've ever encountered was the buying a 914 part.
I should add that although I haven't ported and polished a gazillion sets of heads, this is #2, I have spent many years shaping metal and body filler and piloting a die grinder around the garage.

[jaybird840]

Hey SWW,
Can you explain (like, dumb it down) how you enlarged the manifold to match the 36mm intake ports? My engine build is almost identical to yours... 2.7 euro pistons, free flowing exhaust, 40mm Webers... I had thought about having CGarr enlarge the ports to 36mm from 35mm based on the Jerry Woods engine combo in Bruce Anderson's book. I don't know how much that extra mm would help me on a street car? Anyway, back to my original question... How is the best way to get my intake runners from their current 32mm to the 35mm I need to match my intake ports? Do you just go at it real slow with a rotary tool? small flapper sanding wheel? My biggest concern is keeping the intake runner round.... Thanks for any help!

[sww914]

I didn't machine the intake manifolds because they aren't straight like the heads, they're all at a different angle and it was too much work to set up 3 different times for each manifold in the mill. I bought one of these- http://store.summitracing.com/partdetail.asp?autofilter=1&part=SAI%2D260001&N=700+400430+115&autoview=sku
I blued the flat surface and centered the gasket with a 36mm hole to scribe a line on the flat. I started with the 40 grit cylindrical tool and took it almost out to the scribed line and then switched to the 80 grit cylinder and worked it all the way out. I really took my time studying the shape of the holes and planning how to shape the new areas without screwing anything up. As I've said above, I've spent many years shaping body filler so after the 80 grit and making it look good, I ran my fingers around the hole and in and out feeling for any irregularities and smoothing them until it felt perfect, too. I finished it off with the 120 grit flap wheel. The manifolds were way easier than the heads because there's nothing in the way like the valve guides are in the heads, even though I was going from 32mm to 36mm rather than 35mm to 36mm on the heads.

[sww914]

BTW, Henry Schmidt has said here that there is some advantage to having the intake port a bit smaller than the head port. It seems that he was talking about a smoother idle and better off idle response, the step will keep the air/fuel mixture more pressurized in the port because it will help to slow it from bouncing back as the valve closes.
I don't care how it idles so long as it does idle, and I don't really care about how it runs at part throttle or at low RPM's. I'm just concerned with getting the most HP at 4000 to 7500 RPM's.

[cgarr]

Yes it is important to not open them up too big, you should keep a venturi effect going to keep the speed of the fuel air mix up otherwise your idle will suffer, you can have the same problem if you try to use too large of chokes in the carbs also.

[jpnovak]

While we are on the subject... Does anyone know the optimal shape for the valve guide boss? Who has pictures of "properly" ported and flowed heads. I suspect that the smoothing of the boss is the most important factor.

What about the inside radius of the port? It is common knowledge that when incoming air turns the corner its momentum concentrates the density on the outside wall that contains the valve guide. This means that the air slows down on the inside radius of the curve. My question is how bad is this on a 911 head? I know they are pretty straight through so it shouldn't be much of an issue. but I also think that someone has done a lot more work than I have on the subject.

I will reference This nice looking and reportedly well flowing port design from Extreme Heads posted in an earlier thread. Notice the machine marks around the valve guide and support. What is hard to see is what was done to the inside radius at the bottom of the picture.

http://forums.pelicanparts.com/showthread.php?t=409810&highlight=extreme+heads

[JFairman]

It would be nice to see the short side radius in that nicely done port.
One thing I've learned from experience is don't remove much material from the short side radius, just radius it a little to make a smooth curve.

If you remove too much there, the airflow doesn't straighten out as much and hit the back side of the valve as evenly and detrimental turbulance will be created right at the backside of the intake valve as air hits the back of it at a slight sideways angle on the short side radius.

[304065]

How about a mold, like the one Jack made? Would be way cool.

[sww914]

That's perverse.

[304065]

Gives new meaning to "being into Latex."

[rs911t]

Damn Jamie. That picture is simply pornographic