CNC markings

[fekbmax]

So, I have heard so many different opinions on this but I figure theres some of the very best Fe engine builders right here on this forum so I have to ask.
To smooth out the CNC marks in heads and manifolds or leave them as they are ?
I remember reading countless articles back in the day about dimpling runners and combustion chambers and its benefits and effect on fuel/air tumbling.  I would think if it was any real benefit that it would be a more common practice these days but you don't see much about it. I'm wondering if that's why some head guys leave the CNC markings in the ports but smooth and polish the combustion chamber's. 
Hoping this isn't to dumb of a question.. uggh.. 
I'm just curious

[Autoholic]

Drawing from my experience with fluid dynamics, not engines specifically, I would say you want everything as smooth as possible. The surface friction will effect how the air moves from the intake to the combustion chamber and then to the exhaust. The least amount of resistance, the better. I know OE's today will dimple the sides of the skirt with lots of tiny holes, to reduce friction as these tiny holes will trap a spec of oil. They also are using super smooth and very hard coatings as well to reduce friction. Tumbling would only be beneficial if it was caused by a ram air scoop of some design, where incoming air slows down and increases pressure, ~1 psi. A rough air passage won't increase pressure all that much and you would benefit more from a smooth surface allowing the air to quickly get to the engine, in a laminar flow or as much as possible.

[jayb]

I always smooth them out.  I got a set of heads once and flowed them with the marks still in them, and they missed the claimed flow target by 30 cfm.  Took a 40 grit cartridge roll and smoothed them out, and they picked up 15 cfm (from 330 to 345).  Still were a little short of claimed, but much improved.  There is always some discussion about the benefits of leaving the marks, but I've never seen them on any competitive Stock Eliminator or Super Stock car.  You'd think that in those very competitive classes, if there was an advantage to leaving the CNC marks they would do it...

[Dumpling]

CNC marks are different than dimpling.

Golf balls are dimpled, for a reason, but one with a scored surface, something like a CNC mark, would be considered flawed.  Airflow is disrupted by scoring, but properly sized dimples helps.

[Autoholic]

Dimpling runners might have a benefit but that would require either some very elaborate CNC work that will take hours if not days or to cast them into the part. Either way, you have have to create the part in a CAD program and do a lot of CFD work to find the best design. Trying to find the best design by creating multiple prototypes would get stupid expensive in a hurry. The only area I could see taking a look at this would be F1 where every tiny advantage helps but their runner lengths are probably too small for any of this to matter. NASCAR could benefit but if you look at how much time you would spend on this compared to the gains (maybe 5 hp?), it isn't viable when compared to just smoothing out the runners.

[Jim Comet]

Smoothing the combustion chamber and getting rid of any sharp edges in the combustion chamber helps to eliminate hot spots that may cause detonation. Jim

[faulkdaddy]

Keith, did you get your heads from Kuntz?  I have a set of his CNC'd heads also. Can you give me a call please?  2 five 4- 5 seven seven - five 4 six two.  I have a couple questions.  Thanks

Bruce Faulkner
SSG U.S. ARMY  (RET)

[cjshaker]

I'm certainly not a professional, but I'd have to agree with Jay and Jim, look at anything from Top Fuel, F1, Nascar, Pro Stock, Pro Mod, pretty much any class you want to. They're all ported very smooth and many of the chambers are polished to a near mirror finish. Dimples MAY help with a less than ideal head in CERTAIN spots, but that hardly applies to todays heads and most likely would hurt most old heads if not done perfectly with a lot of testing. If there was anything to gain, it would most certainly be done by at least one of those classes of racing, but every Pro class you see has very smooth runners.

[fekbmax]

Keith, did you get your heads from Kuntz?  I have a set of his CNC'd heads also. Can you give me a call please?  2 five 4- 5 seven seven - five 4 six two.  I have a couple questions.  Thanks

Bruce Faulkner
SSG U.S. ARMY  (RET)

PM sent.

[blykins]

You all need to try a dimpled intake some time. 

[machoneman]

Apparently, it is still being done: https://www.google.com/search?q=dimpled+intake+ports&rlz=1T4GGHP_enUS533US533&tbm=isch&tbo=u&source=univ&sa=X&ved=0CB4QsARqFQoTCNuLvfiepscCFYmODQodF8oANw&biw=1097&bih=494

Long ago in the 70's, I do know many BBC shops (sorry!) were doing 2 runners on each head. IIRC, this came about since due to the GM layout, 2 runners were 'slow' and the effort was an attempt to equalize as much as possible all 4 runners. More common was some dimpling in the combustion chamber on 1st gen. SBC's (good pic in the link). Had no idea in the intervening years that dimpling was still in vogue in certain applications since I had thought modern CNC porting and flow benches took care of many head imperfections. Guess I was wrong!       

[Joe-JDC]

The dimples question is still not resolved in real life.  I can tell you one of the manifolds in those pictures is a real let down.  There was a huge torque loss, and the ports have a variation of 65cfm between the highest and lowest port flow.  Will be doing some dyno testing soon on that principle.  Will report the findings later this fall.  Joe-JDC

[Autoholic]

I'm gonna toss another golf ball into the mix with what I found...

[Autoholic]

I would love to hear from the major engine builders on this topic. Dimpling a smooth intake port or sanding it to a consistent rough surface. The below link is an interesting read.

http://mototuneusa.com/homework.htm

[Nightmist66]

I'm gonna toss another golf ball into the mix with what I found...

Wow, a whole 1-2 extra horsepower that needs to be serviced in only 20 hrs at who knows what cost. I think I would stick with a traditional valve and look for an extra gain in ignition/cam timing, fuel, whatever, before I drop a small fortune in "disposable" valves. Interesting though. Does have that golf ball look. A quick look on their site shows that their prices start at $99.95 per valve. Ouch, that could get expensive.