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dimpled vs polished port job


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I've always seen cylinder heads with a "polish". However, I was just turned onto an alternative from some v8 friends of mine who are drag racers, which the port is dimpled like a golf ball or roughed up with a carbide bit, on american V8 engines.

 

Doing the intake manifold rough up the same way with carbs installed, I am told allows the air/fuel mixture to better atomize since the fuel droplets are bigger rather than if your ports are smoothed out. Their is debate on the exhaust side since its "dry" not wet like the intake side is with Fuel. the intake port has to deal with wet flow which can cause fuel to stick to the port walls. dimpling or roughing up the walls coaxes these fuel deposits to continue to flow through the port or evaporate because of the turbulence it creates. From some research online there seems to be varying degrees of what should be dimpled and what should not.

 

Has anyone experimented with doing dimples on the cylinder head or intake manifold on an L6? 

 

Article I found on v8 engines:

 


 

"I took one of my old chipped up and slightly bent Alumina Burrs and rough ground just the entire intake ports on a big block Chevy Dart head all the way to the bottom valve job angle cut. Back on the dyno the engine gained almost 15 HP on a 950 HP engine. We kept going with the rough carbide finish everywhere in the intake, exhaust ports, chambers and intake manifold from that point on. So far every engine has responded with 15-25-plus horsepower increases, a wider power curve, sometimes 100 to 300 higher RPM point of peak horsepower, less fuel consumed on the Dyno ( lower BSFCs ) and dryer exhaust ports. On the flow bench, basically no flow gains I can measure from roughing up the entire heads/manifold surfaces. On the dyno and down the dragstrip, we consistently see more HP and quicker ET/MPH times"

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It affects the boundary layer and will free up power that is lost to poor atomization.  I would imagine that for an engine that doesn't get rebuilt every week, a polished exhaust runner and chamber would be much less prone to deposits.  That is the way I have most often seen it practiced- somewhat rough intake, polished exhaust and chamber.  Oh and it is nothing new, Dad's old 283 that was built in 1973 was done that way.  They even went so far as to rough some surfaces more so than others.  Think of the way a river flows around a bend... so a little more rough on the short side and smoother on the long side.  That little engine did 498 bhp (1.71hp per cu in) on the engine dyno 10 years before I was born.

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They did all that by hand with a die grinder?  That looks like a LOT of hours!

 

I think this makes sense on intake all the way up to the valve, but I'm curious about the combustion chamber surface.  Seems like the effect would be different there.

 

I used a ball-end bit in my die grinder to add some dimples on the short side radius on my Mikuni manifold.  I honestly can't tell if it made any difference, but it doesn't seem to have hurt anything.  As roger280zx mentioned above, I think the effect might get negated by carbon deposits pretty quickly on a street engine.

Edited by TimZ
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Yeah it doesnt seem like anything new, it was that I just have always heard of a polish job whenever someone quotes me. I have never heard it "rough' so to speak.

 

Anyone see it done on an L6 cylinder head?

 

Im shipping an N42 head out to Slovers today, and may have him do some changes in lieu of a polish job...

Edited by AZGhost623
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So talked with Slovers a bit, and he is going to leave the intake side rough for now. Once he evaluates the cylinder head more when he gets it will go from there. But for now the intake side will be roughed up with 80 grit and the exhaust side will be polished. Not going to do the combustion chamber, as roger mentioned, I think it would load up on carbon deposits rather quickly and negate any positive effects. Slovers said they saw early detonation when it was done that way, which contradicts what the article above said. But im not going down that road anyways.

 

This is going to be a monster head, 12:1 CR,  seeing 9k rpm. Big ports, big lift. 

Edited by AZGhost623
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A sphere without dimples has a large wake, and the drag slows the ball down. Dimples help the flow stay attached, reduce the drag, and the ball goes further. In the port, you would think that flow attachment would be most important on the short side radius, but I haven't been able to find pics of a head dimpled only on the short side. Airplane wings (and some Z cars) do use vortex generators to keep flow attached to wings and make the flaps (and spoilers) more effective, FWIW, but there are big differences between the air flow in an enclosed port and on a wing or a golf ball in free air.  Not sure if the idea works or not, but "they do it on golf balls and airplane wings" isn't necessarily proof of concept inside a cylinder head.

 

The dimples on the ball also help to generate lift from the ball's rotation, which is shown in this really cool video, but that part doesn't relate to what we're doing. Still worth the watch:

Edited by JMortensen
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Speed Secrets (capitalized).  PipeMax version 4.0.  Magical dimples.  All of the below is true?  Dryer exhaust ports?  Because the fuel is burned so efficiently?  Can't believe it.  

 

" 15-25-plus horsepower increases, a wider power curve, sometimes 100 to 300 higher RPM point of peak horsepower, less fuel consumed on the Dyno ( lower BSFCs ) and dryer exhaust ports. On the flow bench, basically no flow gains I can measure from roughing up the entire heads/manifold surfaces."

 

There's a psychological factor involved when people do these "experiments" and odds are that other things were changed that affected horsepower in those hot rod magazine tests.  "Oh yeah, I advanced timing 10 degrees, but it shouldn't matter" kind of stuff.  Subconscious.  That's giving the guy the benefit of the doubt.  It might just be pure BS, to sell Speed Secrets.

 

 

Just saw that Naptown Dave already commented re the golf ball.  I didn't see the actual "theory" either, that better atomization caused all of the described benefits.  The best explanation for something that can't be explained is in terms of something that can't be measured. 

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The only time dimples or texturing matters is where the fuel enters the tract far away from the valve... since most fuel injected vehicles have a relatively short distance between fuel and valve, you won't notice a thing. On older cars with old TBI injectors or carbs that spray poorly, I'm sure there's some recovery of inefficiency, but again this wouldn't be the case with modern injection hardware.

 

The other side of the equation is the air characteristics, texture changes the way the air hugs and tears from the wall (boundary layer). If you have lots of texture, you also have lots of directions that impede or cancel each other, which is why it will alternately behave like a larger cross section or smaller cross section, depending on density and speed. At best you could see it like a resonator in an exhaust, in most cases it doesn't affect anything, in a few cases it helps, in a few cases it does hurt flow, but ultimately it won't make things perform as well as a bigger pipe.

 

Visually the best way to visualize it is a ton of little aerofoils with their own laminar flow and shock fronts in different directions. Venturies are always smooth, carbs are always smooth, it's not an accident.

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The only time dimples or texturing matters is where the fuel enters the tract far away from the valve... since most fuel injected vehicles have a relatively short distance between fuel and valve, you won't notice a thing. On older cars with old TBI injectors or carbs that spray poorly, I'm sure there's some recovery of inefficiency, but again this wouldn't be the case with modern injection hardware.

 

The other side of the equation is the air characteristics, texture changes the way the air hugs and tears from the wall (boundary layer). If you have lots of texture, you also have lots of directions that impede or cancel each other, which is why it will alternately behave like a larger cross section or smaller cross section, depending on density and speed. At best you could see it like a resonator in an exhaust, in most cases it doesn't affect anything, in a few cases it helps, in a few cases it does hurt flow, but ultimately it won't make things perform as well as a bigger pipe.

 

Visually the best way to visualize it is a ton of little aerofoils with their own laminar flow and shock fronts in different directions. Venturies are always smooth, carbs are always smooth, it's not an accident.

 

Well it might be an interesting test to take my Kameari intake manifold and rough it up with 80 grit, and re-dyno the setup and see if anything changes. I already have a baseline and everything properly jetted from when it was done back in December. 

 

According to my tracking info, Slovers will be receiving the head on Tuesday, so I dont really have a lot of time to have a proper decision made only from what my V8 friends tell me and what I have read online. From everything I read, having the walls a bit turbulent when its fuel/air is a good thing and it helps break up that laminar flow along the edge. The idea is that at the surface walls, the wet air moves more slowly than when its not at the surface. 

 

Venturies and carbs are always smooth, because its BEFORE the air gets mixed with fuel. That goes back to the fact that when its dry you want it polished/smoothed out. 

Edited by AZGhost623
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11:1 which wouldn't run the best timing without 95 or 96 octane gas.

I know a few modern day cars that run 12:1. I assume this must come down to a more efficient combustion chamber design or something then. Talking to a few people (who don't run carbs) said not to go higher than 12:1 for pump gas.

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