Polished intake manifolds are VERY BAD!
 

Hello everyone, I was talking to someone educated in intake systems, and contrary to what I often hear, told me that polished intake manifolds are actually detrimental to the performance of your vehicle, primarily because rather than the airflow injecting directly into the engine, the loss of friction causes the air to "bounce" around. Any comments on this?

I don't know squat about boundary layer physics, but VERY BAD? That's probably an overstatement. If that were the case why would they be machined at all. They'd be a lot less expensive to make.
Stick around, I'm sure there will be some slide rule pushers here shortly to argue about this...

actually the whole reason you get it polished is to take out imperfection so the air doesnt bounce around and have a smooth flow to the engine thus in some cases increassing air flow plus they look really nice

Are you saying the outside of the manifold being polished affects air flow INSIDE it? I don't see how that would affect anything at all. The only discussions I've read discuss the possibility that polishing reduces heat transfer out of the manifold.

It's entirely possible that this person is describing the difference between laminar flow (flow without any mixing - think about the molecules marching in order) and turbulent flow (flow is mixing as it travels). As a fluid flows further thru a system, it becomes more turbulent, and if the system has rough surfaces, it become turbulent faster.

In some situations, turbulent flow actually flows better and more consistently than laminar flow. So if you want turbulent flow at the engine, then if the piping surfaces are too smooth it will still be laminar. The dimples on a golf ball change the airflow over the ball in this manner. Certain aerodynamic features on cars do also. I suspect this person was referring to this kind of concept.

Considering that an FD is turbocharged and intercooled, I guarantee the air is pretty well stirred up (turbulent) before reaching the engine. So this line of thinking probably doesn't apply to an FD very well.

Dave

A smooth surface will reduce friction, and reduce turbulence. Reducing friction and turbulence will result in increased flow. I don't believe your "expert" ;)

I didn't believe him either, which is why I made this post. I think Dave makes a good summary of the situation.

Good post. If anything, polishing the outside of the intake might make for more heat to be able to build up because of less surface. However dave is correct. look at the underside of some cars, and they will have hundreds of dimples. That is because air will get stuck circling in the small holes and the outter air will flow over that pushing it. The outside of the intake will not effect the air inside.

Yeah, at the end of the day especially in an FI application you're looking for the most smooth surface possible.

Sorry, will not effect the air turbulence.

my friend actually got a masters in this area for building and designing submarines (he was a PC freak and was into watercooling and what not,)

lets see if he can apply what he learnt to engines and ill let u guys know

I have NEVER seen a manifold polished on the inside, but as was previously stated it does affect heat transfer slightly, probably not noticably.

Agreed, and his probable idea is that while: ...is true, increased turbulence leading into and inside the combustion chamber is desirable, because it enhances combustion.
Exactly.

For everyone that thinks an intake manifold should be as smooth as possible, or as "smooth as glass," take a look inside Hogans Racing manifolds, Wilsons manifolds, port jobs, etc. You'll notice they'll keep a semi rough finish in the runners/plenum/ports. Typically something your finger nails will catch, but still feel somewhat smooth to the tips of your fingers.

As for it being VERY BAD, not exactly. Typically, a casted surface is to rough, therefore even polishing it to a "smooth as glass" finish will help performance. However, if polished with a semi rough surface, it'll help squeeze out some more power. I'm not saying your going to gain loads of power with a semi rough surface, but when it comes down to it, it'll yeild greater results than a perfectly smooth finish.

Something else you want to keep in mind is the fuel. As well as injecting water/alcohol. A small amount of turbulence will help them mix with the air better. However, injector placement has a large effect on this as well.

Bottom line, a "smooth as glass" finish is typically better than a casted or extremely rough surface, and a semi rough finish will usually yeild the best results.

Oh, and before someone brings up the extrude hone process, most racers only use this process to enlarge runners. They'll usually go back and rough the surface up some.

-Alex

if the inside of an intake is polished to a miror shine it will have a reasonable amount of surface tension but it will flow better than a rough cast intake or mismatched ports.
The best porting and finishing on the inside of the intake is a very slight texture like a mill finish or a light beed blast. This is called a vortex generator.
If the intake manifold is the restrictive part on your air intake system then smoothening it up may help, but don't buff and polish it to a shine.
On a non turbo intake you would be able to measure the differences between polished and not easier than a turbo intake as boost pressure overcomes a lot of turbulent air.

Ian.

The builders I spoke with also claim that the rougher finish is to promote air swirl to better mix the air/fuel. But as previously stated, in a boosted environment it IS better to have a smooth surface to maximize flow since the pressure will already compress the mixture together.

So do you have any facts to back that up, or is this just opinion? There's more than enough facts to back up a semi rough finish.

Just because a FI vehicle creates turbulance from both the turbo/supercharger and intercooler doesn't make totally smooth runners the best thing to have. You have any reasoning as to why it would?

I will say that a totally smooth runner on a FI vehicle won't create as much of a loss, if any, as it would on a N/A vehicle. However, that doesn't mean it's the best finish to have, or performs better than a semi rough finish.

-Alex

I'm gonna have to ditto that entire statement... Nailed it... :)

Unfortunately, I'm not a lab...do you have any recorded data to prove otherwise? I confess what I'm reiteratie is hearsay but the source (and I"ll sound like I'm just name dropping) was Darrell Drummond...THE engine builder for Mazda Comp. Perhaps you can challenge whether this empircal data is bunk. ;)

There's data listed all over the internet, in which I posted sources to see for youself. Give Hogans Racing, or Wilsons Manifolds a call. Look at their runners and plenums. Do a search on JimLab's Hogans Racing intake manifold. You'll clearly see how the runners and plenums have a semi rough finish. Hell, speak with all the top FI rotary drag racers. Abel has a custom intake manifold, and I can almost guarantee the runners and plenum have a semi rough finish, as well as the ports on his engine. He can be quite anal on what he'll actually tell you, (for good reason) but I'm sure you can atleast get this info out of him. Do a search on extrude hone. You'll find MANY independent studies of this process being done on intake manifolds, and they all typically lost power after it was done. If all that is still not enough, speak with every top engine builder you can find. Most of them will all tell you a semi rough finish on the intake runners, plenum, and heads will yeild the best result. As for rotarys talk to A-spec, Gotham, Rx7-Store, Racing Beat etc. I bet every one of them will tell you they get the best results with a semi rough finish when porting engines. Sean has a custom intake manifold on his car as well, send him a PM about it. I'm sure he'll tell you if he has it polished to a perfectly smooth finish, or if he has a semi rough finish.

-Alex

Edit: BTW, wasn't Carbon Man contracted by Mazda to do some CF intake work? Go back and read his post! ;)

Holy crap...tenacious aren't you. Learn to read before you jump all over me...I never said the best finish is a mirror polish. A smooth finish is better...and I did look at some sites. Extrude hone still claims to be smooth but they also say that the unidirectional "graining" from the abrasive extruding helps direct air flow.

I'm no builder so you can say all you want to disprove me...it's not very hard. I'm just stating what I heard/was told. BTW, Formula 1 and most high formula racing uses CF plenums and intakes...pretty sure they don't have a rough surface.

Not jumping all over anyone. Alot of people posted FALSE info in this thread, and there's no need for it to continue. I for one would hate to build an intake manifold, thinking that having the runners as smooth as possible would yeild the best results, when in fact it doesn't.

You asked me about data to prove what I'm saying, so I posted it up, that's all! :)

The issue I have is everyone seems to be spreading "hearsay." If you think a smooth runner will yeild better results, then post why, and data to back it up, not that so and so told me. Just about everyone I listed has facts to back up a semi rough finish, tested on computers, flow bench, and dyno's.

As for CF plenums and intakes, I can't really comment on that. However, I'm sure Carbon Man can shed some light on the subject.

-Alex

as for carbon fibre intakes, most of my customers want shiny parts. I know it is not as good as a matt surface but they like the look of it.
I do a lot if intake design work and if the intake has to be shiny I just change my dimentions to compensate for the slight surface tension.
If I am making an intake that has a restrictor size as part of the rules like ALMS then I will scuff the intake track near the smallest part to create a vortex and release the surface tension on the air.
The photos are of some OLD spec intake trumpets for a ford V8 for a cross over manifold (I can't show any currant spec trumpets)

Ian.

if you could build a manifold that was straight or had very gentle bends a polished surface will give better flow. The rough surface is not a vortex generator but it does trip a laminar BOUNDARY LAYER into a turbulent boundary layer (you are not going to get laminar flow in an inlet manifold). Now, a turbulent boundary layer is benificial in most inlet manifolds because the boundary layer will stay attatched to the surface over larger pressure gradients (such as bends in the runners) than a laminar boundary layer. This is the reason that carbon mans roughing up of the at area changes and bends works

I posted a similar question about polishing the inside of the intake piping/manifolds in the Tech and Performance area and didn't get a single response...

https://www.rx7club.com/showthread.p...ight=polishing

Here's the post for those too lazy to click on the link...

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I was reading about intercoolers here...

http://www.are.com.au/feat/techtalk/intecoolersMR.htm

...and it sais...

"A few factors should be remembered with air flow. Definitely don't polish the inside of tanks or pipes. An ultra smooth surface causes capillary tension on the surface layer of air with the wall & will increase drag so increasing pressure drop."

I was under the impression that polishing the inside of tubing and manifolds would result in less friction so more air flow. Not polishing the end tanks would allow for more heat dissipation but I believe the author is talking about the tubing as well. I did a search in Yahoo and most of the finds on capillary action deal with liquids although air can sometimes be considered a liquid, I didn't find anything about tension on a polished surface? How polished does a surface have to be for this capillary tension to be a problem to the flow?

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I have smoothed up the inside of my UIM, Greddy elbow, and Efini y-pipe with crossover tube. They're not highly polished but much smoother than the rough casting textures. My reasoning is that the smoother surfaces will reduce the pressure drop from turbos to engine which is very important to us guys trying to make power with the stock turbos that aren't reliable at higher boost pressures. I will leave the lower part of the LIM casting roughness to cause turbulance and help fuel atomization. I'm starting to tune my car so I can't say how much of a difference it makes but the 7 runs very smooth and has a lot of low end torque in vaccuum.

True... but low to no turbulence will ahve a negative impact on fuel mixing with air in the intake and in extreme cases let fuel condensate on the intakes walls, causing a lean spots in the mixture.

Don't see how polishing would affect outside temperature. To have metal disapate heat to air you need a lot more than the graining on the manifold.

I have never heard of applying term surface tension to enclosed air flow. Surface tension applies to liquids, not gases. In fact, it is the one major difference between them.

Cheesey makes an interesting point - the more turbulent the flow, the better that it will turn corners. By the time the air flows from the turbos to the LIM, a little extra stirring can't be all bad.

The rest of these intake examples don't appear to be related to turbocharged engines, although it could be said that increasing the turbulence as the air approaches the fuel mix zone is a good thing. But I'd have to guess that since this is a turbocharged engine with injection taking place almost inside the combustion chamber, the improvement will be much smaller than with piston engine or NA rotary engines.

Dave

Right on. Saved me some typing. :)

What I'm not seeing is how all this effects the rotary since the fuel injectors are damn near in the intake chamber and the only thing were moving is the air itself. I can see all this being an issue with fuel as well as air moving through the runners but it's not the case with our motors. We do everything in our power to increase air flow, larger exhaust, freeier intakes, larger TB ports, gasket matching ports, larger intake and exhaust port timming, it seems air acts very much like a fluid when moved through tubing so there would infact be less friction/drag with polished intake runners and therefore more flow. Someone explain how that would effect our cars with our fuel and air setup. ?

it's more about the bling bling faktor

A rough finish helps keep atomized fuel in suspension, which is especially important with the intake reversion common to naturally aspirated engines with higher duration cams. In other words, air with fuel suspended in it coming back out of an intake runner into the common plenum, then entering another intake runner. This keeps the fuel from pooling in the runners or the plenum, promoting better combustion no matter which intake runner it eventually goes down. :)

As Alex mentioned, the interior of my Hogan's intake had a "220 grit" finish. Relatively smooth to the touch, but not perfectly smooth or anywhere near polished, obviously. Hogan's builds intakes for many of the most powerful naturally aspirated racing engines on the planet, so it's safe to say they know their business.

https://www.rx7club.com/attachment.p...chmentid=73818

He speaks! :p:

https://www.rx7club.com/3rd-generation-specific-1993-2002-16/polishing-uim-bad-heat-dissipation-398617/ ;)

The runners will casue a swirl throughout the intake runner and into the combustion chamber. Hence why intake ports have a certain shape. The shape should improve the intake flow and spread the mixture as optimal as possible in the combustion chamber, partially enhancing the flame front/movement art combustion.

Look at piston engines where different manufacturers have different shape inlet ports and piston shapes. Both to optimalize the combustion and to spread the mixture.


You need to see the bigger picture and look at it from intake manifold to runner with injector to intake port to combustion chamber.

FDNewbie ;-) twisted minds think alike :-):-)

Again I agree to a point (where fuel is mixed with the incomming air before it hits the combustion chamber) our motors are not very good at burning all the fuel as it is, I just don't see how increasing the flow is a bad thing or we wouldn't spend all the money we do to do exactly that.

Just be very careful assuming that if it's for sale on the aftermarket, that it's an improvement. Auto aftermarket IMO is richly loaded with products for sale that look effective, but don't actually do anything. They get away with it because the buyer either wants to believe it works, or is unable to prove that it doesn't work.

Dave

My head hurts.

I believe in collecting DATA!
Having enough information before a modification and then after tells a big story.

I had a customer 2 weeks ago that had run 2 different engines in his RX7 from 2 different engine builders and both engines had similar peak power outputs on the dyno and he thought that one engine felt faster on the track than the other, but from the data we could see that the engine that he felt was slower actually made more touque lower in the rev range with a flatter torque curve and was actually faster on the track. The engine that he felt was faster actually came on harder and felt like it had more power.

Becarefull doing modifications and just feeling like it is faster, collect data.

Fixing only one part of your air intake may not have any effect if that part is not the bigest restriction on your intake.

Ian.

Exactly. To futher expound on that... https://www.rx7club.com/3rd-generation-specific-1993-2002-16/when-does-uim-lim-tb-become-restriction-371413/ ;)

~Ramy

I have been useing a MoTeC ADL Dash logger to do most of my development work for about 10 years,
for example using pressure and temperature sensors before and after a lot of the intake components, like measuring the air box before and after the air cleaner element, before and after the IC and after the throttle body when coupled to the rest of the engine and car data tells the true story especialy when you can collect the data in the conditions the vehicle has to be used in. With this data I can see clearly when it works and why.
When Mazda Australia developed the RX7-SP we used Pi Research data logging, we had to produce a competative race car out of road going RX7 with a minimum amount of modifications, like the modifications for the air intake, the main restrictions we found were in the air inlet to the box, air box to turbo plumbing and the intercooler. We were able to test the modifications at the race track that we were going to race the car at.

Ian.

I see your point, but increasing flow and the behaviour of the flow are 2 different aspects.

Opening up an intake port will increase flow. The shape of the port and the finish on it's walls will determine the swirl of the incoming air.
They have little to do with eachother when it comes down to the volume of air being transferred.
I would prefer smooth polished exhaust ports and rougher intake ports than the other way around.

So where you polish would be more important than what you polish ? I.E. work done in the TB, UIM, and LIM area would have more effect than say the Elbow, IC pipes and turbo ? I can't see where turbulance would be any bennifit prior to the TB.

This pretty much covers my point, it's more important/more crittical with N/A where the fuel is mixed/flowing with the air than with direct port injection (which our engines appear to be)

i dont think intake manifold texture has anything to do with mixing fuel and air. the air that "sticks" to the rough metal helps the air touching only air move smoother, and quicker

this is a pretty common opinion, and one that i agree with entirely. given a different setup, the super-smooth flow could definitely be bad for the fuel and air mixing. but think about how much the air tumbles going through the turbo, intercooler, and 3 throttle-body butterfly plates, not to mention the crazy plumbing route.

just my .02

Intake manifolds themsevles are fine when polished. He meant polishing the ports of the intake themselves I'm sure. THAT is detrimental.

I understand he was referring to the "inside" of the manifold, and i understand the consept of needing the turbulence for mixing the fuel in the intake runners (N/A) but i fail to see the rough texture promoting a higher flowing condition than a smooth one. Rough causes turbulence, eddies, or whatever you want to call them and I believe the air will flow smoother and faster if all the air is indeed going in the same direction not all over the place, were talking strickly air flow not air fuel mixture. I can garuntee you a pipe with smooth walls will flow water faster than a tube with rough walls and air acts very much like water when it comes to flow. Less restriction, friction, or turbulence will give you higher flow, you can even prove this theory with a mass air flow meter.

Airflow over any surface creates a boundary layer, which in very simplistic terms is like a thin film of air that follows the surface to help the rest of the air flow around it. As was stated before, if your intake is fairly straight without any bends or protrusions, then you want the inside surface to be as smooth as possible, because it helps maintain the initial laminar boundary layer, which in a perfect world is the best and smoothest layer. Unfortunately, intakes are rarely completely straight and smooth, and a laminar boundary layer is extremely fragile; it doesn't take much aerodynamically to upset a laminar boundary layer, and the numerous bends and twists in an intake basically make it impossible for a laminar boundary layer to exist.

The next stage of boundary layer airflow before it separates from the surface completely and tumbles into a wake (which slows down airflow drastically and creates pressure drag) is called a turbulent boundary layer. The advantages of this stage of boundary layer is that it has more energy, which gives the airflow momentum (and speed) to follow gentle curves and bends that the laminar boundary layer cannot.

The rough internal surface basically "trips" the airflow into the turbulent boundary layer stage more quickly, so that the airflow can follow the bends of an intake with more speed. It's the same concept as the dimples in a golf ball; the dimples create an earlier formation of turbulent boundary layer that allow the airflow to maintain speed and cling to the trailing surface of the ball longer, reducing drag and allowing the ball to travel farther.

The boundary layer is also the reason the rough surface helps keep the fuel droplets in suspension. If airflow speed drops low enough, the boundary layer separates from the internal surface, and then the fuel droplets fall out of the intake charge airflow and condense on it.

I'm probably a little off technically on this (any aerodynamicists out there can correct me), but I'm pretty sure the basic premise is correct.

Amazing :)

cool.....polished on straight, bumpy on cures. Now I get it. (-:

Reynolds number
 

http://en.wikipedia.org/wiki/Reynolds_number

Turbulent flow in a fluid occurs when the inertial forces of the fluid overcome the viscous forces. It is easiest to visualize with a lit cigarette in a still room. The smoke will ascend smoothly, and then start tumbling over itself. That is the speed at which the fluid viscous forces become smaller than the inertial forces. The fluid kind of trips on itself and rolls into eddies.

Kento is exactly right. Turbulent flow over a concave surface will separate later than a laminar flow, causing a smaller low pressure zone (which is why golf balls have dimples - the smaller low pressure zone behind the ball means less drag). I am guessing it is these low pressure zones that condense out the gasoline (like the contrails behind a jet airplane formed by condensed water), so keeping them smaller would be good, hence the rough surfaces helping on the curves. But that's just an educated guess.