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.