I have noticed that engines that make a lot of power, have huge amounts of air exiting the exhaust pipes. feel the exhaust emitting from you tailpipes at idle. if it really pounds out your most likely making some good power.
The accepted theory today is that roughness in the walls aids turbulence and assists fuel mixing with the air, which is much more important for power.
Terry Sturgeon.
So maybe instead of smoothing out the inside, you can roughen it up.
Mike Moore
Well, dang!!! My ported head is a little rough on the inside. I always figured I would smooth it a bit next time I remove the head. Now I guess I better leave it alone.
The things I don’t know could fill a book!
LLoyd
I bet they polished them, that was the in thing then, all the advice I got with heads and ports was…polish them…I suppose they likened it to water flow, the theory being if it flows more water, it will flow more air, spent hours getting a shine on side valve Ford heads, and I had a thing about valve springs, I was always taking them out of my air rifles and stretching them for more performance, tried it with some Singer le mans springs and ruined them !!!
Smooth it. Don’t smooth it. Don’t polish! Polish! Polish the outside instead. Make it rougher! Stick some straws and epoxy in it. Turbulence bad, turbulence good. Add a decal or two while you’re at it. Suck, blow, fettle, CFM, meters/second, dyno, seat-of-the-pants, radius the pistons each side of the bifurcated bafflegarb, Pintos, Dolomites, Fords, tightwad Lyons …
Ask an E-type owner/expert for the time and he’ll tell you how to build a watch. Put 15 of them into a room and ask them a question and you’ll get 19 different answers!
Is it too early for a drink?
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That means only four of us are in two minds, surely that is a low estimate.
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What I was taught was leave roughness on the intake, but polish the exhaust side.
Sounds to me like the only reasonable solution is to smooth HALF of them, leave the other half rough… 
Regards,
Ray L.
So what do you do with an intake manifold intended for port (or direct) fuel injection?
This is a fun and interesting topic but the reality of modifying your intake manifold is that it is something you do at the end of your quest for horsepower, not at the beginning, and only when you are looking for the absolute maximum performance. Understanding fluid flow, including air is not intuitive and the probability of doing more harm than good is very real. At best you are talking about very minor gains - not at all noticeable in day to day driving. Better to do the big valve thing, increase ignition timing, run richer needles, install a better air filter etc. It’s a fact - the stock head, intake and exhaust manifold flow air extraordinarily well. My dyno guy, who builds all makes (not Jaguar though) racing engines was astonished when he put the setup on his air flow bench.
Terry Sturgeon
Newfangled fads. They won’t last. What could possibly be better than an SU carburettor?
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Too early? I don’t know about you, but it’s 10 PM over here!
Cheers!
Pekka T.
Fin,
That puts you seven hours ahead, Pekka. I’ll just have to drum my fingers for the next couple of hours till the sun’s over the yardarm. Or, I can pretend I’m in Helsinki. Better still, Oz. It’s already tomorrow morning there, so I can experience the drink and the hangover simultaneously.
Three of them in a row.
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More decals = MORE horsepressure!!!
Thank you!
Been saying this for YEARS.
Remember, it’s always happy hour somewhere.
I wondered why and found this, from hotrodders.com:
Re: polishing intakes
Quote:
Originally posted by Max Keith
I highly advise against polishing intake runners both in the intake and the head. The problem with polishing them is that it allows the fuel air mixture to become unsuspended, costing a lot of HP. What you want is a florentine finish. Something to do with laminar air flow, and since Im not an aeronautical engineer, I wont try to explain it. What I do understand is that you need a certain amount of roughness in the surface of the runners and ports to keep the fuel air mixture suspended.
You are correct. To make a short story long, gasoline in an intake manifold likes to condense into a liquid (@ its dew point in the manifold at many temperature/pressure conditions) and liquid/air don’t distribute equally plus liquids do not burn - only gasses burn. To keep the gasoline vaporized or at least well atomized and in suspension in the air stream, there must be turbulent flow in the intake manifold runners. Turbulent flow is a precise technical term but it’s meaning is pretty plain in non-technical terms.
There are several ways to induce and sustain this turbulence. One is to have the gas go around a bend which induces turbulent flow. Most manifolds don’t have sharp bends so the only other alternative is to add roughness to the passages. Envision the gas flowing in a conduit and the molecules in direct contact with the conduit wall are by definition not moving. The next molecule out is moving a little but due to friction with the stationary one, it is not as fast as the next molecule out. Speed of the molecules increases as they are spaced away from the wall. This virtually no-flow region is termed the boundary layer and the thicker this is, the worse it is for an intake manifold. Continue looking at the molecules as you go to the center of the flowing stream and you will find they get faster and faster until the very center ones are fastest of all.
There is a definite break-over point in any flowing stream which is a function of fluid viscosity, velocity, and conduit roughness where fluid goes from ‘laminar flow’ with a very large boundary layer where there is no turbulence to the desired full turbulent flow which nearly eliminates the boundary layer. In the former case, gasoline will tend to condense and drop out of the stream causing many problems so it is good practice to design the intake runners with significant roughness.
That said there probably is some value to port matching.
Ok. I’m convinced. Leave 'er be.
But Pete, can you explain the payback from your straw/epoxy patch to match in/out vacuum orifices?
I’m not about smoothing.
I’m about matching the intake to the head and usually, I’ll dowel pin it for repeatability. No use getting it matched and letting the stud hole slop mismatch your work.
This entails matching the gasket to the head and the head to the gasket.