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I found this short article and thought I'd throw it out for discussion. Â What surprised me, is that it claims that polishing can actually hurt performance.

After skimming the article, he seems to have 2 objections to port/polish jobs.

1) fuel dropout due to overly-smooth surfaces in air/fuel's path. Â My cure? Â roughen it up a bit, once you're done porting.

2)loss of air/fuel mixture velocity due to higher cross-sectional area of the runners. Â No way around that, but if your heador manifold or whatever is restrictive, improving flow capacity to an extent surely overcomes the cost of losing velocity.

EDIT: Since the link is dead I've posted the article content here.
Engine Intake Porting
Know the facts before you modify your engine.

Smooth porting of the intake tract, in nearly every instance (save a few engines designed to run at full throttle all the time), COSTS HORSEPOWER. That's right.. that nice mirror finish that makes those ports look so sexy, is in most cases causing you to lose power, and in more extreme cases, make an engine non-streetable. Lets look at what scientific research has to say about the mechanics of engine breathing and power production.

Atomization and Vaporization
Before gasoline can be burned, it must be vaporized. Vaporization is the process of a liquid changing to a gas state, and this change only occurs once the liquid has exceeded it's boiling point.

In the case of water, it boils at 212F when at sea level. At a lower atmospheric pressure, it boils at a lower temperature. Thus, the lower the pressure within the intake tract at the point of fuel introduction, combined with an acceptable air stream temperature, the better the vaporization of the fuel, and efficiency of the burn.

In most passenger car engines, heat is applied to the intake manifold to raise the air stream temperature for better vaporization. However, this does cost some top-end power, as the higher temperature mixture, also has a lower density (and thus, less fuel/oxygen). It's a trade-off between street drivability and maximum power. The power loss in passenger engines is considered acceptable due to the smoother running of the engine, at part throttle. In a true race engine, every measure is taken to remove heat from the intake tract, because a cooler, more dense mixture means more horsepower, at the expense of low-end power and drivability.

There are number of factors that can cause poor fuel vaporization;

Intake tract pressure too high (raises boiling point of the fuel)
Air stream temperature too low (below the boiling point of fuel)
Air stream velocity too slow (fuel will drop out of the stream)
Any of these will reduce power output, drivability (throttle response), and efficiency (mileage).

Mixture Speed and Turbulence
Let's focus on the mixture velocity (speed) and turbulence, as these are directly affected by the design and modification of the cylinder head's intake ports.

The 'art' of cylinder head port design, is often considered a black art, and as such, many myths exist regarding cylinder head design and porting. One of the most common is that polishing the ports is the trick to make a head work. In reality, NOTHING is further from the truth. A polish does nothing to increase the power of an engine, and in fact, can reduce power output.

This can be proven only on a dyno. A test conducted by engine builder David Ray, using two cylinder heads, proved this point. Head #1 was highly worked over, and sported a very nice looking set of polished ports. Head #2 was worked over only slightly in the valve bowl area, but flowed teh same amount of air as Head #1 on the flow bench. When put together, and run on the dyno, the results where amazing. The rough port Head #2, produced it's best horsepower with a secondary barrel main jet two sizes smaller than that required for maximum horsepower using Head #1. The engine also produced more horsepower on less fuel!

What is happening in the above case? The engine is suffering from fuel dropout.

Fuel Dropout
Fuel dropout is the loss of fuel from the air stream, due to either condensation (temperature drops below the boiling point), or loss of speed and turbulance. In both cases, droplets of fuel will literally 'fall' out of the air stream onto the walls and floor of your intake port. If your head and manifold retain enough heat, it will once again boil and attempt to re-enter the air stream.

This is basicly a case of poor vaporization (the three primary causes of which we looked at, above). Poor vaporization means less power output per once of fuel consumed (not necessarily burned, read on to find out how this happens).

Why a polished port, costs you power
There are a number of reasons a nice looking polished port can make your engine produce less horsepower than it did in stock form, but here are the two that are most relevant to this discussion.

Loss of velocity. Over-working the port and making it too large is not a good thing.
Fuel will not properly puddle on intake walls or floor.
In both cases, the end result is the same. Fuel dropout causes the collection of liquid fuel on the walls/floor of the intake tract, where it will now easily flow down the port, directly into the cylinder as a liquid, which the engine will not be able to properly burn, causing lost power, wasted gas, and more emissions (that can clog your catalytic converter, causing even more power to be lost).

So why does a rough port work better? It's really very simple. As fuel drops out of the air stream, it collects on the walls and floor of the intake port, and puddles. However, since the port is not smooth, the gas is far less likely to flow into the cylinder as a liquid, before it can be re-vaporized by heat in the head/intake, or be picked up by the turbulent air within the port.

So, as I'm sure you can now see, even though it 'seems' like a polished port will get you that power you so badly want, in nearly every case, it's all show, and no go.
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