1.8 Vs. 2.0 Revisited AKA why 1.8 instead of a 2.0 head

[treed2@u.washington.edu (T. Reed)]

I feel like I should interject here..

> And yes, I do agree that smaller ports mean a faster air velocity, but look
> at the big picture and take into consideration Volume of air moved.  When
> the exhaust valves are closed, the slight increase in "how quick" the air
> reaches the combustion chamber isn't as important as "how much" air gets in
> there.

In a normally aspirated motor, velocity is just as or more important than
flow. The speed of the air coming in is what mixes the air and fuel
together for good combustion. Accounting for this is what makes the
difference between a good port job and a bad port job. People that just
hog out as much material as possible to get the biggest opening will
probably get more flow but they will decrease the air velocity so much
that the car will actually make less power because the air doesn't swirl
around in the chamber when it "hits a brick wall" and mix properly with
the atomized fuel.

If you don't believe me take a look at the numbers for the 50mm intake
manifold. Bolted right on to a stock engine, it results in a motor that
makes less horsepower and less torque than before (with the 40mm intake).

If you have forced induction, though, you have positive pressure going in
to the head so there is plenty of air velocity even with very large
openings.

In these statements I haven't even accounted for "flow" in the flow bench
sense of the word. As in, not simplified 9th grade physics flow. An
idiotic port job will also introduce counter-productive eddys, etc.

> But I do agree with you when you say the shape is important.  However, when
> you mention cooling, it seems that the more material in head the more it
> would insulate right?  Well yes, but maybe too well, just remember, it may
> take longer to heat up (resist heat longer), but that just means it's gonna
> take a whole lot longer to cool down.

You're correct, but I think you're missing part of the picture here. How
long it takes to cool down doesn't matter because it is only cooling down
when the engine is off. You're not worried about making power when the
engine is off.

When you blow air through the ports in the head, it heats up (no matter
what). This is bad, but in the process of heating up, it cools down the
head a little bit (think about it, just like blowing on a hot piece of
pizza). If the flow is high enough and stays high, over a period of time
the metal is going to be cooled down more and more as air flows past it.
The cooler it gets, the less it heats up the air in turn. That's good for
power.

The insulating point is pretty moot, though. Metal doesn't insulate well.
I was just trying to explain why it would help IF it was insulated. You
could ceramic-coat the inside of the head all the way down to the valves.
I bet it would help noticably to get cold cold air that far in to the
engine before warming it up (but also hurt flow because of the thickness
of the coating).

My 2 cents..

-Toby