there's a simple enough explanation. EFi stuff - the inlet manifold is
dry flow. There's no fuel to drop out of the inlet flow or condense on
the runner walls, or puddle in low spots. Injector placement on efi
production cars is relatively close to the head, and therefore the inlet
valve. It's like this not for power, but for the lowest emissions at
part throttle, low-mid rpm operating. For absolute best power (and it is
utilised on stuff like formula 1 partially, and on aus supercars) as
more remote injector placement = more power. I'm told that supercar
injectors are placed as far out as at or above the bellmouth of the
intake trumpet.

Even with efi, injector location too remote from the inlet valve, and
the port will still result in some level of fuel dropout at lower rpms.

WIth a long runner inlet, you can get high velocity at lower rpms. You
can also tailor the length so that you get sonic tuning effect down low.
The difference is (with some carbed hypothetical setup with similar
runner cross section area and length etc) that the fuel isn't introduced
until near the inlet valve, and may even be aimed at the valve back
'face'. This would be the point of maximum shock and turbulence and
would theoretically affect the best mixture distribution/dispersion and
suspension. so for all intents and purposes, the inlet tract can be
considerably lengthy for better low rpm volumetric efficiency. But in
this scenario the boost to volumetric efficiency and cylinder filling is
no longer overpowered by the losses resulting from poor mixture
suspension, distribution and suspension.

You could achieve _some percentage_ of the results with carbs (esp side
draught webers - hypothetically on an inline 6) by having the carbs not
too far from the head - a modest inlet manifold runner length, but then
adding custom 'trumpets' to the carb mouth that effectively extend the
runner much further. Then you'd get a lot of the effects without
suffering fuel dropout etc.

Having said all that - you could theoretically get excellent part
throttle economy and low-mid rpm output using a tunnel ram. Most of the
ready made ones would be designed for higher rpm use however. What would
be called for is either a custom made one - tig welded together (which
wouldn't be cheap, but is an option) specifically configured to suit, or
you'd need to modify an existing one.

The (massively abbreviated) short list would include :

Filling of the port floor etc in the head.

filling the tunnel ram runners so that they too have a reduced cross
section (and to make sure there's no 'step' at the head face due to the
port floor in the head being higher than the tunnel ram runner it mates
to.

a total 're-think' of the plenum section that sits on the top of the
tunnel ram runners (usually bolted - a removable top section) to firstly
make sure that the front and rear plenums are separate (generally not
the 100% best option for these goals, but due to idiosynchrasies of teh
trams I've come across, of more benefit due to the overall considerable
reduction in total plenum volume = a better signal strength at lower
rpm)

In general you'd want to further minimise plenum volume if possible - by
filling in any ballooned out areas above the runners. Blending it to
make a smooth and most direct transition from plenum to each runner.

You'd want the smallest pair of carbs (or mixers) consistent with still
meeting peak power requirements (of your choosing). THe smaller the
better - potentially higher velocity. more velocity through carb or
mixer = best chance of finitely setting the best part throttle a/f
ratio.
--
more on the heads specifically. similar to the intakes, the exhaust
ports have a huge dead area on teh floor (and it fans outward as you
move further along the port, so the dead area grows. This creates poor
exhaust flow, encourages reversion, and thereby can pollute the intake
charge for the next power cycle. BY building up the port floor
(different methods mandated - tongues made from iron or something and
attached via bolts or cap screws through holes drilled in the port wall
(I have a web link with details and pics as to what was done to
pro-stock cleveland heads a couple of decades ago). This ups exhaust
velocity, actually improves exh flow, drasticaly reduces reversion and
contamination of the incoming charge. All add up to greatly reduced
emissions, but also it drastically reduces the intake contamination -
bottom line - for the same amount of ingested air/fuel - it will
experience a much better quality burn (more complete, and quicker - so
the maximum amount of 'push' is delivered to the piston while it's still
high enough in the bore that the expanding gases can push hard. Once it
travels too far down, the same gas at the same temp will have a lot less
pressure (due to exponentially greater volume. SO getting the majority
of the burn at a more opportune time will produce more power - even if
the amount of total heat energy created is identical.

At the risk of way oversimplifying - burnt out valves can be caused by
poor combustion. If you get the burn quick and complete, then there's
far more power developed from the same amt of air/fuel.

IF the chamber etc produces a poor burn, and an incomplete one, then the
tail end of the mixture will still be smouldering/burning as it travels
out the exhaust port. The exhaust valve cools when it's on the seat.

Modern chamber design biases as much mixture toward the exhaust valve
area on the compression stroke. then the burn will initiate there etc.
It may sound counter productive - as it's mroe heat at that spot. BUt it
ends up producing a more complete and relatively quicker burn - so that
by the time the exhaust valve opens, there's less buring still occuring,
it's done, and hence it actually heats the exhaust valve a lot less.

Just thought I'd throw that in there since some might wonder why if high
exhaust valve temps lead to accelerated valve/seat recession, it would
ever be advantageous to deliberately 'concentrate' the mixture in that
location just prior to spark initiating the burn.