08/22/2003

Boost and acoustical tuning

Assume an engine of 100% power.  With acoustical tweaking and peaking, you can
reach about 115% power.  With a low boost system, you can go up to about 175%
power and 5 to 7 psi and the two will complement.  Silly me has been saying
that for years - but what the heck would I know.

With an open plenum design, you can start with 100% power and get orders of
magnitude like 300% to 500% with SUFFICIENT HIGH BOOST PRESSURE.  Emphasis
repeat Emphasis SUFFICIENT HIGH BOOST PRESSURE.  Strange but bruce and I have
been saying that with qualifications for years.

But once you get above 5-7 psi, everything acoustical is simply backpressure.

If you are building for high boost pressure, small end power increases are not
worth the effort - a gear change or transbrake or .... can double your power -
well past what acoustical could possibly ever provide - instantly.  But if
those little knick nack power bumps are important to you - go for it but they
mean diddlysquat on the full power output.

If you chose not to run boost above 5 to 7 psi, then the acoustical/kinetic
tuning will work very well.

The acoustical tuning that is so very helpful at low boost begins to seriously
affect power and response at HIGH BOOST and using the Cosworth as an example
is a short wide funnel like bell mouth to simply assist in filling the port.
And its does not have to even function below major boost and 11,000 rpm.

But in this case the discussion was with high boost engines and designs and
with the ability to quickly move into high boost regions.  

Where do you want to spend your time and money?  Every possible benefit to
acoustical tuning/helmholz/runners etc do not make up for a short hard stab on
the gas and a rapidly spooling HIGH BOOST turbine.  But, if you wish to only
occasionally see boost, drive at 55 and obey the law, then you do not need or
want a high boost engine.

Bruce's engine has no acoustical tweaks and peaks and has no spool up issues
and response issues.  Could acoustics help? Maybe - if Bruce did not have such
big feet.  But he does and he spends way too much time grinning.


-----------------------
Around 6 psi maximum maximum maximum is the range for peak pressure on a well
made ram tuned runner.  Realistically, you see closer to less than three psi.
These are not contiguous figures, they are peak figures at the end of the
intake.

If the compressor is capable of pumping a certain amount of air against a
certain amount of back pressure, once you exceed the pumping pressure over the
back pressure, the back pressure begins to lose its meaningless to flow.

It is easiest to visualize with a roots/eaton/lyscholm supercharger.  Each
turn of the screw forces so much charge into the engine.  When the screw is
being driven hard enough, it does not matter what happens downstream - that
chunk of air is going into the cylinder period.  1000 cfm off a roots means
1000 cfm is going into the cylinder (100%).  The only other place that air can
go is through a major explosive leak.  

The screw forces a fixed amount in.  All that anything downstream does is vary
the back pressure - the amount of charge is essentially fixed.

Now, look at a centrifugal compressor.  If the blade is fixed rpm, then the
air that will be pumped is related to both the capacity of the pump and the
back pressure.  Changing pressure waves downstream will set up a cycle of low
and high pressure - BUT - when the BACKPRESSURE peaks and the air flow slows
ways down at the valve closing and then when the null hits elsewhere and the
back pressure drops, the flow will seriously increase.  Flow does not diminish
with a reduction in back pressure like NA but greatly increases.

Note that the "hole" that the acoustics create to make a peak at the end of
the event - the compressor fills in with a very large charge of air.  In fact,
all of the peaks where the acoustics do their thing HURT charge flow into the
engine ( greatly increased back pressure and significantly lower charge flow
), and the valleys - to be avoided like the plague on NA - allow the
compressor to pound more charge into the cylinder.  In fact, everything about
acoustic tuning starts to get turned around under continuous boost.

NOTHING the reciprocating assembly does "pumps" air into the engine once the
pressure is positive.  Everything is simply resistance to how much charge the
compressor is going to pump in.  And acoustics simply varied the resistance.

The area of 5 to 7 psi deals with where there is sufficient pressure to fill
the runner and the cylinder regardless of what the acoustics are doing at
whatever rpm and point.  

With a mechanical supercharger, to maintain the flow against a resistance, it
is necessary to increase the impeller speed.  A turbo OPERATING WITHIN
WASTEGATE increases its speed as needed to maintain the one way flow of charge
thru the system until either the resistance ( back pressure ) gets too high or
the compressor moves out of map and "stalls".


-----------------------------

Lyscholm, Miller and Fueling ( at the least I can remember ) have all used the
exhaust pressure in or near blow down to raise the pressure in the intake
plenum.  

Each of them at various times, proposed cracking the intake port at about the
same time as the exhaust valve cracks.  Properly done, it will "shoot" a jet
of superhot exhaust out the intake port and draw most of it back in around bdc
- with the normal end of blow down in cylinder depression.

Note that the volume of the "exhaust charge" will be about 6 to 9 times the
volume of the intake charge because of the pressure.  Most engines will be
lucky to clear 95% of the used charge as exhaust.  This reverting percentage
allows a large slug of gas to run backwards up a runner - shoving fresh charge
in front of it.  

A proper system has a one way fluidics device to trap reversion.  This
reverting exhaust shoves charge against the fluidics device and raises delta p
nicely.  Timed in conjunction with another cylinders valve opening, the delta
p can be significantly raised and the overall super-atmospheric effect can be
increased.  

A cross ram or a tunnel ram would be excellent manifolds to apply this
super-atmospheric trick to.

Ford's Tom Ma developed another tactic to get continuous "ram" tuning
throughout the rpm range.  Rather than dick with the length, he used a
programmable interrupter.  This was a rotating cam in a barrel that when it
rotated around to the cylinder - it blocked the air flow.  This blockage was
near midstroke and generated a massive pressure spike.  By timing when to
generate the spike, he was able to acoustically reflect the spike and have a
significant charge improvement just as the valve was closing.  No need to
tweak the cam when you could dial in the acoustical tuning over a 2:1 region
or more.

Just remember that are a thousand ways in the transition region to get maximum
power and only a very few once you go above it.

--------------------------

The discussion about "runners" being effective was originally and completely
about manifold's under sufficient boost pressure.  In the internet special
olympics, virtually all forms of acoustical tuning regardless of any relevance
to high boost pressure were brought out to shove up my arse to prove acoustics
work.
 
Whoop de pharking de.  Silly me had already laid out the fact that they were
effective and useable up to 5-7 psi.  Did not matter - I was now branded
anti-acoustic - not fully following the pronouncements of bombast John and
everyone pile on.  It was even detailed that the pressure was compressor
pressure.  And discussion were held were the delta p on turbos was discussed
and the criticalness of using a buchi or birmann type exhaust to establish
this delta p.

But, to win the special olympic argument - nothing I had said was understood -
jest get dat boy cause I, not having thought it thru, just know he be full of
shiiite.  Everybody wins - not another new technical thought to hit these
pages.

Old issue.

There has never been a question of the effectiveness of proper air flow
shaping - whether at the mouth of a carb/throttle body to various transitions
and to the entry of the port.

A "runner" or tuned length generally has an angle of dispersion or entry of a
maximum of one degree.  Its purpose is to establish the temporal relationship
of acoustical or give acceleration to kinetic tuning or both.

Bell mouths, funnels, etc like I thought I saw on the last generations of the
Cosworth engines ( about 30 degree divergence angle from port casting to edge
of Bell mouth ) are not acoustical tuning.  They are air flow shaping devices
and they do make significant power - but not from acoustical effects.  They
make it possible to put more air into the engine by reducing the back pressure
needed to fill the cylinder and are effective from idle to full load.

Vizard covers bell mouths and similar air flow Shapes.