07/02/2003

Turbocharging and CR

An econ-box pushrod 2 valve v-6 with a piddling 231 cubic inches that was
designed to replace an inline 6 in the least demanding of normal american car
usage - and then simple turbo'd and one day EFI makes an 11 second quarter
car, and more E-ticket rides than you can imaging.

Truthism:  Any engine larger than a 231 that cannot make an eleven second
ride, turn 130+mph, lose traction at all speeds and brake your back under
boost in a 4000 lbs car is a piece of doggy doo doo.  If you have 400 inches
and can not keep up with a larger heavier black car - your engine stinks.

That engine has already proved it is incapable of passing first loser any
time. 

Keep thinking whatever that led you to first loser and keeps you failing - or
try to understand why 231 kicks your buns and eats your lunch and happily
blows corncobs off plants in fields.  Facts is Facts

If whatever you are doing with whatever excess of cubic inches does not run as
quick or fast in a lighter car - you are not doing it right.

Secret 1.  The engines displacement has nothing to do with airflow thru a
turbo/centrifical equipped engine.  The compressor does it all.  750+ from a
90 cubic inch miller, 1000+ from 1.5 liter F1 all day long in competition.
Translation - 5000 hp from a chevy v-8 for the distance of an Indy 500 on
alcohol.  Get the picture - if you can't match the performance of an indy car
or F1 with their dinky engines, maybe you better learn how they do it.  BOOST
- airflow from an external compressor.  

Secret 1 REPEATED.  The external compressor determines air flow thru the
engine.  The engines displacement merely determines back pressure behind the
compressor.  That is all displacement is good for - back pressure.  It lost
its ability to have any effect on power when the intake manifold goes
positive.

Secret 2.  An effective combustion chamber.  Even n/a an open chamber is worth
50 to a hundred horsepower LOSS over a closed chamber.  Boost makes it worse.
Loss of quench, swirl, tumble, squish will make less power and rapidly and
early led to detonation and self destruction.  Turning a closed chamber into
an open pie gives you major reductions in power under boost.  The Buick V-6 is
about 8.5 to one with good basic design.

Secret 3.  Low compression makes MORE POWER - not by LOW COMPRESSION - but by
better time and pressure relationships of the piston and clearance volume near
TDC.  Nothing these relationship changes make will ever compensate for a
crappy combustion chamber shape and will turn a OPEN chamber design into a
worthless soggy piece of dinosaur door.  Historical note.  By the 1920, when
the available swill limited compression ratio to under 5:1, an open pie shaped
chamber was known to be weak, slaggardly and too prone to detonation to even
be considered for use on a HIGH COMPRESSION 5:1 engine or even on a cheapo
production engine.  In 50 years, we forgot, did it again and turned the
reputation of 454 and 460 into extremely low performance truck engines -
beatable by any V-8 half its size without even trying.  Care to repeat it 30
years later?

Secret 4.  Lower compression higher boost is more resistant to detonation.
Back to combustion basics 101.  The MASS of the charge is a prime factor in
the burning rate of the charge.  Shoving more mass into a cylinder increases
the mass of the charge ( btw  Power is directly proportional to mass ), which
means the total mass will take longer to consume.  Simply theory - explains
why it does not all go bang at tdc and the characteristic of similar pressure
but significantly longer pressure pulse from a supercharged engine.

Atomic ratio is the same - CHON.  The greater the density of the charge, the
less the heat loss to the metal.  This counters the greater mass - but not
perfectly - still leaving a longer burn time.  Ceramics eliminates this issue.

But, the total energy of the charge - the heat - is lower under lower
compression - because the piston can't shove as much energy into the charge!!!
The pistons compression is almost adiabatic - there is not much time for heat
loss or addition.  The turbo's initial compression is abiabatic - but the long
manifolds, plenum etc give sufficient time for significant heat loss.  Plus,
the turbo is more heat efficient at compression that the piston.  End result -
same mass similar pressure but much less heat with a turbo over a high
compression piston.  Detonation equal Heat x time x pressure.  

The lower the piston compression - the greater the charge can be without
detonation.

Bob W is very close to having it right, a long rod 302 can be higher boosted
and make more power with a decent head and chamber than a higher compression
350.  The lowered compression gives bettor geometry and detontation resistance
and the reduced displacement allows the retention of an excellent piston and
chamber.  

The seventy ( 1/4 larger engine over a 231 ) extra cubic inches will make
little difference on the response or performance of the engine and even going
to 350 - an extra 120 cubic inches won't be squat in performance over a well
done 231.  It will simply mean that the centrifugal blower he wants to use
will have the air flow capacity within his budget.  In reality, it will be
much harder to make the higher compression 350 produce the same results as the
231 than for the 302.

Secret Next.  Displacement x rpm.  A 20% reduction in stroke can be matched by
a 20% increase in rpm and the total air of the engine is the same.  You get
better rod relationships, less inertial energy, less acceleration losses, a
faster quicker engine and still flow the same amount of air.

There is only so much you can do with a given engine combination.  It would be
difficult to turn an 11 to 1 engine into a high powered 6 to 1 engine - but
you may be able to get it down to 9 to 1 and put some decent boost on it. Or
you may be able to de stroke it, spin its power band out of the detonation
range and get ridiculous with boost.

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

Right.  A 20 % difference in displacement is easily compensated for with more
pressure and a better chamber.  Each engine gives you a range to work within.

Example 1000 CFM will develop about 675 to 700 hundred hp with about 325 to
350 lbs of fuel an hour.  

A given external compressor with the proper drive can deliver 1000 cfm thru a
broad range of back pressure.

Non turbos require gearing changes to compensate for back pressure - but as
long as a turbo is in its delivery curve and its turbines waste gate is open
can compensate with changes in its power by changes in the wastegate.

The approach would be to determine the power you want for your performance
level.  This gives you the air.  Pick a compressor that delivers that air in
its performance range.

Next, we develop the reciprating assemble.

Normally we deal with Volumetric Efficiency.  This is how effective the piston
is operating against the atmospheric well.  100% VE means that the piston
makes a pump that exactly fills the cylinder to atmospheric pressure.  Side
valves operate around 75% VE, 2 Valve OHV engines around 85% ( the mystery
number that appears in all simplified calculations ), and 4 valve pentroof
engines can reach 115%.

The manifold pressure can be considered an online current indication of the VE
when NA.

When the compressors output is below atmospheric ( throttled ) VE determines
how much total charge the engine can get.

When the compressors output matches the VE and the cylinder filling is
identical to peak VE, then VE is no longer a relevant number or consideration.

The VE becomes a resistance to the charge.  The lower the VE, the more the
resistance to air flow.

Thus, until the flow across the valves becomes sonic and limited ( critical
pressure and flow ), the difference between side valves and 4 valve pent roofs
is the amount of back pressure they introduce.

With a turbo operating within wastegate to deliver the desired flow, side
valves or quad valves - within back pressure limits make no difference to air
flow.

Note:  When the air flow begins to choke the valve - the better the basic VE
the higher the ultimate performance will be.  There is no way a side valve
engine will ever keep up with a 4 valve when the air flow exceeds what the
side valve can accommodate.  This point can be a lot higher that propaganda
will tell you thought.   Think things thru.

Now at a given fixed flow - the boost pressure ie back pressure is dependent
on VE.  Ok.  Now lets look at displacement.

Once the pressure exceeds the atmospheric filling of the cylinder,
displacement does not affect power.  Power is determined by the compressor
because the compressor determines the airflow.

The displacement becomes simply a means of back pressure.  The cylinder will
be filled with a fixed amount of air or fuel regardless of the displacement.
The displacement can range from 90 cubic inches to 600.  As long as the
compressor pumps 1000 cfm - regardless of the back pressure - about 700 hp
will be developed.

This makes displacement within a reasonable range a convenience.  The only
affect of displacement is a variation in back pressure.  Within the waste gate
range - it cannot affect power.

Now consider chamber efficiency.  Under the best of circumstances,
displacement can have an effect until the compressor is delivering more air
than the reciprocating assembly can provide and then presto - its back
pressure.  OK

Chamber efficiency however affects power from idle to max power.  The amount
of power per pound of fuel and air depends on how efficient the engine is.
Chamber efficiency seriously affects the overall efficiency of the engine.
This seriously affects power all thru the band.

Thus the absolute insistence on getting and maintaining a good chamber.
Displacement is back pressure and easily compensated for.  Loss of efficiency
affects the entire spectrum unfavorably.

Within reasonable limits compression ratio affects two thing - power
efficiency and knock.  A lower compression ratio gives a moderately lower
expansion ratio which can effect low end economy - however - it gives
significant advantages at converting pressure to power.  It is significant but
not enough to get insane about conversion to.

This leaves knock as the issue.  Knock is temperature, pressure and time
related.

A common component for knock reduction and power increases is the intercooler.
If an engine is Knock Limited and the intercooler raises the knock limit by
cooling the charge, Knock Limited power will increase.  Heresy next.  Power
can be increased by intercooling only to the point the engine is no longer
knock limited.  An intercooler will always increase the back pressure of the
intake and raise the temperature of the charge prior to entering the
intercooler.  The more effective an intercooler is, the higher the back
pressure it will cause and the higher the inlet temperature.  Too small and it
does nothing.  Too large and the increase does nothing for the engine.  Think
it thru.  Again apply lessons learned in Thinking 101.

A lower compression means that less energy is shoved into the charge during
compression.  Higher boost shoves more density MASS into the chamber which is
indifference to knock.  The larger mass slows combustion and the higher
pressure raises temperature - but the same mass at tdc is cooler with high
boost low compression than with low boost high compression.  Knock being
partially dependant on temperature will go down with the identical mass.

Why do we mention same charge?  Because the compressor will deliver the same
charge in mass at varying temperature and back pressure as long as the
compressor is within its map and waste gate.

Now that I've thoughly confused the issue, I've got to hang up and get ready
for dinner at Pinnacle Petes with Liz.  Liz is young, almond eyes, dark
chocolate skin, long black hair, slinks like a cobra, with nice che che's and
is kind a of wild and crazy.  Lots of steak, doing my salsa lessons afterwards
and a friendly good nite kiss.  She no wants to hooochy coo with gramps - but
she keeps me introduced to her friends who like to hooochy coo with gramps.
Don't even think of that word about my young delightful friendly eye candy and
ego builder.