[DeTomaso] Re: HP limits for 351C Block & Rotating Assembly, Turbo vs Non-Turbo

[Dave Doddek]

This could be a good reason why the Jap cars run tons of boost on a little
2 litre 4 cylinder and get 800 hp on stock block and heads and crank.  

Really what boost does is not give you more compression, it just increases
atmospheric pressure.  It is no different then operating your car several
miles blow sea level.  Sea level gives you 14.7 psi of air pressure to fill
the cylinder.  More air pressure means more air.  More air means more fuel.
 More fuel means more power.

By gas laws, pv=nrt.  P is pressure, v is volume, n is the number of moles
of air (quantity of mollecules), R is the constant representing air, and t
is the temperature in Kelvin.  Since r and v is constant and t is near
constant, P becomes proportional to n.  If you double the pressure, you
have double the air.  Since fuel is burned at a specific ratio between air
and gasoline, double the air means double the gasoline. Power is directly
related to the energy released during the burn per time.  If you burn
double the gasoline, you get double the BTUs and double the power.

Using the above equation, the only way to get more power from a naturaly
aspirated engine is to make the burn more effecient (thermal effeciency) by
increasing compression, or make the volume appear larger (voulmetric
effeciency) by carefull tuning of intake and cam.  A non supercharged
engine can also burn more fuel per unit time by making more firing cycles
per unit time.  If you turn 6000 rpm and get V of air.  To get double the
Horsepower you need 2V of air.  To do this you need twice the rpm or 12000
rpm.

Thus, forced induction while being somewhat expensive for a small
improvement, is far more cost effective for large increases in horsepower.
One caveat of forced induction is the added advantage of water injection.
Since compressing air causes it to heat up according to thermodynamic laws
and when air heats, it expands, it can benefit from cooling.  Water
injected into a hot air stream will not only cool the air, but when the
combustion chamber will do four things.  It will turn to steam which will
tend to clean the combustion chamber.  It will cool the combustion chamber
hotspots to reduce detonation.  The steam acts to give additional pressure.
 And because water takes high amounts of energy to convert from liquid to
gas, it tends to reduce peak cylinder pressures and increase the average.
Water injection was used in WWII aircraft when under high boost.  It has
been used in racing.  I had a friend use it on his street SVO Mustang.
Stock engine and modified turbo.  No wastegate, pump gas, 27psi boost.  NO
detonation.  The car ran for 10 years this way and never broke anything.
He sold the car still running.  It has been found that if you run enough
boost, you can run equal parts of water and gasoline.

Dave "turbo believer" D

At 11:44 AM 2/3/2007 -0800, you wrote:
I re-read the discussion in Maximum Boost by Corky Bell on an engine's
ability to handle HP on a naturally aspirated versus turbocharged engine,
and he seems to make some valid points.  Basically, he states that the
design point for rods is TDC after the exhaust stroke because the rod is in
maximum tension and the power stroke places less stress on the rod than at
this point.  He goes on to say that with a normally aspirated engine, you
have to increase displacement or RPM to substantially increase power.  A
large bore isn't an option with a Cleveland, so we're left with stroking
and/or RPM.  Both of these increase the stress at the design point.
  
     He also looks at the peak stress on the power stroke, and it looks
like you can double the HP with turbos and only increase the max load on
the power stroke by 20%, but doubling the HP with natural aspiration would
double the stress (or worse because RPM really increases stress quickly).
This probably is an over simplification, but it does suggest that a lower
RPM stock stroke 600 HP turbo motor would place less stress on the block
and rotating assembly than a stroked and/or higher RPM 500 HP naturally
aspirated motor.  
  
     If this is correct, it may be a cheaper to build a reliable 6,000 RPM
600 HP turbo motor then a higher RPM or longer stroke 500 HP non-turbo
motor because you don't need a high dollar rotating assembly, valve train,
heads, etc.
  
     Does anyone have experience with the reliability of a stock short
block turbo motor?  Can Dave, Dick, ect. comment on this please?  It seems
like a carefully thought out motor could be built with a stock Patera long
block, and provide great performance and reliability at a reasonable cost.
Of course, none of us seem to be satisfied anything reasonable .
  
 Ken