VW Reality

*******************************************
"...the great little VW
conversion with a 2.0:1 reduction producing 103 HP."
---------------------------------------------------------

To All:

In the context of an engine converted for flight the figures above are
wildly fallacious. At best, they represent a 'dyno blip,' at worst
they may be an out-right lie. Here's why:

The maximum SUSTAINABLE power available from ANY air-cooled engine is
determined by the engine's ability to cool itself -- to couple its
waste-heat to the atmosphere. And with a carburetted, spark-ignited,
gasoline-fueled engine there is a LOT of waste heat to be managed
since such engines are no more than 25% efficient when it comes to
converting the heat of combustion into torque at the crankshaft. That
means that for every horsepower measured at the crank you must
generate at least four horsepower's-worth of heat in combustion.
These basic rules of thermodynamics are made even worse by two
additional factors, the first being 'Economy of Scale' in that smaller
engines are LESS thermally efficient than larger engines, and the
basic definition of Standard Day conditions -- 59.9 degrees on the
Fahrenheit scale and an atmospheric pressure of 29.92 inches of
mercury -- a fairly cool day.

With those laws of physics as preamble the next factor worthy of note
is the physical dimensions of the Volkswagen cylinder head and the
fact that ALL VW heads have the same exterior dimensions. This is
because they must fit under the stock VW engine shrouding. No
manufacturer of VW heads, either stock or after-market, offers a head
having more fin area. Indeed, most after-market heads have LESS, due
either to thickening of the combustion chamber wall or even
eliminating one of the eight fins -- and in a few cases they have done
both.

All -- ALL -- Volkswagen heads in common use today are derived from
the heads developed for the 1300cc engine; their external physical
dimensions remained exactly the same for the later 1500 and 1600
engines. The output of the 1300 engine was approximately 40hp and
could SUSTAIN that level of output indefinitely under Standard Day
conditions. This engine was bored-out to 83mm to produce the 1500
engine, then over-bored to 85.5mm to create the 1600 (actual
displacement 1584cc), the maximum output of which was 57bhp for
carburetted models, achieved in the 1971 model year. But that level
of output could only be sustained for a bit less than FOUR MINUTES,
until the cylinder head temperature exceeded safe levels, again under
Standard Day conditions.

So what's this 'safe level' of CHT? About 450 degrees on the
Fahrenheit scale. This reflects the fact that VW heads are made of
CAST aluminum (as opposed to a forging) and the fact aluminum is a
'white short' metal, meaning it becomes frangible when its temperature
enters the 'plastic' range. A characteristic of white-short metals is
that when heated they fracture like a cube of sugar when subjected to
stress. The floor of the frangible range is a bit higher for a
forging -- about 550F according to Pratt-Whitney -- but can be as low
as 400F in a casting, depending upon the alloy.

A common thread used to impress technologically naive buyers is tales
of driving a Volkswagen bug or bus for hours on end with the throttle
wide open. The fact the engine was was probably producing LESS THAN
TWENTY HORSEPOWER goes unsaid. This involves the Horsepower Myth and
generally leaves a large black question mark hanging over the heads of
those without an engineering background but it needs to be touched
upon since ignorance can be as deadly as a machine gun when it comes
to aviation.

The Horsepower Myth was create by James Watt in order to sell his
modified Newcommen steam engine to mine owners. To do so he added the
element of TIME to the power equation and from that day to this the
general public has been comfortable with the idea that 'horsepower'
represents a given quanta of energy... which it does... but only
within a defined unit of TIME. And from that day to this, that
arithmetical loophole has been used by those eager to prey upon
technologically naive consumers.

Indeed, in the early days of aviation those predations cost so many
lives that government agencies had to step in, requiring the
manufacturers of aircraft engines to justify their claims of power and
durability.

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

All of which tends to leave the average homebuilder with more
questions than answers. Fortunately, the ENGINES themselves are
incapable of lying, especially when it comes to FUEL CONSUMPTION.

The Specific Fuel Consumption (SFC) of all -- ALL -- air-cooled,
gasoline-fueled, normally aspirated Otto-cycle engines is clumped near
the 0.5 mark, meaning it takes about 0.5 POUNDS of 'gasoline' (*) per
HOUR to produce ONE HORSEPOWER'S worth of torque at the crankshaft.
For aviation gasoline that works out to about 12bhp per gallon per
hour. For a 103hp engine that works out to 8.58 gph.
--------------------------------------------

(*) -- Thanks to additives and dilutants (such as alcohol) gasoline
intended for automobiles has LESS potential energy.

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

So when someone tries to sell you their Whiz-Bang 103hp VW engine,
simply ask about its fuel consumption. If they give you an honest
answer, such as 'nine gallons per hour' your next question should be
'For what TBO?' (And if they try to feed you the usual '3gph'
bull****, simply walk away.)

The truth is, by simply spinning an engine faster you can claim an
impressive amount of 'horsepower' -- up to 1500bhp for some 'VW'
powered dragsters (but with a TBO measured in MINUTES). Some years
ago turbosupercharged VW engines were all the rage... until people
learned they needed a valve job about every ten hours, no big deal if
you're only SELLING such engines -- but of some importance to the
folks who actually FLY them :-)

-R.S.Hoover

Well said. Thanks for the sanity.

bildan

wrote in message
...
*******************************************
"...the great little VW
conversion with a 2.0:1 reduction producing 103 HP."
---------------------------------------------------------

To All:

In the context of an engine converted for flight the figures above are
wildly fallacious. At best, they represent a 'dyno blip,' at worst
they may be an out-right lie. Here's why:

The maximum SUSTAINABLE power available from ANY air-cooled engine is
determined by the engine's ability to cool itself -- to couple its
waste-heat to the atmosphere. And with a carburetted, spark-ignited,
gasoline-fueled engine there is a LOT of waste heat to be managed
since such engines are no more than 25% efficient when it comes to
converting the heat of combustion into torque at the crankshaft. That
means that for every horsepower measured at the crank you must
generate at least four horsepower's-worth of heat in combustion.
These basic rules of thermodynamics are made even worse by two
additional factors, the first being 'Economy of Scale' in that smaller
engines are LESS thermally efficient than larger engines, and the
basic definition of Standard Day conditions -- 59.9 degrees on the
Fahrenheit scale and an atmospheric pressure of 29.92 inches of
mercury -- a fairly cool day.

With those laws of physics as preamble the next factor worthy of note
is the physical dimensions of the Volkswagen cylinder head and the
fact that ALL VW heads have the same exterior dimensions. This is
because they must fit under the stock VW engine shrouding. No
manufacturer of VW heads, either stock or after-market, offers a head
having more fin area. Indeed, most after-market heads have LESS, due
either to thickening of the combustion chamber wall or even
eliminating one of the eight fins -- and in a few cases they have done
both.

All -- ALL -- Volkswagen heads in common use today are derived from
the heads developed for the 1300cc engine; their external physical
dimensions remained exactly the same for the later 1500 and 1600
engines. The output of the 1300 engine was approximately 40hp and
could SUSTAIN that level of output indefinitely under Standard Day
conditions. This engine was bored-out to 83mm to produce the 1500
engine, then over-bored to 85.5mm to create the 1600 (actual
displacement 1584cc), the maximum output of which was 57bhp for
carburetted models, achieved in the 1971 model year. But that level
of output could only be sustained for a bit less than FOUR MINUTES,
until the cylinder head temperature exceeded safe levels, again under
Standard Day conditions.

So what's this 'safe level' of CHT? About 450 degrees on the
Fahrenheit scale. This reflects the fact that VW heads are made of
CAST aluminum (as opposed to a forging) and the fact aluminum is a
'white short' metal, meaning it becomes frangible when its temperature
enters the 'plastic' range. A characteristic of white-short metals is
that when heated they fracture like a cube of sugar when subjected to
stress. The floor of the frangible range is a bit higher for a
forging -- about 550F according to Pratt-Whitney -- but can be as low
as 400F in a casting, depending upon the alloy.

A common thread used to impress technologically naive buyers is tales
of driving a Volkswagen bug or bus for hours on end with the throttle
wide open. The fact the engine was was probably producing LESS THAN
TWENTY HORSEPOWER goes unsaid. This involves the Horsepower Myth and
generally leaves a large black question mark hanging over the heads of
those without an engineering background but it needs to be touched
upon since ignorance can be as deadly as a machine gun when it comes
to aviation.

The Horsepower Myth was create by James Watt in order to sell his
modified Newcommen steam engine to mine owners. To do so he added the
element of TIME to the power equation and from that day to this the
general public has been comfortable with the idea that 'horsepower'
represents a given quanta of energy... which it does... but only
within a defined unit of TIME. And from that day to this, that
arithmetical loophole has been used by those eager to prey upon
technologically naive consumers.

Indeed, in the early days of aviation those predations cost so many
lives that government agencies had to step in, requiring the
manufacturers of aircraft engines to justify their claims of power and
durability.

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

All of which tends to leave the average homebuilder with more
questions than answers. Fortunately, the ENGINES themselves are
incapable of lying, especially when it comes to FUEL CONSUMPTION.

The Specific Fuel Consumption (SFC) of all -- ALL -- air-cooled,
gasoline-fueled, normally aspirated Otto-cycle engines is clumped near
the 0.5 mark, meaning it takes about 0.5 POUNDS of 'gasoline' (*) per
HOUR to produce ONE HORSEPOWER'S worth of torque at the crankshaft.
For aviation gasoline that works out to about 12bhp per gallon per
hour. For a 103hp engine that works out to 8.58 gph.
--------------------------------------------

(*) -- Thanks to additives and dilutants (such as alcohol) gasoline
intended for automobiles has LESS potential energy.

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

So when someone tries to sell you their Whiz-Bang 103hp VW engine,
simply ask about its fuel consumption. If they give you an honest
answer, such as 'nine gallons per hour' your next question should be
'For what TBO?' (And if they try to feed you the usual '3gph'
bull****, simply walk away.)

The truth is, by simply spinning an engine faster you can claim an
impressive amount of 'horsepower' -- up to 1500bhp for some 'VW'
powered dragsters (but with a TBO measured in MINUTES). Some years
ago turbosupercharged VW engines were all the rage... until people
learned they needed a valve job about every ten hours, no big deal if
you're only SELLING such engines -- but of some importance to the
folks who actually FLY them :-)

-R.S.Hoover

wrote:

-R.S.Hoover


So how does one start the slow clap on USENET?

Well written.

" wrote:
....
The Horsepower Myth was create by James Watt in order to sell his
modified Newcommen steam engine to mine owners. To do so he added the
element of TIME to the power equation
....

It makes no sense to claim "time" was added to the "power" equation because
"power" already is energy per unit time. Perhaps you meant to say time was
added to the "energy" equation?

I'm also not sure what the nature is of the "myth" you are complaining
about. You don't actually make it clear - at least it isn't clear to me.
Just so you know what level to respond at, I managed to somehow get a BSc
in physics, so feel free to use technical terms.

On Jan 31, 3:33*pm, Jim Logajan wrote:
" wrote:

... The Horsepower Myth was create by James Watt in order to sell his
modified Newcommen steam engine to mine owners. To do so he added the
element of TIME to the power equation

...

It makes no sense to claim "time" was added to the "power" equation because
"power" already is energy per unit time. Perhaps you meant to say time was
added to the "energy" equation?

I'm also not sure what the nature is of the "myth" you are complaining
about. You don't actually make it clear - at least it isn't clear to me.
Just so you know what level to respond at, I managed to somehow get a BSc
in physics, so feel free to use technical terms.

I don't get that statement either but the SFC is the more important
part. Those numbers add up, such that 3 gph means 36hp. Which veeduber
reports as indefinitely sustainable, which means I guess set the TBO
at what you want (barring other bad signs).

veeduber did I miss it somewhere, or what is the usual CHT of a 1300
sustaining 40hp on a standard day (or any day, that you might have
measured)? That would give a nice reference point to the max temps
given by pratt & whitney.

Is it fair to say that with a particular engine type, that SFC can be
used to estimate HP? (because that's what the equations are doing
seems to me)

Now check this related stuff out from a supplier's website:

Continuous HP, 3400 RPM 80 (note this is 6.6 gph, no?)
Bore (mm) 92
Stroke (mm) 82
Displacement (cc) 2180
Compression Ratio 8.0:1
Idle RPM 700-900
Cruise RPM 3200 +/- 200
Maximum RPM 4000
CHT @ Cruise 350-375 degrees F
CHT @ Climb (5 min) 420 degrees F
CHT Max 450 degrees F

What's the gph figure?

Hmm. Missing ... as is the amount of time that the engine was run to
reach those CHTs ...

Very slick.

On Jan 31, 1:33 pm, Jim Logajan wrote:

I'm also not sure what the nature is of the "myth" you are complaining
about.
-----------------------------------------------------------------------------

The one where an articled engineer insists 65hp @ 7200 rpm is exactly
the same as 65hp @ 1800... and equally appropriate for powering a
Piper 'Cub.' (You'll have to go back at least ten years on Usenet if
you want to pick up this particular thread, and to eScribe/Yahoo for
the articles posted to specific Newsgroups.)

In explaining why this was not true -- and in numerous other articles
about VW conversions -- I've used terms appropriate to the reader's
level of understanding... always sure to set an engineer's hair on
fire :-) If you'll take the trouble to read the foundation articles
you will see where the use of valid terms lead to conflicts with what
the typical American homebuilder 'knows.'

-R.S.Hoover

On Jan 31, 3:44 pm, wrote:
what is the usual CHT of a 1300
sustaining 40hp on a standard day
---------------------------------------------------------------------------------------

I don't know.
The only quantified engine parameters provided by Volkswagen were for
their industrial engines and then only for OT & OP... and only for
color-coded gauges. By comparing the VW gauges to standard gauges
allowed me to define the limits of the arcs. I've posted that
information somewhere (probably on the AirVW Group).

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

(or any day, that you might have
measured)?

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

VDO offered a dealer-installed instrumentation package but the
readings couldn't be trusted due either to improper installation or
depredation of the installation over time. VW issued a couple of
Service Notes pointing out that after-market instrumentation should
not be used for diagnostics unless they had been calibrated.

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

That would give a nice reference point to the max temps
given by pratt & whitney.

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

With 50W oil and forged heads (vs cast) I don't think such a
comparison would be very wise.

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

Now check this related stuff out from a supplier's website:

Continuous HP, 3400 RPM 80 (note this is 6.6 gph, no?)
Bore (mm) 92
Stroke (mm) 82
Displacement (cc) 2180
Compression Ratio 8.0:1
Idle RPM 700-900
Cruise RPM 3200 +/- 200
Maximum RPM 4000
CHT @ Cruise 350-375 degrees F
CHT @ Climb (5 min) 420 degrees F
CHT Max 450 degrees F

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

There's nothing wrong with the numbers. Dig around the AirVW Group
archives you should find dyno pulls posted by myself and others.
Indeed, the figures are fairly typical for a VW hot-rod engine. Which
of course is the problem.

What's missing are a host of apparently unimportant details, such as
how certain things are measured. By simply locating the CHT sensor to
a cooler location on the head you can 'prove' the engine runs cool.
Ditto for oil temp. A lot of flying VW's are actually measuring the
temperature of the crankcase rather than the oil inside of it. (VW
measured CHT at various locations, depending on the purpose. For
engines converted to flight the best location is the threaded boss
adjacent to the exhaust flange, as provided on a particular model of
cylinder head, which I've shown in several articles. Oil temperature
was measured at the inlet to the oil pump.)

But as I've said before, this is Old News. This ground has been
tilled many times before.

-R.S.Hoover

Ted Barker fitted one of his engines to a J3 in the late 60's. It was
a bit sluggish with two on-board but more than adequate when flown
solo. Unfortunately, an ATR with a poor grasp of energy management
augered it in whilst doing aerobatics.

-R.S.Hoover

wrote:
On Jan 31, 1:33 pm, Jim Logajan wrote:


I'm also not sure what the nature is of the "myth" you are complaining
about.

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

The one where an articled engineer insists 65hp @ 7200 rpm is exactly
the same as 65hp @ 1800... and equally appropriate for powering a
Piper 'Cub.' (You'll have to go back at least ten years on Usenet if
you want to pick up this particular thread, and to eScribe/Yahoo for
the articles posted to specific Newsgroups.)

In explaining why this was not true -- and in numerous other articles
about VW conversions -- I've used terms appropriate to the reader's
level of understanding... always sure to set an engineer's hair on
fire :-) If you'll take the trouble to read the foundation articles
you will see where the use of valid terms lead to conflicts with what
the typical American homebuilder 'knows.'

-R.S.Hoover


Robert,

You will not believe this one. Or - maybe you will?

A VW on a Cub...

Not flying, of course.

http://www.youtube.com/watch?v=BlqVT...eature=related

wrote in message
...
'Economy of Scale' in that smaller
engines are LESS thermally efficient than larger engines

Back before I got thrown out of engineering school, I was taught that if you
double all of the dimensions of something, the volume increases by a factor
of eight, but the surface area only increases by a factor of four.

The question posed was "Is there an upper limit to the size of a bird?" The
answer given, as I remember it was yes. Because the lift of the bird's wings
is proportional to area, but the weight of the bird is proportional to
volume, at some 'size', the wings become overloaded.

The same reasoning was applied to body cooling. The birds cooling was
proportional to surface area, but the heat produced was proportional to
volume.

What did I miss in EE102 after I transferred?