vs. water cooled
Engine vs. Mid-engine
vs. water cooled
Air cooled engines go through extreme temperature variation because
they are entirely dependent upon ambient air temperature for their
cooling. The engine can only efficiently dissipate heat in direct relation
to the difference between ambient and engine surface temperatures. This is what
engineers call delta T. It is obvious that cooler air will
cool a hot engine faster than hot air. However, nature does not
always give us cool air for this task. As a result the assembly
tolerances for air-cooled engines must be built "loose" to
compensate for the inevitable expansion of the mechanical parts
subjected to high thermal expansion. When they are forced to these
high temperatures the oil viscosity diminishes and clearances are closed
which can result in bearing or piston failures. Loud air
moving fans necessary to cool the engines are a dominant sound. These
engine failures are usually heat related.
Water-cooled engines are literally bathed
in a blanket of thermostatically controlled water so the temperature
fluctuations are more controllable and less dramatic. Liquid
immersion is a more efficient heat removal method. Radiators are
oversized to give a high degree of safety margin. The liquid
blanket serves as a sound absorber as well and the fans are
significantly quieter and further from the occupants.
Engine vs. Mid-engine
Engine location in the chassis is a direct contributor of performance
capabilities as well as braking and traction. Rear engine cars are
"tail heavy" causing the rear of the car to tend to swing out
in a curve or turn. The faster the car is traveling the greater the
tendency. More weight on the rear means less on the front.
Since the primary brakes are on the front and the direction of the car
is controlled from the front loss of weight there means less braking and
steering control. The shorter wheelbase of the Speedster exacerbates
this tendency. The more balanced a chassis the less swing out and
the greater steering and braking on the front. Ideally, although
unrealistic, a (50/50) bias
would be preferred. Rear engine VW Speedsters typically have about
38% on the front and 62% on the rear (38/62), but our mid-engine
Speedster is (45/55).
Even Ferdinand Porsche knew the best
handling setup was mid-engine. The first two 356 Porsches were
mid-engine, but as we all know there is no backseat for the kinders.
Family usefulness over ruled performance so the 356 became a rear
engine exactly like the VW Beetle and for the same reasons.
Porsche immediately introduced a true performance car the 550 Spyder
which was mid-engine and has historically been lauded for its
handling acumen. The rest of the Porsche performance history is full
of record setting mid-engine cars.
The point is best made by noting that virtually all world class high
performance racing or road cars are mid-engine, and that includes Lamborghini, Ferrari,
Jaguar, Maserti, Lotus, Porsche Boxster, Formula 1, Indy, and CART to
name a few. The few that are not are deeply
rear set front engine cars like the Corvette and Viper. The
water-cooled Porsche 911 rear-engine car has alleviated the worst of the
rear engine tendency by taking advantage of 40 years of design effort
and making it an all wheel drive system with
individual computer controlled wheel power distribution.
Front/rear weight bias affects many aspects of performance. The
most significant is the responsiveness of the car. Front biased
cars tend to under steer heavily while rear biased cars tend to over
steer heavily. Both of these types have a high polar moment of
inertia, which is an engineering measure of the resistance of a rotating
object to change direction.. These cars are usually altered to
alleviate these tendencies, but those alterations cannot affect the
responsiveness. Cars designed and built for performance
responsiveness are typically mid-engine configuration which creates a
low polar moment of inertia. Weight bias also has a large impact
upon braking efficiency and traction. Front engine cars have most
of the braking resistance carried by the front brakes. Although
the greater weight is on the rear wheels of rear engine
cars they still rely on the front brakes for controlled braking. The
front brakes must be dominant in all cars or the car will tend to lose
control on hard braking efforts. Since rear engine cars already
have this tendency in turns great care must be taken to get the braking
bias forward so as not to exacerbate an already sensitive
situation. Mid-engine cars distribute the
braking effort more evenly to front and rear allowing the brakes to run
cooler, wear slower, and provide a balanced feel when braking.
The finished weight of a car can greatly influence the
performance characteristics. The lower weight allows the engine to
deliver its horsepower and torque for greater acceleration and
pulling. However, most importantly a lower finished weight can
allow the car to be negatively affected by cross winds, choppy road
conditions, and noise control. One must decide whether they are
willing to sacrifice a more comfortable and quieter ride desired for 99%
of their driving needs in order to be able accelerate more quickly
during 1% of their driving.
A true measure of performance is the ratio of weight to
horsepower. As this number decreases the perceived performance of
the car can increase due to the ability of the engine torque to overcome
the inertia required for acceleration. Obviously one can achieve
this favorable ratio by decreasing the weight of the car or by
increasing the horsepower. There are advantages for both.
Heavier cars have better penetration and ride characteristics. More
powerful engines cost more and consume more fuel. Modified engines
consume more fuel, generate more heat, are usually more noisy, and can be fragile and prone to failure. One must weigh the advantages
and disadvantages carefully. Light cars are sensitive to cross
winds and turbulence created by highway traffic and passing large
vehicles as well as a choppier ride caused by normal road aberrations.
Horsepower and torque ratings can be very deceiving since they are
usually advertised at one rpm level which is typically the maximum
rating. If this rating is achieved at a very high rpm then you
will never approach the rating at normal driving ranges
Another significant factor is how the power and
torque are achieved. If an engine normally rated at 60 hp is
modified to produce 120 hp the engine can become very fragile and
suspect for heat and loading failure. If the necessary alterations
and additions are made to the 60 hp engine to safely achieve the 120 hp
the costs will be very high and in many cases the heating and loading
concerns are not entirely removed. A true test of an engine
builder's confidence in his modifications is his willingness to warranty
the engine. Most modified performance engines have no
warranty. Some of these type engines are warranted but one should
read the conditions very carefully because of limitations and exemptions
The most secure method of achieving reliable horsepower and torque is
to design the engine from the beginning to accommodate the power
provided. Modifications and alterations create questions about
reliability. All original equipment engines (OEM) are designed,
tested, and warranted to reliably deliver the power advertised.
Brakes are an obvious important consideration in a
performance car. Their effectiveness is greatly influenced by
what engineers call normal force. Normal force is the force the
tire is exerting directly against the ground and is created primarily
by the weight of the car. Dynamic and aerodynamic influences also
affect this force, but for a normal road car the weight is dominant.
Formula 1 racing cars create aerodynamic forces that increase the
normal force at speed as much as 3 times. Many characteristics of
handling, braking, acceleration, and even tire wear are directly
linked to this force. Aside from the obvious requirement of the
braking system having adequacy for the platform, such as swept contact
area vs. power and weight of the car, the normal forces prevail.
The ability of a tire (not the brakes) to stop the car
in a controlled manner is more often than not overlooked. The
cheapest and most expensive brakes in the world are both limited by
the traction created by the tires. If the tires lose traction
then braking and steering control is lost. This is a phenomenon
that is frequently experienced by drivers who find themselves on icy
road conditions. The brakes work but the tires have no traction
with a subsequent loss of control. This extreme example can be
used to describe the same results even under dry conditions.
Wet, sandy, or oily roads only exacerbate the traction loss phenomenon. Modern
automobile design incorporates Anti-lock Brakes (ABS) which
controls the sliding tendency of the tires thereby increasing
Tire traction is complicated and is influenced by the
compound used to make the tire, tread design, sidewall flex,
temperature, and road surface conditions. Coefficient of friction
is an engineering term that describes quantitatively the resistance
two surfaces exert when you try to move one over the other.
There is a static and dynamic value of this coefficient. The
static value describes the effort to break the two surfaces free
from each other and the dynamic value describes the effort to keep
them sliding. The coefficient is a constant for a given set of
parameters. Traction is a product of Normal Force and
coefficient of friction so as you can see the normal force
becomes a variable of extreme consequence.
Front brakes and traction must dominate the stopping
of the car. Braking bias is adjusted so that the front brakes
dominate the requirement. If the rear brakes are allowed to
dominate directional control is easily lost. All effort should
be given to maximizing the traction on the front tires in order to
maximize the effectiveness of the front brakes. If all things are kept
constant it becomes obvious that the more weight you have on the
front tires the greater will be the Normal Force and that force
multiplied by the coefficient of friction will define the traction
From this presentation comes the obvious reason why a
rear engine VW Speedster with a 38/62 weight bias has less front tire
traction force (normal force) than our mid-engine Speedster with a
45/55 weight bias. For an 1800# car this means 810# on the front
tires vs. 684#. Our mid-engine Speedster has 19 percent more
weight on the front wheels. In order to keep from losing
traction on the front brakes of a VW Speedster but at the same time
not let the rear brakes dominate the entire system has to be
diminished to the ability of the front traction.
It should be noted that in addition to improved
braking the normal force and subsequent traction force greatly influences
directional control and high speed stability.
Fuel delivery to a normally aspirated (not super or
turbo charged) internal combustion engine has probably had more impact
on engine performance than any other single engine system. It is
very complicated and must be integrated with the other engine systems to
achieve a balance for proper use. Two methods of delivery have
been pioneered since the invention of the internal combustion engine -
carburetion and fuel injection.
Carburetion is totally dependent on ambient
conditions, attitude (hills, curves, etc) and paths of delivery to the
cylinders. Carburetion is not dynamic but rather statically set
up to provide acceptable delivery over the normal range of use of the
car. It cannot accommodate changes in altitude, barometric
pressure, humidity, or temperature. It just delivers a generally
correct fuel/air mixture for general driving conditions. To
exemplify this limitation: professional drag racers have as
normal equipment devices to measure temperature, barometric pressure,
and humidity so they can immediately change jet setting in their
carburetors when conditions change at the race track. This
allows them to achieve the maximum power out put for that engine, at
that track, under those immediate conditions. This is not
something you can do while driving to visit grandma in the country.
Fuel injection has been around for a long time
and has been primarily mechanically controlled. These mechanical
systems are used on diesel engines as well as famous cars of the past
such as the Mercedes, 57 Corvette, etc. These systems were a
great leap forward in that they were able to deliver precise amounts
of fuel over a broad range of conditions thereby maximizing power out
put. However they did have limitations.
Welcome to the 20th/21st century. Electronically
controlled fuel injection found on just about every car on the
road today started appearing in great numbers in the early
eighties. With the miraculous advances in computer chips and
miniaturization fuel delivery today is truly state-of-the-art.
The Engine Management system monitors every conceivable condition
which could affect proper fuel delivery ratios and instantly makes
adjustments to every cylinder while driving. In other words your
engine is producing the maximum power for the conditions monitored at
just about every instant of use. This is why fuel injected cars
can produce more horsepower while at the same time deliver greater
fuel mileage. Try driving from the coast to Pikes Peak with a
carbureted car versus an electronically controlled fuel injected car
and you will immediately see the reasons why carburetion is no longer
a viable method.
Our 356A replicas all have electronically controlled
Engine displacement is a surefire way to gain
horsepower, but it is done at sacrifice of other desirable
features. Large displacement requires large engine blocks which
add weight, higher fuel consumption, and often aerodynamic losses due
to the large shape of the body required to house them. There are
circumstances where displacement is advantageous, but for a high
performance sports car one must consider more than brute power.
Fuel economy, space, handling, braking, noise levels must be balanced
with power to achieve the "perfect" compromise. This
argument has never been nor will ever be settled amongst car
Emission compliance has become an issue of concern for
kit cars, replicas, specially constructed cars, and others falling
under this general description. The past practices of buying an
old title for an abandoned car or using the title of the donor car
which in no way looks like the new replica are successfully being
attacked by the state DMV offices. They are viewed as fraudulent
registrations that often times are used to avoid the proper taxes due
as well as to disguise the driveline in the car thereby avoiding
proper emission evaluation.
EPA has issued a requirement for emissions compliance
for these type cars.
and I have covered this issue in detail at
One should be concerned about building an expensive
car that does not address these requirements since it can subject the
builder/owner to EPA fines, cause the car to fail registration
requirements, and/or prevent the car from being transferred in the
future to second owner for the same reasons.
All Specialty Auto-sports, Inc. 356A and XK120
replicas are delivered with conforming drivelines that meet the EPA
requirements "Replica definition".
There are two basic types of
steering control found on most vehicles. A link type or rack and
pinion. Link type steering was found on almost all early vehicles
up to the 1960's. Since that time the industry as almost
completely evolved into rack and pinion. The reasons for this
evolution center around the advent of front wheel drive cars, weight,
cost, space, and the very positive steering feel provided by rack and
Link type steering is plagued by
having too many connection points each having a required degree of
looseness, and each being further loosened by wear. The end result
is a sloppy feeling at best and a dangerous reaction at worst. The
link steering has a steering gear box with a complicated recirculating
ball bearing, and at least 5 tie rod connections. This is further
complicated by the extraordinary angles the links are required to
achieve during normal driving. The rack and pinion has a gear driving a
flattened cog gear and 2 inner ball connections and 2 tie rodends.
This method has proven to be the only used on racing cars and is where
the technology evolved. Most all cars today have rack and pinion
steering. The VW Beetle uses a link type steering. The last
Beetles imported into this country called Super Beetles had rack and
pinion steering, so even VW eventually recognized the importance of rack
and pinion steering. Super Beetles are not used to build VW
Speedster replicas. All Specialty Auto-sports, Inc. cars have rack
and pinion steering control.
There are generally three types of frames used with
replica automobiles. Ladder rail, pan, and structural tube.
Ladder Rail Frame
this is a frame that is found on most cars and trucks prior to the
mid-60's. It is composed of two parallel main members and
cross braced with diagonals or cross beams. Attached to these
members are the connections for the suspension members, drive line,
and body components. The body contributes little to the
structural integrity of the car, and the loads and forces are borne
by the frame structure. Remove the body and you have an
non-drivable vehicle. These frames have almost been completely
replaced by uni-body construction where the engineered body
structure contributes the major structural integrity of the
vehicle. A fiberglass replica body cannot serve as a uni-body
type structure because fiberglass does not have the structural
properties required for enduring the loads and fatigue causing
inputs from automobile driving.
Pan Type Frame
- this is a pseudo-frame in that all of the suspension and
driveline components are bolted to the pan, but the pan has
insufficient integrity for its intended use without the VW body
attached. The pan
requires the additional structure provided by a welded steel body.
Although all of the driving components are on the pan that allow it
to be driven it does not have the strength or rigidity to serve as a
stand alone chassis. All VW Beetles were built with pan/body
construction. When the Beetle body is removed in order to use
the pan for a replica Speedster foundation all of the required
structural integrity afforded by the body is lost. Most
Speedster replica companies attach a perimeter sub-frame to the
which serves as a stiffener and an interface between the fiberglass
body and the pan. This is not a structural tube frame.
It is a perimeter reinforced pan. It is obvious that this sub frame does not provide any impact protection.
This type VW sub-frame is found under virtually every
VW speedster replica.
Structural Tube Frame -
This is the most abused term used in the replica industry.
People with no engineering education or background seem to have the
belief that if you cut up some tubes and weld them together you have
a structural tube chassis. Well as you can see from above the
ladder and sub-frame have welded tubes but they are not stand alone
chassis. The true test of whether a chassis is structural tube
or not is simple. If you could wave a magic wand over the car
and remove all of the fiberglass body components and then walk up to
what remains, open the door, sit down, start the car and drive away
knowing the chassis could be driven anywhere the finished car could
be driven.... you have a structural tube chassis. (Be sure to take
your raincoat) The real challenge is the engineering
required to ensure that it is of sufficient strength and has fatigue
input factored into its design. This is not body shop type
compare these SAS frames with the VW sub frame above. The
protection should be obvious
steel reinforced doors for strength and safety -
you won't find this on VW speedsters
All Specialty Auto-sports, Inc. Speedsters, Cabriolets, and Coupes are built on a true Structural tube frame not a sub-frame that is misrepresented as a structural frame.