v8 engine seminar
TRANSCRIPT
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TYPES
There are two major types of V8 engines, which differ by crankshaft.
Flatplane
The flatplane V8 is similar to two inline four cylinders sharing a single crankshaft. Whenviewed from one end, the crankshaft appears to form a flat shape.
A flatplane crank with the typical 12-6-6-12 o'clock crank pin locations, when viewed from the front of
the engine. This configuration is identical to an inline-four cylinder engine.
Crossplane
The other, much more common type is the crossplane V8, which Cadillac came up with in
1923. The first and fourth crank pins are 180 apart, and the inner two are 180 apart
from each other, and 90 apart from the pins on each end.
A crossplane crank with the typical 12-3-9-6 o'clock crank pin locations, when viewed from the front
of the engine.
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BALANCE
First Order Balance
Flatplane
The flatplane V8 has first-order balance, that is, the rocking moment that one piston
would cause is directly cancelled by another at the other end of the engine.
Crossplane
The crossplane V8 would have first-order imbalance because of how the pistons are
moved by the crankshaft, but the use of full-weight counterweights prevents this.
Full-weight counterweights on the crankshaft are shaped and weighted in such a way
that they counter the forces from the pistons and connecting rods on the nearest pin at
the same time. Regular counterweights only counter the forces from the connecting
rods and the crank pin. Only an engine with a 90 v-angle can utilize full-weight
counterweights.
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Full-weight counterweights are placed on both ends of the crankshaft to oppose therocking motion that would come from the forward most pistons moving in the opposite
direction to the rearmost pistons. The middle two crank pins do not need full-weight
counterweights because the piston motion forces are somewhat cancelled by the pistons
on the other side of the engine. These counterweights are still very large compared to
regular counterweights, however.
The counterweights on a crossplane V8 decrease in size from each end to the centre because the
inner cylinders partially balance each other, and the counterweights on each side of a main bearing
have a similar effect. The counterweights on each end must be very large because they alone areopposing the forces from two pistons and their connecting rods, and the crank pin.
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Second Order Balance
Flatplane
The flatplane V8 has the same type of imbalance that an inline four cylinder does, which
is known as second-order imbalance. At any given instant, the velocities of the pistons
traveling upward does not equal the negative velocities of the cylinders traveling
downward. The net velocity of all eight cylinders is therefore not equal to zero, which
causes vibrations.
Inline-fours and flatplane V8s have second-order imbalance.
Crossplane
The crossplane V8 has second-order balance, and that fact is the main reason for its
existence. The odd-looking crankshaft moves the pistons in such a way that the net
velocity of all of the pistons is always equal to zero, meaning that no second-order
vibrations will be present.
Engines that are out of balance are somewhat limited to small displacements. As parts of
the rotating assembly get larger or move faster, the vibrations they would tend togenerate become stronger. Strong vibrations can cause much greater wear on all parts
of the engine, and even parts of the vehicle itself. For this reason, flatplane V8 engines in
production cars have not exceeded an overall displacement of around 4.5L. Crossplane
V8 engines can handle any displacement; the largest regular production version being
the 500CID (8.2L) Cadillac V8 built from 1970 to 1976.
FIRING ORDERS
With so many cylinders, a V8 engine has many different firing order combinations. Some
may cause less stress on the crankshaft than others, and some may allow for better
breathing on the intake or exhaust sides.
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Flatplane
The firing order of a flatplane V8 is ideal, that is, each cylinder in the sequence will be on
the other cylinder bank from the previous. This allows the exhaust gas pressure on each
side to stay somewhat even, allowing for good scavenging from the cylinders.
Crossplane
The crossplane V8 does not have an ideal firing order. Because of how the pistons aremoved around by the crankshaft, there must always be a cylinder fired on the same side
as the previous at least twice, once per bank. Not only will that, but at least once in the
firing order, two adjacent cylinders fire in sequence, which is undesirable because
thermal and mechanical stresses become higher at the two adjacent cylinders.
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With a crossplane V8, it is actually possible to fire all four cylinders on one bank in
sequence, followed by all four on the other bank. These firing orders are always ignored,
leaving only four practical firing order choices. Within those four, they are chosen based
on which one will put the least stress on the crankshaft, and which will allow the best
breathing. Below are the three firing orders that have been used on the Chevy small
block engine over the years.
Three common firing orders for Chevy V8s.
In light of the fact that Ford uses a different cylinder numbering scheme, the firing
orders used by Ford are not listed in the diagrams above. Below is a digraph showing
the mapping of one cylinder numbering scheme to the other.
A digraph showing the cylinder numbering conversion for Chevy and Ford V8 cylinder numbers.
It should be noted that the "old" Ford V8 firing order (1-5-4-2-6-3-7-8) is the same as
the original Chevy small block firing order (1-8-4-3-6-5-7-2), and the "new" firing order
(1-3-7-2-6-5-4-8) is the same as the GM LS series (1-8-7-2-6-5-4-3). Ford was first to
make the switch in 1969, but not with all of their models. GM followed with the LS firing
order in 1997. Cadillac used the LS firing order a long time ago as well, in the form of (1-5-6-3-4-2-7-8).
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EXHAUST SOUND
Flatplane
The flatplane V8 is often described as sounding similar to two inline fours running at
once, although there is a slight distinction between the two which depends on theexhaust setup.
Crossplane
The very distinct exhaust sound made by a crossplane V8 is from the irregular firing
order. Each time two cylinders fire on the same side in sequence, the two exhaust pulses
create high exhaust pressure and noise which can be heard out the tailpipe. This repeats
later in the firing order on the other side of the engine.
Often times, balance pipes are used to equalize the large exhaust pressure difference
between each side of the engine. The pressure equalization improves exhaust
scavenging, especially at low RPM.
PACKAGING
Packaging is one of the greatest strengths of a V8 engine. When fully dressed with all
accessories, a V8 engine has a roughly cubic shape, which means they fit very well in
most rear-wheel drive vehicles, and even some front-wheel drive vehicles, such as the
late model Chevrolet Impala SS.
A V8 is slightly longer than an inline-four with a similar cylinder bore spacing, and much
shorter than an inline-six. The short length puts the engine's centre of gravity well
behind the front axle, which improves handling. It also allows the hood of the car to
slope down at the front for improved aerodynamics. Inline-six engines, which are very
long relative to their displacement, have been mostly replaced by V6 engines for the
aforementioned reasons.
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The short length of a V8 is a significant packaging advantage over a comparable inline-
six or V12, which means that the V8 is essentially the smallest engine type that has first-
order and second-order balance, allowing very large engine displacements, which can
help give very high power-to-weight ratios. A V8 engine can still be more compact than
a V12 with the same overall displacement, and has the added benefit of being cheaper
to manufacture. However, a V12 of equal displacement would likely be able to make
more power, at the expense of added size and weight. It is for this reason that some
auto makers, such as Jaguar and Mercedes-Benz, are using supercharged V8s to achieve
V12 performance with V8 size and economy.
Another benefit of the short length of V8 engines is that they can be front-mounted in a
car, but located entirely behind the front axle. Cars with this layout are described as
having a front-mid engine (FMR) layout.
While the 90 v-angle can make a V8 too wide for use in smaller cars, the use of
pushrod-actuated overhead valve (OHV) heads helps make the engine narrow enough
to fit into many cars. The space and weight saved by OHV cylinder heads leaves room
for a slightly larger block to provide more displacement.
Even with narrow OHV heads, there is often little room on the sides of the engine for the
exhaust manifolds. Coupled with the need for an h-pipe or x-pipe, the exhaust systems
for a car with a V8 end up being a compromise between making power and keeping
costs down.
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CONCLUSION
The V8 engine is a popular choice in high performance road cars and race cars because
of its compact size and light weight relative to its power output. The crossplane V8's
ability to support very large displacements, coupled with small, low-cost OHV heads
makes it one of the most affordable high performance engine types in use today.
V8 engines are also used in many luxury cars where their smooth running characteristic
is an asset. The exhaust sound is considered pleasing by most people, but the uneven
exhaust pressures which make the unique sound are detrimental to performance and
add complexity to the exhaust system.