And now we can discuss the most interesting part of the Seville STS: the unbelievable Northstar System. The Northstar System is more than an engine. It is a tuned three-fold, consisting of engine, transmission and suspension. These units are constantly in interaction with one another, as if they were talking to each other. Located all over the car are electronic sensors, monitoring conditions that are of relevance to the Northstar System. The system is tuned for optimal engine performance under all conditions , taking into consideration almost all thinkable factors.
The Northstar Engine
For the non-tech people who
are reading this, it may be interesting to give some basic
information on engine design. So what makes the Northstar engine
so very special ? First of all it is of the Dual Overhead Cam
(DOHC) design. That means that camshafts are not located
centrally, as in a classic engine. Instead the camshafts are
mounted above the valves. This means that there is no longer need
for pushrods or rocker arms, eliminating a lot of internal
friction and power loss. "Hey", I hear you say. "You
are constantly talking about camshafts (plural ), why's that
?". You are right, "normal" engines only have
one camshaft, which is centrally located in the V-block. The
Northstar (and some other American and European V8's) has
multiple camshafts. Each cylinder head has two camshafts, mounted
above the valves. The reason for having more than one camshaft
per cylinder head, is the amount of valves that are involved in
the Northstar. Again, "normal" engines only have two
valves per cylinder (one intake and one exhaust). In order to
allow better engine breathing and better cylinder filling, the
Northstar has two intake valves and two exhaust valves per
cylinder, totaling 32 valves. Having only one camshaft per head,
would result in this camshaft operating on 16 valves. Not only
would this require a special valve layout, but it would also
result in an enormous loss due to unnecessary friction. With a
DOHC design, one camshaft operates the intake valves, the other
one the exhaust valves. So as a matter of fact, the Northstar has
four camshafts, compared to one as in a classic V8 (like
the Cadillac 4.9 liter). Another reason why the Northstar is so
special, is that is made completely out of aluminum. This makes
the engine considerably lighter than one in cast iron. The
Northstar engine weighs about 230 kg, about the same as the 4.9,
despite the additional valves, camshafts and related chain-drive
components. When you make a car lighter, you have less dead mass
to drag with you, and you get an overall more performant machine
(the so-called horsepower to weight ratio). The Northstar engine
has a host of other improvements over the 4.9, amongst them a
special modulating intake manifold and a special kind of
distributor. A normal engine uses a device which contains a rotor
or cam to close contact points and create a spark. This always
implies using moving parts, subject to wear. The Northstar Engine
does not have any moving parts in its iginition system. Magnetic
sensors are positioned at the crankshaft and camshafts. Those
sensors are triggered by notches in the crankshaft and camshaft.
These generate an ON-OFF pulse wave, similar to the open-closed
position of classic breaker-points. This wave is interpreted by
the ignition module, containing 4 ignition coils (1 coil per 2
opposed cylinders). As the wave signal enters the ignition
module, a high energy voltage is generated in the appropriate
coil, resulting in two simultaneous sparks in the opposing
cylinders. Since one cylinder is at its exhaust stroke, this
spark will have no effect. This system is called waste spark
ignitionThe 4T80-E Transmission
The 4T80-E tranmission is the latest generation of the GM's Turbo-Hydramatic (THM) family of automatic transmissions. These transmissions are world renowned for ruggedness and reliability. THM transmissions are being used in cars and trucks from different makes, including Rolls Royce and Ferrari. Starting in 1966, GM has to make a drastic change to some of its THM transmissions, as they introduced front wheel drive Oldsmobile Toronado. Cadillac got the new tranny in 1967 in the front wheel drive Eldorado. Drivetrain layout on front wheel drive cars is such that the driven wheels are located under the powerplant and not behind it. Hence, the transfer of power must make some twists and turns before it can reach the front wheels. GM solved this matter ingeniously by actually folding the transmission. The tranmission parts behind the convertor were put in the folded part under the engine. A special chain linked the upper part (actually the convertor) with the lower part and thus accomplished the "power transfer twist". Initially GM was laughed at by other engineers who claimed the chain would never last long, especially with the high torque engines the transmission would be mated with. As experience has shown, the chain is extremely durable and one of the most reliable parts of the transmission. The 4T80-E still uses this kind of chain to transfer power. However, it is completely governed by electronic components which guarantee unnoticeable shifts and high performance operation. The Northstar system takes care of the interaction between the engine and the transmission.
For instance, just before an upshift
takes place, the engine power output is reduced in order to avoid
power transfer during the shifting phase. Furthermore, the
transmission has a special adapt function. As transmissions are
being used, their internal components (clutch plates, seals,
springs) are subject to normal wear. As these components wear,
the line pressure set at the factory is no longer valid for the
optimum performance and comfort. That is why the 4T80-E
constantly monitors its own shift harshness and slippage. Line
pressure is adjusted accordingly. The Northstar Suspension and Ride Control
This system contains two subsystems: Road Sensing Suspension (RSS) and Traction Control System (TCS). RSS is governed by sensors located at each wheel. These sensors monitor how the car behaves on the current road surface. The shock absorbers can be adjusted in to three settings: Comfort, Normal and Firm. The RSS also reacts to changes in car speed. Acceleration /deceleration sensors measure changes in speed. If a sudden acceleration is detected, RSS goes to firm in order to provide anti-lift. Likewise , the system goes to firm to provide anti-dive on sudden deceleration. TCS is particularly useful on high-torque engines like the Northstar. It requires very little throttle to make the Northstar wheels spin on slippery surfaces. That's when TCS comes into action. When front wheel spin is detected, engine power output is reduced and the appropriate brakes are applied to counter the wheel spin. With light throttle application, the Seville can easily accelerate on an icy road. ABS brakes guarantuee safe stopping.
