The GM 5.7-litre LS1 engine - known in Australia
as Holden's Gen III - has made a huge impact on performance cars. Here we take a
look at the original design goals and outcomes of the engine, which was first
released for the pictured 1997 (US model year) Corvette.
The Goals
While performance enthusiasts tend to think power,
power, power when considering the goals of a new performance engine, GM in fact
had two primary goals:
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Improved manufacturing and assembly efficiency
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An engine that would meet the new requirements of
the Corvette, specifically in terms of its packaging, mass and performance
targets.
It was for purely marketing reasons that the 5.7
litre capacity was maintained - the all-alloy engine had virtually no components
in common with its LT1 predecessor. But with effectively an all-new design, why
stick with pushrods? GM took into account mass, packaging, cost and low-end
torque requirements before again plumping for pushrods.
However, pushrods might've been retained but the
chance to introduce other high-end engine technologies was embraced. The deep
skirt block used cross-bolted main bearing caps and cast-in cylinder liners. The
nodular iron (nope, still not a forged steel crank!) was subjected to less
stress through the use of a new firing order. The alloy sump - cast using the
'lost foam' process - formed a part of the engine structure, adding
rigidity.
New cylinder heads were used. Over the previous
design these gave better combustion air motion and flow with reduced
cylinder-to-cylinder variation. However, most interesting on the top half of the
engine was the new design inlet manifold assembly, which comprised an
'Integrated Air and Fuel Module' (IAFM). This was made up of a composite plastic
intake manifold, a sequential port injection system and an
electronically-controlled throttle.
The Outputs
Over the 1996 LT1 engine, the 1997 LS1 developed
15 per cent more peak power, an increase in peak torque of 5 per cent
(indicating that better breathing at higher rpm was responsible for most of the gain in
peak power), a reduction in mass of 12 per cent, and Brake Specific Fuel
Consumption improvement of 4 per cent.
Specifically, the new engine developed 257kW at
5600 rpm and 476Nm at 4400 rpm.
GM also claimed that quality and noise/vibration
characteristics were improved. Being in the game of mass manufacture, they were
proud of the fact that where feasible, sealing surfaces were single plane and
along with sensors, were located away from submerged areas. Welch plugs weren't
used in the block or front and rear covers. A "very limited number of threaded
oil plugs" was used in place of conventional welch plugs, and neither water nor
oil came in contact with the plastic intake manifold.
Reduced vibration came about through increased
block stiffness, the 6-bolt main bearing caps, the structural sump, and
direct-mount accessory drives.
In Detail
The block was cast from 319-T5 aluminium with
cast-in iron cylinder liners. The 99mm bores were spaced on 111.76mm (ie 4.4
inch) centres, allowing the presence of cooling passages between adjoining
cylinders. The cylinder bank offset of 24.1mm allowed the use of a flat con-rod
and enabled a single piston/rod assembly, simplifying assembly. The oil pump was
moved to the front of the engine, allowing the shortening of the block by
34.2mm. The nodular iron crank was internally balanced; the use of a 234.7mm
deck height giving sufficient space for the weights. The cam was driven by a
9.525mm pitch full roller chain that didn't use a tensioner; the crank-to-cam
distance was 124.08mm to cater for the longer stroke.
The block weighed just 48.85kg, some 48 per cent
lighter than an equivalent iron block. The crank was held in place by five
cross-bolted main bearing caps which were formed from powder metal. Four
vertical M10 and two horizontal M8 bolts were used in each cap. Cylinder head
and bearing cap bolt threads were roll-formed for strength. At the top of the
block a structural die-cast valley cover tied the two cylinder banks together,
further improving rigidity.
The nodular iron crank used variable radii
undercuts (which increased the width available for bearing surfaces) and rolled
fillets for improved fatigue strength. The mains were 65mm in diameter while the
big-ends were 53.328mm. Number Three bearing was designed to cope with
longitudinal thrust and was positioned thus to minimise the effect of the
different thermal expansion rates of the iron crank and alloy block.
As mentioned, the firing order of the engine was
altered over the previous design. Specifically, it was changed from
1-8-4-3-6-5-7-2 to 1-8-7-2-6-5-4-3. This reduced crank stresses by 7 per cent
and caused an increase in bearing oil film thickness of 13 per cent. In parts
the crank was hollow - 25.4mm holes were drilled through the centre of main
journals 2, 3, 4 and 5. This reduced the mass of the crankshaft by 650 grams.
The con-rods were hot forged powder metal PF1159M.
They utilised a 'broken' big end and 9mm bolts. Piston pins were press-fitted
and used 24mm diameter pins.
The pistons were cast eutectic aluminium of a
closed skirt, strutless design. Each had a mass of 434 grams and a compression
height of 34mm. The top land (the distance from the upper surface of the top
ring to the piston crown) was a small 4.5mm to reduce crevice volume and so
hydrocarbon emissions. The rings - all low tension to reduce friction -
comprised 1.5mm thick compression rings made from barrel-faced, chrome-moly
filled 9254 steel. The second ring was cast iron.
The lubrication system was required to perform
adequately when being subjected to 1g lateral acceleration and under maximum
acceleration and braking. One problem to be overcome was that the deep-skirt
design restricted the flow of air between adjacent cylinders within the block.
This in turn slowed the return of oil to the sump. To encourage this flow,
28.5mm ventilation holes were added to the cylinder block bulkheads, the
previously mentioned holes in the crankshaft through the centre of main journals
2, 3, 4 and 5 were added, and 'side-pods' extending out from the block were
added to allow air to flow around the main bearing caps.
In addition to the ventilation changes listed
above, aeration of the oil and windage were reduced by the use of a crankshaft
deflector (mounted on the main bearing caps) and a stamped steel baffle in the
sump. The shallow sump's capacity was increased by the presence of ears which
were added either side. The 356-T6 allow sump also had dams cast into its floor,
slowing the movement of oil both fore-aft and laterally.
The oil pump was capable of flowing 22.7 litres
per minute at an engine speed of 6000 rpm.
A design aim of the heads was to have better fuel
injector targeting and equal airflow direction and energy for each cylinder. The
valve angle was 15 degrees from the vertical and valves were positioned to
provide better geometry with the pushrods. To improve the motion of the air/fuel
mix around the sparkplug, a sharp fillet radius was added to the inlet side of
the combustion chamber. This provided better swirl to the incoming mixture. Tall
(73 x 27mm) inlet ports were used to reduce restriction and provide for better
injector aim.
The hollow camshaft was made from 5150 steel
billet with induction-hardened lobes. Closing ramps were angled to provide
slower valve closing velocities, so reducing noise over the previous engine.
Needle bearing roller rockers with a 1.7:1 ratio were used in conjunction with
hydraulic lash adjusters. The geometry was such that pushrods angles were "less
than one degree" relative to the centreline of the lifter bore. Mass
optimisation resulted in a reduction in effective inlet/exhaust valve moving
mass of 20 per cent relative to the previous LT1 engine.
The valves used polished 8mm stems and were made
from "basic low cost materials". Valve springs were of the beehive design,
constructed from Cr-Si 4.6mm diameter round wire.
The preassembled Integrated Air and Fuel Module
comprised the intake manifold, fuel system and electronic throttle body. It
weighed 7.4kg. The intake manifold was made from glass-filled nylon and alone
weighed 3.8kg. This represented a major weight saving over the previous LT1 cast
alloy intake manifold, which in one form had a mass of 8kg. The plastic LS1
manifold used a 10 litre plenum chamber and equal-length, 260mm long intake
runners which each decreased in cross-sectional area between the plenum and the
intake ports. In addition to its reduced mass, GM quoted the plastic intake
manifold as having significant thermal advantages, both in keeping intake air
cooler (and so denser) and also in the reduced heating of the fuel.
Fuel pressure was regulated to 400 kPa at the
entrance to the H-style, stainless steel fuel rails. The injectors, fired
sequentially, were rated at 3.6 grams/second flow.
A DC brush type motor was used together with a
two-stage reduction gear to control the motion of the 75mm electronic throttle.
Prior to that, the induction path included a 74mm mass airflow meter and a
rubber-mounted filter box containing a slide-out cartridge filter. The maximum
pressure drop through the whole intake system (from atmosphere to the rear of
the intake manifold) was less than 5 kPa at peak power.
Conclusion
At the end of their SAE engineering paper on the
new engine, the GM engineers said: "The LS1 engine was created from a clean
sheet of paper utilising some of the best engineering experts in the industry to
meet the challenges of today's competitive market. We are all very proud to add
this engine to the legendary line of 'Small Block' V8 engines."
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1997 GM 5.7 Liter LS1 V8 Engine - SAE paper
970915
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