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Anatomy of the GM LS1 V8

The goals and outcomes of the design team behind the LS1 engine

by Julian Edgar

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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:

  • Improved manufacturing and assembly efficiency

  • 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.

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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

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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

  • Block

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.

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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.

  • Crankshaft

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.

  • Con-rods

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.

  • Pistons

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.

  • Lubrication

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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.

  • Heads

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.

  • Valvetrain

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.

  • IAFM

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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.

  • Injectors

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.

  • Induction

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."

1997 GM 5.7 Liter LS1 V8 Engine - SAE paper 970915

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