Aluminum blocks, or “cylinder cases†as engineers say, are becoming common for production engines. Why? fuel economy and exhaust emissions regulation have forced vehicle weight reduction and aluminum blocks weigh less.
Aluminum has downsides. For a given chunk of metal, its strength-per-mass is less than that of cast iron. With liquid-cooled, aluminum block engines, corrosion and porosity are durability concerns.
GM’s first attempt at a high-volume, aluminum V8, a 4.1L pushrod engine in Cadillacs in the early-’80s, had many problems. Stripped head bolt-hole threads are so common that, today, many dealers keep thread repair kits in stock. Engine failures due to coolant-contaminated oil are, also, common.
It’s taken years of work and not just a few angry customers, but GM has developed aluminum block reliability and durability to where its comparable to iron engines. For its part, the General improved its aluminum engines such that by the late-’80s/early-’90s, later versions of the pushrod Caddy along with the DOHC engines, LT5 and premium V8 (the “Northstar†4.9L and the Olds “Aurora†4.0L), proved excellent designs. Problems of the type that plagued the old 4.1 are unheard of with LT5s and Northstars.
By the mid-’90s GM was ready to try another aluminum, pushrod engine. One of the first challenges was to design the case. John Juriga told us, “Overhead cam engines are simple from a block standpoint. (The LS1) made for a complicated block design. The deep skirt, six-bolt bearing caps, deep-threaded head bolt holes, camshaft and tappet locations and other features made it challenging to engineer.â€
If packaging wasn’t problematic enough, due to aluminum’s lower strength-per-mass and higher coefficient of expansion, dealing with block distortion, noise and vibration gave design engineers fits. Burning fuel generates the torque that makes a car go. It also makes heat and stress which cause an engine to distort. This distortion of a few thousandths of an inch might seem trivial; however, it causes increased friction, cylinder bore distortion and degraded piston ring seal, all of which negatively impact fuel economy, exhaust emissions, durability and, of course, performance.
If you could measure a running engine in real-time, you would note that the block “quivers†like a big tuning fork as a result of stress to the block by the engine’s power impulses. This makes for noise and vibration, two more customer satisfaction issues.
Considerable design resources went into making the structure of the LS1 case both lighter and more rigid than that of the iron block engine it is replacing. Examples are: 1) many, external, stiffening ribs, 2) six-bolt, steel, main bearing caps and 3) the “skirt†that extends below the crankshaft centerline. These features make an extremely rigid case. Chevrolet refused to quantify this rigidity, but we suspect that it is significant. The pay-off is less noise and vibration, better fuel economy, reduced emissions, improved durability and higher performance.
“Once we had the design,†Juriga continued, “there were casting and manufacturing issues that had to be resolved. First, what casting process to use. We went with a process that was relatively conventional in that it wasn’t lost-foam or die-cast.â€
The LS1 block is made of 319 aluminum heat-treated to the T5 standard by the Montupet Corporation of Ontario, Canada. It is cast using the semi-permanent mold technique which Juriga described as “….a cross between die-casting and sand-casting.†The case weighs 107 lb. Compared to the Gen II’s 160 lb. block, that’s a significant weight saving.
The engine uses centrifugally-cast, gray-iron liners. The liners to be quite thin, but very strong due to centrifugal force increasing the density of the iron during casting. The sleeves are, then, cast into the aluminum block at the foundry. When asked in June of ’96 about these sleeves’ tolerance of overbore during rebuilds, Juriga said that a thickness figure was unavailable and that, to date, GMPT had not addressed the service overbore issue. In a second interview, in March of 1997, John Juriga told me that the LS1 engines for MY97 and 98 will tolerate only about a .005-in. overbore which really amounts to just a clean-up hone. In 1999, the sleeves will be revised such that service overbore of .015-.020 is possible without compromising durability.
The liners are finished with a bore size of 99 millimeters (3.8976 -in.). That, with a stroke of 92 mm (3.6620-in), makes the LS1’s displacement 5.665 liters or 345.69 cubic inches. Obviously, it won’t fly with the Chevrolet marketing folks if, upon opening a C5’s hood, people holler, “Hey, Vern! I got me one of these new, three hundert ‘n’ forty-six inch motors, here.†so Chevy wants you to call it a “350″ or a 5.7L engine.
John Juriga on working the bugs out of the manufacturing process: “Getting prototypes made without porosity, without cracks and on time was the next difficulty. To make a producible component–if I look back–was a steep learning curve for us. It was a challenge. It took us a while to get to where we could produce castings for our prototype builds in quantities that we needed.â€
At that time I also learned about the LS1 development via a good source on the C5 team. “We had problems with leaks due to sealing and porosity,†I was told, “Both coolant in the oil and oil in the coolant. Another problem was getting engines. Any new engine program is going to have failures. Powertrain was working to solve them but there were times where vehicle development slowed because there were not enough engines. We even tried to fit LT4s in some C5s so we could push ahead with testing not related to powertrain, but the LS1 is shorter and the steering rack had been relocated rearward, so an LT4 just wouldn’t fit.â€
Getting the manufacturing technology of the Gen III’s aluminum block right proved a daunting task that took a couple of years. Clearly, people at GMPD working to make the LS1 reliable and durable put in a ton of overtime and had to make some tough choices. One of those had GM discontinuing its relationship with the initial block supplier, Alcoa. Some of Alcoa’s manufacturing process were based on aerospace techniques. In this case, the attempted application of aerospace technology to automobiles was unsuccessful. Alcoa’s failure to perform drove the GM decision to transfer the casting to Montupet. Other hard decisions came later upon discovery of a problem with the LS1’s oiling system which is covered later in this article.
While these problems were traumatic, they are an expected part of the high-stakes business that is bringing a new engine to production. GM designs, develops, tests, then develops some more based on that testing. Eventually, this process results in a Corvette powerplant that is reliable, durable, drivable and capable of best-in-class performance. If my road test experience in a pilot cars along with the experiences of other magazine test drivers is any measure, GM has a home run in this engine.
Plastic Intake
How ’bout that plastic intake manifold, eh? Okay, we’ll use the marketing buzz word “composite†once, but we common folk find the stuff of which that intake is made closer to plastic than anything else. Specifically, it’s a Dupont material called “Nylon 66″ that is a mixture of a Nylon and glass fiber reinforcement.
Plastic manifolds are easier to manufacture, weigh less, run cooler and better lend themselves to intricate designs. We wonder how long it will take C5 design chief, John Cafaro, to discover plastics can be dyed to get stuff like fluorescent-pink intakes? Ok. Just kidding. Ah….but don’t get any ideas, Cafaro.
The first question about a plastic intake is, “Do they, like…melt when the engine overheats?†Well, if overheating means setting your C5 on fire; then the intake will probably melt; however, this manifold will withstand the heat of operating temperature, even that of an engine stuck in Death Valley in mid-August with a coolant overtemp situation.
This intake manifold uses some cleaver packaging. The plenum is beneath the runners allowing the runners to be long, but also to curl smoothly from their junction at the plenum, up and over to each intake port in the heads. The smooth curves, also, enhance airflow. The plenum occupies space in the valley, making the engine as short as possible. The intake is well-integrated with the intake ports, because, in the early stages, the same person had design responsibility for both.
Look closely at the throttle body on the front of the intake manifold and you’ll see a major innovation. Instead of a throttle bell crank there is an electric throttle control (ETC). LS1 will be the first throttle-by-wire application in a GM car. The connection between your right foot and a C5 will be via a wiring harness. Throttle-by-wire has been used in aircraft for many years and on a GM light-truck, diesel application since 1995, but C5 will be its first performance car application outside of motorsports.
The LS1’s sequential electronic port fuel injection (SFI), is similar to what has been used since 1994. Each cylinder has its own ACDelco Multech injector to meter fuel. A mass air flow (MAF) sensor, meters the air. The injectors are controlled by the PCM. It sets the fuel delivery schedule by applying data, such as crankshaft position, mass and temperature of intake air, engine speed, coolant temperature and a few other parameters, to fuel “look-up†tables in the PCM software or “calibration.†Based on those look-up tables, each cylinder’s injector is “pulsed†in the engine’s firing sequence such that a precisely metered amount of fuel is shot down the intake port just before the valve opens.
Under many driving conditions, the PCM uses a “feedback loop†to trim fuel delivery to optimum levels. Free oxygen in the exhaust is an accurate measure of fuel mixture. The feedback comes from oxygen sensors (O2S) screwed into the exhaust manifolds. They measure the oxygen content and send that information to the PCM. When the engine runs in this “closed loop,†combustion is optimized for best performance, exhaust emissions and drivability.
One interesting aspect where LS1 departs from the Small-Block is that the whole induction system, intake manifold, throttle body, injectors, fuel rails and wiring, is assembled by an outside supplier, shipped to the engine plant as one piece and simply bolted in place.
LS1 uses a tuned, intake port length as did the L98 of 1985-’91; however, LS1’s 15-in. runner length is tuned for top-end power whereas L98s 21-in. runner was tuned for mid-range torque.
The Rest of the Basic Engine Story
The crankshaft material is cast, nodular iron, the same used for Gen II and many Gen I cranks. It is noticeably shorter than that of a Small-Block and the main bearing size is larger than that of all except the old 400. The rod bearing journals are the Small-Block “large journal†size. In fact, the only part in the whole darn engine that carried over from the Small-Block are the rod bearings. For improved strength, the crank uses the rolled-fillet journals introduced with LT4. The crank weighs a bit more because of the larger main bearing journals and an ignition trigger wheel that is part of the casting. In another departure from the Small-Block, to reduce the effect of crankshaft expansion on alignment of internal engine parts and external accessories; the crankshaft thrust is taken by the center main bearing rather than the rear unit.
The LS1 uses a sintered, forged, PF1159M steel connecting rod. Also called “powdered metal†or “PM,†this technology was introduced in Corvettes for MY96. The basic, Small-Block rod currently in the GM Performance Parts catalog is also PM.
To make a sintered rod, a mold is filled with steel powder which is “briquetted†or compressed under extremely high pressure. Then, the rod is “sintered†which heats the metal just to its softening point causing the steel molecules bond and making a dense, very strong part. Next, the rod is put through a conventional forging process. Lastly, it is shotpeened. The combination of these manufacturing techniques results in a rod with “net shape,†which requires no machining for profile or balance and is more consistent in mass than rods of traditional manufacture.
The LS1 rod is also known as a “cracked rod†because the big-end is fracture split. During the finishing process, to split the big-end; a stress riser is cut into its inside diameter. The rod is stressed such that it fractures at that riser. The jagged surface left on both pieces precisely locates and locks the rod cap in place once the rod is assembled. For simple assembly and mass reduction, the LS1 rods use a 9 mm. capscrew rather than a rod bolt and nut to hold the big-end together.
Rod length is 6.1 in., .400-in more than the LT1/4 rod. The extra rod length reduces rod angularity and piston speed which decreases friction and noise and increases durability. LS1 rods have no balance pads making for less overall mass and allowing the engine to rev quicker. Undoubtedly you’re asking, “Hey, wadaya mean ‘no balance pads.’? How do they balance the rods, then?â€
Well, they don’t.
Small-Block rods were held to a weight tolerance of ±5 grams, per end, after balancing. The LS1’s PM rods are manufactured to a tolerance of ±3 grams for the small end and ±4g for the big end without machining for balance. Such are the advantages of a net shape.
How good is this connecting rod? Many stock rod Small-Blocks, after lengthy time in severe duty, will display fretting corrosion of the inside diameter of the big end. This is due to the big end flexing a tiny bit under the bearing shell. The LS1 rod, under similar operating conditions, shows virtually no fretting. Bottom line: The LS1 rod is the strongest connecting rod ever used in a GM, production, mid-displacement V8.
The new engine has of cast aluminum pistons. Their compression height is 34mm and they weigh 434 grams each. Unlike production Small-Block pistons, they have no steel reinforcement strut. The old engine needed that feature to control piston expansion because the manufacturing process controls used previously and bore distortion due to the old engine’s having head bolts threaded into the block decks, made for wide variation in bore sizes. To keep piston-to-bore clearance such that acceptable durability would come even with the smallest, expected bore size; piston expansion had to be restricted and that was done with the steel reinforcement.
With the Gen III engine family, process controls at the block machining stage are tighter and there is no bore distortion due to head bolts because their threads are very deep in the block. With bore variation significantly reduced, piston expansion control is not an issue, so the steel strut was eliminated making for a lighter piston that is less costly to manufacture.
The biggest visual differences between pistons for the new engine and those for LT1/4s are 1) LS1 units have no valve reliefs, 2) they have 6mm. less compression height which allowed the longer connecting rod and 3) the top ring was moved up 1.5mm.
There is a ton of technology in piston and rings aimed at reducing friction. The rings use the same basic materials as before but the design is different. The LS1 top and second rings have 1.5mm faces vs. the 2.0mm rings used in LT1/4. The tension of all rings have been reduced by about 30%. Reduction in ring face widths and tension would never have proven reliable from a cylinder sealing and oil consumption standpoint, if process control improvements did not result in reduced bore variation and improved consistency in individual bore diameters.
The LS1’s pistons are lighter than a LT1/4 piston. A bore size 2.5mm smaller, 6mm less compression height and the lack of a steel strut make this possible. Improvements in LS1 rods and pistons have reduced the weight of each rod/piston assembly by 120 grams compared to the same LT4 pieces. That is a significant decrease that guarantees the engine will rev quicker (and it does!!) and be more durable at high engine speeds