Gen III’s ignition system is evolutionary. In 1990, with the LT5 engine, Chevrolet introduced a distributorless ignition system (DIS) to the Corvette. The next step is the LS1’s coil-per-cylinder idea.
The ignition hardware is mounted atop the valve covers. Each cylinder has its own coil and coil driver assembly and a short plug wire connects each a spark plug. The reasons for moving the coils to the covers are simple: 1) less spark energy is dissipated by short spark plug wires so more energy available at the plug, in fact, it increases nearly 100% and 2) shorter plug wires reduce radio frequency interference with on-board computers and the sound system.
The other big ignition story with Gen III is a different firing order. Gone is the time-honored 1-8-4-3-6-5-7-2 sequence. This new engine fires 1-8-7-2-6-5-4-3. The cylinders are numbered the same: left bank: 1-3-5-7 and right bank: 2-4-6-8. The reason for the new firing order is better idle stability and less vibration.
The rest of the ignition system is conventional. The ignition advance is controlled by the PCM based on manifold pressure, air temperature, engine speed, coolant temperature and a few other data. The PCM computes the optimum trigger point then sends a trigger impulse to the coil driver at the appropriate cylinder.
Detonation protection is similar to what’s been used in the past. There are two knock sensors (KS) working in a feedback system with the PCM. When a KS “hears†detonation, the PCM retards timing a set amount and for a set time, then waits for additional sensor input. If the detonation stops; timing is gradually reset to the value called for in the PCM calibration. If detonation continues, timing is retarded an additional amount.
Spark plugs are an AC, platinum-tipped plug of a type similar to that introduced in 1992 on the LT1.
Exhaust, emissions control and accessories
LS1 exhaust manifolds are double-walled and welded-up from hydro-formed, tubular stainless steel. The double-walls prevent heat loss between the head and catalytic convertors which is a big factor in how quickly the cats. start catalyzing the exhaust. Late cat. “light-off†is a significant contributor to exhaust emissions during cold starts and early warm-up. Unlike the LT1/4/5 engines, the LS1 cats. are not attached right at the exhaust manifold outlet. They are a bit farther downstream and because of that, it was necessary to add measures to reduce heat loss and preserve quick cat. light-off.
The rest of the LS1 emissions controls are similar to what has been used on Corvette since about 1990. There is an electric air injection reactor (AIR) pump that runs after start-up for a short length of time set by the PCM. Interestingly, the LS1 is the first Corvette engine since the early-;70s that can pass exhaust emissions standards with out an exhaust gas recirculation (EGR) system. LS1 has the second generation, on-board diagnostics (OBD-II) used, in part, since 1994, and, in entirety, starting in 1996. The LS1 PCM has more computing power than the ’94-’96 units which allows the new engine to be OBD-II-compliant in a more “seamless†manner. Additionally, it allows the C5 platform to meet more stringent emissions regulation due in the late-’90s and early-’00s.
OBD-II is one of those wonderful Federal mandates that’s raised the cost of cars but will have little practical impact on air quality. Nevertheless, it has satisfied the vote-getting needs of politicians who pander to the environmental lobby and it has driven some pretty amazing technology from car companies.
The biggest difference between so-called, “OBD-Iâ€, used on Corvette from the late-’80s to 1993, and OBD-II is that the current system requires the PCM to predict potential failures of emissions controls as well as notifying of failures that have already occurred. Two significant features enabling this prediction are catalyst monitoring and misfire detection.
The catalytic converter makes a large contribution to reducing emissions. OBD-II monitors cat. performance by taking oxygen readings from a second pair of oxygen sensors downstream of the cats. When the downstream readings begin to mimic the upstream readings, the PCM assumes cat. performance is starting to degrade and turns on the malfunction indicator light (MIL).
Misfire detection is the most complex engine management problem faced by the car companies in a decade. It demands a very fast, powerful and sophisticated processor in the PCM, very accurate crankshaft position data and some technically innovative software. The PCM reads very small and extremely rapid variation in crankshaft speed as the engine accelerates and decelerates in reaction to power impulses. Inconsistent variations in those accel. and decel. rates are indicative of engine misfire. Specific types and durations of misfire can be a sign of other emissions system problems and will turn on the MIL. The trick comes in accurately determining what’s misfire and what’s not. That has stumped some of the automobile industry’s biggest players, especially with manual transmission powertrains.
GM Powertrain Division leads the industry in diagnostics. Since full-OBD-II compliance became law (generally, with the 1996 model year) there have been cases of car companies discontinuing manual transmissions on some models due to failure to meet the misfire detection challenge. The most notable example occurred in 1996 when there was no manual version of Toyota’s flagship Supra Twin Turbo. It has also been rumored that the Mazda RX7 TT’s departure at the end of 1996 was also, in part, because of Mazda’s inability to address OBD-II. The 1997 Corvette has a manual transaxle available because GMPD has successfully answered the misfire detection challenge with the LS1.
Most of the accessories used on the LS1 we have seen before. LS1 uses a dependable, ACDelco, CS-series alternator. A geroter pump supplies steering power assist. The water pump, of course, is new as is the air conditioning compressor. As with several recent GM engine programs, most notably the Gen 1E for trucks; noise and vibration inherent in accessory mountings was carefully researched, then reduced by designing very rigid accessory mounts, quiet running accessories and an accessory drive that uses two serpentine belts. One drives the air conditioning compressor and the other drives the rest of the accessories.
In Closing
The various PR apparatus at Chevrolet, GM Powertrain and GM Midsize/Luxury Car Division would like everyone to think that Corvette development is as simple, quick and trouble-free as is bringing to market a new type of vegetable slicer, hair dryer or whiffle ball racquet.
Not even.
Development of an all-new engine is a monumental task requiring hundreds of individuals to work tens of thousands of man-hours. It is complex, costly and filled with surprises. It is a credit to the team at Powertrain that it addressed each challenge with effective solutions.
Well, this new medium-displacement V8 is quite an engine, don’t you think?
LS1 generates 345hp at 5600 rpm and 350 lbs/ft. torque at 4400 rpm. Maximum engine speed is 6200 rpm. Compared to the LT4, it generates 15 more horses with peak power 200 rpm lower. It produces 10 more lb/ft. torque with peak torque 100 rpm lower. Chevrolet refused our request for a chart of LS1’s torque curve, but we believe it’s a bit flatter than that of the LT4. All this performance comes from a package weighing 66 pounds less and measuring half-an-inch shorter than a Gen II Small-Block. Some of us still compare today’s power ratings to the gross power figures of the 1960s. If that was still being used, LS1 would put out about 390-400hp. Most notable is that this engine is the first two-valve V8 to reach the one-net-horsepower-per-cubic-inch plateau. That is a monumental engineering achievement.
I recently received a very interesting insight to the power of the LS1. A high-level Corvette Development executive told me that he personally back-to-back tested both a stock, 1995 ZR-1 and a 1997 prototype. The ZR-1, with 60 more horsepower, was a mere second-a-lap faster than the C5. That says much about the ’97’s new engine, lighter weight and better handling.
Bottom line…the new LS1 is all Corvette and one hell of an engine.
. I’ve posted a subset of this note in another string, but felt it deserved to be dealt with as a separate topic. This is meant to be a primer on the subject, which may lead to serious discussion that fleshes out this and other subtopics that will inevitably need to be addressed.
