This article discussed the technical aspects of the LT5 engine. It was the second part of a "double-feature" that ran in an early 1989 issue of Corvette Fever. The story was developed, in-part, from information dispensed to the press in a July, 13, 1988 press briefing Chevrolet staged in Riverside, California in conjunction with it's long-lead media preview of the 1989 models. The briefing was unique in that it was the first time GM had staged a major media introduction for just an engine.

Power Fit for a King

Chevrolet's LT5

©1988, 1999 by Hib Halverson

LT5!!!

Like LT1, L88, ZL1 and LS6, it's an alphanumeric you'll not forget.

It labels a brand new engine from Chevrolet, its first new V8 since 1965, a production engine of such revolutionary design for an American manufacturer, it leaves even the techies nearly speechless in amazement. Of course, I'm talking about what's been rumored since 1986: the all-aluminum, 5.7-liter, 4-cam, 32-valve V8 that powers the Corvette ZR-1. The King is coming, motivated by LT5!

Competitors best beware the Red Bowtie.

The LT5 story starts in the spring of 1985. Even though the in-house turbo program was canceled, Chevrolet's requirement for a 400 horsepower powerplant survived. Corvette Chief Engineer, David McLellan, next turned to Group Lotus in Hethel, England, initially to have them develop a set of dual-overhead camshaft cylinder heads for the existing, 5.7-liter, small-block V8. After looking at Chevrolet's needs, Tony Rudd of Lotus counter-proposed a totally new engine. This idea was subsequently approved by Chevrolet and, quoting Chevy's Chief Engineer, Fred Schaafsma, the program's goals were:

"Create a car that is second to none in acceleration, nothing less than the fastest production car in the world (my italics, an astonishing public statement from a Chevrolet Chief Engineer).

"Achieve that kind of performance without sacrificing drivability, not only at the high end, where you expect fast cars to drive well, but at the low end, too.

"Package all this leading-edge performance and drivability into an engine that could still deliver great fuel economy.

"Design this engine to fit between the rails of the existing Corvette's engine compartment, a brand new engine but not one that would require a totally new car."

In 1985, this looked like an impossibility. However, in about three months, Chevrolet is set to make the impossible happen with the debut of the LT5.

The engine design is first rate. The cylinder case is an aluminum casting with lots of webbing and gussets making a very rigid structure. It's fitted with removable, Nikasil-plated, aluminum wet liners. The only thing LT5 shares with the L98 5.7L small-block V8, is a bore center-to-center distance of 4.40-inches. This quirk was, apparently, a "gimme" to the marketing people. This bore center restriction and the aluminum liners' thickness demand that the LT5's bore be reduced to 3.90-inches (.010" less than L98). The stroke is 3.66-inches (.018" more than L98) and block deck height increases because of the longer stroke. The iron main bearing caps are an integral part of an aluminum girdle that is cast with the caps in place. Studs screwed in the block hold the caps and girdle. The oil pump is a new, "gerotor" (similar to automatic transmission oil pumps) design, is crankshaft-driven and bolts to the front of the block. The oil pan, containing the oil pump pickup and a windage tray, bolts to this girdle.

The crankshaft, like high performance Corvette cranks of the past, is forged steel. It's cross-drilled for centrifugal oiling. Oil is routed from the pump, through the nose of the crank to the rod journals and then to the mains. To accommodate the oil pump inner rotor mounted on the crank snout, the LT5 crank is just a bit longer than a traditional L98 unit. Main bearing diameter is 2.76 in. and rod journals are 2.10 in. The connecting rods are a brand new forging that mimics the heavy-duty, Bowtie rod and has a center-to-center length of 5.74 inches. The pistons are Mahle, cast aluminum, dish-type units having full floating pins. Rings are special cast iron units designed especially for use with the Nikasil-plated cylinder liners.

Is LT5 starting to sound a bit like a race motor?

An engine is only as good as its cylinder heads and on the LT5, they're a work of art and science unheard of until now on an American V8. They have four valves per cylinder, hard valve seats, press-in valve guides and centrally located spark plugs. Valve angle is a narrow 22 degrees that results in several desirable features: 1) shallow combustion chamber with good squish characteristics 2) straight ports and 3) relatively narrow head allowing less overall engine width. Valve sizes are 1.54-inches for intakes and 1.38-inches for exhausts. The shallow combustion chambers and the dished pistons make for an astounding 11.25:1 compression ratio! LT5 could live with that high figure on, so said Chevrolet at the time, 87 octane regular unleaded gas due to high-swirl intake port and combustion chamber design along with digital control of the fuel and ignition schedules. Later, once the ZR-1 was in production, Chevrolet revised its fuel requirement for the LT5 to at least 91 octane, premium unleaded fuel.

There are two intake ports and two exhaust ports per cylinder. Intakes are individual but subdivided into primary (front) and secondary (rear) ports. The secondaries are all slightly larger in diameter and each contains an auxiliary throttle plate. The exhaust ports are all the same diameter, are individual for most of their lengths but pair-up just at the port exit. The four camshafts ride on bearing surfaces machined in the head and are retained by the cam covers. Each cam has two different intake profiles, mild (252°) for the primary valves and aggressive (272°) for the "secondary" units. All exhaust durations are identical (252°) and valve lift of all lobes is the same (.390"). Direct-acting hydraulic lifters work the valves and dual valve springs are used.

Most DOHC V8s have a wide cross-section because of the camshaft drives. Not so with LT5. The crankshaft drives an idler, located at where the cam gear is in a pushrod engine, at a 2:1 ratio via a traditional, double-row chain. This half-speed idler runs the cams through two more chains and sprockets. Those two cam chains are kept tight by hydraulic tensioners on their slack sides. This system cuts the diameter of the cam gears in half and reduces engine cross-section.

The induction system is described as "three-phase" and looks like a sci-fi version of the tuned port injection we've known since 1985. There are three throttle bores, 16 runners and 16 Multech injectors. The smallest throttle plate, 22mm, supplies air at idle and very low speeds. Next, the two main throttle bores, 59mm each, open. Below half-throttle and 3500 rpm, the engine runs with the auxiliary throttle plates in the secondary ports closed and on eight of the injectors.

Once the driver gets his boot into a LT5 V8, things begin to happen. When both 3500 rpm and 1/2 throttle are exceeded, the auxiliary throttle plates pop open and the second set of injectors come on line. The extra punch of the secondaries is further augmented by their more aggressive intake valve timing. This three-phase TPI maintains high intake flow velocity and turbulence along with tight control of air/fuel ratio over a wide range of engine speeds. The result of this is a flat torque-curve and that, along with digital engine controls, is responsible for the engine meeting Schaafsma's goal of smooth down low and real nasty on the top end.

A most interesting feature of the LT5 TPI is the power key, or "valet switch" mounted inside the car. Turning it off, disables the secondaries and protects Its Royal Highness from the abuses of valet parking services, children trying to impress their peers or detail shops who aggressively "road test" each job.

The engine exhausts through two free-flowing, tubular stainless steel exhaust manifolds. A catalytic converter mounts right at each manifold outlet making for rapid cat light-off and cleaner emissions. Under normal operation no smog pump is necessary. To pass emissions certification on cold starts, the LT5 has an intermittent, electric smog pump that comes on only when needed. To make way for the engine hugging cats, the starter moves to the valley under the injection system. The ignition unit also mounts there. It is a direct-fire (distributorless) system consisting of four coils (each firing two plugs), a crankshaft trigger and an ignition module. The trigger wheel is inside the engine on the crankshaft just aft of the 3-4 throw. It contains nine notches on its circumference: eight firing notches and an index notch to tell the ignition when to start the firing order. As each notch passes a Hall Effect pickup on the bottom of the ignition module a trigger voltage is created. The ignition module interfaces this signal with ignition schedule data from the car's electronic control module (ECM). The result is each spark coming at exactly the right time.

The power steering pump, water pump, air conditioning compressor and alternator mount on the front of the engine and are driven by the customary serpentine belt. Maximum safe engine speed is an astounding 8000 rpm, however Chevrolet engineers tell us the accessories are short-lived at that speed. Thus, there is a fuel cut-off program in the ECM software that stops all the action at 7200 rpm.

LT5 was designed and developed by Group Lotus. Chevrolet-Pontiac-GM Canada (CPC) Engineering provided concept direction and design input. Interestingly, during the LT5 program, GM bought Group Lotus so, in the end, the design of the LT5 ended up an intercontinental, in-house effort. The combination of Lotus' dual-overhead cam racing engine technology and Chevrolet's expertise in building millions of production line engines made for (quoting Chevrolet's technical information) "a synergistic effort that resulted in a design that was better than either group could have done independently." Interfacing the LT5 with the Corvette chassis, testing and Federal certification is being handled by CPC and the Corvette Development Group.

There have been unconfirmed reports in the automotive press about development difficulties with the LT5 project. We've heard about problems early in the program that necessitated a redesign of the camshaft drive to the configuration discussed above. Other anomalies with the cam chain tensioning system have been rumored. There was a lot of talk six months ago about serious overheating. We hear discussion about continuing problems with durability and economy causing reductions in rated horsepower. Supposedly, the car's exhaust system may bring further reductions in power. All this may or may not be true.

Even if it is, so what?

Let's look back 20 years. The last new Chevrolet V8 was the Mark IV, big-block introduced mid-year, 1965 in the Corvette as a 396. How easy we all forget that it took about three years to develop that engine. The LT5, like the big-block, is a clean sheet-of-paper engine. Anytime a project like that is undertaken, everything–right down to mundane things like gaskets, thermostat housing shape and spark plug heat ranges, must be developed and tested. My own opinion is that compared to what we taxpayers have gotten with the Air Force's B1B bomber; development of the LT5 must be a piece of cake. Jim Ingle, (interviewed elsewhere in the magazine) when asked about this subject once said, "(With LT5)…all of it had to be developed. You know what? We'd have to be the luckiest guys in the universe to have everything in a new program like this finished at the same time and with no problems."

So, let's leave it at this: Yes, there have been development problems with the LT5. However, none of them are anything out of the ordinary for a brand new engine nor are any unsolvable.

There are a few who seem to feel this shows the engineering process at Chevrolet is flawed. For the most part, that view is idle chit-chat on the part of a few automotive writers who feel themselves qualified to armchair-quarterback a highly technical program they in fact probably know little about. Or maybe they know just enough to be dangerous.

As a result, even our best sources at Chevrolet are unwilling to talk about the specifics of the LT5 development. This is too bad because it deprives us all of what is undoubtedly an excellent story about what is certainly one of the automotive engineering feats of the decade. Of course, I can sympathize with Chevrolet, when a some of my colleagues have a "National Enquirer" attitude; one can expect no other reaction than a bunker mentality.

The LT5 is built under contract by the Mercury Marine Division of the Brunswick Corporation. At face value, this may seem strange but upon examination it appears a wise step. The reasons for Mercury's participation are two: 1) this engine will be a moderate production item (10-50 units per day) and, to attain quality goals, will require race-shop style assembly. Chevrolet would have trouble with costs and quality in a small run like that whereas Mercury, who normally deals in those production rates, is ideal for it. 2) The engine has many precision-machined aluminum die-castings and uses anaerobic (gasketless) sealing in many places. Mercury Marine is a world-leader in these technologies because of their experience with aluminum, anerobicly-sealed outboard motors and outdrives. Mercury will subcontract much of the foundry work but 95% of the machine work and all of the assembly tasks will be done in Mercury's facility at Stillwater, Oklahoma. At this point, the plan is not only break-in each engine on a test stand but each engine's maximum horsepower will be verified before it is shipped to the Corvette assembly plant in Bowling Green, Kentucky. Lastly, Mercury's agreement with Chevrolet includes a clause that gives them the right to adapt the LT5 for marine use approximately one year after it's introduced in the Corvette. Undoubtedly, in 1990, the hip, discriminating yuppie will need a tow hitch on his ZR1 to pull a day cruiser powered by, say, two marine LT5s.

What about servicing this new engine? The American manufacturers' dealer service networks have a reputation of being slow to respond to new technology trends in their products. Chevrolet recognizes this and also understands that ZR1 customers who pluck down nearly sixty thousand dollars are going to want something more than a puzzled service writer telling them "Ah, yea, well it's gona be six weeks until we can figure out how to fix it."

One thing the dealers will have is a new diagnostic tool called The GM Computerized Automotive Maintenance System (GM-CAMS). It is programmed with vehicle specifications and all current service bulletins, can test the entire computer engine management system, assists the mechanic with adjustments to the engine management system, and can transmit data on the specific car being serviced to other destinations such as GM divisional technical assistance centers or even the engineers in Detroit.

However, the question still remains: once an LT5 problem is diagnosed as being other than with engine management, what framework is in place to acquaint dealer service departments with the revolutionary technology of the LT5? At the press introduction of the LT5 at Riverside, California last June, that question was raised. Chevrolet said that training would commence a few months before the introduction of the ZR1 and until servicing ability is up to speed, any LT5 problem requiring the opening-up of the motor will be handled with an engine exchange. The dealer will remove the offending LT5, replace it with a known-good unit then ship the bad motor to Mercury for repairs.

In theory, this is a cleaver way to sidestep the learning curve problem but we have additional questions. Does the customer have to wait while Mercury ships out a fresh motor or will replacement units be stocked at GM Service Part Operations warehouses around the country? Then, what about the customer who wants to keep his original engine in the car? Will he have to wait while his motor is shipped to Stillwater for repairs? Chevrolet told us right at deadline that with the debut of the LT5 still five or so months away they had not completed the specifics of the engine exchange program but that they will do everything possible in an engine exchange situation to "respond to the customer's needs with the least possible down-time."

Anyway, that's the story of the LT5 V8.

"Oh yea, I almost forgot horsepower and torque! Please understand the figures we have at deadline are preliminary and, as LT5 durability testing and Federal certification (both driveby-noise, fuel economy and emissions) are still under way at Chevrolet; they could change as much as 5%. With that disclaimer out of the way, how about 375 horses at 6000 rpm, 370 pounds/foot torque at 4800 and, confirming that this motor does in fact have a lot of grunt down low, 300 lbs/ft. at (choke, gasp!) 1500 rpm!

LT5 is indeed power fit for a King, wouldn't you think?