Development History of the 1990 - 1995 Corvette ZR-1 LT5 Engine

We have compiled a collection of interesting notes and historical facts on the development of the LT5 engine by Lotus and Corvette Powertrain Engineers.


Graham Behan:  Graham Behan was an engineer who joined Lotus a couple years after the LT5 engine project began in the mid-1980s. He was eventually in charge of overseeing the enhancements for the 405-hp "Gen II" LT5 used in the 1993-'95 cars. Today, he is the Chief Engineer at Lingenfelter Performance Engineering.

"For 1990 - 1992 production engines, our durability target was 200 hours of GM's Corporate Durability test cycle, which consisted of cycling the engine between peak torque and peak horsepower rpm at WOT, with a 2.5 minute ramp up and down between the two speeds. For the 1993 and up engines the requirement was 400 hours on the same cycle. The biggest single improvement from the 4 bolt blocks was actually a process upgrade made at Central Foundry which eliminated porosity in the main bearing webs. When we actually started to run the first 4 bolts, with cases from Dennison Foundry, they split up the #2 & 4 main bearing webs. Since Dennison went chapter 11, we moved to Central Foundry and reaped the benefits of better process control leading to less gas entrapment." - Graham Behan

Regarding catalytic converter failure on the LT5 engine:  "The failure mechanisms are either a pure overheat of the cat, during testing we found stuck open injectors could cause this or a serious misfire, 2 cylinders. Secondly within the catalyst itself the hottest area is approximately 1" into the brick, this is caused by the exothermic reaction within the cat, as the cat ages a crack can initiate in the brick in this region, this is known as a "ring off crack". With an LT5 running at 7000 rpm WOT, the vibration level measured at the catalyst is approaching 50g, as the packing around the brick(monolith) deteriorates with age the two pieces of the brick can move relative to each other and generate ceramic particles. Under certain engine operating conditions the particles can be transported into the engine on the 90-92 exhaust manifold, not good. In so far as a means of early detection any noise from the cat is a sign but may be too late." - Graham Behan

"The production LT5 engine is an internally balanced rotating assembly. Some of the early prototypes were externally balanced, in that a proportion of the balance mass was carried in the TV damper and the flywheel. However with the production engines, the crank was redesigned to have sufficient mass to perform the balance internally and the flywheel and the TV damper were zero balance. This balance of the rotating assembly assumed a nominal pin weight, to represent the piston, rings, retainers and rings and an assumed amount to cater for the oil flowing within the crank pins (due to the through crank oiling system. Since the pin weights were based on a nominal mass condition for the parts mentioned above, a final balance was performed on the running engine and the remaining minor adjustments were made by adding Carbide bars to either the damper or the flywheel or both. When we made the changes to the 1993 MY engine specification, we machined the rod forging to gain the mass reduction, this in turn reduced the variation from the nominal mass specification. Since we were controlling the rod mass variation more acurately, this reduced the amount of additional balance that was added on the assembled engine. In fact from the 1993 MY on, only a few engines received any additional mass at all. All of the packages mentioned above from LPE have the balance completed with pin weights representing the actual measured mass of rod, piston etc, therefore no mass is required to be added for the production variation which Mercruiser corrected on the stock engine." - Graham Behan

"The breather system of the LT5 is certainly not one of its strong points, stories from the dark early days of development were akin to the striking of an oil deposit for the first time. There were many strange contraptions which were intended to keep the LT5's running, one of which was a four inch diameter tube connected to the oil filler cap, which when under wide open throttle, a column of oil would raise three feet into the air. Since this would have been a severe packaging problem, other solutions were sought for the vehicle. Basically there is insufficient crankcase volume and the system which exists is a serious compromise. So all LT5 engines carry a small amount of oil through the breather and it is perfectly normal to see oil in the plenum. With increasing blowby, this situation will get worse, but typically not enough to cause detonation problems. The engine in the World Record car was a stock motor which was shipped out of Mercury. In most race applications we recommended running a quart over full to minimise the effects of oil surge." - Graham Behan

Regarding ZR-1 Windshields:  "The standard Corvette windshield is identical except lacking the metallic heat shield. This was used because the AlC evaporator in the ZR1 was made smaller than optimum to accomodate the engine. Chances are that only in the hottest areas of the country would you have problems. In any event, it would probably be better than a broken windshield." - Jim Ingle - Corvette Powertrain Engineer

Regarding a special machine built to manufacture the LT5 engine:  "It now resides in a warehouse in Ohio I believe and is pretty close to becoming scrap. It is a neat piece though, diamond tipped tooling on a tungsten boring bar that was held in tension during the cutting operation. All of which made it able to control a five micron (metric) tolerance with a cpk (for closet statisticians) of better than 1.33. Terry Stinson of Mercruiser was credited with the invention of the beast and was actually voted the young production engineer of the year for the USA in 1989 or so." - Graham Behan - February 1999

"The tale of the stalling ZR1s" - by Ian James - June 10, 2020

"A few days ago, I wrote that I'd write the story of the stalling ZR1's when we got to the right time. I've just found my 1991 calibration review report which tells me that this happened in September '89. All the early ZR1's off the Bowling Green line had a stalling problem and so I ended up back at MPG (Milford Proving Grounds) to sort it out.

As I said earlier, this was serious.

It was particularly serious as my key partner on the calibration team known for his ability to think in a crisis had been stolen away to the mired Elan project months earlier as the LT5 was all done and finished wasn't it?

I ended up on the wrong end of writing daily reports to Roy, Larry, Jim and the Bowling Green Plant Director to explain the problem, the analysis, the immediate fix, the root cause and what I was doing about it to make sure it never ever happened again.

It was caused by a green engine effect that we'd not picked up through the development program. It showed up a weakness in our closed throttle ignition/ transition into idle speed control strategy. Cars were shipped to MPG, and harassed hours spent with Jerry Kortge again, to test, document and come up with a solution. Again he wrote the prototype code, we validated the solution and got it through DE to release in very rapid time.

There were two software guys at DE who's names need recording. They were in the background but still did vital work. I think they were David Gray and Eric Holm. I've begun a search for my old notebooks to find them but I've only got back to 1998 so far.

While we'd been at the DPG in the summer of '87, Brian Green the CPC V8 Group Calibration Manager visited and one of the conversations considered our use of PCM 'level control' software. What software? You know, level control software that's on the controlled path to a production release. That was an Aw sh*t moment! We had no clue and it turned out that our PCM's were running development software written by a very clever man, but it wasn't in the proper software control system.

I guess Brian went away and made some calls. Soon he was back and said something like ‘as soon as you're done here, go to Milford, find a guy called Jerry Kortge and work with him to sort this out!'. Having finished up at the DPG and Denver, I got to MPG during October, and found the man (as Mark has mentioned) who became the father of the LT5 PCM software. Jerry had the wonderful ability, in answer to any question to be able to say, "I have no idea!" with such authority that you felt a complete fool for daring to ask. I quickly realized though that just because he had no idea this afternoon didn't mean he wouldn't have the answer by tomorrow. I explained what we wanted – independent closed loop Lambda control for each bank. He explained that it had never been done inside CPC! When he outlined the software logistical nightmare of keeping track of individual bank fuel corrections as applied to each sequential injector pulse time I just about died. But he wrote the development code, we proved it and off it went to Delco Electronics to be written up correctly, tested and released back to us (with a level control reference!). It was a real privilege to work with Jerry.

The release came on a South East Michigan day at the beginning of February '88. The weather forecast was freezing rain. The software went into 081. It ran fine and the first drive was going to be the run home that evening. We looked at each other and said we don't want to do this! But we were under pressure to have level control software in the cars. Failure wasn't an option! The rest you've read, though I have to publicly acknowledge a major driving feat to get around the F150! It really could have been a lot worse."

GM corporate driveability ranking table for 1990 model year vehicles

"With apologies for the quality, it's the only copy I've got. This (above) is the GM corporate driveability ranking table for 1990 model year vehicles. The driveability auditors drove and rated every car that GM made that year using the GMUTS cold start and drive test procedure. You'll be able to make out the important point! To top that list from a standing start with a new engine meant a lot. I was 3700 miles away so it's a genuine result!" - Ian James