Thank you to Jerry Watts and the ZR-1 Registry for the following trivia information:

The 1 to 4 lockout was GM's way of meeting government mandated CAFE (Corporate Average Fuel Economy) mileage considerations and avoiding a gas guzzler tax for the ZR1. 1990 through 1994 requirements included the following:

• Coolant temp.: at least 120 degrees
• M.P.H. - 12 to 19
• Throttle Angle - Less than 35% In 1995, these requirements were reduced to the following:
• M.P.H. - 15 to 19
• Throttle Angle - Less than 13%

• In the first half of the 1990 model year, the CD radio came with a lock-out feature (Delco-Loc II). Loc II was printed on the face plate. The owner was instructed to program a six-digit number into the radio. Whenever the radio lost complete power, the number had to be used to unlock the radio, otherwise the CD and cassette players were locked. If the numbers were lost or forgotten, the radio could be de-programed.
• There was no low oil option on the 1990 model, but the bulb position was the DIC (Driver's Information Center).
• The half-shaft was changed during the 1990 model starting with VIN # 800621.
• The little triangle buttons on the seat switches for the 1990 and 1991 models can be ordered with the following part #'s: Left - 12514822, Right - 12514823.
• There is a Kent-Moore tool #J38845 to do diagnostics on 1989-91 low tire problems. The tool will plug into the cigarette lighter.
• The center caps and lug caps were installed at the factory before shipment for the 1991 model year. In 1992 these parts were shipped loose for the dealer to install beginning with VIN #800154.
• The C-note horn was replaced with an F-note beginning with the 1991 model.
• In 1992 the master cylinder bore was increased. In addition, the front brake calipers were changed to increase the brake pedal height.
• In 1993 the horsepower was increased. A good percent of this increase was do to a reduction of restrictions in the exhaust system.
• From 1993 on, there are only four grease zerks on the entire car- upper and lower ball joints.
• The 1990-1992 LT5 engine would stay at a high idle longer than was desirable, sometimes up to 20 seconds after coming to a complete stop. New E-proms were issued to reduce this idle time to 3 seconds.
• On 1994-95s, hesitation was noticed between 1700 and 2500 rpm due to the timing being reduced. A new E-prom was issued.
• The Bowling Green emblem under the hood was replaced by the Mobil 1 emblem on Sept. 28, 1993.
• The first 1993 ZR-1 sold to the public was VIN #800024.
• The part # was changed on the ZR-1 transmission from 10186865 to 10255122 starting on VIN #800232 in the 1993 model year.
• For the model year 1993 the BG plant began putting VIN's on the glass tops. The number is located on the driver side header and can be seen by looking at the the top from the front.  (Note: It also seems that this is maybe true on other years. One owner found it on his '90.)
• Sometime between '93-94 model years. The advanced computation power of something called the "P8" ECM first came into existence in the ZR-1. A full blown upgrade to the earlier 8 bit microcomputer, aka the "P4" design.With the latter cars, LT5 engine RPM could now be resolved into +/-1 rpm increments. Prior to 1993, ZR-1 ECMs could not resolve any better than +/-25 rpm using the "P4" CPU architecture. Thanks to Dennis Duchmann for this info.

• In the first year the ZR-1 was out, 1990, manual AC was standard(C60) with electronic control AC (C68) as an option. Only 124 1990 ZR-1s  produced were equipped with manual AC. Manual AC went away after the first year with electronic control AC becoming the standard.

From Geoff Jeal a past member of the Lotus development team that help design , build , test, and calibrate the LT5 engine and now a member of the ZR-1 Net:

Where did you find all these LT-5 engines and parts?

When GM ordered Lotus to destroy engines, they were taken to a scrap yard nearby and hit with a sledgehammer. This engine has a broken crankcase and inlet castings but is otherwise intact and can be rebuilt. It is an early phase2 development engine, note the metal plate behind the flywheel.

What was that for?

You have to understand that the early motors had severe crankcase oil breather problems. In the dyno cell, we had pipes going up the wall to a 25 litre drum to try and contain the oil thrown out at high rpm. We needed extra crankcase volume to be able to run the engines at high load/ RPM so we cut a slot in the back of the crankcase and sealed the area behind the flywheel with a plate. This was a temporary measure while we sought a solution to the problem.

How was the problem solved?

The solution was clever, we had to find a way of creating extra volume for the oil vapours. An oil separator box with baffles was added on top of the crankcase and the inlet manifolds were cast hollow. If you look at the route taken by the vapours, they pass from the separator through rubber hoses into the inlet manifolds before being routed to the PCV valves so we made use of the dead volume inside the manifold castings.
Then you added the separator box, you had to cut a big hole in the top of the crankcase. Didn't this weaken the structure of the engine?
The crankcase was re-designed to cope with this. We had so many other problems at the time, believe it or not this was not our biggest problem!  We were testing with nodular cast crankshafts which were flexing excessively causing real serious stresses to the bottom end. We dismantled one engine to find the crankcase had split completely into two parts right down the middle, so instead of a V8 we had two straight 4's lying on the bench !

How do you regard the strength of the bottom end now?

Everyone who worked on the LT-5 has tremendous respect for the huge strength and mechanical integrity of the bottom end. We have seen close to 1,000 bhp from the Indycar twin turbo engine built on it. I am in the process of building a special 5.4ltr 800 bhp naturally aspirated engine for a client right now which will use different con-rods but otherwise the same bottom end.

How do you achieve this kind of power output?

You have to realize that, in road engine terms, 100 bhp per litre is nowhere like the threshold it was some years ago. The LT5 cylinder heads flow enough air for 150bhp per litre at 8000rpm so there is loads of potential within the 5.7 litres we have. Cam design, inlet breathing and exhausts are the areas to look at.

Why then do guys in the 'States go for increasing cubic capacity?

Historically, English engineers have refined smaller engines where the Americans go for extra cubes, a different engineering culture perhaps. It is obviously a quicker development process just to add more cubes so perhaps ultimately cheaper for the customer.

From Christopher Mudd Calibration Engineer
Historically English Engineers have refined smaller engines because: In the UK around the 1920's, horsepower was taxable, the tax scale was derived from the RAC (Royal Automobile Club) horsepower rating. The formula used to calculate the horsepower rating of an engine included the number of cylinders and the bore diameter, but not the stroke length, so the larger the bore size and the more cylinders you had the more tax you paid.
To get by this ruling, people such as William Morris, proprietor of Morris Garages, built long stroke small bore four cylinder engines and fitted them into lightweight chassis's, these cars were later known by the initials MG.
American cars with large multi-cylinder engines were made even more expensive at this time by hefty import taxes being imposed by the UK. GM under the steward ship of A.P.Slone had highlighted Europe as a major growth area but found it was not cost effective to ship cars over from the US and then have to pay import taxes.
To get around this problem GM identified three strategically placed vehicle manufacturers in Europe and approached them with buy-out offers, the three companies were Citroen in France, Adam Opel in Germany and Austin in the UK. Austin was thought to be too expensive (GM later bought Vauxhall Motors instead), the Citroen deal didn't go through (GM to this day do not have a manufacturer in France), Adam Opel became part of GM and now supplies Cadillac.
Now that GM made cars in Europe it didn't have to ship them from the US, leaving the Brit tuning guys with an even more remote chance of getting their hands on the big American motor ...That was until the LT5 ................ When 60 years or more of bottled-up engineering frustration was unleashed on an unsuspecting V8.

Once the engine has gone closed loop, do the Oxygen Sensors always dictate the Air Fuel Ratio?

No, the LT-5 has two completely different conditions of AFR. The first, around 14.7:1 (stoichiometric), is under light load with no secondary injection, and ECM software is constantly adjusting the AFR in real time. This design is intended mainly to achieve emissions compliance. The second AFR is about 12:1, under power with secondaries operating, and is not closed loop, the ECM ignores the oxygen sensors and uses a different fuel "look-up" table.

How easy is it to modify the ECM calibrations?

You must remember that ten years have passed since we began the LT-5, and engine management systems have advanced dramatically. Modern systems can be examined and modified much more easily using PC compatible systems. By comparison, a calibration that would have taken us several weeks in the old days would take perhaps two days now. It is possible to make improvements, but you need to be intimately familiar with the LT-5 software, how it works and why before making any changes.

What was the next generation LT5?

The 96 model year was to have been:- 450bhp/450lbft with full OBD2 compliance. The changes were to have been, new cylinder heads with new porting and a change in valve angle plus changes to the combustion chamber. New cam profiles and timing. New inlet manifolds including heater pads to aid fuel vaporization on cold start ( to improve cold start emissions). New exhaust headers with new heated catalysts. Twin plenum intake system with the adoption of twin mass air flow sensing for fuel calculation in place of the speed/density system that the engine currently uses.  All of the parts had been drawn and procured and the prototype engine builds were under way when the project was cancelled. All the parts(enough to build 20 engines) were literally thrown away. Yes really!!

[What was the "octopus"?

The "octopus" you refer to was the air shroud harness from a Phase 1 LT5 circa 1986. The original idea was to feed extra air around the primary fuel injectors on cold start to aid emissions - this didn't work so the idea was dropped- thankfully as they were a real pain to fit up. These harness were only fitted to the first 6 or so engines.

The "GOLD" Plate:

Somewhere out there is a 199? ZR1 which ended up at a eastern U.S. dealership (possibly in Pennsylvania). Rumor also has it that this plate can be seen by looking through the right side runners of the plenum, possibly between 2 & 4 cylinders.
For information the plate which is on the first 91 MY LT5 is actually signed by Barry Eller (of Mercruiser) and Jim Perkins (of Chevrolet). I guess it would have been nice to have Harry sign it but GM thought the two people that actually did were more appropriate. Due to the way the production of cars/engines were scheduled it did not go in the first car, but it was early. This plate can be seen without stripping down the engine but only just.

Cylinder Head Differences:

The production cylinder heads for the LT5 engine were originally produced by Birmal, all engines right up to the last few of the 1995 MY were fitted with Birmal heads. The machined port matched and hand blended heads were all Birmal parts.
Originally when I returned from being Resident Engineer at Stillwater the changes for increased output for the 93 MY were intended to be increased lift and duration on the secondary cam profile from what was known as the 'B' profile to the 'RB' profile which had a further 10 degrees duration. This resulted in engines which achieved the power target but paid the price in terms of reduced low end torque, something for which the LT5 had always been criticized. Since this was now my responsibility we made a change in the design direction ie. we would stick with the same cam profiles, optimize the cam timing and devise a simple machining and hand blending operation for increased port flow. So the 1993 MY engines started to have a revised throat cutter diameter an increase in the primary and secondary ports and a simple blend at the junction of the two machine cuts. I would not really call the operation CNC porting because to me that implies some change in the fundamental port design which this operation certainly did not. Now the time frame for all of this was early 1991, Birmal advised us that they did not want to produce heads for the future model years, infact they wanted to get out of the semi-permanent mould business. So we looked for alternative sources. A.L. Dunn were selected and the tooling was transferred from Birmal. At this time because of the original production schedule and the cut back in the volume of cars produced there were sufficient Birmal heads for a large portion of the remaining engine builds. We decided that since the port core box needed refurbishment, we would take this opportunity to re-design the port so as to eliminate the need for machining and hand blending at Mercruiser. The final cast port actually flows better and more consistently than the machined part. This change was incorporated with the production fix to the right hand side chain tensioner reservoir for engines late in the 95 MY build. All A.L Dunn heads had these features incorporated. There were about 200 head sets cats by Dunns and I believe about 130 engines were fitted with them.
Above info provided by Graham Behan

Cylinder Head Differences: Part 2

The following information provided by Mark Broin.
The latest two head designs had recast pockets for providing better oil retention for the cam timing chains. This reduced even further the possibility of chain rattling during startup, providing better dry start lubrication, especially after long periods of the engine not running. These heads were are referenced as the "A.L. Dunn Cylinder Head, First Design and Second Design" ( The latest Second Design Lotus Part Number was 550.4005.878A-RH and 550.4005.877A-LH) These second design heads were a design year change occurring sometime just before the final '95 designated engines were completed. The model year change specification documents were "ECH=3D 550.2680; MY=3D95A; ECA=3D1.0285". These heads did not include the CNC machining of the port runners when installed on the motors. The improved port matching technique was the same as the first design heads. GM's part numbers are 10228866-RH and 10228865-LH (Service part numbers are 10168655-RH and 10168654-LH. These are the numbers used internally for the construction of the motor).
The first design Dunn head included the CNC machining and better port matching technique to the manifold for '93-'95 applications, and also the revised oil retention pocket.
Again, the second design heads did not have CNC machined runners, and the casting walls were thicker. GM argued there was very little difference in flow rates between the cast runner and CNC machined runner sets of heads. Most of the horsepower increase in stock applications, anyway, was supposed to be from better port matching. When replacing heads, GM may inadvertently send one of each rather than a matched pair,ie., both machined or both cast runner. These heads are not uniquely distinguished by GM part number, so be careful when you buy to at least check for comparable runner treatment. The second design Dunn head is not uniquely part numbered from the first. Note: the thicker casting walls on the second design heads might be advantageous for machining out to even greater flow rates than achievable with any of the other head designs?
The high flow head work done early on by the fellows at Mercury consisted of refinements on the cast runners and port matching done by hand. From this work, patterns were developed for the horsepower bump work marketed through the rebuilders. This early modification work was all sub contracted to the two fellows at Mercury, Scott Skinner and another fellow named Greg VanDeventer, returned and then installed on customer motors.

3:54 Rear Ends

The following information provided by Gordon Killebrew.
It was originally released for cars going to the European market because of low noise level requirements.
It was not an option you could order. It was automatically a 3.54 axle instead of a 3.45 when going to a Country that required this.
From the start of production in 1988 (MY89) to 01/01/89, all 6-speeds had a 3.54 axle. After 01/01/89 the ZR-1s used a 3.45 axle. The L98 6-speeds used a 3.31 axle. The 3.54 axle was not EPA certified in 90-92 because it went to the European area market. In 1990 the L98 & LT5 6-speeds used a 3.45 except the GHO code with a 8.5 ring gear.
The are no records for how many, and there were no break points. They were produced as ordered.
I doubt that any went to the country of Switzerland due to their noise level requirements. Most ZR1's did not meet their requirements. There were some sent to England.

Chain System

The reason the chain system on the LT5 is at best a compromise is because one of the design criteria was that it should be loaded from the underside of the car. What this actually did was to give us a width criteria which basically dictated the max diameter of the camshaft timing sprocket which in turn limits the engagement of the chain, which in turn causes high stress concentration in the engaged links leading to fatigue failure. This system could have been bullet proof without the imposition of the width limit.
Graham Behan

LT5 Badge

The first 15 pilot ZR-1's carried the "LT5" badge on the rear fascia. Later on the production ZR-1's GM starting putting on the "ZR-1" badges.
Will Rupprecht

LT5 Polishing and Performance

The fact is, the cylinder heads were the only castings to receive any "fettling" as Lotus referred to the process of removing metal from the ports with a hand grinder. They only got a very small area of the intake ports blended just above the valve seat in the short side radius on the inside wall where the bowl transitions into the runner. Yes some could have received a little more attention than others, but believe me, there wasn't enough time to get carried away, and no way to do a "special set" to go on an engine as they were picked randomly to build the engine many steps further in the process. This "hand fettling" process wasn't a huge factor in the final engines performance.
A more likely culprit of poor airflow would be core shift especially in the injector housings. Core shift in other castings could have had a part in the losses also.
Another suspect cause of variability that has already been addressed is the cam timing. The production cam timing method did have more variability than "degreeing" them in using an indicator and degree wheel.
This leads me to respond to another statement regarding the Dynomometer testing on the LT5 engine. Every LT5 was dyno tested and met the tolerances before it was shipped. The dyno sheets and tons of other data were given to the museum, in hopes it would be shared with the car owners.
Greg Van Deventer