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


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.


The following information provided by the ZR-1 Net Registry

        In 1992-93 GM started running out of the Birmal heads. They found another manufacturer, Dunn, to make heads for the ZR-1. As GM ran out of Birmal heads for the 1994-95 production model of the ZR-1 they started installing the new Dunn heads except they ran out of the left side Birmal heads first so some '94 ZR-1s have the new Dunn heads only on the left side. Eventually GM started installing Dunn heads on both sides but only later built '95 ZR-1s got Dunn heads on both sides.

For '95 cars:

All '95 cars have the Dunn heads on the left side: #10225121
Most of the '95's still have the Birmal heads on the right side: #10174390
In '95 is when they started installing some of the Dunn heads on the right side: #10225122


        All the engines were produced substantially before the 1995 and about 1/3 the way into the 1994 production periods (i.e.., Fall of 1993). The motors were stored at the plant and assembled into the cars by some methodology known only to the guys who brought them to the line. There is no guarantee the last 1995 cars built got the latest engines built, or that some of those engines may not have found their way into earlier serial number cars. Also, some motors went to SPO for replacement or sale requests while cars were being built. As we all know, many other engines made their way into the public's hands. So, the only way to determine if a car has A.L. Dunn heads, is to be able to recognize them when you examine the car. Vehicle production S/N is no guarantee. To see if you have Dunn heads using a mirror look for a triangular shaped tensioner cover on the passenger side heads with 3 bolts and a center adjuster that lies under the cam cover. Look for a little casting mark with a mirror just under the cam covers of A.L. Dunn heads - left of the dipstick as you look through the wheel well on the passenger side. You will see a sort of rolling scroll (a similar font is bauhaus 93 if you look in WORD or AOL) that reads "dunn" in all small letters; and then what appears to be an "l" on the left side head, and an "r" on the right side head. The Dunn casting mark is also present on the driver's side in about the same position. Also, it is possible to get one head of each design from SPO if you order a replacement pair. They don't open the boxes when the heads are shipped to see if the surfaces are machined or cast. There is also no part number difference between the two heads. GM's rationalization is there is no performance difference.

Thanks to Mark Broin for this info.