AT/MT Cam Profiles and ECU Data

The AT and MT versions of the NSX differ not only in their transmissions, but also in their engine cam profiles.
I therefore checked valve lift and valve timing in the Service Manual's Design and Construction section and compared the AT and MT data in the factory JDM ROM. I examined how fueling and ignition timing are calibrated for the different cams.
I also compared this with the factory USDM ROM (ACURA NSX) to see whether the JDM and USDM calibrations differ.

Note: Duration was calculated by reading the opening and closing angles from the valve timing diagrams in "Service Manual, Design and Construction, 11-13: Camshaft," using "Duration = Opening Angle + 180° + Closing Angle."
The measurement reference may differ from that used for aftermarket cam manufacturers' published duration figures, so the absolute values cannot be compared directly.

Cam specifications alone cannot establish actual engine characteristics. I therefore decided to compare the factory ROM data to examine how the ECU calibrations differ between AT and MT.
I could not find JDM ROM data online, so I purchased it from GRID.
The ROM data I obtained was from AT ECU "37820-PR7-J12." Analysis with TunerPro showed that the AT and MT ECUs use the same ROM image, so I used this data for the comparison.
For the analysis, I used the "Acura_NSX_and_Legend.xdf" definition file available for download from the TunerPro website.
The XDF defines five fuel maps, each consisting of 20×16 cells (320 cells in total).

Comparing the Full VTEC Fuel Map

I compared all 320 cells in "FUEL MAP 5 - Power Enrichment and WOT, VTEC Active" for the JDM AT and MT versions. This map has 20 engine-speed breakpoints and 16 load breakpoints ranging from 0 to 100%. The table below shows the MT map values minus the AT map values.

Note: The table shows fuel map value differences calculated as "MT − AT." Negative values mean the AT map value is larger, and positive values mean the MT map value is larger. The table also includes cells at engine speeds where VTEC is inactive.

AT/MT Differences by Load and RPM

Overall, at high load, the AT map values tend to be larger at mid-range engine speeds, while the MT values tend to be larger at high engine speeds. At low load, many cells have larger AT values; at 0% load between 4,000 and 6,200 rpm, the MT values are approximately 30% or more smaller. However, this is a ratio of map values and does not mean that the actual fuel injection quantity differs by the same proportion while driving.
At the high-load breakpoints of 87.8%, 94.1%, and 100%, the AT values are consistently larger from 4,600 to 6,200 rpm. At 5,200 rpm, the average difference across these three columns is approximately 7.5%. The difference nearly disappears around 6,400 rpm, and the MT values become approximately 4% larger on average from 6,600 to 6,800 rpm. Although the AT values briefly become larger at 7,000 rpm, the MT values are again approximately 6–7% larger on average from 7,200 to 7,600 rpm.

Note: Differences in map values cannot be interpreted directly as differences in intake airflow, torque, or power output.

Next, I compared ignition timing.
The XDF defines three ignition timing maps. Like the fuel maps, each consists of 20×16 cells (320 cells in total).

Ignition Differences Only with EGR

Comparing the AT and MT ignition timing tables, all 320 cells match in both the normal-operation table (Ignition Timing Table 1) and the VTEC-active table (Ignition Timing Table 3). In contrast, the EGR-active table (Ignition Timing Table 2) differs in 54 cells spanning 1,460–3,400 rpm and approximately 25–63% load.
Although the VTEC cam profiles differ substantially between AT and MT, the ignition timing table used with VTEC active is shared. The differences occur at low-to-mid engine speeds with the throttle partially open. These may be adjustments to suit the cruising conditions of the AT and MT versions. In the tables compared here, all other ignition timing settings were shared.

VTEC-Active Ignition Differences

I compared the VTEC-active ignition timing table (Ignition Timing Table 3) for JDM and USDM MT cars.
The JDM version has more ignition advance toward the high-load end. At 100% load from 4,600 to 7,600 rpm, it has 3.50–5.75° more advance than the USDM version at every engine speed.
However, at low and medium loads from 4,600 to 5,600 rpm, some cells have more advance in the USDM version. Which version has more advance therefore depends on load.

Note: The table values are calculated as "JDM ignition timing − USDM ignition timing." Positive values indicate that the JDM version has more advance.

Now consider these findings for a car converted from AT to MT. Even with an AT engine paired with an MT ECU, the non-VTEC WOT fuel map and normal-operation ignition map are shared, so the AT and MT map values do not differ in this region.
Once VTEC engages, however, the MT Fuel Maps 4/5 are used with the AT high-speed cams. In the JDM versions, the VTEC-active ignition map is also shared. Thus, among the main maps examined here, the clearest difference is in the VTEC-active fuel maps.
It is also important to note that using an MT ECU while retaining the AT valve springs allows the engine to rev beyond the AT setting of 7500 rpm up to the MT rev limit.

Considering the cam profiles and fuel maps together also reveals differences in AT/MT engine characteristics. The AT cams have slightly greater low-speed intake lift than the MT cams, but less lift, duration, and overlap with VTEC active. The fuel maps show a corresponding pattern: after VTEC engages, the AT values are larger from 4600 to 6200 rpm, while the MT values are larger at 7200 rpm and above.
This suggests that the AT specification favors the mid-range after VTEC engages, while the MT specification places more emphasis on higher engine speeds. It is interesting that both the cam profiles and fuel maps show these trends. However, map values alone cannot establish the actual differences in torque or power output.