OBD0 and OBD1 VTEC RPM Calculation Formula
Calculate VTEC engagement and disengagement thresholds for OBD0 and OBD1 Honda ECUs using the standard RPM conversion formula.
VTEC engagement and disengagement thresholds in OBD0 and OBD1 Honda ECUs are stored as single-byte hexadecimal values in the ROM. These values must be converted to decimal and processed through a specific formula to determine the actual RPM trigger point.
Calculation Formula
Use the following formula to convert the ROM byte value to engine RPM:
RPM = (Decimal_Value - 128) * 62.52
Variables
- Decimal_Value: The base-10 integer representation of the hexadecimal byte found at the VTEC activation or deactivation memory address.
Important
Ensure the hexadecimal value is converted to decimal before performing the subtraction. If the resulting value is negative, the threshold is effectively set below the engine's idle range.
Reference Table
| Hex Value | Decimal Value | Calculated RPM |
|---|---|---|
| 0x80 | 128 | 0 |
| 0x90 | 144 | 1,000 |
| 0xA0 | 160 | 2,000 |
| 0xB0 | 176 | 3,000 |
| 0xC0 | 192 | 4,000 |
| 0xD0 | 208 | 5,000 |
| 0xE0 | 224 | 6,000 |
| 0xF0 | 240 | 7,000 |
Tip
When tuning, always ensure the VTEC engagement RPM is set higher than the disengagement RPM to prevent "VTEC hunting" or rapid cycling of the solenoid.