What a dual intake runner does
Long intake runners help cylinder filling at low and mid rpm; short runners help at high rpm. A switchable manifold gives you both by opening a valve or flap, usually driven by a vacuum or electric solenoid, once the engine reaches a certain speed. Some designs switch once and stay switched, while others switch on for a band in the middle of the rev range and back off again near redline.
The Nexus Dual Intake Runner (DIR) function controls that solenoid based on engine rpm, with switch points that can shift depending on engine load. Because it's essentially an rpm-and-load based on/off output, it's also handy for other jobs that just need a simple rpm switch.
How the switching logic works
At its simplest, the output turns on above an On RPM. Optionally it turns off again above a higher Off RPM. The twist is that each of those rpm points is defined at two loads, a low one and a high one, and the ECU blends between them:
- Below the Min Load, the ECU uses the min-load rpm values exactly.
- Above the Max Load, it uses the max-load rpm values exactly.
- In between, it interpolates, so the switch point slides smoothly as load changes.
That lets you switch at one rpm on a light cruise and a different rpm at full throttle, which is often what the engine wants.
Settings
| Setting | What it does | Our advice |
|---|---|---|
| RPM Hysteresis | An rpm margin applied as rpm falls back through a switch point, so the output doesn't chatter on and off | Enough that holding rpm near the switch point doesn't make the valve flutter |
| Off RPM Condition | Enables a second, higher switch point that turns the output off again | Leave disabled for manifolds that switch once and stay switched |
| Min Load | The load at or below which the "Min Load" rpm values apply | Set it around light cruise load |
| Max Load | The load at or above which the "Max Load" rpm values apply | Set it around full-load, wide open throttle |
| Min Load On RPM | Rpm above which the output turns on at low load | Find it on the dyno (see below) |
| Max Load On RPM | Rpm above which the output turns on at high load | Find it on the dyno |
| Min Load Off RPM | Rpm above which the output turns off again at low load (only with Off RPM Condition enabled) | Only for band-type manifolds |
| Max Load Off RPM | Rpm above which the output turns off again at high load (only with Off RPM Condition enabled) | Only for band-type manifolds |
A worked example
Say you set Min Load to 40 kPa and Max Load to 100 kPa, with a Min Load On RPM of 3,500 and a Max Load On RPM of 4,500. At light cruise (40 kPa or less) the runner switches at 3,500 rpm. At full load (100 kPa or more) it waits until 4,500 rpm. At 70 kPa, halfway between the two loads, it switches at 4,000 rpm, halfway between the two rpm values.
Now add a hysteresis of, say, 200 rpm. If the output switched on at 4,000 rpm, it won't switch off again until rpm drops about 200 rpm below that point. That stops the valve from toggling back and forth if you happen to cruise right on the switch point.
Those numbers are only an illustration. The right values depend entirely on your manifold and engine.
Finding the best switch point
The ideal switch point is where the torque curves cross. On a dyno, we make one pull with the output forced to stay in the long-runner state and another with it forced to the short-runner state, then overlay the two torque curves. Where the short-runner curve overtakes the long-runner curve is where the switch should happen. Repeat at part throttle if you want to tune the min-load rpm point too.
Switch too early and you'll see a dip in the torque curve just after the switch. Switch too late and you're leaving power on the table in the rpm range where the short runners were already better.
Log the DIR output state alongside rpm, load and lambda. Changing runner length changes how the engine breathes, so you'll often need a small fuel and timing adjustment right around the switch point.
Wiring
The DIR function needs an output assigned to drive the valve's solenoid. See our dual intake runner wiring article for the details. If you're on an Elite ECU instead, the ESP version is covered in our Elite dual intake runner article.
Troubleshooting
| Symptom | Usually |
|---|---|
| Valve flutters on and off at steady speed | Hysteresis too small |
| Valve never moves | Output not assigned, wiring fault, or no vacuum supply on a vacuum-actuated valve |
| Output turns off unexpectedly near redline | Off RPM Condition enabled when the manifold doesn't need it |
| Flat spot right after the switch | Switch point too early, or fuel and timing not adjusted around the switch |
FAQ
Can I use the DIR function for something other than intake runners?
Yes. Anything that needs to switch on above a certain rpm, and optionally off again at a higher rpm, can use it.
Do I need the load-based switch points?
No. If you want a single switch point regardless of load, set the min-load and max-load rpm values the same. For more Nexus functions, browse the Haltech knowledge base.
