How a sequential system works
Two turbos, used one after the other. At low RPM, all the exhaust goes to the primary turbo so it spools fast. As RPM climbs, the ECU gets the secondary turbo spinning, then opens it up to feed the engine alongside the primary. A set of vacuum-operated valves, switched by solenoids, controls where exhaust and intake air go at each stage.
The hard part isn't the concept. It's the handover. Open the secondary too early and boost sags; too late and the primary chokes. The Haltech function gives you control of each valve and the RPM points that trigger the change.
Mazda RX-7 FD3S (13B-REW)
Which ECUs can run it?
NSP's Sequential Twin Turbo function handles the FD system on the Nexus R5, R3 and S3 and the Elite 2500. The Nexus S2 and Elite 1500 have the function too, but you may run out of outputs. A plug-in adapter kit, for example, uses Ignition Outputs 5-8, which those two ECUs don't have. With a custom wire-in harness you pick the outputs yourself, and you might free some up, for example if the oil metering pump isn't being used.
For a plug-and-play FD install, see the Elite 2500 RX-7 FD3S S7/S8 kit or the Nexus S3 FD S6 adapter kit.
The five valves
| Valve | Type | Primary only (below switch point) | Both turbos (above switch point) |
|---|---|---|---|
| Pre-Control | Duty | Sets primary boost like open loop boost control; more duty, more boost. Exhaust it bypasses is sent to the secondary's turbine to pre-spin it | Boost control passes to the wastegate valve |
| Exhaust (Turbo Control) | Switched | Off, so exhaust doesn't reach the secondary | On, opening exhaust flow into the secondary |
| Charge Relief | Switched | On, holding the secondary's bypass open so its compressor spins freely and spools up | Off |
| Charge Control | Switched | Off, keeping the secondary's outlet shut so primary boost doesn't leak back through it | On, letting the secondary feed the engine |
| Wastegate | Duty | Not the main control here | Normal boost control for both turbos, set up in the Boost Control function |
The settings you tune
- Secondary On RPM: above this, the secondary is brought online.
- Secondary Off RPM: below this, the system returns to primary only.
- Pre-Control Duty Cycle: sets boost while the primary is working alone.
Leave a gap between the on and off RPM. If they're equal, an engine cruising right at the switch point will flip back and forth between modes, which you'll feel as surging.
Toyota Supra JZA80 (2JZ-GTE)
The Supra system needs a six-cylinder capable ECU, such as the Nexus R5, R3 or S3 or the Elite 2500. You can run it through generic outputs, or use the same Sequential Twin Turbo function with the Charge Relief output simply left unconnected. Toyota's valve names are different, but they map straight across:
| Toyota valve | Type | Equivalent in the Haltech function |
|---|---|---|
| Intake | Switched | Charge Control |
| Exhaust Bypass Control | Duty | Pre-Control |
| Exhaust Gas Control | Switched | Exhaust (Turbo Control) |
| Waste Gate Valve | Duty | Normal boost control via the Boost Control function |
Plug-in options include the Nexus S3 Supra JZA80 adapter kit.
How we tune the handover
- Check every valve first. Old vacuum lines, check valves and solenoids are the most common cause of sequential problems. Test each one with the ECU's output test before blaming the tune.
- Set primary boost with Pre-Control duty. Treat it like open loop boost control: start low and add duty until you have the primary-only boost you want.
- Find the switch point from logs. Log boost and RPM through the transition. A dip means the secondary is coming online before it's spooled; a spike or a primary that's run out of breath means it's too late.
- Tune twin-turbo boost with the wastegate. Once both are online, it's regular boost control.
Troubleshooting
| Symptom | Usually |
|---|---|
| Big boost drop at the transition | Secondary brought on too early, or charge relief/exhaust valve not pre-spinning it |
| Weak boost on primary only | Charge Control leaking, letting boost escape through the secondary, or Pre-Control duty too low |
| Surging at steady cruise | On and off RPM too close together |
| Not enough outputs | ECU too small for the harness in use; consider a wire-in harness or a bigger ECU |
FAQ
Can a Nexus S2 or Elite 1500 run an FD sequential system?
The function is there, but on a plug-in harness you'll likely be short of outputs. With custom wiring it can work.
Should I convert to a single turbo instead?
It's a common path, but a healthy sequential system on a good ECU drives beautifully. Book a free consultation and we'll talk it through for your car. Wiring details are in sequential twin turbo wiring.
