Executing cut and blip in your ECU

Applies to: IOcan · Standalone controller. What this means

The transmission asks for a torque reduction during an upshift and a rev-match during a downshift. The device only asks — your ECU does the work. This page is how to set up that half.

If the ECU ignores the requests the transmission still shifts, but shift quality suffers and manual downshifts arrive without rev-matching. It is one of the most common causes of a car that "shifts, but badly".

What the device actually sends

Two outputs, Cut request and Blip request, each a percentage on the device's normal 0–1024 full-scale range:

Value Meaning
0 No intervention requested
1024 Full intervention requested

Route them to your ECU over CAN or on a pin, the same as any other output.

The request is a percentage, never an angle. The device does not compute ignition timing, and it has no view of your engine's timing map. How much retard, or how much pedal, that percentage corresponds to is entirely your ECU's calibration — which is exactly why it has to be set up deliberately.

Where the percentage comes from is worth knowing, because it explains why torque calibration matters here: on the OEM gateways the factory TCU requests an absolute torque limit, and the device converts that into a proportion using its own engine torque estimate. A wrong torque model therefore changes how deep the cut is — see Has your tuner broken your torque model?.

Cut — ignition retard driven by the request

The standard approach. In the ECU, drive an ignition modifier from the cut request received over CAN, so that a rising cut percentage retards timing proportionally.

  1. Create a CAN input in the ECU for the device's Cut request output.
  2. Point an ignition retard / ignition modifier table or function at that input, so retard scales with the request.
  3. Set the authority — how much retard corresponds to a full-scale request.

A typical full-scale authority is on the order of 30° of retard, but treat that as a starting point rather than a specification: usable retard depends on the engine, the fuel, and how much timing is in the map to begin with. Start conservative and increase it while watching shift quality.

⚠️ Retard makes heat. Sustained ignition retard puts energy into the exhaust. Shift cuts are brief, so this is normally a non-issue — but a misconfigured strategy that holds retard, or one triggered continuously by a stuck request signal, is a different matter. Verify the request returns to zero between shifts.

Blip — pedal position added on top of the driver's

Rev-matching is done by adding to the throttle pedal position, not by touching ignition.

  1. Create a CAN input in the ECU for the device's Blip request output.
  2. Drive a pedal position modifier from it, adding pedal opening in proportion to the request.
  3. The addition sits on top of whatever the driver is asking for, so a blip works whether the driver is off the throttle or partly into it.

This is a drive-by-wire strategy — it needs an electronic throttle. On a cable-throttle engine there is no blip path of this kind.

⚠️ Treat blip as a dangerous feature and tune it accordingly.

A blip opens the throttle. Every other adjustment on this page reduces engine output; this one increases it, and it does so at a moment when the driver is braking or lifting and expects the car to slow down.

Get it wrong and the failure is not a poor shift — it is the car accelerating when the driver did not ask it to.

Why it bites

  • It adds to the driver's pedal, it does not replace it. A blip that felt right at closed throttle is a much larger opening when the driver is already partly on the throttle.
  • It fires while braking. Downshifts happen on the brakes and into corners. That is the worst possible place to discover the blip is too large.
  • One setting has to cover every downshift. The engine-speed step varies with gear and road speed, so a value tuned on a low-speed 3–2 can be badly oversized for a high-speed 5–4, or push the engine into the limiter.
  • A stuck request means held throttle. If the request never returns to zero, the added pedal never goes away.

Tune it up from zero

Start at zero and creep up. Never start from a guess.

  1. Set the blip authority to zero. Confirm the whole path works with no added pedal at all: the request appears on a downshift, and clears afterwards. Downshifts will be unmatched and slightly harsh — that is expected at this stage.
  2. Verify the request clears every time, before you allow it to move the throttle at all. A request that stays high is the failure mode that matters most, and it is safe to find while the authority is still zero.
  3. Increase by a small step. Small enough that the first road test cannot surprise you.
  4. Test it deliberately — a straight, clear road, low speed, one downshift at a time, no traffic, off the brakes. Not in a corner, not in traffic, not on a track session.
  5. Check three things after each step: engine speed rises toward the matched speed but does not overshoot it; the engine does not reach the limiter; and there is no perceptible push from the car.
  6. Repeat steps 3–5 until the rev-match is clean. Stop as soon as it is — more is not better.
  7. Re-verify across the range. Once low-speed downshifts are right, work up through higher gears and higher speeds. The requirement is not constant, and a value that suits one shift can be wrong for another.

If in doubt, run it too small. An undersized blip gives a slightly harsh downshift. An oversized one accelerates the car while the driver is braking. Those are not comparable outcomes.

Worked reference — Ecumaster EMU Black

The reference build uses an EMU Black and does exactly the two things above:

Function ECU-side mechanism Driven by
Cut Ignition modifier, sourced from a CAN input Cut request
Blip Throttle pedal position modifier Blip request

See Ecumaster EMU Black for the rest of that integration.

If your ECU cannot do this

Some ECUs have no torque-management or modifier facility to hang these requests on. Two options:

  • The standalone controllers can execute the cut themselves, via the DBW Modifier, by overriding the pedal signal directly. It was built precisely for ECUs that cannot handle a torque-reduction request. It is not available on the OEM gateways — see Stated limitations.
  • Accept degraded shift quality. The transmission will still shift. Upshifts will be harsher under load and manual downshifts will not rev-match.

Verifying it works

Log the request outputs alongside the ECU's own ignition and pedal channels:

  • [ ] Cut request rises at the start of an upshift and returns to zero afterwards — a request that does not clear is the dangerous failure.
  • [ ] Ignition retard follows the cut request in shape and timing.
  • [ ] Blip request rises on a downshift, and returns to zero afterwards.
  • [ ] Pedal position rises with it — and returns to the driver's own demand.
  • [ ] Engine speed rises to meet the lower gear without overshooting it, and without reaching the limiter on the highest-speed downshift you can make.
  • [ ] No perceptible acceleration from the car during a downshift on the brakes.
  • [ ] Upshifts feel unloaded rather than abrupt; downshifts arrive without a driveline thump.

If the requests look right but the engine does not respond, the problem is the ECU-side mapping. If the requests themselves look wrong — too deep, too shallow, badly timed — check the torque estimate first.