I was thinking of replacing the air filter of a bike into a custom free flow filter.

I understand that I have to mod the exhaust to free flow as well to accompany the increase in exhaust due to the new filter.

One of my friends suggested that i need to add something called as a "Piggy back" ECU before the existing ECU so that it can compensate the changes.

My question is why? its the job of the ECU to monitor and compensate why do we need a piggy back ECU.

  • What bike is it?
    – chilljeet
    Commented Jun 9, 2015 at 13:45
  • Yamaha R15 old one
    – Shobin P
    Commented Jun 10, 2015 at 5:46
  • You don't need to mod the ECU or get a piggyback. You don't need to get a custom exhaust either (as the answer below explains) . Though, you should get them anyway! A good piggy back ecu designed for your bike could give you substantial performance gains.
    – chilljeet
    Commented Jun 10, 2015 at 5:50
  • I need to look at the map of the R15 ECU like Rory suggested. dont want to run lean :-) . @chilljeet do you know of any good piggybacks??
    – Shobin P
    Commented Jun 10, 2015 at 5:54
  • 2
    the difference is not going to warrant that. The stock sensor+ecu will easily compensate for that. It's designed to work in all sorts of altitude and temperature (i.e varying air densities and temp). It'd be a different story if you were running any sort of forced induction.The piggyback ecu uses the inputs from the existing sensors and is only 'tricking' the stock ecu into effectively running different maps.
    – chilljeet
    Commented Jun 10, 2015 at 6:52

2 Answers 2


The exact answer to this is - It Depends...

The reason for this is that your existing ECU, whether it be on a bike or in a car, has a range of inputs and a range out outputs, and a mapping between them. As long as your inputs are within the ranges expected, it may well be able to cope with the changed architecture, however there are two common problems:

  • If the type and number of inputs aren't sufficient to let the ECU know the new architecture sufficiently accurately, the outputs may just be wrong - either inefficient, or potentially damaging
  • If the inputs stray outside the values expected, the ECU will not know what to do, and although ideally will cope gracefully by reducing fuel flow etc to 'safe ' levels, it could theoretically deliver unexpected and dangerous outputs, which could damage the engine.

For the majority of cases, point 1 is irrelevant, as you aren't changing the mapping requirements dramatically, but point 2 can happen relatively easily if the stock ECU is limited in its mapping permutations.

  • I would think in most cases a custom air filter alone would not be enough to change mapping in the ECU. A custom air filter will not allow that much more air to go through the system to cause issues, even on larger engines. Usually you wouldn't need to worry about remapping (unless you wanted to), until you start on the major mods (headers, heads, cam(s), etc). Stuff that really makes a difference. Commented Jun 10, 2015 at 1:40
  • @Paulster2 The stock filter is like really restrictive , the custom filter is going to be almost free flow. I am going to do this on a 150cc motorbike so do you think its unnecessary.. ?
    – Shobin P
    Commented Jun 10, 2015 at 5:47
  • @Anarach - Really, I don't know. I'm not sure the method your bike (or any bike for that matter) measures and adjusts for variations in intake flow. Without thinking, my comments were based more towards a car with a much larger engine and much larger air flow. If the percentage of air flow difference is large, it could go beyond the scope of what the bike ECU can handle without some kind of tuning effort. Commented Jun 10, 2015 at 11:10
  • 1
    @Paulster2 : I got hold of some data which should help me to flesh out an answer. Stay tuned...
    – Zaid
    Commented Jun 10, 2015 at 11:29

There is no need to reprogram the ECU

This is for a couple of reasons:

The engine barely feels a difference

Contrary to what the name implies, a "free-flowing" intake impacts the pressure at the valve inlet and not the volume of fresh inlet air per cycle.

Here are the numbers to back it up:

K&N provide detailed test data for the Yamaha R15, which serves as a basis for the following assumptions:

Pressure before the air intake (at max. flow) = 14.636 psi
Pressure drop across K&N intake (low dust)    = 6 in. H2O
                                              = 0.217 psi
Pressure at intake valve inlet                = 14.419 psi

Let's assume that the stock air intake is really bad and drops pressure by 14 in. H2O (similar to pressure drop across the K&N filter when it is really clogged with dust). Under similar inlet conditions, the stock filter would yield the following:

Pressure at intake valve inlet                = 14.130 psi

So that's less than 0.3 psi difference. In terms of mass flow, the engine sees roughly 2% more air1.

In the absence of fueling correction, a stoich AFR would increase from 14.7 to 15.0. Lean, but not exactly anorexic. Plus...

Fuel-injection management corrects for minor changes anyway

Most fuel-injected systems are capable of making fueling corrections to ensure that AFR's are what they ought to be. What this means is that leaning out should not even be a concern.

1 - based on values from Wolfram|Alpha, assuming 25 °C ambient temp

  • Your conclusion isn't true for all engines. WRX motors are known for running too lean with some intakes. That's bad in a forced induction engine. You either get too much air (possible detonation) or the ECU panics and adds to much fuel (too rich = lower power and worse emissions). The engine will be fine within some bounds but if you exceed them, you need a tune.
    – Bob Cross
    Commented Jun 10, 2015 at 21:09
  • @BobCross This is what my friends said .. Now again i am confused :-(
    – Shobin P
    Commented Jun 11, 2015 at 6:19
  • @Anarach, Rory is still right. It depends on the specific application.
    – Bob Cross
    Commented Jun 11, 2015 at 9:18
  • @BobCross : This answer is specific to his motorcycle.
    – Zaid
    Commented Jun 11, 2015 at 9:46

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .