Do I need to reprogram ECU when adding custom Air filter

Question

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.

Answer 1

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.

Answer 2

There is no need to reprogram the ECU

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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.

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