OZ ATV :: The Australian ATV Forum

[Tazz]

Hi all,

I have an 08 KFX450R and dont want to spend heaps on go fast parts but would like to open up the airbox and exhaust to get it to breath better as the standard system is very restrictive.

Talking to people I am getting different opinions from quad parts supply shops than from a few mechanics I know.

Talking to the quad parts suppliers and they all say you must upgrade the EFI with any changes - however talking to the few dirtbike mechanics and general mechanics they say you will not.
These guys are saying that the computer will register the increase in airflow inwards and change mixtures just the same as it does with altering altitudes and as long as you open up the intake and exhaust there is no need to upgrade the EFI and this does make some sense.

They have said that it isnt necessary, though could definatley be a benefit to fine tune for racing etc or if you are putting on a full exhaust sytem.
A few have said that people saying that YOU MUST just dont know enough about EFI, as its fairly new (in sports quads).

My question is - are there any EFI gurus on here or people in the know from the bike or quad industry that can shed some light, I am not looking at a total new exhaust system - just opening up the standard airbox and exhaust.

Please no guessperts, but it would be great to get some good tech from people in the know....

Sorry for the essay...

[Quad Squad]

The EFI on current sport quads are different to the "closed loop" self adjusting EFI that you would find in the automotive world. What we have is an "open loop" system, in laymans terms it means that there isnt an O2(oxygen) sensor in the exhaust to return information to the ECU so that it can adjust fuel delivery accordingly.

They are only self adjusting for altitude and air temperature changes, via sensors on the intake side. These changes are minor compared to changing the exhaust or inlet flow.

So to answer your questions, any mods will need a modified fuel delivery to properly take advantage of the mods, the same way you would if you were to rejet a carb. If you give the engine more air and let the exhuast gasses escape better without increasing the amount of fuel then it will no doubt run lean. In some cases you will end up with less power than before, even though it still runs somewhat smooth.

The way to do this is with a fuel controller. Whether you need one for the mods you have planned depends on how far you derestrict the muffler. Taking the lid off wont do much on its own because its still restricted by the muffler.

Sometimes EFI is its own worst enemy because it will still allow an engine to run, albeit not efficiently, even if it is way out of tune. This gives riders a false sense of security in thinking that because its running well (on first impressions) that all is ok when in fact you can be very lean on the air/fuel and will cause premature engine wear.

Think of it this way, if you take the lid off and bung out of a YFZ450, you wont be able to ride it(possibly even get it to run) untill you rejet it (change the fuel delivery)
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[greenhuman]

Plus most mechanics know squat about ATV's. They are a different ball game than most bikes.

[Tazz]

Thanks Quadsquad, thats exactly the sought of info I was after - thanks for sharing the knowledge