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All 650's Buying a tune when you have an o2 sensor?

Reesefool

2020
CB650R ABS
May 11, 2023
Riding Since
2021
Hey guys! I own a 2020 cb650r and I am going to be buying a new full system exhaust for it, I’ve been told by people I need a tune and by people who say I don’t need a tune? This is the part I don’t understand, I have a o2 sensor on my bike that is placed before the cat, obviously the point of the o2 sensor is to measure the amount of air exiting the exhaust to then send to the ecu which then adjusts your AFR to run at the optimum setting, so I don’t understand why people say I would need a tune if I were to get a new exhaust when I have the o2 sensor, wouldn’t the o2 sensor automatically change the AFR for me to make the bike run properly so I wouldn’t have to get a tune? I’ve been looking everywhere and haven’t really found a solid answer to the question so I thought I could ask in here. Also if I were to get a wideband o2 sensor wouldn’t that be as good as getting a perfect tune for the current modifications sense wideband o2 sensors can find perfect AFR for any engine at all rpm numbers and in any gear?
 
Last edited:

miweber929

2014
CBR650F
650 Alumnus
Staff
Feb 13, 2015
Woodbury, MN
Riding Since
1975
Hey guys! I own a 2020 cb650r and I am going to be buying a new full system exhaust for it, I’ve been told by people I need a tune and by people who say I don’t need a tune? This is the part I don’t understand, I have a o2 sensor on my bike that is placed before the cat, obviously the point of the o2 sensor is to measure the amount of air exiting the exhaust to then send to the ecu which then adjusts your AFR to run at the optimum setting, so I don’t understand why people say I would need a tune if I were to get a new exhaust when I have the o2 sensor, wouldn’t the o2 sensor automatically change the AFR for me to make the bike run properly so I wouldn’t have to get a tune? I’ve been looking everywhere and haven’t really found a solid answer to the question so I thought I could ask in here. Also if I were to get a wideband o2 sensor wouldn’t that be as good as getting a perfect tune for the current modifications sense wideband o2 sensors can find perfect AFR for any engine at all rpm numbers and in any gear?
We have covered this multiple times so please make sure to search the exhaust and tuning threads already in this forum.

In a nutshell, your o2 sensor only adjusts in the lower rpm ranges and not the entire rev band. Also Honda’s ECU will only allow parameters to be adjusted per their specs and only to a certain level before it will no longer adjust, so if you drastically change something like exhaust flow it can only adjust so far then it can’t. As far as a wide and o2 sensor goes, the same applies: unless you change the limits of the ECU parameters, it can only adjust so far. So it won’t do you any good.

Having said that, you can run the bike forever, and it will run just fine that way, without a fuel tuner. It won’t be 100% to top spec it could be but will not harm the motor in any way or be unrideable.
 
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