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Discussion Starter #1
Hello. my Fireblade is almost a stock bike. no major modification done. but when referring some forums and videos, i noticed that many people do a carburetor/EFI tune up after an exhaust system change.

so does the exhaust system directly affect our air+fuel mixture ? apparently it does. could someone explain the technical aspect of it please :)

for instance let's say we change our stock exhaust to a Yoshimura or something. after that can't we just ride with it. will it do any harm to the bike? thank you very much :)
 

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In layman's terms the fuelling is reactive to different pressures within the exhaust system. A Yoshi, Akra, or whatever, as a free-flowing system will reduce combustion chamber pressure by virtue of exhaust gases escaping quicker. The ECU (Engine Control Unit - sometimes referred to as EMU, Engine Management Unit) will not recognise this change and therefore maintain the same fuel delivery through the injectors. This is why some bikes will run fluffy or lumpy at idle.

The dyno or remap will fix this by changing the fuelling tables (mapping) to match the new changes.

There is no evidence (to my knowledge) of any engine damaged being caused. Most bikes will invariably run a tad richer rather than leaner. Hope this helps.
 

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Discussion Starter #3
thank you very much @nigelrb . useful information there. i can understand that the free flowing exhaust reduces the combustion chamber pressure.

but i can't figure out how it affects the mixture. the exhaust stroke occurs after the intake, compression and power strokes. which means when the exhaust stroke occurs the mixture has already been combusted. only the combustion gas is remaining there.

sorry if i said something stupid/wrong above. I'm not a mechanically inclined person but have seen many people tune up their engines after the exhaust system modification. just trying to understand the importance. thx again :)
 

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Discussion Starter #6 (Edited)
ok :) read some discussions and articles about the back pressure. it seems that it's a complex theory.
from what i understood, there's a time where both intake and exhaust valves are open for fraction of a second. if the exhaust flow is high, it creates a vacuum which sucks some amount of fesh air from the intake. due to that when the intake stroke happens the mixture becomes lean.
if the back pressure is high it will send some exhaust gas to the intake which could be problematic.
I'm not sure how free flow exhausts lead to an erratic idle. as far as i know faulty ignition, out of sync carbs and vacuum leaks are the reasons. but many have experienced erratic idle with free flow exhausts. thank you for your help :)
 

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. . . I'm not sure how free flow exhausts lead to an erratic idle. as far as i know faulty ignition, out of sync carbs and vacuum leaks are the reasons. . .:)
And herein is the issue. We are not dealing with carbs but with fuel injection. Whole different ball game. With carbs, one simply increases the jet size - job done. ;)

With injection though, the issue is extremely complex, hence the reason why manufactures like Dynojet UK - Power Commander, Automotive Dynamometers, Motorcycle Dynamometers, ATV Dynamometers, Jetkits, Wideband Commander, Wideband Commander 2, WB2 produce the Power Commander, and other companies such as Woolich Racing - ECU Flashing for Suzuki, Kawasaki, Honda, Ducati and Yamaha and ECU Studio thrive with their products to extract the best tuning from our bikes.;)
 

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Discussion Starter #8
thx a lot @nigelrb
yea mine is a carbureted one. i have no experince in EFI. saw some videos and it seems pretty complex compared to a carbureted system. thx for the links too :)
 
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