This dymo chart, from the Norton website, shows what happens to the air fuel ratio (AFR) curve when a full Akrapovic exhaust (which replaces the OEM header and muffler, and removes the CAT) is installed:
![19670]( "19670")
As you can see, the Norton notes on the graph says that "With the Akrapovic full system and no tune, the entire power band is above 13.5 AFR, that's bad".
WHY is that "bad"?
The implication is that it is bad because any AFR above 13.5 is "bad", but that is not so.
Yes, it is true that an AFR in the range of 13.5 is viewed by many professional tuners as perhaps "optimal", but any AFR between about 13.0 and 14.7 is considered pretty good, and of no danger to the engine. If you examine this graph carefully, you will see that the AFR with the Akrapovic system never gets above 13.8 even right at the 12,000 rpm rev limit. In fact, even at the rev limit, the AFR doesn't exceed 14.7, which is the theoretical "perfect" AFR (the so-called stoichiometric point) where the amount of air and the amount of fuel are in perfect balance.
Rather than being "bad", I would argue that what this AFR curve actually shows is that:
- The Kawasaki Ninja 400 / Z400 OEM tune is REMARKABLY ideal in its ability to tolerate exhaust system changes withOUT requiring an aftermarket ECU tune. In fact, I have never before seen a motorcycle OEM ECU that is better in its tolerance for an aftermarket exhaust
and
- The OEM exhaust, including especially its CAT, is SO restrictive that instead of an aftermarket system causing the engine to run "lean", it actually "uncorks" the overall exhaust airflow enough to actually make the engine run COOLER, as evidenced by the lower temperature gage reading (3 bars versus 4) that me, and other owners with full exhaust systems, are seeing. In fact, the OEM exhaust system is SO restrictive that even the large 6% peak power increase that this Akrapovic system (and others, including the Delkevic system that I and others on the forum have installed) are delivering, is staying within acceptable AFR limits AND is HELPING the engine stay COOLER than OEM while doing so, DESPITE making more power.
I know that others on the forum have tried to claim that owners who do not buy a $300 tune are endangering their engines, but there is nothing dangerous about a 13.8 PEAK AFR in a STREET motorcycle that is not running at 8 or 10 thousand rpm constantly.
Sure, an actual ECU tune can OPTIMIZE the AFR, and can also make other desirable changes (e.g. reduce closed throttle deceleration for those who WANT that). But, it is not "essential" for the safety or durability of your engine.
I have often purchased aftermarket ECU tunes for many of my project motorcycles, but only if they are really needed or add enough advantages to make them worth the cost. When I had any motorcycle where I made enough exhaust or intake mods to lean the AFR a bit too much, I spent the money on the tune because I wanted the enhanced power, throttle response, cooler running, specific features, or longterm reliability that a tune could add IN THOSE CIRCUMSTANCES. But I see no compelling need here - unless you are a racer. I view this specific OEM ECU tune as remarkably good and flexible as it comes from the factory! I think it is very hard to argue that an OEM tune that can accept mods that yield 6% more power without any problems, and that actually cools the engine better with an aftermarket exhaust, can be called "bad".
I hate to be a cynic, but perhaps the loudest and most persistent calls for "a proper ECU tune" might be coming from those who make money on selling such tunes, and for whom the racer market is not big enough. Those folks hurt their credibility in making such claims.
Jim G
As you can see, the Norton notes on the graph says that "With the Akrapovic full system and no tune, the entire power band is above 13.5 AFR, that's bad".
WHY is that "bad"?
The implication is that it is bad because any AFR above 13.5 is "bad", but that is not so.
Yes, it is true that an AFR in the range of 13.5 is viewed by many professional tuners as perhaps "optimal", but any AFR between about 13.0 and 14.7 is considered pretty good, and of no danger to the engine. If you examine this graph carefully, you will see that the AFR with the Akrapovic system never gets above 13.8 even right at the 12,000 rpm rev limit. In fact, even at the rev limit, the AFR doesn't exceed 14.7, which is the theoretical "perfect" AFR (the so-called stoichiometric point) where the amount of air and the amount of fuel are in perfect balance.
Rather than being "bad", I would argue that what this AFR curve actually shows is that:
- The Kawasaki Ninja 400 / Z400 OEM tune is REMARKABLY ideal in its ability to tolerate exhaust system changes withOUT requiring an aftermarket ECU tune. In fact, I have never before seen a motorcycle OEM ECU that is better in its tolerance for an aftermarket exhaust
and
- The OEM exhaust, including especially its CAT, is SO restrictive that instead of an aftermarket system causing the engine to run "lean", it actually "uncorks" the overall exhaust airflow enough to actually make the engine run COOLER, as evidenced by the lower temperature gage reading (3 bars versus 4) that me, and other owners with full exhaust systems, are seeing. In fact, the OEM exhaust system is SO restrictive that even the large 6% peak power increase that this Akrapovic system (and others, including the Delkevic system that I and others on the forum have installed) are delivering, is staying within acceptable AFR limits AND is HELPING the engine stay COOLER than OEM while doing so, DESPITE making more power.
I know that others on the forum have tried to claim that owners who do not buy a $300 tune are endangering their engines, but there is nothing dangerous about a 13.8 PEAK AFR in a STREET motorcycle that is not running at 8 or 10 thousand rpm constantly.
Sure, an actual ECU tune can OPTIMIZE the AFR, and can also make other desirable changes (e.g. reduce closed throttle deceleration for those who WANT that). But, it is not "essential" for the safety or durability of your engine.
I have often purchased aftermarket ECU tunes for many of my project motorcycles, but only if they are really needed or add enough advantages to make them worth the cost. When I had any motorcycle where I made enough exhaust or intake mods to lean the AFR a bit too much, I spent the money on the tune because I wanted the enhanced power, throttle response, cooler running, specific features, or longterm reliability that a tune could add IN THOSE CIRCUMSTANCES. But I see no compelling need here - unless you are a racer. I view this specific OEM ECU tune as remarkably good and flexible as it comes from the factory! I think it is very hard to argue that an OEM tune that can accept mods that yield 6% more power without any problems, and that actually cools the engine better with an aftermarket exhaust, can be called "bad".
I hate to be a cynic, but perhaps the loudest and most persistent calls for "a proper ECU tune" might be coming from those who make money on selling such tunes, and for whom the racer market is not big enough. Those folks hurt their credibility in making such claims.
Jim G
