OK, lets discuss this.
Carburetors don't measure air flow. Fuel flow varies some what versus air velocity through the carburetor bore, but mostly fuel flow is determined by how wide the slide is open, because that opens the needle.
Open loop fuel injection actually measures the air going into the engine and delivers fuel accordingly. The measurement on the WR450F is based on engine RPM, intake air temp, throttle position and manifold pressure. Basically, the ECU looks the readings for all these things and says there must be X amount of air flowing into the engine. It then meters out the fuel for X amount of air.
So when you change the muffler, more air flows. The FI computer senses more flowing based on a lower manifold pressure and delivers fuel appropriately.
A carburetor doesn't do this. A carburetor only knows slide position and velocity through the carburetor bore. The fuel that gets pulled into the air stream doesn't change linearly with the air velocity and thus jetting changes have to be made when the air flow for a given slide position changes.
MAP changes on the Yamaha fuel injection systems serve to tell the computer to add or subtract fuel from what it calculates should be needed for the calculated air flow. Effectively it is richening or leaning out the mixture from that the ECU thinks is ideal.
So when one changes the air flow through the engine by running an aftermarket exhaust or installing a cam, the ECU knows that the engine is flowing more air and delivers fuel accordingly. As long as it is accurately measuring how much air is going into the engine, the mixture is going to be fine across changes to the engine. And if it isn't, that is what the GYTR tuner is for.
Like any engine, richening or leaning out the mixture affects how the engine runs. I'm not saying the stock map is perfectly optimal, but it has never been so bad that I had the impetus to change things, mixture wise. FWIW, this has saved me a lot of time not having to rejet a carburetor. I ride with guys who are rejetting with ever engine mod and spring and fall to boot. And they are mentioning that their jetting is less than optimal when the temperature and elevation changes. I'm not feeling that. Aside from the lean idle mixture, I'd give Yamaha a 9/10 on the WR450F fuel injection system.
I've run the stock MAP in my bike from stock until now. I am now running an FMF muffler and the 08 exhaust cam. I use it at elevations from 2,000 feet to 8500-9,000 feet at temperatures from near freezing to about 90F. In these conditions the mixture has been really good. I can't ever say that its run poorly.
The off idle response and engine heating was a different story. Changing C1 made a big difference on my bike, but that (sub 3,000 RPM) isn't affected by the map.
I had the plug out when I changed the cams. Its a gray bown color, perfect in my books.
FWIW, people make a point to say that the WR450F needs to be uncorked for optimal performance. They go on to list removing the throttle stop screw, installing the competition ECU and an aftermarket exhaust system. What those people don't realize is that the WR450F exhaust cam is part of the restrictive system and it too needs to be addressed to get the full potential from the engine. While map changes can affect how the engine runs, no map change is going to affect how the engine breathes and it turns out that changing how it breathes makes a huge difference in its character.
At the start of this thread certain people jumped all over the notion that changing the cams in a 2012 WR450F could do anything to improve it, especially in a tight woods situation. After all, who needs more power than what a WR450F has ?
It turns out that there is way more to a cam swap than peak power. It affects every aspect of the engine's character. Engine braking, overheating, throttle response, both part and full throttle, lugging ability, over rev, peak power, etc.
I'm very happy to say that installing an 08 YZ cam improves the entire character of the engine, especially for woods riding. I verified this yesterday first hand by completing our usual 20 mile test ride way, way faster than I ever have, while using way less energy than I ever have before. I'm usually pretty tired at the end of the 20 miles. Yesterday I was ready to ride a second lap. If that doesn't verify that the 08 YZ exhaust cam makes the WR450F a better bike for tight woods, I don't know what does.
I also lost my buddy riding a KTM 200XC twice. Usually we are the same speed.
I think the biggest change was in snotty and really technical conditions. The bike is so much easier to control.
Edited by MidlifeCrisisGuy, September 08, 2014 - 07:42 AM.