Preface: I'm gonna shoot in the *** with a pellet gun the first person who tells me I need to put my bike on a dyno to get this right! lol...

In an engine that seems to otherwise be running normally, is AFR - as measured in the exhaust stream by wide band O2 sensors - affected by such things as compression imbalance? Or is it a done deal once the mixture is in the combustion chamber of an engine that otherwise seems to run normally?

 

 

I have had a couple of cars that had something reset to do with the fuel injection system. The engine ran quite poor for about 10 miles or so and then sorted itself out. The system was "learning".
In an ideal setup the EFI system should keep learning what is required and make the necessary changes to the fuel air mixture based on the response from the O2 sensor. This is what a closed loop system is, taking care of atmospheric changes as well as changes in the engine.

 

 

Jbarr1 so anything that effects the airstream into and head and past the intake valve would would impact it? Makes sense. But one cylinder being weaker than the other should not?

I got a chuckle at your s$no comment! Thanks for that!

yeah, sorry Andy, I should have pointed out I'm on an EFI dyno-saour! Open loop only but thanks for the effort

 

 

interesting, I'd not have expected that. Would the weak cylinder be lean or would it be rich?

 

 

Obviously, this question is related to my previous one about different AFRs front to rear in spite of having identicalfuel maps.

As @marcodarq educated me, you'd not expect to see a full point difference between two cylinders with an MM system because there is no common plenum but instead a dedicated intake runner for each cylinder.

My front cylinder tests more than 10% lower compression than my rear ***but*** I have Easy Start cams and had to test by starting the engine on one cylinder while testing the compression of the other (with the injector disabled) and I've read that this method isn't very acccurate.

Finding a solid relationship between a 15%+ compression difference and a 1 point variance in AFR would drive me to look closer at that compression difference.

The mixture is richer in the weaker cylinder, which may be nothing more than a red herring.

 

your pc has the same map in each table front and rear correct? from the start we were chasing a temp issue, one being much hotter than the other, by 100 dergrees if i remember correctly.
in the base programming, both cylinders have the same DESRIED AFR, which translates to what comes out of the pipe at the end (assume for the moment that we are getting the same from each pipe, say from a dual exhaust system, with no crossover or possiblility of mixes gasses from one cylinder to the next). each cylinder will have different fuel requirements to get that same DESIRED AFR, because of differences in intake tract and pipe design, and compresssion, all of which influences cylinder fill and flow, along with cam, etc.
if each cylinder has an identical fuel map(in the pc3) it is very unlikely that they will both hit the same desired afr. that is why each cylinder fuel table can be adjusted, in order to come as close to hitting that value. On the dyno, this is measured in real time, at specific conditions and can be recorded, logged and adjusted and re-tested.( i just had to say it like that, don't get sparked).
ignition timing also plays its part and is just as important. the change in compression(up from stock) the difference in cam( again not stock) and the Thunder header are going to make the engine want differences in the fuel and ignition tables front to rear. Your 02 sensor set up that you have installed is great for getting the afr value of what is going on, real time. that is how it is measured to answer the question in the beginning of the post. Now the hard part is trying to get a value on what your throttle position is at what RPM and under what kind of load. It is very hard to ride down the road with your laptop on the tank, with the PC control center software open, and trying to watch what your throttle position is, rpm is and keep an eye on the afr guage all at the same time.
Hence the data logging software to try to record the data and make changes.
adjust each cylinder, for the sake of arguement, a midrange, ok value of 13.5 afr,,,,try to make all the adjustments that you can to get it to read that in as many road conditions as you can. once you get there, see if your temps have evened out. i'll bet that you see that you no longer have identicle fuel map tables for the front and the rear cylinders AND that your bike will run pretty good.
The value i'm suggesting is not something to tune for, it is just a safe value, that wont be too lean or to rich and shouldn't get you into any trouble anywhere. then if your temps show a change, do the same with making timing adjustments, however these are almost more important!!! too much or too little can land you back where you started with this whole deal!
hope that helps some.
m