Fuel Map Differences

[M. Hausknecht]

We know from the Yamaha shop manual that the cylinder pressure for cylinder 2 (rider's right) is less than for cylinder 1, by roughly 10-15 psi (approximately 10%). Some have hypothesized as to why, perhaps as a consequence of the uneven firing cycle and how it is experienced at cranking rpm, but my thinking is even if this is the explanation, it ceases to be relevant once the motor is running on its own. But I don't know.

In any event, my current curiosity surrounds the fuel map differences between the two cylinders in the aRacer fuel map for the 07. Its a map intended for a stock motor. The number 1 cylinder is used as the fuel base standard, and then cylinder 2 can be varied, to provide more or less fuel than provided to cylinder 1. Here are the differences in the aRacer provided fuel map, in summary: at lower rpms (up to about 5000 rpm), cylinder 2 gets between 12.5% and 2% more fuel (more at low rpm and /or low intake pressure (so, roughly, low throttle opening/low load)); between 5000 rpm and 7400 rpm fueling is identical; above 7400 rpm and to rev limit, cylinder 1 gets more fuel (2.5%) at lower intake air pressure and the same fuel  at higher intake air pressures. Any one know why the differences? Are these differences evident in the fuel maps provided by other tuners/flashers for the 07?

Since the intake tracts are separate, and the cam lobes and throttle bodies are identical, I'm struggling to understand why there would be differences in volumetric efficiency that manifest themselves at different rpms and intake air pressures. Is it that the ports aren't identical or nearly so, or is it something about the stock airbox and its significant restrictions that cause the differences (my hope, since I'm using Hordpower airbox).

My hope here is that I can get enough insight to begin to build the fuel maps for a recently assembled race engine and, ideally, be able to avoid using separate O2 sensors or adding bungs to each cylinder downpipe to track individual cylinder AFRs.

[shinyribs]

One cylinder has lower compression while cranking? I would bet it's a result of an auto decomp feature nudging the valve open on that cylinder. 

[M. Hausknecht]

Both cylinders have the same decompression on one exhaust valve in each cylinder. But even weren't that so,  the decompression function ceases to "function" once the motor is running. I can't see how it can explain differences between the two cylinders once the motor is running. Thanks for trying!

[AP996]

I also wondered about this when I first looked at the stock fuel maps and mentioned on another thread that I would make both fuel maps the same as I had removed the stock airbox and had identical intake tubes on each cylinder, however another member had the opinion that the difference was to do with one cylinder being first to receive the cooling water from the pump and there being a difference in temperatures between the cylinders, I don’t know if this is the reason for the fuelling difference but I decided to take his advice and not make both cylinders the same.

I think it’s likely that the only way we will know is to as you say weld a bung in each down pipe and use a wideband sensor to log AFR on each cylinder individually, I might eventually try this but if you do first please let us know.

[M. Hausknecht]

37 minutes ago, AP996 said:

I also wondered about this when I first looked at the stock fuel maps and mentioned on another thread that I would make both fuel maps the same as I had removed the stock airbox and had identical intake tubes on each cylinder, however another member had the opinion that the difference was to do with one cylinder being first to receive the cooling water from the pump and there being a difference in temperatures between the cylinders, I don’t know if this is the reason for the fuelling difference but I decided to take his advice and not make both cylinders the same.

I think it’s likely that the only way we will know is to as you say weld a bung in each down pipe and use a wideband sensor to log AFR on each cylinder individually, I might eventually try this but if you do first please let us know.

Interesting idea, AP. Thanks for your thoughts. Have you carefully examined your plugs to see if they look the same, mixture-wise, with the different fuel maps to confirm?

[AP996]

23 hours ago, M. Hausknecht said:

Interesting idea, AP. Thanks for your thoughts. Have you carefully examined your plugs to see if they look the same, mixture-wise, with the different fuel maps to confirm?

Sorry, no I haven’t checked the plugs yet but I will when I install my reground exhaust cam.

[AP996]

On 2/8/2022 at 9:46 PM, M. Hausknecht said:

Interesting idea, AP. Thanks for your thoughts. Have you carefully examined your plugs to see if they look the same, mixture-wise, with the different fuel maps to confirm?

Looks pretty similar to me 

Edited March 1, 2022 by AP996

[M. Hausknecht]

Thanks very much. Yes, they look very similar. You might have a smidgen too much spark advance, based on where the color change appears on the ground strap (beyond the 90 degree bend). Here is detailed description of plug reading that, based on my research, appears as well-thought out as any I've seen. 

Reading spark plugs

 

[AP996]

Just now, M. Hausknecht said:

Thanks very much. Yes, they look very similar. You might have a smidgen too much spark advance, based on where the color change appears on the ground strap (beyond the 90 degree bend). Here is detailed description of plug reading that, based on my research, appears as well-thought out as any I've seen. 

Reading spark plugs

 

Thank you, that’s a good article, very informative and I see what you mean about possibly having a little too much ignition advance, I’ll reduce it a couple of degrees to be safe.

[AP996]

After having 18mm bungs welded into each down pipe, I’ve now had a chance to log a/f ratios (only at wide open throttle) and map each cylinder individually, I found that at low revs any fuelling differences were relatively small (3% or less) however from 7000 rpm up the differences became much more pronounced.

cyl 2 %fuel less than cyl 1 at wide open throttle.

7000rpm         5.3%

7250rpm         6.3%

7500rpm.        6.5%

8000rpm         6.8%

8500rpm         6.5%

9000rpm         5.3%

9500rpm         3.6%

10000rpm       1%

These results of a/f data logging are on my engine with re ground cams, bored throttle bodies etc. and will probably bear no relation to a stock motor but may help others work out what causes the difference in fuelling between seemingly identical cylinders.