04 RT Overrun Fuel Cutoff Observations

When I first rode my 2004 R1150RT, one of its most startling characteristics is how rapidly it decelerates from cruise when you closed the throttle quickly. I have read that this function goes by various names but I'll call it Overrun Fuel Cutoff (OFC).

It is written that this function aids in fuel economy, recharges the Cerium layer in the CAT with oxygen, improves deceleration (you can say that ) and in some motorcycles, reduces exhaust system backfiring.

I was reading through my GS-911 data lists last night, a slow manual process, and made some observations:

--When you drop the throttle from cruise or acceleration, within a second, the fuel goes full off--nothing.

--The fuel stays off until the RPM drops to 1800 RPM and then the fueling pulses come back on.

--Fast idle, which is TPS=1.28 degrees on my bike and about 1500 RPM when warm, does not block OFC.

--There was no data above 2 degrees TPS that resulted in OFC coming on.

--When the engine is cold, OFC is still operational.

--If the throttle were dropped to 2 degrees open (as opposed to 0.32 degrees which is close throttle on my bike), it looks like OFC would not occur.

--If you drop the throttle, and then open it a couple degrees, OFC is canceled. (But feels a bit jerky to me.)

--Many times when we shift, the OFC function comes on for 3/4 second, give or take.

I think I've adapted to OFC fully with my driving style but I was used to motorcycles that lacked this "benefit".

Roger 04 RT said:

I was used to motorcycles that lacked this "benefit".

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You were used to carburetors.

FI (fuel injection) is how my 1200RT has 40 more horsepower than my old R100 Airhead and returns better fuel economy and pollutes less. Lots of pollution on overrun, hence the air injestion system on the last Airheads designed to ignite unburned fuel in the exhaust system. Not necessary with FI.

Yes, it's a bit "snatchy," but nobody ever said driving a manual transmission smoothly would be easy. If bikes had auto transmissions, this would be a nonissue.

lkchris said:

You were used to carburetors.

...

Yes, it's a bit "snatchy," but nobody ever said driving a manual transmission smoothly would be easy. If bikes had auto transmissions, this would be a nonissue.

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I think you hit the nail on the head. I was used to carbs and did find the function snatchy on the bike, not noticable on my automatic cars. After a few months, I've learned how to modulate releasing the throttle so that I get a gentle decel, followed by a more aggressive decel when I'm ready for it.

Kind of a fun challenge, actually.

I took up lkchris's challenge today ...


I should have titled this thread, "Overrun Fuel Cutoff, Friend or Foe."

Now that I know how it works, and I went out and practiced, and now my shifting is as smooth as on my former carburetor fueled bikes. Since I've only owned this motorcyle for 8 months, I'm probably the last person that should explain how to manage the Overrun Fuel Cutoff (OFC) feature but now that I see how it operates I'll give it a go.

It's worth mentioning that it was a while before I realized that the Motronic regularly SHUTS OFF the motorcycle while I was driving it--no fuel equals SHUT OFF to me.

Recapping the OFC algorithm:

If RPM is falling but above 1800 RPM, and
If the throttle is closed to less that 2 degrees
The Motronic SHUTS OFF the motorcycle until:

Either the throttle is opened up more that 2 degrees (just a bit more than the Cold Idle lever), or
Until the RPM has fallen to or below 1800 RPM

Deceleration
When you want to decelerate by closing the throttle, relax the throttle a bit till all the backlash in the drivetrain is taken up and the motorcyle justs starts to slow. Then for fast deceleration you can then drop the throttle to the stop without feeling like you've been thrown forward. Or you can drop the throttle almost to the stop (2 degrees open) and get a more gentle slowing. Or you could start out gently for a while and then drop the throttle. This gives you two deceleration speeds.

Accelerate after deceleration started:
If you've reached OFC mode, crack the throttle so the engine can RESTART, and then open the throttle as fully as you want keeping in mind takeup of the drivetrain backlash.

Shifting
If you drop the throttle to the stop as you pull in the clutch (above 1800 RPM) the Motronic SHUTS OFF the bike. After you shift and as you let the clutch back out, first crack the throttle 2 degrees so that the motor is restarted, and the finish the shift normally.

After an hour of driving with this new idea in mind, my shifts were smooth and on-par with a non-OFC vehicle.

Even the fairly crude analog LJetronic ECU in K75 and K100 bikes did this if the throttle position switch was set right. Closed throttle above 2000 rpm shut off the injectors.

Roger 04 RT said:

Shifting
If you drop the throttle to the stop as you pull in the clutch (above 1800 RPM) the Motronic SHUTS OFF the bike. After you shift and as you let the clutch back out, first crack the throttle 2 degrees so that the motor is restarted, and the finish the shift normally.

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I don't think the engine ever goes below 1800 when I shift, unless I'm truly doddering around town without earplugs in. And in that case I probably just roll off the throttle slightly, avoiding the cut-off.

Normal shifting for me means leaving one gear at 4000+ and getting the next at 2500+ (and sometimes the + is pretty big). Dropping to idle between gears? No wonder you need this kind of analysis!

AntonLargiader said:

I don't think the engine ever goes below 1800 when I shift, unless I'm truly doddering around town without earplugs in. And in that case I probably just roll off the throttle slightly, avoiding the cut-off.

Normal shifting for me means leaving one gear at 4000+ and getting the next at 2500+ (and sometimes the + is pretty big). Dropping to idle between gears? No wonder you need this kind of analysis!

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To me the more interesting OFC occurs when you are in a long deceleration. I mentioned the shifting OFC because I was surprised to see it.

It's not that the engine gets to idle during shifting (unless you take a nap) but that when you shift, as you describe, IF the throttle gets below 2 degrees and it does sometimes even on a quick shift, that the OFC kicks in. It kicks in because the rpm stays above 1800, not that it gets down to 1800. In that sense OFC is ON because the shift is quick.

For another reason today I was looking at the ignition circuit and I noticed that the driver for the secondary ignition coil was also connected to the tachometer. This led me to conclude that the spark must stay on during OFC. So I went back and looked again at my data, sure enough I had been looking at the wrong column--the spark does stay ON during OFC. The spark timing though shows a firing angle that is between 3 and 6 degrees, lower than idle. Sorry for the mistake, I have gone back and edited the affected posts.

I was wondering...if it did cut off the ignition, wouln't the engine be dead if you pulled the clutch in at the same instant?

I played with that yesterday on a R12S...still running after a complete snap of the throttle and clutch engage at 30MPH

Anyways...

henzilla said:

I was wondering...if it did cut off the ignition, wouln't the engine be dead if you pulled the clutch in at the same instant?

I played with that yesterday on a R12S...still running after a complete snap of the throttle and clutch engage at 30MPH

Anyways...

Click to expand...

The Motronic reacts in thousandths of a second so the engine wouldn't stop. OFC just means that the engine slows rapidly due to the injectors being shutoff.

If I understand your example when you dropped the throttle and pulled in the clutch, the injectors would stop firing and the RPM would quickly drop to 1800 but then immeditely the injectors would start delivering fuel so the engine would seamlessly keep running.

Now that I know when OFC is starting and stopping, it's a pretty good way to get two stages of deceleration and I like it. Although there are times in the low gears that you can feel the injectors kick back in after a deceleration.

Normal shifting for me means leaving one gear at 4000+ and getting the next at 2500+ (and sometimes the + is pretty big). Dropping to idle between gears? No wonder you need this kind of analysis![/QUOTE]

This is great reading! Just finished a book called "Total Control" and have been trying to apply some of the techniques which call for swift and very subtle shifting up and down-very interesting--

I finished a tank of gas today and mileage is back to normal--don't have a clue why it dropped during the interim--around 43-44 mpg again--seems like there is a lot more low end torque also--love this bike!

Tony

This equates well with the recommended practice on Oilheads to not "chop" the throttle between shifts, especially at lower RPMs. I found long ago that barely no throttle chop, and minimal clutch action between shifts produced smoother shifting.

Big twins have a lot of large parts spinning about, lots of momentum, which is smoother if kept spinning. I used to think it was just the momentum of all this mass that I strived to keep moving, that produced the smoother shifts. Now, learning this, I can see that excessive throttle chop makes matters worse if the TPS is allowed to go to its zero/rest position because the fuel flow actually, briefly, stops.