Airhead Air Fuel ratios

[supershaft]

Originally Posted by Rob Farmer

I'm an Engineer at a University with some of the best measuring equipment available. Do you think it hasn't been double checked for accuracy?

Unfortunately some of the finest automotive minds in the world cannot figure out the jetting for an airhead

What good would double checking your equipment for accuracy do if your probe is too short? My point was that your photo might tend to get people thinking that the probe in your photo actually works without contamination issues. My experience is that is needs to be a lot longer than you might think.

[supershaft]

Originally Posted by Rob Farmer

That will be using a narrow band sensor. These sensors are designed to fit into the header of an exhaust and need to be very hot to work properly if you put them into the silencer they will not be hot enough and not read correctly. The wideband sensors have their own heater to keep the sensor at the correct temperature so can work in cooler parts of the exhaust or even away from hot gases. They also produce a linear output of 0-5V

The narrow band sensors produce their own voltage so a simple LED display is easy to construct but the output is non linear and temperature dependent. Essentially your mates sensors, if they are using narrow band, are about as much use as a chocolate fire guard.

I don't know my band widths. I guess sense most EFI systems I have worked on have to have their lamba sensors warmed up before they work they are narrow band? They work pretty well what ever band they are and they are heat sensitive.

The setup my mates used worked damn well for them. I rode the Rotax's before and after and it there was a big diff. We were guessing a little over 5hp. Both my friends dialed their bikes in pretty quick gauging that melted chocolate.

Remember that it doesn't matter how accurate your setup is if it is getting a contaminated sample.

Good luck!

[Rob Farmer]

Originally Posted by supershaft

What good would double checking your equipment for accuracy do if your probe is too short?

That probe in the picture is quite short, others are much longer. There is some contamination at tickover and during throttle closure but the rest of the time the sensors sitting in a positive pressure of hot gas. there's no way there's going to be any contamination under load.

[Rob Farmer]

Originally Posted by supershaft

Both my friends dialed their bikes in pretty quick gauging that melted chocolate.

There's a sentence in that DIY article above -

"You must understand that using stock lambda probe you won’t get accurate air to fuel mixture readings. For engine tuning special wide band lambda probes are used."

[Lornce]

Wish I lived closer to Loughborough.

Cheers, Rob. Great info you're sharing.

[supershaft]

Originally Posted by Rob Farmer

That probe in the picture is quite short, others are much longer. There is some contamination at tickover and during throttle closure but the rest of the time the sensors sitting in a positive pressure of hot gas. there's no way there's going to be any contamination under load.

I am just sharing my experience and hoping it might help some. IMO, thinking that their is no way to get contamination under a load can get you just that. My advise is to run a much longer probe than that if you can. That's all.

[Rob Farmer]

I've settled on the 2nd notch from the top on the needle. 2.66 needle jet and 158 mains for the time being. The bike runs great.

Unfortunately the 308° cam in my wrecked engine turned out to be scrap when I pulled it this morning. So I need to track one down.

[supershaft]

Originally Posted by Rob Farmer

There's a sentence in that DIY article above -

"You must understand that using stock lambda probe you won’t get accurate air to fuel mixture readings. For engine tuning special wide band lambda probes are used."

I am glad he doesn't tell EFI systems that! At any rate, I have seen the setups I described work plenty well enough to help dial the bike right up. No contamination issues and the setup is so small you can race the bike with the setup on for a totally accurate sample. I am just trying to help. I didn't read any articles about it. I just watched and helped friends jetting their bikes with them. I have tried using a sniffer on the road like you are myself and didn't have much luck. I would rather use what I have seen my friends use successfully on the road and leave the sniffers to the dyno room. Just trying to help somebody out there.

[supershaft]

Hey Rob. I figured out where that DIY was. Isn't he saying that what I am talking about works real well just like I am? Stock Lamba sensor? I never said my friends were using a stock lamba sensor. They were using whatever came with the setup that looks just like the setup he is making.

[pommie john]

I've dug up a couple of dyno figures for A/F ratios.

The only change between the two is jetting. I went down a few sizes on the main jets.

As you can see it makes more power with the A/F ratio in the mid 12s rather than mid 11s.

[supershaft]

Look at that airhead go PJ! That thing is a pulling with the right jets in it!!

Almost 5 hp out of a couple of jets! I try to tell people. That kind of gain rarely happens going the other way!

My bike was a bit leaner than your second run and then I leaned it out a bit more into the low 13's for a bit more power.

[RecycledRS]

While it is true that leaner (closer to an ideal burn ratios) the power output goes up, however, with an air cooled engine the survival of the piston is dependant on the cooling the fuel provides the engine.

When racing air-cooled engines (karts) you can lean out the engine until critical engine temperatures are approached. At that point more fuel must be added to stop the piston from melting. The use of a cylinder head temperature instruments is used to monitor this critical temperature (either exhaust or under plug temperatures).

Short period dyno runs will sometimes show high "flash" hp but longer runs more than 5 or 20 minutes) require richer mixtures for survival. Care should be taken to ensure the engine is jetted to survive for track/road use.

My name is Bill.

[pommie john]

Originally Posted by RecycledRS

While it is true that leaner (closer to an ideal burn ratios) the power output goes up, however, with an air cooled engine the survival of the piston is dependant on the cooling the fuel provides the engine.

When racing air-cooled engines (karts) you can lean out the engine until critical engine temperatures are approached. At that point more fuel must be added to stop the piston from melting. The use of a cylinder head temperature instruments is used to monitor this critical temperature (either exhaust or under plug temperatures).

Short period dyno runs will sometimes show high "flash" hp but longer runs more than 5 or 20 minutes) require richer mixtures for survival. Care should be taken to ensure the engine is jetted to survive for track/road use.

My name is Bill.

Yeah. the dyno operator reckoned that mid to high 12s was OK for an airhead, but if it was a watercooled four, he'd be running it in the low 13s.

[RecycledRS]

Good stuff john, I always enjoy seeing the actual data. Bill.

[Rob Farmer]

Excellent! Thanks John that's just the information I was looking for

Shitty weather here at the minute otherwise I'd be seeing what AF mines running now. Hopefully It's in the mid 12's. They definitely feel much better slightly rich.