Two Stroke Fuel Tuning

[whittlebeast]

Years ago, I tuned a two stroke with an MS1 running fueling only using a Ford MAF. Recently, I resurrected an old laptop and found some of the old data logs. I opened these old logs and looked at then in the way I now tune. Here are some screen shots on what I found.

http://www.nbs-stl.com/RxpxTuning/Scatt ... e%20VE.png

http://www.nbs-stl.com/RxpxTuning/Scatt ... e%20PW.png

http://www.nbs-stl.com/RxpxTuning/Scatt ... e%20TP.png

http://www.nbs-stl.com/RxpxTuning/Scatt ... TPsqrt.png

http://www.nbs-stl.com/RxpxTuning/Scatt ... %20RPM.png

http://www.nbs-stl.com/RxpxTuning/Scatt ... %20MAP.png

What I am finding is these 2 stroke motors tune out very similar to what I find with ITB motors. What really stands out is how narrow the MAP signal range is. About 80 to 100 KPA. Keep in mind that I had to run a pair of 800 cc injectors and the max duty cycle came up right at 70. This motor is a 800 cc motor at about 125 HP.

Feel free to ask questions

Andy

[whittlebeast]

I then used the MegaLogViewer HD table generator to build a Alpha-N VE table. It looks like this could have been tuned out with throttle based tuning.

http://www.nbs-stl.com/RxpxTuning/2%20S ... 0Table.png

when I did the same trick to generate a Speed Density table, I got this mess

http://www.nbs-stl.com/RxpxTuning/2%20S ... 0Table.png

This motor would have tuned out better with AN. In those days given the tools I had at that time, I went MAF.

Andy

[dontz125]

This is a jetski, correct? The problem I forsee using this technique for bikes & ATVs is chamber temperature. A jetski is used in warm weather in a fairly enclosed environment, with some models using water injection to adjust effective header length. A bike has the chamber out in the breeze, subject to continuously -and wildly - varying amounts of sun, heat, rain, wind and insulating mud. I can easily see header and belly temperatures varying rather widely from day to day. Competition bikes are often re-jetted multiple times during the day to maintain peak performance; street bikes and recreational sleds just suck it up and live with the reduced performance.

MAF, and the XAP I've muttered about a few times, could compensate for that. Is there any way of doing that with A-N?

[whittlebeast]

If I had it to do again, I would first run the motor on MAF and use MLV HD to build my base AN VE table. Then I would to blended tables to give me a MAF correction if it was needed to compensate for exhaust temp and weather changes messing with the tune.

My gut feeling is the jetskis and the ever changing amounts of boiling water and steam in the expansion chamber is even worse that what the bikes are fighting. The water flow into the exhaust chamber is a function of pressure in the jet pump.

Andy

[whittlebeast]

For the people reading this stuff and have not seen the MAPxRPM vs Duty cycle trick, In MLV, if you go to optional fields and turn on MAPxRPM you will have a new tuning trick. On most motors, this scatter plot will give a straight line. These are motors that will tune out just fine on Speed-Density.

Here is an example of a motor that when tuned, it proved to be a Speed Density motor.

http://www.nbs-stl.com/motec/MAPxRPM%20 ... c%20DC.png

ITB 4 stroke and 2 stroke motors look more like I posted in the first post of this thread.

Andy

[Nath88]

Awesome info Andy.
The MAF to Alpha-N converted table is interesting. I'm mostly interested in the decel area, since it can't be tuned with lambda on the 2 stroke, correct? I've manually adjusted this area of table by feel, based on throttle response from various RPM while decelerating. I change the values in the 0% field then interpolate to the no-load line. I'll post up my VE table later.

I think EGT would shift the VE curve along the RPM axis, since it only changes the resonant frequency.
RPM bins as a function of EGT would allow the tune to follow the pipe frequency, in regions where the pipe is influential on the airflow.

Don, by header and belly temps varying, do you mean the external pipe surface or gas temperature?

I was thinking EGT would be consistent at when load/rpm/lambda/ignition timing are the same. I will be datalogging EGT soon, will find out!

[whittlebeast]

With all the water and oil in the exhaust chamber, I never attempted to install a wide band on that motor.

I had a button on the bars that would increase the fueling about 10% that I could touch at any time to see if the motor wanted more fuel under some condition. As I recall. I had a tach and a volt meter on the MAF that I could see to know where I was in the tune.

The fuel increase was done by screwing with the CLT and the WUE curve. I could see in the log when I had my finger on the button.

Even now, I don't think I would try tuning a two stroke without a MAF. It may be possible to remove the MAF after the tuning process is done. Most motors I am tuning now have a base SD VE table with a blended AN correction table. Most of the low power tuning is done SD and the top end tuning is done on the AN table.

Here are my B18C ITB motor fueling tables just for the reference to see how I am tuning it. Below 3000 RPM and below 30% throttle, it is all tuned on the SD table. The Vtec hits at 5450 RPM

http://www.nbs-stl.com/CrxTypeR/CRX%20T ... ueling.PNG

Andy

[TheSilverBuick]

This is a conveniently timed thread! I just bought an ATV with a 38HP 2-cylinder 2-stroke engine and was thinking what will be needed to EFI it. It currently runs poorly on a sad looking carb, so I'm going to rebuild the carb first then down the road see about EFI'ing it, but I can install an MS1 and datalog much sooner than installing the fuel system.

Quick question, do you run pre-mix oil/fuel through the injector? If so, any issues?

[whittlebeast]

I ran 16 oz to 5 gal of fuel pre mixed and had no issues.

In Amazon, search for

Pennzoil 550022757 Premium Plus Outboard 2-Cycle Marine

[Nath88]

I think if anything the injector and fuel pump would appreciate the lubrication.
The tricky part as I've found recently, is having the engine transition from closed throttle to accelerating when at high rpm (ie; gear changes). When the exhaust pipe resonates (due to combustion) the airflow increases drastically, depending on how effective the exhaust chamber is. So you need a step change in fuel when the airflow increases on the next cycle. MAF sensor would be the easiest way as you are measuring the actual airflow.

[dontz125]

Try using the Sequential Shift Cut feature, for WOT shifts. Throttle doesn't close, airflow isn't disrupted.

[Nath88]

I will definitely be using that in future, just need to put in a shift switch.
I will test a multiplied secondary load table, with exhaust pressure as the input, to detect combustion/airflow and correct the main table accordingly.

[whittlebeast]

Are you planning on running a wide band or MAF?

[Nath88]

I'm running wideband only, can't fit a MAF in my intake layout. I'm trying to keep as many stock/off the shelf parts as possible.

[whittlebeast]

Will a wide band survive two stroke oil?

[dontz125]

Not as long as it would running straight gas. Assuming this is a pump-fed oiler with a throttle cable, the leaner mixtures (IIRC, the RZ350 approached 100:1 at closed throttle) will help at part-throttle operation. If you're running premix at 30 or 20:1 for sustained WFO survival, you're going to load it up pretty quickly. A top-mounted extended bung will probably help.

Start-up is gonna be interesting, with the richer fuel-air mix needed to light off combining with the puddled oil in the crankcase blowing onto a cold sensor. The need to preheat the sensor to prevent rapid fouling conflicts with the risk of cold condensate hitting a hot sensor element and shattering it.

Positioning the sensor is another question. 4-strokes typically mount it pretty high up in the header, but the very nature of a 2T means that alternating fresh mix and burned exhaust is going to be blowing through the header into the chamber. While I await the wisdom of those who've actually done such things, I would suggest the end of the belly, right where the reflector starts to narrow back down, might be a good choice.

Unlike the poisoning caused by leaded fuel, I would suggest that an oil-fouled sensor might very well be restored with a lean acetylene torch, much like spark plugs can be burned clean.

[whittlebeast]

My bet is temporarily mounting a MAF and then using MLV HD to develop a AN tune is the only way you will ever get the motor tuned.

[Nath88]

The exhaust pressure experiment worked, still a bit rough, but it's definitely proof of concept. I can run the engine at 8000rpm and slowly roll on and off the throttle consistently with no bogging. VE1 table is tuned where there engine is firing, then extrapolated into the low load area where it doesn't fire. VE1 is multiplied by VE2. VE2 = 40% when the exhaust pressure is below 102% of baro, and 100% when above. I need to tune that pressure threshold value and the multiplier below the threshold for best response.

My wideband is in the belly of the pipe, I've read having the wideband in the converging cone is ideal, didn't fit in my set up, belly will have to do. There should be enough bends in the header to catch stray oil particles, I've probably got 4-5 hours on the sensor, about 1 tank of fuel, still working as expected. I'm running 32:1 premix. There is an area where the wideband shows lean, but the motor is actually rich around 5300rpm, probably due to the extra air problem, anti-resonance or something, I don't let autotune tune there (since it keeps getting richer and richer). The rest seems to tune ok. Even around low RPM and low loads it tunes well. Getting the low load, low rpm area right was dead easy with auto tune.

Bike.jpg

Wideband.jpg

[TheSilverBuick]

I have a Ford MAF sensor on the way, including the large mounting tube, but did I read in another thread somewhere you mounted the Ford sensor in a honda intake tube? I'm guessing with the flexibility of the MS, I can just mount the sensor in any size tube that'll it physically fit and tune from there? I'm still likely several months out from actually EFI'ing it as I just rebuilt the carb and hope to use it for the summer+.