Run in stand/dyno build.

[Terry Keeley]

A little update, I've trimmed the wheel to 5" and am getting run times where I want them. Having some issues with vibration on the oiler at the coupler but I've isolated it now and will try it tomorrow.

I recently got the EngMod2T software and am digging into it, it's very detailed and will take some time to get ports mapped out and other data entered. Once done I'll run a simulation on this stock CMB 67 and compare it to the dyno, there are tweaks that can be done to match the two then it's supposed to be very good at simulating changes. I'll use it to design pipes etc. then try them on the dyno and finally in a boat.

At least that's the the plan.

 

[Steve White]

I gotta ask. Peak HP is at a lower RPM than I would have guessed. I'd be inclined to shorten up the pipe length and give it a go. Is the port timing optimal for the RPM range below 20K?

 

[Terry Keeley]

I gotta ask. Peak HP is at a lower RPM than I would have guessed. I'd be inclined to shorten up the pipe length and give it a go. Is the port timing optimal for the RPM range below 20K?

Ya, this pipe is long and the needle is rich, just giving the unit a shake down after some mods and getting a baseline.

 

[Terry Keeley]

Made some more pulls today, fixed the oiler issue and got a good base line for starting on the EngMod2T simulation software.

This thing is repeating very, very well, I actually got three runs back to back to back within 0.01 HP of each other. Most others were within 0.05 or 1% or so.

 

[Terry Keeley]

Got an AFR/Lambda sensor installed to help find the right needle easier between changes, seems to be working so far.

 

[LohringMiller]

I posted a lot more in another forum. Your results are the first that I know of measuring this. However, lambda measures oxygen in the exhaust. For gasoline at an ideal air/fuel ratio of 14.7:1 lambda is defined as 1. However, with oxygen containing fuels the air/fuel ratios are very different. Methanol's is 6.4/1 and nitromethane's is 1.7:1. How an oxygen sensor reacts to a mixture of these has not been tested as far as I know. I also believe that really rich mixtures don't lower power that much especially if you can raise the compression ratio.

Lohring Miller

 

[RaceMechaniX]

Years ago I had fantastic mentor who helped Yamaha develop their 2-stroke GP engines. He mentioned that measuring Lambda was not a useful data collection point for 2-strokes. EGT and CHT were far better indicators compared to lambda which was subjectable to fouling from 2-stroke oil.

 

[Norm Doerr]

BMEP is a good KPI to look at.

 

[Terry Keeley]

Years ago I had fantastic mentor who helped Yamaha develop their 2-stroke GP engines. He mentioned that measuring Lambda was not a useful data collection point for 2-strokes. EGT and CHT were far better indicators compared to lambda which was subjectable to fouling from 2-stroke oil.

Ya, that can be an issue with 2T for sure, I saw a few kart/bike guys that made fittings so the tip was flush with the tube so I copied that. Performance trends even sells a filter box that pulls exhaust in and separates the oil (2/3 down the page): https://www.performancetrends.com/wide_band_uego.htm

I didn't want to put that much effort into it until I saw it gave good info, for now I'm still puzzled why it starts out about right at 0.8 but goes rich to 0.65 by the end of the run. I know for sure the needle is bang on.

 

[RaceMechaniX]

This is your next step Terry: https://www.tfxengine.com/NitroEngineData.html I think I still have some sensors left over from years ago when I used this system on a 2-stroke diesel project. The one I know for sure has an M8x0.75mm thread. I might have an M5x0.5 somewhere in the stash.

 

[Terry Keeley]

This is your next step Terry: https://www.tfxengine.com/NitroEngineData.html I think I still have some sensors left over from years ago when I used this system on a 2-stroke diesel project. The one I know for sure has an M8x0.75mm thread. I might have an M5x0.5 somewhere in the stash.

Lol, I'm already down the rabbit hole as far as I can go at this point, the EngMod2T software I'm playing with has all kinds of simulation data available including pressure traces, guys on their FB page are talking about stuff I had no idea about just a few months ago.

Pretty cool idea though, ENGMod2T predicted around 175 BMEP (12 bar) for a stock CMB 67.

https://vannik.co.za/EngMod2T - Out...ces - this allows the,pipe and past any valve

 

[LohringMiller]

The more you play with EngMod2T, the more you will find out about how a two stroke works. That plus what the best two stroke guys have learned will really help. Pipes and port design for scavenging are the keys to power. However, you are one of the few people who have investigated our small, glow ignition engines. Keep on posting what you are learning.

Lohring Miller