Another chainsaw dyno...

[Terry Syd]

I'm wondering what the difference in the HD-11 and HD-17 is. I would expect, looking at the graph and the increase in power lower in the powerband, that the HD-11 has slightly larger air bleed/transition holes.

If someone had those two carbs it would be informative to find out the size of the transition holes.

 

[Red97]

Again, Joe, I’m not critiquing you at all! I am extremely envious and quite humbled that you got a dyno together and running. Please don’t take it the wrong way!! If i knew how to help you, believe me I 100% would no questions asked!! I’m just an “outside looking in” trying to help. I watched the videos and saw how you work the valve, it is consistent. I don’t see how you could do any better than what you are (without spending a *s-word-ton of money) and was thinking out loud. Kudos to you for building the dyno, kudos to you for sharing it with others and kudos to you for sharing the results with all of us. This is by far my favorite thread. Please don’t take me the wrong way, I was NOT being critical of you in any way.

Thank you for the support.

I do have a few things to in the setup address, and hopefully the arm will live a long happy life.

 

[Red97]

What a good run looks like ^

Flawed run ^^

You can see those skips. Came up on the load too fast, and the rpm dropped quick. Thhe data won't show in those instances.

Like it omits anything abnormal. A good run is typically within the same output rangw as in the previous run.

As soon as I push the test button the previous run is overwritten.

Those were some of the very first dialed in runs I did.

Some saws take 5-10 pulls to get a good smooth reading. I'm sure by that time heat soak may start to play into the numbers. But nothing drastic has been seen.

 

[Deets066]

I'm wondering what the difference in the HD-11 and HD-17 is. I would expect, looking at the graph and the increase in power lower in the powerband, that the HD-11 has slightly larger air bleed/transition holes.

If someone had those two carbs it would be informative to find out the size of the transition holes.

Your suppose to know this stuff Terry!

 

[Terry Syd]

It's a question that may lead to an interesting answer. The other alternative I see is that Deets may have used a bit more time/area on the transfers.

However, if the carb is what is allowing that extra power down low, then a carb swap could quickly clarify it.

If it was the HD-11, then if we know the size of the transition holes, everybody could drill whatever HD they have.

 

[Bigmac]

I'm wondering what the difference in the HD-11 and HD-17 is. I would expect, looking at the graph and the increase in power lower in the powerband, that the HD-11 has slightly larger air bleed/transition holes.

If someone had those two carbs it would be informative to find out the size of the transition holes.

Me looking at it I see more cylinder pressure to peak and the pressure is hurting overrev, but you feel it’s a richer fuel curve creating more torque

have been wrong many times!

 

[Terry Syd]

The more cylinder pressure to peak and then affecting the overrun doesn't twig with me (been wrong before).

For example, if you increase the ignition timing it may help with top end power and the overrun, but it can dampen the power down low where the pressure comes up too fast. - Hmm, which could also be a possibility in the difference in the graphs.

This is where the dyno is so kewl. If Deet's saw's ignition timing was retarded a couple of degrees and then the two curves matched!

 

[Bigmac]

The more cylinder pressure to peak and then affecting the overrun doesn't twig with me (been wrong before).

For example, if you increase the ignition timing it may help with top end power and the overrun, but it can dampen the power down low where the pressure comes up too fast. - Hmm, which could also be a possibility in the difference in the graphs.

Was thinking that too, except timing hurts over rev past peak usually in piped applications.

 

[Red97]

Was thinking that too, except timing hurts over rev past peak usually in piped applications.

The over rev isn't really visible on these runs. Unless a saw is way lean. Unless told otherwise I hate to go much if any over 14k. Which is about where the cfb saw was vs the 15.5 ish of deets. That is why the graph is lacking.

I may be wrong, but I don't believe any other 2 strokes are rpm limited by fuel?

As in no load they would run away?

Maybe I'm thinking wrong.

 

[Terry Syd]

It's when the 'volumetric efficiency'/'cylinder charging' drops off that an increase in timing can help bring back the power. Past the peak the cylinder charging is dropping off.

One of the weirdest spark curves I ever saw was for a KTM motorcycle. They had actually published the ignition curve for the 'black box'. The timing was initially retarded for starting, then came up a bit, then jumped up a heap (in that flat zone just before the pipe comes in), then as it came on the pipe the timing was rapidly pulled back, at peak it was surprisingly low compared to the rest of the curve, then after peak it started increasing again, then finally it hit 'red-line' and quickly dropped off.

It was truly a work of art that I wished I could have programmed into my bikes when I was racing. I had only used a fixed ignition timing and I had to work around those problems. A heavier flywheel helped down low, but a proper ignition curve would have been great on an enduro bike.

 

[Bigmac]

This is some dyno runs on my banshee, all kinds of changes, but the curve with overrev was 4° less timing

here are some modern-ish 250 Honda Mx 2t timing curves

 

[Terry Syd]

I may be wrong, but I don't believe any other 2 strokes are rpm limited by fuel?

There may be some others, the factor that matters is whether the carbs have an air bleed jet on the main jet. The way it works is that both the main jet that the fuel goes through and the air bleed jet that allows air into the main flow are working in parallel.

It is much easier for air to get though a jet (it is compressible) than the in-compressible liquid going through the main jet. As the revs continue to climb (the suction increases on the jets) the air bleed starts feeding more air into the system compared to the fuel from the main jet. Thus, the carb doesn't go rich like on the fixed jet of the saw carb.

 

[Bigmac]

The over rev isn't really visible on these runs. Unless a saw is way lean. Unless told otherwise I hate to go much if any over 14k. Which is about where the cfb saw was vs the 15.5 ish of deets. That is why the graph is lacking.

I may be wrong, but I don't believe any other 2 strokes are rpm limited by fuel?

As in no load they would run away?

Maybe I'm thinking wrong.

I consider anything past peak as overrev

 

[Terry Syd]

here are some modern-ish 250 Honda Mx 2t timing curves

Interesting curves, where is the power peak on the 250 Honda?

 

[Bigmac]

Interesting curves, where is the power peak on the 250 Honda?

7.5-8k depending

 

[Red97]

I consider anything past peak as overrev

Yep, and that is most effected by the tune

Saw that is spinning 16kcan get a lot more load applied by 14k vs one just starting the pull and still blubbering rich at 14k.

 

[Bigmac]

Yep, and that is most effected by the tune

Saw that is spinning 16kcan get a lot more load applied by 14k vs one just starting the pull and still blubbering rich at 14k.

True, but peak isn’t effected much by tune based on what you have said, over rev is past peak, and can fall off quick if rich, and hang on if lean. In my testing, piped banshee, timing pas peak is a killer, the timing cure falls off at about 6k but really hurts after peak. I have a programmable cdi now so I need to hit the dyno again!

 

[Terry Syd]

7.5-8k depending

The 2000 Honda curve is close in concept of what the KTM was doing. After peak the timing increased, then fell off when it was hitting red-line.

EDIT: You can see the development of the engine with those ignition curves. The '98 model had a hard hit as it came on the pipe and had to back the ignition down quickly. Then had to stabilize, then drop off again. When you look at the ignition curve of the 2001 model, it looks like the engineers had worked out the 'music' of the pipe and ports and had a nice even steady decrease as the engine revved to full power. - Gotta love dynos.

 

[Bigmac]

The 2000 Honda curve is close in concept of what the KTM was doing. After peak the timing increased, then fell off when it was hitting red-line.

Yes, agreed a fall off, but a small peak it peak power and fall off again after peak, it’s interesting!
I am going to duplicate something similar in my banshee, have 4 tunable curves that I can load, so I am going to test a few things

 

[Terry Syd]

A piped two-stroke is a complex bugger to tune to get the broadest powerband. A non-piped saw can get fairly close, but I would still like to have a tune-able ignition. Here's a document from Stihl on the 'ideal' curve versus the compromise of a set ignition timing (page 11).