Max RPM vs Load RPM

[timberwolf]

Using a minature USB data logger, lots of different products. Though I'm not 100% happy with the one I am using, it has some bugs with how the data is stored. I had hoped to use it for the dyno, but am going to go have to spend a few more bucks (hundreds) to get better software and hardware. None the less it does fine for basic readings temperature, RPM ect.

Here is the EGT Probe and RPM pickup.

 

[bitzer]

The stock and ported strato tunings I have done sound altogether different and had different timing results. The stock tuning, just a hair or two into four stroking made the best times and the burble sounded fairly normal compared to a classic two stroke saw. The ported strato needed to be set richer than a hair or two to make the best times, nearly an 1/8 of a turn or so back from two stroking. The saw also now "jumps" in your hand at four stroke, almost like it is hitting a limiter, but the coil is not limited. I originally attempted to make timed cuts for tuning (not by ear out of the cut) and the leaner I went the faster it cut until I knew I was way too lean.

It seems to also take a second for the saw to calm down when its out of the wood. Like the extra strato air needs to be evacuated after the butterfly shuts and returns to idle again.

Porting these saws also give more air to a system that already has a lot of air in them, so it would stand to reason for the saw to need to run richer. These saws are already tuned rich stock and the strato air makes up for the correct mixture. When porting more air is allowed to flow with more air than a classic two-stroke. Its almost a turbocharger effect.


I also agree with RPMs in the wood. It is difficult if not impossible to reproduce accurate readings, at least with the sendac model I have.

 

[bitzer]

Using a minature USB data logger, lots of different products. Though I'm not 100% happy with the one I am using, it has some bugs with how the data is stored. I had hoped to use it for the dyno, but am going to go have to spend a few more bucks (hundreds) to get better software and hardware. None the less it does fine for basic readings temperature, RPM ect.

Here is the EGT Probe and RPM pickup.

Very cool stuff Professor TW! I swear you should open up a school. You've probably forgotten more about saws than most of us know.

 

[timberwolf]

LOL I don't know about that, but I find I am forgeting where I put saw stuff and spend more and more time trying to find it.

 

[troutfisher]

LOL I don't know about that, but I find I am forgeting where I put saw stuff and spend more and more time trying to find it.

Ha! you too? I spend hours stumbling around the garage hunting through drawers and boxes, swearing like a drunk sailor, looking for something I used an hour ago. :censored:

 

[timberwolf]

One other thing on the tach for in the wood RPM is it allowed me to think I could keep the saw within one or two hundred RPM in the cut. That dream is now shattered for me and I know I would need to cut very smooth to keep the saw within a 1000 RPM window.

 

[Paul001]

Using a minature USB data logger, lots of different products. Though I'm not 100% happy with the one I am using, it has some bugs with how the data is stored. I had hoped to use it for the dyno, but am going to go have to spend a few more bucks (hundreds) to get better software and hardware. None the less it does fine for basic readings temperature, RPM ect.

Here is the EGT Probe and RPM pickup.

Thank you. While the uses for me in the shop would be limited. It would be useful for finding the best settings for my customers. Find a happy medium between HP and cylinder temps.

Now if I could just figure out away to sell it to the boss (Wife) as to why I'm about to spend a big chunk of cash that will not net me much in return

If you get a chance, toss me the make/model of the unit you wish you had.

Paul

 

[Justsaws]

I would think that the average operator would be doing extremely good to get any where near a 1000 RPM loaded window.

The situations that seems to cause the most damage is

WOT-cutting-stall-cut-stall-cut-stall-cut-stall-cut-repeat for the short life of the saw.

or

WOT-cutting with the engine continuously but slowly coming to a stall, repeat.

The saws demise is further hastened by no idle time, throttle blips, nothing before being shut down.

With additional tuning compromise that the strat design requires these types of use seem to cause excessive P/C temps compared to operators that are able to maintain a high smoother cutting speed through the duration of the load.

 

[Andyshine77]

The stock and ported strato tunings I have done sound altogether different and had different timing results. The stock tuning, just a hair or two into four stroking made the best times and the burble sounded fairly normal compared to a classic two stroke saw. The ported strato needed to be set richer than a hair or two to make the best times, nearly an 1/8 of a turn or so back from two stroking. The saw also now "jumps" in your hand at four stroke, almost like it is hitting a limiter, but the coil is not limited. .

That's exactly how my 362 acts, very odd.

 

[blsnelling]

The saw also now "jumps" in your hand at four stroke, almost like it is hitting a limiter, but the coil is not limited.

It seems to also take a second for the saw to calm down when its out of the wood. Like the extra strato air needs to be evacuated after the butterfly shuts and returns to idle again.

That's exactly how my 362 acts, very odd.

That's exactly how Dan's, aka The WestSpartan, ported 441 acts. It will not turn the kind of no load RPMs that WKs does. It honestly acts like it's rev limited. But it turns excellent RPMs in the wood, with torque to spare. I don't know exactly what the in wood RPM is, but it's not lacking.

I also noticed that it has a tendency to hang after letting off the throttle. This seemed to be cured by richening the L needle.

 

[Andyshine77]

I stick the saw in the wood and let my buddy Andyshine77 handle the screwdriver.

I've said it before and I'll say it again, "He is, without doubt, the best carb tuner I've ever seen".
I know a lot of guys who are good but he just has a gift of hitting the "sweet spot" exactly, even on a bunch of "old junks" like mine.
That, my friends is ONE of the reasons why his stock saws will be nipping at the heels of a lot of guys ported ones!!!


Mike

Mike you're too kind.

It's taken me many years of tuning saws incorrectly to get me where I am now, and I still have a long way to go.

 

[Andyshine77]

I also noticed that it has a tendency to hang after letting off the throttle. This seemed to be cured by richening the L needle.

That sounds like she's getting a little too hot. Maybe the tank vent can't keep up, but if opening up the L speed screw helps that wouldn't make much sense. I think heat is the issue here, I'd try running the saw a little richer to keep the heat down.

 

[RTK]

Wouldn't richening the low also help in total richness at WOT??? The low feeds throughout the whole throttle spectrum......

 

[Andyshine77]

Yes, but very little.

 

[RTK]

Yep, it all depends on the carb. I play with carbs that range from 30 cc engines to 150cc engines. No two are the same in the way they react. Don't really know anyting about the particular one on his saw.

 

[Terry Syd]

Outlet nozzle

RTK, do you have any ideas about the flow characteristics of the High speed outlet nozzle in the venturi? I see numerous part numbers for different outlet nozzles and I wonder how they affect the flow.

It would seem that the High speed screw will control maximum fuel flow, but does the outlet nozzle control the rate of flow at less than max flow. For example, if an outlet nozzle has a larger orifice, does it flow more at the same venturi velocity than a smaller outlet nozzle?

Logically it would seem to flow more as the depression in the venturi could act upon the larger surface area of the outlet, thus creating a larger demand on the High speed needle orifice.

Any tuning tips or ideas?

 

[Wild Knight]

TW,
I am surprised about the tach being incorrect. We as well were using an EDT5. Since it refreshed every 1/2 second, it should be pretty accurate. I know if we pushed on the bar we could get the saw to dog down to 8-9000 RPM. Then again, we try to cut so that the saw stays in the RPM range we want (based on listening to the saw); this generally results in faster cut times than continuously dogging the saw. I am sure if we continuously pushed, then we would observe the same trend as your graph. What is the refresh rate of the unit that produced your graph?

It looks to me that heat may be the killer of power on your third cut on each run. I cannot read your axis very well, but it looks like one temperature line peaks on the third cut of each 3 cut series. The second temp line seems to peak on your third 3 cut series, which was when you observed the lowest RPM in the cut overall. Is this what you make of your graph? Will you explain your axes?

Out of curiosity, did you alter your exertion in the 3 cut series? Did you try to test how pushing on the bar affects cutting RPM, i.e., how well the saw holds its RPM under different loads?

 

[timberwolf]

Sample rate was 40hz but there was some data compression problems that would have dropped it closer to 10hz resolution.

Those were the results of a typical three cut series, it was possible to cut with a more even RPM if not cutting like it was a timed event. Heat did build, but it was still quite a way from maximum and where power would fall off.

When the weather gets better I'll do some more testing.

CHT is lower line degrees on the right, EGT is upper line same scale. RPM in pink.

It's not that the tach is incorrect it is just error from reading slow and maybe it stores the highest numbers durring a period rather than the average?

 

[Wild Knight]

If heat is not the culprit, then why aren't the RPM the same through each cut series?

 

[Wild Knight]

So, theoretically then, which is the best way to test a saw? Do you alternate between pushing on the saw and letting up to hold it in the sweet spot, or do you push continuously (regardless of how dogged down the saw starts to sound) and let the saw show you how much power it can maintain?

Additionally, (and this is not meant to be argumentative) since most of use don't have your set-up to accurately monitor RPM, and RPM from our handheld units in the cut are unreliable, how do you recommend tuning a saw?