Max RPM vs Load RPM

[Wild Knight]

Erick and I recently ported my 441, and the no-load RPM were much higher than expected (16k!). Erick can generally modify the ports on his saws with such precision such that he can dial in the RPM range in which he wants the saw to cut. For this saw, we were shooting for 11,500-12,000 in the cut. We originally set up the saw around 15,500-15,600 (no load) to play around and shoot the after-cuts in our 10" cant. We then richened the saw to 14,700 to play around with the 32" bar. Well, we went ahead and made one more cut with the short bar at the lower RPM setting before playing with the big bar. Surprisingly, the cut times at 14,700 and 15,500 were the same. This, of course, begs the question "why?"

We put the tach on the saw in the cut and sure enough, we were right at 11,400 - 11,900 RPM in the cut. Interestingly, the saw cuts at these RPM regardless of whether we tuned the saw to right on the edge of two-stroking or 1500 RPM under 2-stroking.

Well, we went ahead and did some more formal testing. We tuned the saw to 14500, 15000 and 15500 RPM and made 3 cuts each. Cuts were made by the same operator and obviously the same bar and chain.

Here is the video:

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It seems to me that without the saw under load, the max RPM means very little. In fact, you may be needless putting stress on the piston and crank, as well as increasing your risk of leaning out the saw in long cuts. Leaning to the 2-stroke and coming back a smidge to tune your saw may not be the best method. It seems that there is an RPM that the saw wants to cut, and max RPM cannot affect this. I imagine it is dictated by the port sizes, angles, duration and velocities, and is independent of the maximum RPM you can get out of the saw under no load.

I surmise that when the saw is not under load (i.e., no forces acting as resistance to the piston going up and down) then you can lean out the saw and gain a false sense of the performance of the saw by getting really high RPM. But, it requires more fuel to maintain that power in the cut, and in reality the mods we make are really maximizing the flow dynamics at a different power band than realized. I think the argument that if you increase the maximum RPM, then you increase the power in the cut is rather specious. It seems to me that it would be wiser to modify your saw thinking about the in-the-cut RPM and not maximum RPM.

 

[Justsaws]

Tune the saw for it's use and operate it at it the high end of its operational/loaded rpm and many of the repeated problems that show up on this site will go away.

The strato saws will not hold up to bogging and dogging like the the non strats, it is not in their nature.

Congratulations on some hands on experimenting. It is fun and every bit as engaging as the other aspects of chainsaws.

 

[blsnelling]

I do not have a technical answer for you. However, I believe what you're seeing is based in the fact that it's a strato saw.

 

[bytehoven]

It would be interesting if saws specs would list the HP/Torque curves.

Perhaps it's not as easy to map a small engine specs with a dynamometer.

 

[albert]

I think you might notice some difference if you had more load on it. Say a 24" bar in at 22-24" good hard wood. No load rpm doesn't really mean much.

 

[Paul001]

It would be interesting if saws specs would list the HP/Torque curves.
Perhaps it's not as easy to map a small engine specs with a dynamometer.

Hit the nail on the head there. Without that last little bit of data, we do alot of guessing.

 

[rms61moparman]

Christopher,

I have been saying this for some time now.
17,000 rpm no load looks impressive, but doesn't cut any wood.
It only stands to reason that you want to tune for the max rpm in the cut.

I also wonder how many saws are either fried outright or have their useful lives cut short by going lean in the cut when the load demands are at their peak.
It will be very hard to ever load your saw in an 8x8 cant.
By the time the saw starts to load you will be through the cut.

At what rpm does your saw seem the happiest, and have you put it to any big wood?
Your first two cut times were identical and the third was very marginally faster. I wonder how the times would differ with the exact same tune in 20" wood???
My guess is that the richest setting would produce a cooler mix and when the heat, load and rpm all stabilized the saw would perform its best right there.


Mike

 

[Andyshine77]

Christopher

Like Mike I have been saying and thinking the same for years now, I'm not a Ph.D or anything like that but more fuel should = more power, within reason. I also think heat comes into play, the higher the engine temps are (within reason) the more fuel that engine will require. Correct me if I'm wrong. Honestly to me max rpm means nothing if you can't sustain them in the cut.

BTW your saw looks amazing, and it appears Erick did an outstanding job on your saw. My hat's off to the both of you. When you have time, I, and I'm sure most everyone on this site, welcomes more constructive threads such as this.

 

[Wild Knight]

Andre,

I think you are right, max RPM may be misleading.

I agree, the more fuel into the cylinder, the more energy you have to do work. That said, dependent on the combustion chamber (size/shape/etc), you can only fit so much mix (fuel + air) at ambient pressure into the cylinder. So, the pressure and vacuum of the upstroke and downstroke can only pull so much air into the crank and push so much air into the cylinder at ambient pressure. Thus, once that is achieved, the amount of air that is filled and compressed into the jug once the exhaust port closes is finite. The only way to put more in there is to increase the pressure (PV=nrT), much like we see with turbo- and superchargers.

That said, I think you can lean out the no-load saw and make it turn more RPM. But, when the piston meets resistance (i.e., wood) then that mixture lacks the sufficient energy (fuel) to perform the work required. Thus, the RPM in the cut are not maintained.

As far as heat, it increases power to a point, then begins to rob power. Once fuel is necessary to dissipate (absorb) heat, then your power gains decrease proportionally. For example, with increasing compression, for a 10% mathematical gain in power, you get 3-4% actual power at 15,000 RPM and 6-7% gain at 12,000 RPM (per Bell). This is because at lower RPM, less heat is created and less fuel is being wasted as/minimizing heat.

 

[Andyshine77]

Once again good post.

Yes after a point heat will become an issue, and power will start to drop fast. When I'm making long cuts I normally stop and let my saw idle for a few seconds, the new charge will cool things down. If your saws ported a little hotter than normal I highly recommend doing this. I learned this from Ed.

Funny you mention superchargers, as that's somewhat the the same affect a pipe has, that and it scavenges unburnt fuel.

 

[wigglesworth]

Great post Wild Knight. I have preached quite often but not to the extent of making a thread. WOT means exactly crap. WOT=Crap.

Well, we went ahead and did some more formal testing. We tuned the saw to 14500, 15000 and 15500 RPM and made 3 cuts each. Cuts were made by the same operator and obviously the same bar and chain.

I would venture to say that if the test wood was double the size, that the saw tuned at 14.5k would outcut it tuned to 15.5k. If I was a betting man, id put money on it.

 

[Andyshine77]

The strato saws will not hold up to bogging and dogging like the the non strats, it is not in their nature.

I've never heard of this. Do you have any information as to why you think this is? I'm just curious, nothing more.

 

[mechanizm]

It seems to me that without the saw under load, the max RPM means very little.

YES, i recently rescued a nice saw from the scrap yard. got it running like children from michael jackson. confident, i went to cut some wood and the motor had no power and kept bogging down. adjust the high side of the carb for more gas and now it cuts well. so high revs mean nuthin if the engine isn't under load.

 

[Andyshine77]

I would venture to say that if the test wood was double the size, that the saw tuned at 14.5k would outcut it tuned to 15.5k. If I was a betting man, id put money on it.

I've seen this happen with my own eyes and my own saws.

 

[Justsaws]

I've never heard of this. Do you have any information as to why you think this is? I'm just curious, nothing more.

Based on the damaged strats that I have seen and the people running them. Purely an opinion would have been a better way for me to say it.

Basically, the saws with the dull chain, rear handle tugging, chips is chips, dust is still chips, loving owners seem to be suffering a high failure rate compared to the non strats that they used to run or recently returned to running because of the operator does not have ability to keep the rpms up enough to keep the strat engine cool enough not to suffer damage.

The engines seem to need to be operated at the high end of the loaded rpms in order to not over heat compared to the last generation of non strat saws. 1127s could be abused with better long term results than the current generation of a competitive brand saws.

The failed units that I have seen so far all suffered heat damage, many of units the over heating caused the actual catastrophic failure.

 

[stihlboy]

a little nitrous will cool em down.....

 

[bytehoven]

The failed units that I have seen so far all suffered heat damage, many of units the over heating caused the actual catastrophic failure.

A candle that burns twice as bright, burns half as long?

 

[Wild Knight]

I noticed both ports on the carb are roughly the same size. It stands to reason that if the strato and intake flow equally they could result in different fuel mixtures at different RPM (one is constant, one is mixed), resulting from the differential pressure pulling fuel into the mix. This may result in lean mixtures at lower RPM. If the high RPM were lean out, then the cutting RPM may be dangerously lean.

 

[Andyshine77]

Based on the damaged strats that I have seen and the people running them. Purely an opinion would have been a better way for me to say it.

Basically, the saws with the dull chain, rear handle tugging, chips is chips, dust is still chips, loving owners seem to be suffering a high failure rate compared to the non strats that they used to run or recently returned to running because of the operator does not have ability to keep the rpms up enough to keep the strat engine cool enough not to suffer damage.

The engines seem to need to be operated at the high end of the loaded rpms in order to not over heat compared to the last generation of non strat saws. 1127s could be abused with better long term results than the current generation of a competitive brand saws.

The failed units that I have seen so far all suffered heat damage, many of units the over heating caused the actual catastrophic failure.

Thanks for the input.

I'm also thinking it may be best to tune the strato saws in the wood. I noticed that my 362 four strokes differently than a non strato 2 stroke. It almost acts like it has a rev limiting coil. It doesn't. Must have something to do with the extra intake port. Here's a question for the Stihl techs. Can the strato butterfly valve in carb close at WOT? It almost sounds like that butterfly valve closes or floats at WOT when the saws not under any load.

Well it's late, see y'all tomorow.

 

[wigglesworth]

I've seen this happen with my own eyes and my own saws.

Yep. Ditto.