Max RPM vs Load RPM

[blsnelling]

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.

It's a solid mechanical linkage, directly connected to the throttle trigger. There's no way it can move seperate of the other butterfly.

 

[troutfisher]

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.

I agree 100%. I've noticed videos posted where the saw is screaming until it hits the wood, then it sounds like it drops 2K instantly. All they're doing is making the thing run HOT.

 

[Paul001]

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.

I've seen the same thing. Purely antidotal but it seems to be at a much higher rate than what I seen with conventional 2-cycle saws.

Otherwise, great thread that goes against the typical conventional thoughts on performance. I've always set my commercial saw uses on the rich side, example a unit that max RPM's are 12K I will set for 11K. Both run in the cut within a few 100 RPM's of each other, but over the years, the saws I've set, don't show up at my door fried nearly as often. This month I've done 9 660's needing new P & C's. 1 was a saw I set up 5 years ago (and has never been back to the shop), all the rest were purchased from other shops within the last 2 years (two of the shops I know set their saws to max + RPM).

Again, purely antidotal evidence, but enough so to keep me doing what I've always done. You theory seems to support it as well.

 

[Paul001]

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.

Interesting theory.

 

[sbhooper]

I am not a mechanic at all, but this discussion has me thinking that I am going to drag out the tach and back my saws off just to see what happens when they hit wood. Outstanding thread!

 

[troutfisher]

I've timed cuts to see where the saw was fastest. Set it at 12K & time 3 cuts. Then 12.5K and time 3 cuts. Tne 13K, then 13.5, ect. When the saw starts getting lean it'll lose torque and slow down. My saws are set richer than many I see.

 

[mechanizm]

I am not a mechanic at all, but this discussion has me thinking that I am going to drag out the tach and back my saws off just to see what happens when they hit wood. Outstanding thread!

all you really have to do is adjust your high-side so that it maintains max power under load. running a little rich is better than running a little lean, and on that note; EPA is forcing small engine makers to lean out new engines and then they use non-tamper carb adjustment screws to make it difficult for the owner to richen the mix. many people get frustrated and throw a perfectly good saw away! i've bought many in this "lean condition" sawed screwdriver slots in the mix screws, richened the mix and they work well..... i wonder how much pollution is caused by scrapping perfectly good engines before their time and then buying NEW machines in their stead? it takes a lot of energy (pollution) to melt down an old saw and make a new one.

 

[timberwolf]

I have found that tachs lie when it comes to in the wood RPM, errr at least they don't do a very good job of telling the truth. I suspect there would be a number of people that would not like to see just what the truth on this is either. WOT RPM is near meaningess as mentioned above, and most claims of in the wood RPM I think have a pretty thick coating of BS.

Here is a RPM plot for a well ported 026 worksaw, WOT no load with 16 inch 3/8 was just a hair over 15,000. With light cutting 3 cuts could be made in 11-12 inch wood and the tach (Stihl ETD5) would hardly show a drop below 12k with a good pressure the tach stayed close to 11k even pushing hard it dropped very little below 10k.

Results from data logger at a much higher refresh rate tell a different story, first set of 3 cuts up-down-up show RPM is no more than 10.5k in the wood and dips down to 9.5 hitting change overs. Second set of 3 cuts show RPM in the wood not much over 9.5k, Last cuts were bogged down well below 9k and headed down south of 7-8k when the saw was stuffed.

Other interesting stuff, more loading made for higher cylinder head temperature but lower exhaust gas temperatures.

 

[SawTroll]

..... Honestly to me max rpm means nothing if you can't sustain them in the cut. .....

:agree2:

Andre,

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

........

:agree2:

 

[fredmc]

Thanks for the input.

I'm also thinking it may be best to tune the strato saws in the wood.QUOTE]

Ding, ding, ding... we have a winner!

It doesn't matter what wot rpms you get, under load rpms are where the power and torque are created.

I tune in the wood always and modify accordingly to get the power where I want it to be. Use a nine pin if you want more chain speed.

 

[bytehoven]

It doesn't matter what wot rpms you get, under load rpms are where the power and torque are created.

I tune in the wood always and modify accordingly to get the power where I want it to be. Use a nine pin if you want more chain speed.

Could you or another AS member speak to the issue of properly tuning a saw in the wood?

I assume you might tune H for the highest RPM and then go just a little rich from there.

Thanks

 

[fredmc]

I have found that tachs lie when it comes to in the wood RPM, errr at least they don't do a very good job of telling the truth. I suspect there would be a number of people that would not like to see just what the truth on this is either. WOT RPM is near meaningess as mentioned above, and most claims of in the wood RPM I think have a pretty thick coating of BS.

Here is a RPM plot for a well ported 026 worksaw, WOT no load with 16 inch 3/8 was just a hair over 15,000. With light cutting 3 cuts could be made in 11-12 inch wood and the tach (Stihl ETD5) would hardly show a drop below 12k with a good pressure the tach stayed close to 11k even pushing hard it dropped very little below 10k.

Results from data logger at a much higher refresh rate tell a different story, first set of 3 cuts up-down-up show RPM is no more than 10.5k in the wood and dips down to 9.5 hitting change overs. Second set of 3 cuts show RPM in the wood not much over 9.5k, Last cuts were bogged down well below 9k and headed down south of 7-8k when the saw was stuffed.

Other interesting stuff, more loading made for higher cylinder head temperature but lower exhaust gas temperatures.

good stuff!

 

[fredmc]

Could you or another AS member speak to the issue of properly tuning a saw in the wood?

I assume you might tune H for the highest RPM and then go just a little rich from there.

Thanks

I stick the saw in the wood with one hand and a tuning driver in the other. It's all by ear and feel. Make 4-5 cuts, adjust from there.

 

[bytehoven]

I stick the saw in the wood with one hand and a tuning driver in the other. It's all by ear and feel. Make 4-5 cuts, adjust from there.

I hear ya, but please expand.

You listen for the highest RPMs in the cut?

Do you leave at the highest RPMs or dial it back just a tad rich from there?

 

[fredmc]

Could you or another AS member speak to the issue of properly tuning a saw in the wood?

I assume you might tune H for the highest RPM and then go just a little rich from there.

Thanks

You are really trying to be on the edge of fourstroking -rich in the cut. Then go leaner from there making sure not to go too lean at wot. Somewhere in the middle is a sweet spot, that is what you want to optimize. 1/8 turns at a time.

 

[fredmc]

gotta go make lunch for the spouse...:chainsawguy:

 

[bytehoven]

You are really trying to be on the edge of fourstroking -rich in the cut. Then go leaner from there making sure not to go too lean at wot. Somewhere in the middle is a sweet spot, that is what you want to optimize. 1/8 turns at a time.

Muchas Gracias Senior!

 

[rbmopar]

Everybody needs to read Timberwolf's post, and study his graph a couple of times. Then read it again to make sure it has sunk in. A sustained 12k in a real world cut is usually a fantasy. Excellent info Brian.

 

[Paul001]

I have found that tachs lie when it comes to in the wood RPM, errr at least they don't do a very good job of telling the truth. I suspect there would be a number of people that would not like to see just what the truth on this is either. WOT RPM is near meaningess as mentioned above, and most claims of in the wood RPM I think have a pretty thick coating of BS.

Here is a RPM plot for a well ported 026 worksaw, WOT no load with 16 inch 3/8 was just a hair over 15,000. With light cutting 3 cuts could be made in 11-12 inch wood and the tach (Stihl ETD5) would hardly show a drop below 12k with a good pressure the tach stayed close to 11k even pushing hard it dropped very little below 10k.

Results from data logger at a much higher refresh rate tell a different story, first set of 3 cuts up-down-up show RPM is no more than 10.5k in the wood and dips down to 9.5 hitting change overs. Second set of 3 cuts show RPM in the wood not much over 9.5k, Last cuts were bogged down well below 9k and headed down south of 7-8k when the saw was stuffed.

Other interesting stuff, more loading made for higher cylinder head temperature but lower exhaust gas temperatures.

I'd like to see alot more from that data source. Mucho information you provided...could you expand a bit on what was used to collect the data?

 

[rms61moparman]

I stick the saw in the wood with one hand and a tuning driver in the other. It's all by ear and feel. Make 4-5 cuts, adjust from there.

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