Husky porting: starting the math

[edisto]

Skip to the next post for the short version....

I want to get a 272XP P&C for my 61, but I want to port it before I install it. To start with (I'm following Jennings for now...but I know chainsaws aren't mtorcycles), I need to know the rpm it will be running at. Based for a max piston speed of 3500 ft/minute, a 272 would be turning 15,750 rpm. A more conservative 15,000 rpm results in a piston speed of 3,333 ft/minute.

Can I reasonably expect to get from 13,500 to 15,000 with a good port job?

Does anyone know the center to center length for the 272 connecting rod (or the 268, or the 61...they're all the same)

To figure out the size of the ports, I need to know what the engine will be running at while it's cutting, so I have the peak power at the right rpm. So...rather than ask, I had a little fun (sorry, I like numbers).

To look for an answer, I got all of the working data that I could from the workshop manual for all the models in the book, and used a dimension reduction algorithm (ordination) to group the models based only on those data (equal weighting for each factor). I left hp out, because I have no way to measure it on the saw once it is modded, and i was looking for a predictor of what rpm I should be working with. The variables were: bore, stroke, pitch, idle rpm, clutch engage rpm, max rpm, and rpm for peak power.

The analysis produced some discrete groups: (36, 41) (42, 246, and 242XP)(40,45,51,55,257,254XP,264XP) (61, 268, 272XP) and (281XP, 288XP, 394XP), with the 3120XP, not surprisingly, off by itself.

To condense an afternoons work, a number of variables corresponded with the grouping of the variables, but my interest was in whether there were differences among the groups in terms of the percent of max rpm where peak power should occur (I'll call it %max so i don't have to write it out anymore). I assumed the Husky engineers would put the power band in the right place for a variety of cutting conditions, which is what my saw will be exposed to.

There were notable differences among the groups in %max, and it did explain some of the pattern of the groupings. The strongest predictor of %max turned out to be stroke, which also was a strong predictor of the pattern of groupings.

The graph shows the relationship between stroke and the max RPM/peak power RPM ration (multiply it by 100 for %), the black dots are individual data points and the open circles are the averages for a given stroke.

I'm sure someone would have told me 70% if I had asked.

The hitch is that some of the 32mm stroke saws have high ratios that mess up my line! I know the 40 has points, but can anyone thing of a reason to keep the max rpms low for the 41, 51, and 55 (and maybe the 262XP?). If you cut those out, and just use the 254XP and 257 (and maybe the 262XP) the line looks a lot prettier.

Bottom line, you can get a decent idea of running rpm for peak power as a percent of max rpm by looking just at stroke.

Disclaimer: I'm easily confused by numbers, and have been known to talk out of my butt.

 

[edisto]

Here's the short version:

Is there a consistent percentage of a saws max rpm that gives you an idea of how fast it will spin when it's buried in a log (which should correspond to the peak of the power band for the saw)?

No. Different saws have different values

Do saws of similar construction tend to have peak power bands that are a consistent percentage of the saws maximum rpm?

Yes.

Is there a single variable that you can use to estimate what operating rpm you should shoot for relative to maximum rpm?

Yes...stroke.

And if I could figure out why the models listed above have relatively low max rpms, I could make a simple equation that tells you what rpm should be the peak of the power band from the max rpm and from the stroke.

 

[timberwolf]

I find the RPM a saw turns WOT and In the cut has a lot more to do with how well it flows and how the ports are designed than the bore and stroke.

Example 088 on pipe 15,000
066 work saw 15,200
372bb pipe 17,000 - 18,000
359 work saw 14,400
026 work saw 14,800
026 pipe alky max the tach

a stock saw turning ~13,000 WOT might hold 9500 or 10,000 in the cut. A ported work saw turning 14,000 to 15,000 might hold 12,000 or a little more in the cut. racers can hold over 14,000 in the cut.

All that depends on how it's ported, a work saw with 155 deg exhaust durration and a 15mm carb and choked muffler is not going to turn anywhere near what a saw with 170 deg exhaust duration, 20 mm carb and modded muffler will turn.

 

[edisto]

I find the RPM a saw turns WOT and In the cut has a lot more to do with how well it flows and how the ports are designed than the bore and stroke.

Right...what I'm trying to do is figure out how large to make the ports, but to do that, I need to know how fast it likely will run in the cut. Using the data from a number of saws, and the % of WOT (recommended max) they run in the cut, there is a lot of variation, but most of that variation can be explained by stroke.

In my case, I'm going from a 61 to a 272, and those saws all have peak power at 70% of WOT, as does a 268, all with a 34 mm stroke.

A 3120 has peak power at 75% of WOT, and smaller stroke saws have peaks at 60% WOT.

Basically I need a starting point to start working out the port areas...so I'm thinking I'll shoot for 15,000 WOT (stock is 13,500), and based on the patterns of the other saws, try to max out the power band at around 70% of that.

 

[timberwolf]

Right...what I'm trying to do is figure out how large to make the ports, but to do that, I need to know how fast it likely will run in the cut.

What I'm saying, is when designing a porting plan for a saw the RPM the saw will run in the cut is determined by the porting not the porting numbers determined by the RPM the saw will run.

Where are you getting this peek power @ 70% of WOT no load RPM thing?Again, I find the in the cut RPM / no load RPM has much more to do with how the porting is designed, the bottle necks in the system and the resulting torque curve of the motor. Just take any over ported screamer saw (there have been a few), sound great, turn 15k, 16k, 17k but then drop several thousand RPM the instant they touch the wood. Do the porting work a little differently with lower numbers and the saw can run factory no load RPM, but 1500 or 2000 RPM higher in the cut while producing more torque.

 

[edisto]

Sorry, I missed your initial point because I thought you missed mine. I agree with you 100% too. I hope my explanation is not too long...

I've never done porting before, but have read enough about it for 4-strokes to know that, unless you have a flow-bench, you should probably rely on the work of others. I'm working from Jenning's book on motorcycle engines so I have guidelines about port areas based on the duration of port opening, and how fast the engine will be spinning (so that the power band will peak there).

For cycles, of course, you can figure out a redline based on piston acceleration and the potential for ring flutter, and run the bike at that rpm. For chainsaws, the "redline" rpm is not what it will be running in the cut, which is the number I need to approximate if I want to use those guidelines.

I assumed that the saw engineers would have the peak of the power band at the rpm the saw should average in the cut. The % of max rpm where the peak power occurs is not consistent across all saws (ranges from 60-75%), but if you group saws by their characteristics (at least the models I looked at), those numbers are consistent. For example, the Husky 42, 242XP, and 246 have peak power at 60-64% of max and the 281XP, 288XP, and 394XP are around 72%. For the family of saws I'm working with (268, 272XP, and 61) it is 70%.

Crunching the numbers, it turns out that a pretty good predictor of that percentage is the stroke of the engine, which makes some sense because increasing stroke limits potential rpms because of inertial loadings.

Based on piston speed (I'll need connecting rod length to see if the value for piston acceleration will be more or less than that) the "red line" for the 272 should be around 15,000 to 15,500 rpm. Not sure if I can get that at WOT with a port job, or if i want that. 14,500 rpm might be a safer bet, which means I would base my area calculations on having peak power at 10,500 rpm.

As you say, what the saw actually does will likely be quite different, but I need some idea of what it might do to get started. I needed an objective way to get at the rpm where the power band would peak in order to use Jennings' calculations and, for better or worse, this is what I came up with.

 

[Crofter]

I think you are attempting to extrapolate and create a law out of a coincidence. I think there are many more likely controlling factors than stroke length or bore to stroke ratio, in the advertised relationships percentage wise, between max torque point and suggested WOT carb settings. WOT suggestions are a means of arriving at an approximately rich enough air fuel mixture rather than a red line based on inertial factors. I think you have thrown yourself into a feedback loop.[/B]

 

[edisto]

I think you are attempting to extrapolate and create a law out of a coincidence. I think there are many more likely controlling factors than stroke length or bore to stroke ratio, in the advertised relationships percentage wise, between max torque point and suggested WOT carb settings. WOT suggestions are a means of arriving at an approximately rich enough air fuel mixture rather than a red line based on inertial factors. I think you have thrown yourself into a feedback loop.

I think in my discussion I tossed in WOT, but the ratio was peak power point to maximum rated rpm...sorry for the confusion.

If by feedback loop you are referring to the intercorrelatedness of the variables, you are absolutely right! But if they are intercorrelated, than one variable suffices as an explanation for many.

However, although stroke is the best predictor I found in this data set, it does not explain the ratio for 5 saws in the analysis. If I could find a reason for why their max rpms are relatively low (e.g., if they all had points like the 40), then I would suggest that I had a generalization (the relationship with stroke) that others might find useful...at least for Husky saws.

As for coincidence, I think that 19 models of Husky is a decent sample size. Given the number of variables I looked at, I had to ignore relationships that would be expected to occur by chance more than 1 time in 100. The relationship between stroke and the ratio has problems because I'm using ratios, but would be expected to occur by chance only 3 times in 1000.

As it stands, I know different groups (grouped objectively by construction and running parameters) tend to have different ratios, and I know what group my saw is in, so I have some idea of what ratio to use. For the saws I looked at, the numbers are there for others to use that have those saws. If i could find a basis for the relatively low max rpms out those other 5 saws, I'd know if I had something others could use to extrapolate...if I can't find a basis, then it's not worth pursuing further.

 

[timberwolf]

I agree with crofter, I think you are leading your self into confusion.

Muffler modding a saw alone changes the RPM numbers as much as the spread from the largest saw to the smallest.

Forget all the 4 stroke mumbo jumbo, a lot of it does not apply. Even ring flutter is not the same, in a four stroke the ring must be kept from fluttering under the compression and firing stroke as well as the exhaust and intake. One of those stroke the ring does not have much preasure to keep it seated.

4 strokes are limited by valves and maybe as you say max piston speed. Two strokes are limited by the ability to breath, port timing and flame speed. Ring flutter, ya maybe, but I know I have pushed saws beyond where the rings should have been in theory fluttering like mad.

Even the stuff from Jennings you need to take with a grain of salt, it's written 30 or 40 years ago about bikes, for bikes and assumes a tuned exhaust and roughly square bore to stroke ratio. Look at a highly oversquare short stroke short rod chainsaw motor run on box muffler and things change quite a bit. Rule of thumb numbers for port durations are out the window. 170 degs of exhaust duration on a piped motor is short, 170 degs on a muffler motor is long.

An 80 cc motocross bike will run about 200 degs of exhaust durration, if you tried to run an 80cc muffler saw like that it would be a turd slower than stock. Also a small bike motor needs to pull from 4k or 5k right out to 12k, as saw not so much it mainly cares about a couple thousand RPM band for a work saw and maybe only a few hundred for a racer.

Given Jennings time area stuff was totally reverse engineered from top performing bikes with tuned pipes (he says as much in the book) I would not place a whole lot of faith in it applied to a compleatly different motor configuration with box muffler. On saws it's more times a matter of looking at where the constraints and bottle necks are and then working the design from there. Plug numbers into a computer and look at predicted torque curves and you can gain a sense for what RPM the saw will hold enough torque to cut and design intake, transfers and exhaust to work in together. No point having an intake designed to pull at 12,000 RPM, transfers designed for 13,000 and exhaust for 10,000.

 

[timberwolf]

I don't see the RPM limit on saw motors to be liked to internal forces, yes the biger piston has more mass, and a longer strok makes for more g forces, but the bearing surfaces are larger and wider so pounds per square inch works out much the same. Some saws too have RPM limiters built into ignition or carb, this can complicate matters too.

Big bore diamiters though take more time for the flame and exhaust to get accross and out the port, get to the point where the piston is outrunning the expanding flame front and it just won't go faster. Thats where in general, larger saws run slightly higher exhaust numbers. But even that is not a rule, new 660 has about 180 degrees of exhaust durration, yet simmilar sized huskies about 160.

 

[edisto]

4 strokes are limited by valves and maybe as you say max piston speed. Two strokes are limited by the ability to breath, port timing and flame speed. Ring flutter, ya maybe, but I know I have pushed saws beyond where the rings should have been in theory fluttering like mad.

Well, the limits I'm looking at also came from Jennings for 2-strokes, but as you say, it is for bikes. I'm not taking anything from 4-strokes (except the notion that without a flow bench, I should rely on the advice of someone with a flow bench).

Given Jennings time area stuff was totally reverse engineered from top performing bikes with tuned pipes (he says as much in the book) I would not place a whole lot of faith in it applied to a compleatly different motor configuration with box muffler. On saws it's more times a matter of looking at where the constraints and bottle necks are and then working the design from there. Plug numbers into a computer and look at predicted torque curves and you can gain a sense for what RPM the saw will hold enough torque to cut and design intake, transfers and exhaust to work in together. No point having an intake designed to pull at 12,000 RPM, transfers designed for 13,000 and exhaust for 10,000.

I agree that the applicability is a huge issue, but it is the only starting point I have. I figured the recommendations for road bikes (run all out all the time) would be most applicable for a saw because, as you point out, you don't use a lot of the power band. It just seems to me that I'll have a better shot of producing a usable product if have some objective criteria for what I'm doing, rather than staring at a hunk of aluminum with a Dremel in one hand and a beer in another.

Basically all the math I have done has only been to get to a target number: 10,500 rpm in the cut. If that sounds unreasonable for a ported 272XP, I'll gladly take suggestions! I just thought I'd have a crack at figuring out a baseline for myself.

From there, I'll apply Jenning's recommendations for road bike engines to see how the 272 cylinder stacks up in terms of area, sticking with the stock durations (unless I run out of room). That way, the intake and exhaust will be working together, because they are based on the same target.

All I have to do then is figure out how to get a 3 foot tuned pipe into 3 inches of space!

Thanks for all of your comments...I really appreciate you taking the time to help out. This is going to be a gradual process, and I promise that as I screw up along the way I'll post my mistakes so you can use me as an example the next time a newbie starts sleeping with a copy of Jennings under his pillow.

 

[cpr]

Whatever the result of this, I truly appreciate detailed engineering insight into saw design, even if some of it is above my head (I, too, have to learn to think "saw" instead of 4-stroke race engines).

CPR

 

[timberwolf]

10,500 is very reasonable for 272, a good big muffler mod can likely get you close to that. ported for torque I'd think you can get closer to 11,000 to 11,500. Ported a little more agressivly 12,000 or a little more. As racers guys spin 272s way up so you have no worries about rings WOT at 15,000 rpm as a work saw.

272 is a very short stroke saw for the displacment, it does have a prety restricted intake and transfers though. lots of potential for a fairly wide exhaust.

Just an example of how the books can be missleading, table 3.2 and 3.3 recomendations/ rules of thumb taken right out of of "Two Stroke Performance Tuning", G. Bell 2nd edition, 1999

Transfer Port Duration

RPM-----------Transfer Duration
6500----------120 to 124
8000----------124 to 128
9000----------126 to 130
10000---------128 to 132
11000---------130 to 134
12000---------132 to 136
13000---------134 to 140
14000---------136 to 142

Intake Port Duration

RPM-----------Intake Duration

7000----------150 to 155
8000----------155 to 160
9500----------165 to 170
11000---------185 to 190
12000---------195 to 200

Tell you what, you port a saw up with those reccomended numbers based on RPM somewhere about 11,000, I'll port a saw with numbers for 6,000 or 7,000 RPM. Then keeping all other things equal we'll have a little race, winner takes saws home, loser buys a new jug to go with the saw, they can have old one back as a paper weight.

Seriously thats how sure I am about these numbers being dead wrong when it comes to a chainsaw.


Edisto, I do admire your approach, don't get me wrong, most guys don't think twice before the aluminium chips fly.

 

[edisto]

10,500 is very reasonable for 272, a good big muffler mod can likely get you close to that. ported for torque I'd think you can get closer to 11,000 to 11,500. Ported a little more agressivly 12,000 or a little more. As racers guys spin 272s way up so you have no worries about rings WOT at 15,000 rpm as a work saw.

272 is a very short stroke saw for the displacment, it does have a prety restricted intake and transfers though. lots of potential for a fairly wide exhaust.

Tell you what, you port a saw up with those reccomended numbers based on RPM somewhere about 11,000, I'll port a saw with numbers for 6,000 or 7,000 RPM. Then keeping all other things equal we'll have a little race, winner takes saws home, loser buys a new jug to go with the saw, they can have old one back as a paper weight.

Seriously thats how sure I am about these numbers being dead wrong when it comes to a chainsaw.

Edisto, I do admire your approach, don't get me wrong, most guys don't think twice before the aluminium chips fly.

Never play poker with a guy named 'Doc', and never race saws with a guy named "Timberwolf".

I know you're looking after my saws best interests...and I definitely appreciate it.

I didn't have any plans to change the timing, just the port area (but I'm still not going to race you even if I ever do get finished). Once I actually get the durations documented, get the calculations done, and have my starting point drawn on the cylinder, I'll post with pics before I start grinding.

Thanks again for taking the time to put me on track, and especially for waving the red flag. I knew there might be trouble translating bikes to saws, I just didn't think it would be that far off.

 

[blsnelling]

Don't feel bad Edisto. You just provided a forum for the transfer of information of knowledge to others that read this forum. We all had to start some where. At least you did some homework by readin Jennings book. Not everything translates, but there's still a lot of good info in the book. Listen to Timberwolf and you'll have a runner.

 

[timberwolf]

Ran a couple senarios, 60 cc saws stock with slightly opened muffler and port numbers changed only, one by the book with 136 deg transfers and 190 intake, the other 124 transfer durration and 155 intake.

The dotted lines are torque the solid HP. Red being the low numbers, blue being the high.

Yes the one ported high does make a little bit more horse power up at 12,000 RPM, trouble is neither saw will hold that RPM under load as the torque peek sits well below that and up over 12,000 torque falls off quickly.

The low ported saw will come on torque wise until you load it down to 8,000 RPM, but the high porting will fall on it's face if dropped below 10,000.

Also the high ported saw will have problems with low speed, it needs to get up to about 7000 RPM before it has as much power as the low ported saw at idle. This will result in poor throttle response and slow spool up, in addition to poor idle and difficulties starting.

This is for a work saw, now a race saw numbers can be pushed higher and a smaller higher powerband capitalized upon, however even these numbers would be unacceptable for a cold start racer and would result in slower cut times.

Keeping in mind these graphs are pre tax hp and don't show friction losses, or combustion ineffencies.

 

[edisto]

Now that's a cool toy! Where do I get one of those!

You don't have to convince me about which band is better...that's why I was trying to determine the rpms in the cut.

It seems odd that Bell's recommendations focus just on timing (at least what you showed...I ordered the book last week but haven't received it yet) Jenning's focuses on time-area. Was port area a constant in your simulation when you varied duration, or did increasing duration increase area as well?

I haven't decided on a P&C yet, and I will be needing the one saw I have for the next few weeks, but I'm curious to see whether the stock numbers get you close to stock peak rpm using Jennings' calculations. If not, that should be a solid indication I won't be getting any help from Jennings...unless I get a motorcycle.

Thanks again for all your help. What I really needed for now was an idea of where I wanted the peak to be. I'll post again when the plan is complete.

And I was so looking forward to getting Bell's book...

 

[timberwolf]

Port area was not consistant, port width and profile was. so the higher port obviously had significantly more time area.

I don't see and easy way to predict RPM in the cut, there are just too many variables, looking at the basic geometry of the saw is one of the factors, but the size of the carb, ports and muffler outlets combined are a much bigger factor. Case and point is a 50 cc stock saw that revs 14,000 and cuts at 9500 vs 100 to 120cc ported saws that free rev at 15000 and cut at 12,000.

Something else to considder is the power required to spin the chain at WOT no load. to a 3 hp saw that is a much greater load percentage wise than it is for a 7hp saw. This then moves the point of no load WOT equlibrium downward for the smaller saws. Another way to show the same effect is tach a saw with 16 inch bar and chain, don't touch the jets, put on a 20 then 28 inch bar and measure the WOT no load RPM. All that to say the factory RPM specs are based on different bar sizes too and thats just another curve ball in looking at RPM. Even chain pitch makes a difference of a couple hundred RPM try an 026 with a 3/8 7 tooth gear on 16 inch bar, then .325 8. The lighter .328 chain will free rev higher due to lower acceleration and friction losses.

 

[edisto]

I agree that you have to consider the whole package...porting for one target won't work if you are undercarbed (is that a word?) or the exhaust isn't right, etc. Bar length is a big factor...but I assume engineers design for the average, and I plan on using the middle of the range of recommended bar sizes. At least as a starting point.

I can see the objection to me saying that rpms in the cut as a proportion of redline is determined entirely by stroke, and that might have been the impression I gave, but what I mean is that (for the small sample of saws I have data for, from a single company) stroke is a good indicator because it covaries significantly with many other important variables (max rpm, bore, pitch, clutch engagement rpm, horsepower, and of course the ratio I was looking for). So looking at stroke gives you information about other factors as well.

One question I have about Bell's numbers (still don't have the book) are those specifically for piston ported engines, or is it possible they are for rotary-valve intakes? I know Jennings' time-area numbers are much higher for disc-rotary intakes than for piston ported engines. Bell's book is more recent, and Jennings mentioned that most newer bikes would have disc-rotary intakes.

 

[timberwolf]

They are given as generalities but he makes some quilifications to modify the numbers depending on the application motocross vs enduro vs street race. Still how ever it's tweaked the numbers don't work for a saw type application.

I'm not saying at all that Bells numbers are off, rather they are just out of context for the motors designs we are dealing with. There is lots of good info in his book and he is heavier in the math and formulas then Jennings.