Why so few reed valves these days?

[Lakeside53]

Dolmar.....If I had to say the greatest improvement in recent evolution of saws has been overall reliability...Todays saws can go a year or two without any major repairs beyond maintaince.....not so... a few 5-10 years ago.

I don't buy that... Stihl saws of 20-25 years ago still (today) go for years between any repairs...and often not any maintenance! 011,020, 028, 031, 032, 041, 045, 056 etc etc etc

 

[Lakeside53]

what he said. piston port is famous for huge top end, and very little torque. in a chain saw, you can rev it up to max horsepower, then drop it into the wood.

do you REALLY care how much horsepower it makes at 7500 rpm?????

.

err... "reving up to max hp in incorrect". Reving to Max RPM "produces" or requires very little HP. The HP is only developed when the saw is cutting, and at a lot less than the "max rpm".

And yes, we really do care how much HP is developed at 7500... to say 9500 that's the ability to remain cuttting an not drop out (bog down)...

 

[drmiller100]

err... "reving up to max hp in incorrect". Reving to Max RPM "produces" or requires very little HP. The HP is only developed when the saw is cutting, and at a lot less than the "max rpm".

And yes, we really do care how much HP is developed at 7500... to say 9500 that's the ability to remain cuttting an not drop out (bog down)...

oooooooooo kkkkkkkk then .......

if we want to pick nits, then I will spell it out so eveyrone can understand.

Typically, when I use a chainsaw, I hold the saw above the wood, and rev the engine until it is spinning along merrily. Then I apply the chain to the wood, insuring that the engine is at an RPM above peak horsepower. I then apply full throttle, and let the chain bite the wood.

To be sure, I did simplify the above process by implying I rev the engine until the RPMs are at a state such that Maximum horsepower could be obtained when the engine is loaded.

Further, I assumed that the RPM where maximum horsepower occurs for most modern saws is significantly higher then 7500 rpm. Further, I did assume that if you load the saw to where the engine drops to 7500 rpm or lower, less efficiient cutting will occur.

Was that simple enough? Would you PLEASE correct any mistakes I made?
Thank you.
doug

 

[Lakeside53]

Are you sure you want me too?

Way to many readers don't understand the issues and when you "simplify", it just adds to the confusion and myths out here.

Check the rpm where max torque is obtained on a "modern" saw"... not all are as high as you think, and not all saws cut 6 inch wood.

 

[drmiller100]

maximum cutting occurs at maximum horsepower.

if we wanted torque, we'd run 5 horse briggs motors.

you're the one making weak stabs at my posts.

 

[Lakeside53]

Kind of jumpy for a newbie.. you have a lot to learn grasshopper.... but tonight it's late. Spend the night with the AS site search function to build your knowledge base.

BTW - Max HP on an MS660 is at 9500 rpm.. Max RPM is 13,500... What's the power band?


nighty night..

 

[drmiller100]

You have a lot to learn grasshopper.... but tonight it's late. Spend the night with the AS site search function to build your knowledge base.

BTW - Max HP on an MS660 is at 9500 rpm.. Max RPM is 13,500... What's the power band?
QUOTE]

sigh. i guess u showed me.

show me the horsepower curve, and i'll point out the powerband for you. you can't tell much from max rpm except where the manufactures suggest or put a cap on rpm.

 

[sawinredneck]

You have a lot to learn grasshopper.... but tonight it's late. Spend the night with the AS site search function to build your knowledge base.

BTW - Max HP on an MS660 is at 9500 rpm.. Max RPM is 13,500... What's the power band?
QUOTE]

sigh. i guess u showed me.

show me the horsepower curve, and i'll point out the powerband for you. you can't tell much from max rpm except where the manufactures suggest or put a cap on rpm.

UMMMMMMMM.............. Just a sugestion here, might not want to start a pissing match with the best saw tech on the site, being it is you're first night on. Chill a little, ask nicely, and all will be answerd.
Andy

 

[RaisedByWolves]

I aint even lookin............

 

[sawinredneck]

I aint even lookin............

Why has this thread turned into attackof the newbies? May have to lock down a decent subjet!!
Andy

 

[drmiller100]

you guys are right.

i'm new, and should be more polite.

I went back and edited my comments to take some of the sting out.


I'm not shy. I encourage folks to attack my opinions and show me I'm wrong. For me, it is the fastest way to learn.

So, sorry if I am too abrasive for some. And if someone wants to show me how wrong I am, PLEASE feel free to jump in and start kicking!!!!

My point was that chainsaws are one of the very very few 2 stroke applications where low end horsepower is really unimportant. Blowers, weed eaters, snowmobiles, motorcycles, and boat engines all need low rpm horsepower.

Chainsaws really don't, compared to the above examples. And because of this, piston port engines have ended up dominating the marketplace. Rightfully so.

 

[sawinredneck]

you guys are right.

i'm new, and should be more polite.

I went back and edited my comments to take some of the sting out.


I'm not shy. I encourage folks to attack my opinions and show me I'm wrong. For me, it is the fastest way to learn.

So, sorry if I am too abrasive for some. And if someone wants to show me how wrong I am, PLEASE feel free to jump in and start kicking!!!!

Thank you, it's all good, and once we get to know you prolly won't take such offense to it
This, I find,is more of a knowledge base, I may know, or have a different way, than you have. Lake may have a better way yet, we just try and learn from each other, and have a lot of fn along the way!
Andy

 

[Justsaws]

I would think that when the performance differences between the two types of conctruction got to be similar most manufactures went with the cheapest long term solution. It has been my experience with most products that performance is second to profit. Profit is a long term situation.

I would also think that the large reed engines might carry to much fuel load for EPA regs.

Does anyone know what the first mass produced ported chainsaw was?

 

[Marc1]

Hum...chainsaw don't need low end torque?
I suppose it all depends what do you mean by low end.

Try cutting a 3' gum tree with a nice long bar and no low end torque. And try milling the log afterwards by "revving ip up and them dropping it into the wood".

Hehe love my 1050 ... no low end torque hehe.:hmm3grin2orange:

 

[WRW]

Some good reading here on power. One of the threads has the best description of torque that I have ever read!

http://www.arboristsite.com/search.php?searchid=745957

 

[adkranger]

I don't buy that... Stihl saws of 20-25 years ago still (today) go for years between any repairs...and often not any maintenance! 011,020, 028, 031, 032, 041, 045, 056 etc etc etc

Here, here Lake. I have several "old" Stihls that I've done nothing more over the last 20 years other than plugs, filters, bars, chains and a sprocket or two. Some of those Stihls from that time frame, pro models, are absolutely bulletproof. My 028 supers are still one of my all time favorite saw models to operate, period. Sure the newer models have better filtration, power to weight, AV but have yet find a replacement with the feel in hand. 028, 038, 048, 056, 020 are synonomous with reliability/dependability in my mind.:rockn:

Good thread topic on Porting. I'll keep reading. It is no mystery that most of the small engine mods in recent times is to accomodate EPA restrictions, not just saws. Having said that, there have been some vast improvements in performance IMO over the years.

 

[PES+]

Heck I did myself in by oversimplfying

I did the same thing after I posted the topic. Dean has me in brain lock about the reeds in the tranfers and see what he means but I am mentally hydrolocked on how much actual back flow is there.I am as I said jsut getting back up to speed and am astonished at what I thought I knew has been disproven and am taking a step back on the larger saw and going to beat up on a few piston ports and see what he is saying.

Reliability is a huge factor in saws and the reason for derating from what I am sure the factory knows the prototypes are capable of. The other issue is how much hand work needs to be done in final assembly and that is reflected in most any builders "stage 1" offerings. Skilled labor is the bane of all coroprate structures.
All do it I am not being negative.
Company models are about maximizing profit and needing to pay a skilled laborer his rate is a long term liability due to retirement etc.
I know I was sounding negative to the manufacturers but I am human and personal relations with some corporations can be hard to forget be they real real or imagined.

hehe.......I work for myself for a reason and I am sure most people who come to this site are the same way.

Reading back on my own posts I appeared a know it all and I wish to correct this because the older I get it seems the less I actually know..

fun trip though

Can I have this guy as my avatar?:monkey:

 

[klickitatsacket]

as far as putting the reeds in teh transfers, i think you are missign the theory of the piston pumping air from teh crank case into the combustion chamber. with that action, you don't need reeds in the transfers.

Let me walk you through this real quick. At BDC the intake port is closed and the transfers are fully opened. Now as the piston moves up, it is expanding the crankcase volume and creating vacume. Until the transfers are closed that vacume is filled by the open transfers pulling fuel back down. Now when the transfers close then the leak is shut off and the crank case begins to build vacume again until the intake is opened. Reeds in the transfers could make the pp engine more efficient. Until it was built to see if the advantages out way the flow restrictions; it is just conjecture.

BTW, do not ASSume that every one knows less than you. You will not make many friends that way.

 

[Lakeside53]

you guys are right.

Blowers, weed eaters, snowmobiles, motorcycles, and boat engines all need low rpm horsepower.

Chainsaws really don't, compared to the above examples. And because of this, piston port engines have ended up dominating the marketplace. Rightfully so.

???

Blowers don't need low end HP.

There isn't a lot of difference between a blower engine and a chainsaw engine. The blower motor has more attention to cooling and cylinder construction (thicker coatings, wider rings, etc) as it runs at 100% of it's rated power for long periods, and a chainsaw only has to deal with peaks of full power (few saws last long if run at 100% continuously, such as in milling). The primary difference is in operation - the blower is always under load, the load is proportional to RPM (but non-linear). At low rpm, very little HP is needed or developed. Even at mid-rpm, the HP fairly low. The fan-wheel will eventually absorb the full power developed (obviously). In a Stihl BR400 or 420 (for example) that's about 7500 rpm. If you take off the fan-wheel (or bust off all the fins!), it will run up over 12k in a heartbeat. The blower engines are capable of a lot more hp, but are "derated"... for longer life. You could make the fan-wheel such that it would absorb much higher power, but...

 

[ol'homey]

It seems the main knock on reed vs piston port is the reed is a restriction so the piston port will always outperform on top end HP. Question: Can't you just use a bigger reed or multiple reeds so that the reed is not the limiting factor on top end HP? The old homies used 4 and sometimes 6 reeds in a pyramid.
I've seen a couple of HP/torque vs rpm graphs on piston ported and reed valve engines. I like the milder peaks and broader power band of the reed engines as it allows more flexibility in operating under the variety of loads encountered due to differences in bar length, chain type, wood dia and type. etc. IOW I may not always be able to keep the saw running at or above the peak HP rpm. If I have a piston ported saw it will bog and I'm screwed. If I have a reed valve saw as I drop below peak HP rpm the HP is gradually falling but the torque is gradually rising. So the saw doesn't fall on it's face as quickly. The peak torque RPM might be 1000-1500 rpm less than the peak HP rpm on a reed valve saw. As long I'm somewhere between peak torque and peak HP the saw is happy and the chips are flyin'.