exhaust port timing in a saw vs a motocross bike questions?

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WHSH

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As a new to mx 2 strokes I notice that the bikes exhaust port opening times are an average 10-15 degress earlier than a saw. I know that they have power valves to change the timing at rpms, but why can't a saw get away with 84 deg vs 98-101 on a saw. I have been reading the two stoke tuners literature seaching for a comparison and a hint into why. Stock motos a make power at 7-8k rpm. Stock saw power at 8-10k. It is the giant time and duration on the motocross bike that does it?
 
As a new to mx 2 strokes I notice that the bikes exhaust port opening times are an average 10-15 degress earlier than a saw. I know that they have power valves to change the timing at rpms, but why can't a saw get away with 84 deg vs 98-101 on a saw.


Expansion chamber, nuff said.

Later
Dan
 
got it. So what don't saws have power valves, and some sort of advanced muffler rather than a box with a hole. I know its a size issue. From what I understand there is 5 parts to an expansion chamber and I only see 2 maybe 3 with the saw. All very interesting, and thanks for bieng part of my learning curve.
 
You cannot build a suitable expansion chamber in the size requirements of a worksaw. If you are racing, then an expansion chamber pipe is usable.
 
That clearly makes sense. So how do saw manufactures go about thinking about the mufflers. Some a box with a hole, some have internal baffling, some deflectors and some have diffusors. I am sure they know way more than me, but I do they just build the motor around the exhaust limitations?
 
Totally different application requirements and totally different design parameters...saws are SO focused on size and weight, dirt bikes more focused on power bands and throttle characteristics. Dirt bike two strokes are more square (bore & stroke are close)(a typical 125 IS square 54x54mm while larger displacement move in the direction of more bore than stroke), chainsaw are not even close to square. Most chainsaws are either dual or quad transfer port designs with piston port intake arrangements. Dirt bikes have the weight and space flexibility to have tuned exhaust, much more complex transfer port arraignments, and typically totally different intake arrangement such as case reed systems with huge intake volumes. Reed blocks along with that longer stroke therefore different compression characteristics in the crank case completely change the game relative to intake volume and timing. So while two strokes are... two strokes, these two applications have evolved into different designs from intake through to tuned exhausts including power valves require different timing schemes to optimize the entire system. Other differences in requirements...... dirt bikes shoot for much wider power bands than saws, bigger bore dirt bikes like to make power as low as 3000rpm's, those requirements alter design parameters for that application. Dirt bikes spend most their run time at part throttle....so many differences in what is expected...Apples and crab apples. saw are crab apples. :)

(I find it interesting that one of the BIG design changes from 272 series and 372 series saw was transfer ports and bore & stroke....a little bit more stroke...hummmm. wonder if that's going to be a trend)
 
got it. So what don't saws have power valves, and some sort of advanced muffler rather than a box with a hole. I know its a size issue. From what I understand there is 5 parts to an expansion chamber and I only see 2 maybe 3 with the saw. All very interesting, and thanks for bieng part of my learning curve.

I would actually like to see the manufactures install a power valve setup on saws. It could be a simple setup using the same vane style mechanical setup up like on a 4 stroke governor.

Later
Dan
 
Totally different application requirements and totally different design parameters...saws are SO focused on size and weight, dirt bikes more focused on power bands and throttle characteristics. Dirt bike two strokes are more square (bore & stroke are close)(a typical 125 IS square 54x54mm while larger displacement move in the direction of more bore than stroke), chainsaw are not even close to square. Most chainsaws are either dual or quad transfer port designs with piston port intake arrangements. Dirt bikes have the weight and space flexibility to have tuned exhaust, much more complex transfer port arraignments, and typically totally different intake arrangement such as case reed systems with huge intake volumes. Reed blocks along with that longer stroke therefore different compression characteristics in the crank case completely change the game relative to intake volume and timing. So while two strokes are... two strokes, these two applications have evolved into different designs from intake through to tuned exhausts including power valves require different timing schemes to optimize the entire system. Other differences in requirements...... dirt bikes shoot for much wider power bands than saws, bigger bore dirt bikes like to make power as low as 3000rpm's, those requirements alter design parameters for that application. Dirt bikes spend most their run time at part throttle....so many differences in what is expected...Apples and crab apples. saw are crab apples. :)

(I find it interesting that one of the BIG design changes from 272 series and 372 series saw was transfer ports and bore & stroke....a little bit more stroke...hummmm. wonder if that's going to be a trend)
Great info, and well put.
 
Just as a qualifier, I don't really know much about the saw 2-strokes, learned all I know on places like this. I did grow up living with the evolution of motocross race bikes from the early 1970's until the early 2000's when I had to do other things. So can only speculate relative to saws...built a lot of race bikes though.

I'm one of those who thinks simple it better. Saws are so ..... simple! The more crap you hang on them the more crap the either breaks or you have to clean. Power valves can be a pita when they get gummy. Of course the manufacturer has to assume worst case and some will run a good clean burning synthetic that doesn't gum things up as fast, but others will run bean oil! With the saws being production tools, the less required maintenance the better! So I'm way more in favor of passive mechanical (Port timing & cylinder design & electronics) or active electronic devices to add power to saws. And from the world I was a part of in a prior life.... weight is everything when you have to carry things around all day. So yet again passive vs. addition mechanical things. I see the saw following other motorsport trends with electronic devices to enhance mixture & timing to improve power. Already see this in the new X-torg & auto tune saws a trend. The mufflers will get more sophisticated as well. Look at the layed back cylinders on the new generation Husqvarna's....making room for more space after the exhaust port. Also look at those older 372's intake. Where that butterfly is relative to the intake horn.. there has to be a better way, so I predict changes in places like that as well.

The five parts expansion chamber concept was origionally about rough design parameter's and really suited to chambers built in conical and cylindrical sections. Now the state of the art has stamping dies where there is a continual curvature tweaked for specific design goals. Of course the tooling costs big bucks... Pro Circuit can justify those setup costs as they sell more than a few!

Saw's also don't need the same kind of power band flexibility as bikes do. Just run the things wide open all the time! The power valves in the dirt bikes were an attempt to widen the usable power bands. They appeared in the water cooled bikes in the 1980's. Water cooling was also one of the things that allowed some of the other advances, less expansion & contraction of the castings just one of many benefits of the more stable operating temps, just a different world.

Anyway, there are those with more smarts and insight than me here on this board! Just that's my opinion without any real visibility into the industry.
 
Just as a qualifier, I don't really know much about the saw 2-strokes, learned all I know on places like this. I did grow up living with the evolution of motocross race bikes from the early 1970's until the early 2000's when I had to do other things. So can only speculate relative to saws...built a lot of race bikes though.

I'm one of those who thinks simple it better. Saws are so ..... simple! The more crap you hang on them the more crap the either breaks or you have to clean. Power valves can be a pita when they get gummy. Of course the manufacturer has to assume worst case and some will run a good clean burning synthetic that doesn't gum things up as fast, but others will run bean oil! With the saws being production tools, the less required maintenance the better! So I'm way more in favor of passive mechanical (Port timing & cylinder design & electronics) or active electronic devices to add power to saws. And from the world I was a part of in a prior life.... weight is everything when you have to carry things around all day. So yet again passive vs. addition mechanical things. I see the saw following other motorsport trends with electronic devices to enhance mixture & timing to improve power. Already see this in the new X-torg & auto tune saws a trend. The mufflers will get more sophisticated as well. Look at the layed back cylinders on the new generation Husqvarna's....making room for more space after the exhaust port. Also look at those older 372's intake. Where that butterfly is relative to the intake horn.. there has to be a better way, so I predict changes in places like that as well.

The five parts expansion chamber concept was origionally about rough design parameter's and really suited to chambers built in conical and cylindrical sections. Now the state of the art has stamping dies where there is a continual curvature tweaked for specific design goals. Of course the tooling costs big bucks... Pro Circuit can justify those setup costs as they sell more than a few!

Saw's also don't need the same kind of power band flexibility as bikes do. Just run the things wide open all the time! The power valves in the dirt bikes were an attempt to widen the usable power bands. They appeared in the water cooled bikes in the 1980's. Water cooling was also one of the things that allowed some of the other advances, less expansion & contraction of the castings just one of many benefits of the more stable operating temps, just a different world.

Anyway, there are those with more smarts and insight than me here on this board! Just that's my opinion without any real visibility into the industry.
I like you line of thought. Yes, simple and clever are better than complex.

in my search for what the porting differenced I found this,

"Direct drive motors typically have quite a bit less blowdown than the blowdown of a clutch motor (but not always) and this is achieved by lower exhaust and/or higher transfers. The shorter blowdown results in an engine that has less peak power but the power is usually over a wider RPM range."

I am guessing that since the moto is clutched and gear that you can make a use of a narrower power band.
 
Your comments were very well explained weimedog,I can see you have 2-stroke motorcycle knowledge from earlier days too.:) Motorcycles and saws are very different in design so it's hard to really compare port timing specs,expansion chamber specs,etc.
I like to increase carb size on saws,as they really suffer with miserably small carbs :cry:compared to other 2-stroke engines,stock or modified. A lot of people try to overlook or miss that when modding their saw.
 
As a new to mx 2 strokes I notice that the bikes exhaust port opening times are an average 10-15 degress earlier than a saw. I know that they have power valves to change the timing at rpms, but why can't a saw get away with 84 deg vs 98-101 on a saw. I have been reading the two stoke tuners literature seaching for a comparison and a hint into why. Stock motos a make power at 7-8k rpm. Stock saw power at 8-10k. It is the giant time and duration on the motocross bike that does it?
I agree with your ideas on power valves. 120 cc saw makes 9 hp. 124 cc bike makes 32 hp. biggest difference i see is a cooling system! without coolant, dirtbike would melt, quickly.
 
I know this is a old post but I have some questions pertaining to exactly this. To answer the last post early moto x bikes were aircooled. I have experience porting no power valve 2 strokes in the past I'm not a pro but have had strong results. The motas were banshee quads for drag racing. any 2 stroke porting book I have a few states and I agree from my experience the power is in
the exhaust port. The numbers I've been reading on here don't match up with what I was doing. For example on a stock set of banshee cylinders I would put the exhaust on 192 degrees clean up the intake and raise the transfers to 143 I believe epoxy the sides of the jug hog the transfers out from the bottom and match the case bump the compression to 185psi and with good pipes and a set of 33-35 mm carbs hp would go from stk 48ish hp to around 70hp when I was done. These motas had no power valves. The last one I did got a 4mm stroker crank, a aftermarket set of mono block cylinders the exhaust was 198 on it. Mota made 95hp. All this was on tt111 race fuel. I'm just getting started with saws.
 
Your comments were very well explained weimedog,I can see you have 2-stroke motorcycle knowledge from earlier days too.:) Motorcycles and saws are very different in design so it's hard to really compare port timing specs,expansion chamber specs,etc.
I like to increase carb size on saws,as they really suffer with miserably small carbs :cry:compared to other 2-stroke engines,stock or modified. A lot of people try to overlook or miss that when modding their saw.
On my husky 65's, after a porting job I adapted a bigger tillottson carb.. I wasn't smart enough to measure it first, bu there was a clear difference in the venturi size (3mm probably?) and throttle plate size. There was no downside to it, it doesn't bog from idle and it really works at the top end.
Well, the downside is I need more of these bigger carbs for my other saws and don't know where the donor came from!
 
Moto-x engines make way more power per cc because they have much better port timing,more duration,and bigger carbs. However on a chainsaw you can improve power with better port timing,improve carb size if you can. Of course if port duration or carb size is too much,low speed power will suffer.
 
Good to see you back Mike :)

I think another thing to consider when comparing RPM ranges is that bikes are often MUCH bigger than chainsaws and will have correspondingly lower redlines.. a KTM 85 tachs out at about 12K in stock form.. Not terribly far off of what a built 288 would be running at with a pipe.. Now if you're comparing 250's and up, yeah, their power will be in the 8K range... I think the pipe alone is the single most important difference as far as power adders.
 
yup, a random pipe on a random engine is probably going to hurt more than help if the mismatch is bad enough
 
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