A question about torque, speed, RPMs, and the relation among all three.

[parrisw]

I don't know off hand.....

I was talking to Dan earlier and he said something like 2500 for us.

1500' anybody know the % loss per 1000' of elevation.

 

[blsnelling]

650' here in Franklin, OH. LINK

 

[wi50]

I'd like to see the port mean time / areas of each saw compared to the end results.

 

[blsnelling]

1,050' in West Spart, NY. LINK

 

[parrisw]

1,050' in West Spart, NY. LINK

Thanks Brad. Not a whole lotta difference at all. Not enough for 20psi diff, IMHO.

 

[blsnelling]

Thanks Brad. Not a whole lotta difference at all. Not enough for 20psi diff, IMHO.

The 681 had 190-195 PSI compression before I raised the exhaust. It was 175 last time I checked it. Maybe I had a "good" guage Maybe Dan's reads low. Reguardless, I think the main point Jason was making is that the 7900 has more compression. But I don't think that account for all of the difference in torque. I think the greater exhaust duration of the 681 is affecting the torque more than the 10 PSI of compression. That's also why it turns more RPMs. It's a catch 22 thing with raising the exhaust port. This 7900 I have here now only reads 140-145 PSI compression, yet has more torque than the 681. But the exhaust port is way low with much less exhaust duration.

 

[parrisw]

The 681 had 190-195 PSI compression before I raised the exhaust. It was 175 last time I checked it. Maybe I had a "good" guage Maybe Dan's reads low. Reguardless, I think the main point Jason was making is that the 7900 has more compression. But I don't think that account for all of the difference in torque. I think the greater exhaust duration of the 681 is affecting the torque more than the 10 PSI of compression. That's also why it turns more RPMs. It's a catch 22 thing with raising the exhaust port. This 7900 I have here now only reads 140-145 PSI compression, yet has more torque than the 681. But the exhaust port is way low with much less exhaust duration.

Makes sense Brad. I don't think 10psi will give a saw that much advantage in torque.

How much did you raise the ex to bring it down that much? Reason I ask is, I was thinking of doing that to my 395 but I don't want to lose torque, the thing is a beast in the torque with around 200psi compression, but only spins 12,500 wot, which is ok by me, cuts great, I just don't want to screw it up by raising the ex. What do ya think?

 

[blsnelling]

Makes sense Brad. I don't think 10psi will give a saw that much advantage in torque.

How much did you raise the ex to bring it down that much? Reason I ask is, I was thinking of doing that to my 395 but I don't want to lose torque, the thing is a beast in the torque with around 200psi compression, but only spins 12,500 wot, which is ok by me, cuts great, I just don't want to screw it up by raising the ex. What do ya think?

I'd keep it about 100°.

 

[WoodChuck'r]

100 degrees?? Exhaust duration!?



Now I'm WAY lost!!

 

[spacemule]

After you've made torque, you use a Kleenex to clean up after yourself. Horsepower is what happens when you don't.

 

[wi50]

think, if the exhaust opens at 100*atdc, it would also close at 100* btdc, that would give you the other number for duration.......and there's only so many degrees in the cycle

 

[parrisw]

I'd keep it about 100°.

Thanks, I was thinking that number actually. I guess I should make up a degree wheel. How many thou do you need for 1°?

 

[wi50]

that is going to depend on the phasing, crank stroke, rod ratio, it's different throgh the whole stroke.

for example at 90* the piston is moveing much faster than it is at 10* but at 10* it's moveing the same speed it is at 170* or at 190*

that's where we come up with port mean time, 160* may work awesome in one and not worth a crap in another application depending on the relationship of other things in the engine. But there's alwayse some good sound baseline numbers to aim for,

 

[parrisw]

after you've made torque, you use a kleenex to clean up after yourself. Horsepower is what happens when you don't.

lol dude l o l

 

[Tzed250]

First of all, I've never studied physics, so please excuse my ignorance.

All three of the saws that Jason and Dan have are close in size: 441, 7900, 681. And Jacks 372 was mentioned. So we're not comparing a little saw to a big saw. With that in mind, I noticed myself that you can push harder on the 441 than the 681. They're the only two I've run of the group. But then the 681 will usually out cut it in a timed event. How does that happen? How can you have a 372 like Jacks that's ported even more strictly for RPMs, that is faster in a timed cut, but you push on it much at all, and it stops. You would think that the lack in torque would prevent it from pulling the chain fast enough to cut faster. That's the part that messes with me.

Normally I build strictly for torque, and consequently, rarely raise the exhaust. That was not the case with this 681. I think what we have here is a blend between a torque saw, ie the 7900, and a RPM saw, ie the 372. It's kind of cool that they where built this way since it fits the uses that Jason intends for them, especially considering I did not build the saw with Jason in mind.

The 681 has better cylinder filling efficiency than the 7900 at higher rpms. The 681 is in its sweet spot in the size wood it has been run in. The 7900 wants more load. It is in the same manner that most 90cc saws will not really shine until the wood is over 30".

A good example are two 125cc bike engines. A CR125 will make about 30hp. An RS125 GP roadracer will make well over 40hp. The power deliver of the totally night and day different. The RS has to operate in a 1500 rpm window, above or below there is nothing. The CR is much more flexible. Neither will do the others job.

This is an extreme example, but it shows that the use of a certain amount of cc's has many options.


.

 

[wi50]

do the math on a 7 and 8 tooth sprocket and see what the difference is, now convert that to the RPM of the engine or where the engine makes it's best power or power to speed. Run the saw in that zone and with the proper gearing. It's not that hard for a 7 tooth cut faster than an 8 tooth, but some operators can make either one cut faster than another operator with the same saw, same setup by knowing the setup.

Why will a volkswagen bug go fastert in 2nd gear than it will in 3rd or 4th...... try it if you have one, everyone had one right?

 

[Tzed250]

Thanks, I was thinking that number actually. I guess I should make up a degree wheel. How many thou do you need for 1°?

that is going to depend on the phasing, crank stroke, rod ratio, it's different throgh the whole stroke.

for example at 90* the piston is moveing much faster than it is at 10* but at 10* it's moveing the same speed it is at 170* or at 190*

that's where we come up with port mean time, 160* may work awesome in one and not worth a crap in another application depending on the relationship of other things in the engine. But there's alwayse some good sound baseline numbers to aim for,

Please see the equations here for positioning:

http://en.m.wikipedia.org/wiki/Piston_motion_equations?wasRedirected=true



.

 

[billmartin]

.


I can't believe I'm diving in...here we go again.


Horsepower is a measure of work done.

Cutting is work done.

If two saws make the same torque, but one makes it at a higher RPM, the high RPM saw does more work.

If two saws make the same horsepower, but one makes it with more torque, the one with more torque will be easier to use.

Ever wondered why most devices powered by electric motors don't need gearboxes?

Put 32" bars on each saw, buried in Oak, and see which one does the most work.


This will likely go downhill from here...


.

:deadhorse: So torque is just that good feeling you get when you hit the throttle and it starts tearing it up from the start.

Horse power ahhh horse power It just means your torque producers also work at high rpms.

In conclusion Horsepower is the torque that can be maintained under load.

And that ladies and gental men is either profound speech or pure gibberish

 

[Tzed250]

I can apply 50 lbs/ft to a bolt for 10 minutes, but if the bolt never moves then no work is done.


.

 

[Terry Syd]

Piston crown

Parris, if you put a degree wheel on the saw you can then be sure what modifications you are making. If you have an old piston, then pop in the old piston and nip a bit off the front of the crown next to the exhaust port. The timing will change just like it would as if you had raised the exhaust port.

You can then keep going up in 2 degree increments to see how it changes the powercurve. When you go too far for your preference in power characteristics - then you can port the cylinder to the duration that you did like and put in the good piston.

If the amount off the front of the piston is only a few degrees, then you might even stay with trimming the piston crown and thus retaining the stock exhaust port with the factory chrome at the top of the port. It will also always allow you to go back to stock timing anytime you want.

If you stay with the trimmed crown, you will loose a very small amount of mixture under the squishband from the removed material, but this isn't a race saw that needs every percentage of power possible. The gains you will get from the increased timing will more than offset the tiny mixture loss.