Educate me please...

[Moze]

OK, as I said before, I had the Stihl 044 and loved it. Never needed or wished for more saw.

But now I need some more professional input: What exactly determines which saw cuts faster (assuming they have similar chains):

Is it the horsepower?

Is it the RPM's?

My assumption would be that it's the RPM's. My assumption is based on the following: My DeWalt drill turns at 2,000 rpm's. So if you were to put the same drill bit in the DeWalt and any other drill on the market (DeWalt has the highest RPM's that I know of for cordless drills) the DeWalt would come out on top. So I assume if a Dolmar with the same displacement as a Stihl had a higher rpm, it would cut faster.

Am I on target here, or am I missing somthing...?

And if my assumption is correct, does anyone know what the rpm's are on the Stihls - namely the 260 Pro and the 280? I can't seem to find them anywhere (haven't asked the dealer yet).

 

[glens]

All things being equal, the saw that pulls the more teeth through the cut in a given time-frame is the faster saw.&nbsp; Being able to do that requires power <i>and</i> RPM.&nbsp; Between the MS260 and MS280, for example, the smaller saw makes nearly the same power, and the way to do that is to make it at higher RPM.&nbsp; In practice, both stock saws will probably pull nearly the same speed in the cut, but since the larger saw engine is larger, it will have a more forgiving powerband (range over which it makes power).

Horsepower in terms of engines, is strictly a function of torque at RPM.&nbsp; A smaller engine might make less torque per revolution, but can compensate by being optimized for running at a higher RPM.&nbsp; Any time you let the engine speed fall somewhat below it's optimum range on a more tightly-wound engine, it will fall on its face noticeably more.&nbsp; In this way, the MS260 would be somewhat more finicky than the MS280.

A vastly more powerful engine, which can make its power at a lower engine speed because its larger, can increase the relative chain speed by using a larger tooth-count drive sprocket.&nbsp; Both of the above saws, running 3/8 chain, will use a 7-tooth driver, so pure sustainable engine speed will be the key.

The MS280 <i>should</i> be more graceful to use in an inexpert manner.

Glen

 

[TonyM]

Glens, did you notice the DLG tests showed the MS260 has more torque? I was surprised by that.

Moze, the torque an engine creates has to exactly match the load on the engine. Most engines have a torque curve that starts low at low RPMS, peaks out just under maximun horsepower RPMS, and then falls off as the engine revs to an inefficient level. So when your saw hits the wood, the RPM of the engine changes to a point where the torque the engine creates exactly matches the load required. So, if you want a saw to cut at it's optimum point, you want your load to be equal to the torque the engine creates at maximum horsepower.

Now here's where what Glens said comes in. Obviously conditions when cutting wood make it impossible to match the load on the saw to the engine torque at max hp. If the load is too light, the rpms climb above the max hp point, and you loose power. If the load is too heavy, the rpms fall below the max hp point and you lose power. If the load is above the max torque (which remember is below the max hp rpms) then the engine will stall (or in the case of a saw, the clutch will begin to slip). So the flatter the horsepower curve, the more forgiving the saw is to the variations of the load on the saw. Smaller displacement engines, in order to make Hp, often sacrifice a flat power curve and give way to peaky power curve. There truly is no replacement for displacement.

I like to think about dirt bikes. A typical 125cc dirt bike is making low 30hp now. So a 250cc should make near 60, right? Well, they actually make about 45Hp, but over many more RPMS. A 500cc should make near 120Hp, but they make more like 60Hp almost everywhere. Now a 500cc snowmobile can make 120Hp and use it, because the clutch optimises the gearing to keep the load on the engine at the right point.

 

[Moze]

Thanks to both of you - that makes it a lot clearer. I've pretty much decided to go with the 260 Pro. The power to weight ratio is great and everyone else seems to agree that would be the better saw. Thanks again guys.