.325 vs. 3/8 Technical Comparison. The 'real' Story

[Rudolf73]

I know there have been a lot of discussions/arguments on AS about running 3/8 instead of .325 chain on 50cc saws and the arguement has not been settled as far as I know. Lately I have been designing chain elevators at work which look pretty similar to chainsaw bars (with teeth etc.) and run 3" pitch chain. (Unfortunately I can't include any pics due to patent issues) So as a result I have been doing a lot of chain size and speed calculations. This got me thinking about .325 and 3/8 chain speeds and how they compare.

I did some calculations and without getting to technical here is what I have found...

- The calculations were done at 10000rpm to simplify things.


With a .325 7 pin sprocket at 10000rpm - Chain speed is 19.4m/s or 63.6f/s

With a 3/8 7 pin sprocket at 10000rpm - Chain speed is 22.4m/s or 73.5f/s


So then I converted these numbers to include the number of cutters per metre or foot and got the following results.

.325 - Number of cutters passing any given point in 1 second @ 10000rpm is 587.4

3/8 - Number of cutters passing any given point in 1 second @ 10000rpm is 588


So the actual difference in cutters passing a single point per second is 0.1% (Not a real big difference right?)


It turns out that 3/8 chain runs around 15% faster than .325, but .325 chain has about 15% more cutters per unit length than 3/8.


Now just to clarify - these numbers are all based on my calculations and I may have missed something when converting between imperial and metric and back again. It would be good if other members could check my numbers just to make sure, Thanks.

 

[sunfish]

Interesting. Thanks for the info!

 

[SPDRMNKY]

just had a thought...

what about volume of wood removed per time? (assuming same style of cutter and out-of-box raker height) have to account for different gauges also I guess...be simpler if they were similar kerfs

not sure if this is your pervue...it isn't mine...I'm a V=IR kinda guy

nice calc work btw!!

 

[Stellarex]

Good to hear! Maybe I will switch over to 3/8 instead of the .325 on my 029S

 

[Rudolf73]

just had a thought...

what about volume of wood removed per time? (assuming same style of cutter and out-of-box raker height) have to account for different gauges also I guess...

not sure if this is your pervue...it isn't mine...I'm a V=IR kinda guy

nice calc work btw!!

Assuming cutter style, raker height and gauge are all the same or close - the volume of wood removed per unit time should also be close to equal. This is because you have the same number of cutters passing any given point in any one unit of time.

 

[SPDRMNKY]

Assuming cutter style, raker height and gauge are all the same or close - the volume of wood removed per unit time should also be close to equal. This is because you have the same number of cutters passing any given point in any one unit of time.

I knew there was something simple that I couldn't wrap my brain around...guess that's why I'm a V=IR guy

so it basically comes down to the saw and the knucklehead behind it? ...or do most 50cc saws have a similar hp/torque curve?

(glad to be the whipping boy:msp_wink

 

[MS260 Fan]

Rpm's in the cut

Interesting stuff. I have an 18" .325 on my 50cc saw. If I had an 18" 3/8 I'd check the difference in rpm's in the cut between the two chains. There are more teeth on the .325 but wider on the 3/8. I always assumed the 3/8" would be slower in terms of chain speed but I never thought about it having less teeth. It could be a wash.

Oh well, just another variable in the equation. I should also check my .325 rim. I thought I ran 8 tooth as an equivalent to 7 tooth 3/8. It's the OD of the rim that you'd want to keep constant, wouldn't it? The 7 tooth 325 would have a smaller OD than the 7 tooth 3/8. I'm just rambling here. I hope this isn't taken as picking apart your work. Just throwing stuff out there and too lazy to do the math myself. Thanks for posting!

 

[blsnelling]

You need to compare .325 8-pin with 3/8 7-pin. The diameterof the two is MUCH closer, nearly the same.

 

[Rudolf73]

I knew there was something simple that I couldn't wrap my brain around...guess that's why I'm a V=IR guy

so it basically comes down to the saw and the knucklehead behind it? ...or do most 50cc saws have a similar hp/torque curve?

(glad to be the whipping boy:msp_wink

From what what I know the power/torque curves do vary a bit especially with the strato 261 - It feels like it has a really good torque curve. Some member say that the 5100 runs better with a 3/8 chain and a 346 runs better with .325. But I think when making those comparisons your cutter style/size/shape and raker height need to be exactly the same if not very close to it. As you can imagine this would be very difficult with all the different size/style chains out there.

 

[o8f150]

i have run both,,, i like the 3/8 better

 

[Rudolf73]

Interesting stuff. I have an 18" .325 on my 50cc saw. If I had an 18" 3/8 I'd check the difference in rpm's in the cut between the two chains. There are more teeth on the .325 but wider on the 3/8. I always assumed the 3/8" would be slower in terms of chain speed but I never thought about it having less teeth. It could be a wash.

Oh well, just another variable in the equation. I should also check my .325 rim. I thought I ran 8 tooth as an equivalent to 7 tooth 3/8. It's the OD of the rim that you'd want to keep constant, wouldn't it? The 7 tooth 325 would have a smaller OD than the 7 tooth 3/8. I'm just rambling here. I hope this isn't taken as picking apart your work. Just throwing stuff out there and too lazy to do the math myself. Thanks for posting!

You need to compare .325 8-pin with 3/8 7-pin. The diameterof the two is MUCH closer, nearly the same.

Yes thats right, the .325 8-pin is much closer in size to the 3/8 7-pin and therefore chain speed is almost the same. But .325 has more cutters per foot than 3/8 so you will have more cutters passing any given point in 1 second with the .325 8-pin.

For the .325 8-pin it works out to be 671 cutters passing a point compared to 588 for the 3/8 7-pin. As a result your saw will be under more strain running a .325 8-pin sprocket but will be faster in softer wood.

 

[mactodd]

For the .325 8-pin it works out to be 671 cutters passing a point compared to 588 for the 3/8 7-pin. As a result your saw will be under more strain running a .325 8-pin sprocket but will be faster in softer wood.

Wouldn't the wider kerf of the 3/8" cause more drag, and therefore drag the saw down? Or, even with the increased drag of the wider tooth, given that less teeth passing the theoretical point, would it matter? Going one step further (in theory), would it be wise to step up to a 9 pin for the .325 for even faster chainspeed?

There have been objective tests run here before, with little to no difference when the same size bar, and cutter type are used between the .325 and 3/8. What wasn't tried, at least from what I have seen, is increasing the sprocket size for the .325. If that test has been done, please post the results. i.e. say 16-18" bar with 7 pin 3/8 semi chisel vs same bar length running .325 and a 9 pin. Might be pushing it on a 50cc, so maybe a 60cc would be a better comparison.

 

[w8ye]

I have a Dolmar 111i with a 8 pin rim and .325" X 18"

A Dolmar 5105 with a 7 pin rim and 3/8" X 20"

A Husky 350 with a 7 pin spur and .325" X 20"

They all cut good.

 

[albert]

Wouldn't the wider kerf of the 3/8" cause more drag, and therefore drag the saw down? Or, even with the increased drag of the wider tooth, given that less teeth passing the theoretical point, would it matter? Going one step further (in theory), would it be wise to step up to a 9 pin for the .325 for even faster chainspeed?

There have been objective tests run here before, with little to no difference when the same size bar, and cutter type are used between the .325 and 3/8. What wasn't tried, at least from what I have seen, is increasing the sprocket size for the .325. If that test has been done, please post the results. i.e. say 16-18" bar with 7 pin 3/8 semi chisel vs same bar length running .325 and a 9 pin. Might be pushing it on a 50cc, so maybe a 60cc would be a better comparison.

Some .325 chains have the same "kerf" as 3/8 Mainly Stihl.

 

[Rudolf73]

Wouldn't the wider kerf of the 3/8" cause more drag, and therefore drag the saw down? Or, even with the increased drag of the wider tooth, given that less teeth passing the theoretical point, would it matter? Going one step further (in theory), would it be wise to step up to a 9 pin for the .325 for even faster chainspeed?

There have been objective tests run here before, with little to no difference when the same size bar, and cutter type are used between the .325 and 3/8. What wasn't tried, at least from what I have seen, is increasing the sprocket size for the .325. If that test has been done, please post the results. i.e. say 16-18" bar with 7 pin 3/8 semi chisel vs same bar length running .325 and a 9 pin. Might be pushing it on a 50cc, so maybe a 60cc would be a better comparison.

As Albert said, some .325 chain has the same kerf as 3/8. I also measured the chains I have here and the cutters/teeth on the .325 and 3/8 chain are the same width. But I'm sure this is not the case for all .325 and 3/8 chains.

Yes a 9-pin would result in faster chain speed, but you would need a decent size saw to run it - well above 50cc


With a .325 9 pin sprocket at 10000rpm - Chain speed is 24.9m/s or 81.7f/s

With a 3/8 7 pin sprocket at 10000rpm - Chain speed is 22.4m/s or 73.5f/s

 

[mactodd]

Some .325 chains have the same "kerf" as 3/8 Mainly Stihl.

I get about a .010 difference when measuring my chains with a digital caliper. Both are Stihl full chisel. Not to mention the cutter length and height difference. Not sure it would make a huge difference, but then I'm too lazy, and unqualified to do the correct computation. Just something to think about. But, to make it an objective test, it'd have to be out of the box comparison of the same style chain, from the same manufactuer, with the same raker and cutter angles, same bar length, and to check cut speed, say a 7 pin on the 3/8, and a 8 or 9 pin on the .325.( same saw of course!) That would be the comparison I would like to see. I'm sure someone has done it over the years. For the average guy, there probably ain't a hell's bit of difference. It's just something I am curious about.

 

[SawTroll]

I know there have been a lot of discussions/arguments on AS about running 3/8 instead of .325 chain on 50cc saws and the arguement has not been settled as far as I know. Lately I have been designing chain elevators at work which look pretty similar to chainsaw bars (with teeth etc.) and run 3" pitch chain. (Unfortunately I can't include any pics due to patent issues) So as a result I have been doing a lot of chain size and speed calculations. This got me thinking about .325 and 3/8 chain speeds and how they compare.

I did some calculations and without getting to technical here is what I have found...

- The calculations were done at 10000rpm to simplify things.


With a .325 7 pin sprocket at 10000rpm - Chain speed is 19.4m/s or 63.6f/s

With a 3/8 7 pin sprocket at 10000rpm - Chain speed is 22.4m/s or 73.5f/s


So then I converted these numbers to include the number of cutters per metre or foot and got the following results.

.325 - Number of cutters passing any given point in 1 second @ 10000rpm is 587.4

3/8 - Number of cutters passing any given point in 1 second @ 10000rpm is 588


So the actual difference in cutters passing a single point per second is 0.1% (Not a real big difference right?)


It turns out that 3/8 chain runs around 15% faster than .325, but .325 chain has about 15% more cutters per unit length than 3/8.


Now just to clarify - these numbers are all based on my calculations and I may have missed something when converting between imperial and metric and back again. It would be good if other members could check my numbers just to make sure, Thanks.

You left the gearing out of the picture?

A .325x8 will produce a slightly higher chain speed than a 3/8"x7, as the gearing will be slightly larger.

 

[Rudolf73]

You left the gearing out of the picture?

A .325x8 will produce a slightly higher chain speed than a 3/8"x7, as the gearing will be slightly larger.

What do you mean by the gearing ST?

 

[blsnelling]

Yes thats right, the .325 8-pin is much closer in size to the 3/8 7-pin and therefore chain speed is almost the same. But .325 has more cutters per foot than 3/8 so you will have more cutters passing any given point in 1 second with the .325 8-pin.

For the .325 8-pin it works out to be 671 cutters passing a point compared to 588 for the 3/8 7-pin. As a result your saw will be under more strain running a .325 8-pin sprocket but will be faster in softer wood.

You left the gearing out of the picture?

A .325x8 will produce a slightly higher chain speed than a 3/8"x7, as the gearing will be slightly larger.

I pointed that out and he replied on the previous page.

 

[SawTroll]

Wouldn't the wider kerf of the 3/8" cause more drag, and therefore drag the saw down? Or, even with the increased drag of the wider tooth, given that less teeth passing the theoretical point, would it matter? Going one step further (in theory), would it be wise to step up to a 9 pin for the .325 for even faster chainspeed?
.

A lot of factors actually come into play, so the only real way to find what works best for you, with a given saw and wood, is to try the options. Also, .325 chain vary more in cutter size and resulting kerf than regular 3/8" chain does.

9-pin .325 will loose a lot of speed when you hit the wood, on a 50cc saw - so it isn't really an option on those saws, nor are 8-pin 3/8".