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Big bore=more power?

Jake Dykstra

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So do they really make more power? I think of saws like the 066/064 and wonder why there isn't an 068 using a 56mm bore.
Does the saying no replacement for displacement apply to chainsaws?
If so why does it seems that it's not more popular?
For example I've heard numerous times that the 50mm is preferred over the 51.4/52mm on the 372 platform.
So it's obvious that many things factor into why a bigger bore may or may not make more power and I'd like to hear your experiences and ideas about why?

And Go!
 

Woodslasher

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So do they really make more power? I think of saws like the 066/064 and wonder why there isn't an 068 using a 56mm bore.
Does the saying no replacement for displacement apply to chainsaws?
If so why does it seems that it's not more popular?
For example I've heard numerous times that the 50mm is preferred over the 51.4/52mm on the 372 platform.
So it's obvious that many things factor into why a bigger bore may or may not make more power and I'd like to hear your experiences and ideas about why?

And Go!
51.4mm on a Husky is very slow without a fair bit of work and difficult to find while 50mm is ready to rock right out of the box and easy to come by. The 51.4mm jugs were for low rpm concrete saws, and most people don't want slow chain saws. With the 064/066 saws, I'm midway through putting an oem 56mm cylinder on a 660 right now. I'd guess the reason Stihl didn't make one is because of things like carb flow, crankcase volume, etc. Also, if you build a chassis strong enough to hold up to a 54" bar on a 100cc saw, that'll be a damn heavy 90cc saw. Think 362 vs 372. Same chassis, 10cc difference in displacement. The 372 is legendary, the 362 (edit: Husky 362) isn't. Or 10-10 vs SP81. A SP81 is a lot more expensive than a 10-10 because it has an extra 30-ish cc's in almost the same chassis.
 
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Jake Dykstra

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So is the 51.4 slower because port timing is not ideal? Because that can be dealt with.
I'm thinking of stuff like carb not big enough. Case volume. Timing changes due to extra distance to edge of piston.
Port width in relation to bore diameter. Fuel air charge velocity to make up the longer distance and larger volumes in the cylinder.
More case volume due to bigger bore?
Transfer tunnels are obviously narrower compared to the same cylinder with smaller bore so how to make up for that if that's needed
 

Woodslasher

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So is the 51.4 slower because port timing is not ideal? Because that can be dealt with.
I'm thinking of stuff like carb not big enough. Case volume. Timing changes due to extra distance to edge of piston.
Port width in relation to bore diameter. Fuel air charge velocity to make up the longer distance and larger volumes in the cylinder.
More case volume due to bigger bore?
Transfer tunnels are obviously narrower compared to the same cylinder with smaller bore so how to make up for that if that's needed
The transfer ports are about 1/2 the size they should be, so you'd need all the stuff to grind them out. See the bigger transfer ports in the cyl below? On an XPW they're about 1/2 the size. Since most people can't do their own porting and probably wouldn't know to grind 'em out they'd just have a really slow saw. The rest of that stuff I don't know enough to comment on one way or the other.
https://cdn.shopify.com/s/files/1/0032/0472/products/echoparts_053_1024x1024.jpg?v=1571437854
 

huskihl

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So is the 51.4 slower because port timing is not ideal? Because that can be dealt with.
I'm thinking of stuff like carb not big enough. Case volume. Timing changes due to extra distance to edge of piston.
Port width in relation to bore diameter. Fuel air charge velocity to make up the longer distance and larger volumes in the cylinder.
More case volume due to bigger bore?
Transfer tunnels are obviously narrower compared to the same cylinder with smaller bore so how to make up for that if that's needed
The 51.4 is slow because it has slightly different timing and 365-sized transfer ports
 

Ketchup

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A well designed cylinder of a larger bore with enough carb to feed it will make more power. Most Big Bores are not well designed. The XPW is one example (though it can be ported to beat a 50mm), then the long list of AM big bores with bad ports, timing, tunnels etc (some of which also make excellent power ported).

But there are many examples of larger bore performance. 372 vs 365s, 281 vs 288, 034 vs 036, 290 vs 390, there are many like this. Then Hybrids like the 440/460 or 064/66, 620/460 that have the power of the larger cylinder.

Throwing a $40 Big Bore of eBay into a saw probably won’t improve it over stock, but there are good options out there for many saws.
 

MG porting

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So is the 51.4 slower because port timing is not ideal? Because that can be dealt with.
I'm thinking of stuff like carb not big enough. Case volume. Timing changes due to extra distance to edge of piston.
Port width in relation to bore diameter. Fuel air charge velocity to make up the longer distance and larger volumes in the cylinder.
More case volume due to bigger bore?
Transfer tunnels are obviously narrower compared to the same cylinder with smaller bore so how to make up for that if
So I will admit some saws don't take well to big bores but at the same time finding the right big bore is just as important there's lots of them out there and they can have really pore port timing numbers so bad that I don't care what you do to port them you can't get them to run worth a sheet of paper but at the same time some big bores work and work well.
 

Maintenance Chief

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A well designed cylinder of a larger bore with enough carb to feed it will make more power. Most Big Bores are not well designed. The XPW is one example (though it can be ported to beat a 50mm), then the long list of AM big bores with bad ports, timing, tunnels etc (some of which also make excellent power ported).

But there are many examples of larger bore performance. 372 vs 365s, 281 vs 288, 034 vs 036, 290 vs 390, there are many like this. Then Hybrids like the 440/460 or 064/66, 620/460 that have the power of the larger cylinder.

Throwing a $40 Big Bore of eBay into a saw probably won’t improve it over stock, but there are good options out there for many saws.
40$!!!! Try 18.00$. Lol
 

Traprunner

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The 272xp cylinder on a 268xp crankcase is a fine example of a bigbore on the same case capacity/volume. They made real good power compared to the gains from the 372xp/372xpw. Cylinder design is the key factor?
 

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The 272xp cylinder on a 268xp crankcase is a fine example of a bigbore on the same case capacity/volume. They made real good power compared to the gains from the 372xp/372xpw. Cylinder design is the key factor?

I think so. Good ports matter. The XPW has an inhibited transfer tunnel designed for low RPM. 272 is more of a true big bore: bigger bore, same transfers. Funny though, I don’t feel a major improvement on 272.
Case capacity seems like a secondary factor and often not an issue. My experience is a larger bore with a tighter case will make a more peaky power band but can gain HP at the top end. Longer bars will perform better with greater case capacity.
 

huskihl

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The 272xp cylinder on a 268xp crankcase is a fine example of a bigbore on the same case capacity/volume. They made real good power compared to the gains from the 372xp/372xpw. Cylinder design is the key factor?
To add to what ketchup said, the 51.4 MM XPW cylinder was designed for a concrete saw that made its maximum horsepower below 10,000 RPM. The 272 cylinder was designed for a chainsaw and had performance at RPMs in mind
 

Jake Dykstra

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Is there any limitation or maybe I'd ask is there a bore stroke ratio that is ideal for say 11-12k rpm. Say it's a 34mm crank all other things being equal or close to it would a 50mm bore out perform a 46mm? If so would a 54 be even better? I guess I'm wondering if there's an ideal ratio
 
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