That is correct.

Even if you stayed with a 3.8" blower pulley and just changed the crank pulley from a 6.0" to a 6.5" you would still increase belt wrap by nearly a linear inch. The added belt wrap gained on the 6.5" pulley more than offsets the marginal loss on the 3.8" pulley.

 

No No No it is not correct.

We are really not getting anywhere here are we? Let's try again. You are 100% correct that a 6.5/4.1 combo gives the same speed/boost as a 6.0/3.8 combo. BUT the combo has the same amount of belt wrap, one is not better than the other. The ratio of big to small is the same at .63, that is why you have the same speed/boost. If slip were to occur it would happen at the smaller pulley. Think about the two extremes for a moment. 1st, both pulleys are the same size, they have equal belt wrap, is this case 180 degrees of each pulley would be in contact with the belt. Second go to some extreme crazy size, a 12" crank pulley and a 1" blower pulley. Keep the distance between the pulleys constant and draw it on a piece of paper. With the second example the belt only touches a very small section of the smaller pulley surface and would do nothing but slip. It is about the angle the belt makes at the pulley. So what have we learned?
1) Pulleys with same ratio perform the same. 3.8/6=.63, 4.1/6.5=.63 and have similar belt angles and similar likely hood of slip.
2) Pulleys of same size have the most belt contact.
3) As the relative difference of the size of pulleys increases the less belt contact area on the smaller pulley.
Like I said in the first post making the bigger pulley bigger or making the smaller pulley smaller has the same effect. You increase speed/boost and decrease the contact area of the smaller pulley increasing the likely hood of slip. Since there is a range of small pulley options available people change the small pulley to get more boost until they begin to slip. You can keep the same small pulley and increase boost by getting a larger bigger pulley but you would still get belt slip when you reach the same ratio as decreasing the small pulley.

 

No No No it is not correct.

We are really not getting anywhere here are we? Let's try again. You are 100% correct that a 6.5/4.1 combo gives the same speed/boost as a 6.0/3.8 combo. BUT the combo has the same amount of belt wrap, one is not better than the other. The ratio of big to small is the same at .63, that is why you have the same speed/boost. If slip were to occur it would happen at the smaller pulley. Think about the two extremes for a moment. 1st, both pulleys are the same size, they have equal belt wrap, is this case 180 degrees of each pulley would be in contact with the belt. Second go to some extreme crazy size, a 12" crank pulley and a 1" blower pulley. Keep the distance between the pulleys constant and draw it on a piece of paper. With the second example the belt only touches a very small section of the smaller pulley surface and would do nothing but slip. It is about the angle the belt makes at the pulley. So what have we learned?
1) Pulleys with same ratio perform the same. 3.8/6=.63, 4.1/6.5=.63 and have similar belt angles and similar likely hood of slip.
2) Pulleys of same size have the most belt contact.
3) As the relative difference of the size of pulleys increases the less belt contact area on the smaller pulley.
Like I said in the first post making the bigger pulley bigger or making the smaller pulley smaller has the same effect. You increase speed/boost and decrease the contact area of the smaller pulley increasing the likely hood of slip. Since there is a range of small pulley options available people change the small pulley to get more boost until they begin to slip. You can keep the same small pulley and increase boost by getting a larger bigger pulley but you would still get belt slip when you reach the same ratio as decreasing the small pulley.
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Your sentence is incorrect - "You are 100% correct that a 6.5/4.1 combo gives the same speed/boost as a 6.0/3.8 combo. BUT the combo has the same amount of belt wrap, one is not better than the other."

There is more belt wrap with the 6.5/4.1 pulleys than the 6.0/3.8 pulleys. The drive ratio you are speaking of does not define how much belt slip you will have. Get a measuring stick out and you will easily see the additional belt contact on the 6.5/4.1 vs the 6.0/3.8 pulleys.

crank pulley 6.5 - 6.0 = .5 difference
Blower pulley 4.1 - 3.8 = .3 difference

.5 + .3 = .8 belt gain? No not all of this is in contact with pulleys but some is adding additional grip the belt uses. So the 6.5\4.1 combo adds additional belt wrap and grip helping with belt slip.


Your last sentence sums it up. You do not want a scenario where you are running a small pulley if possible. To avoid this leverage the crank and blower pulley sizing to keep them from being "small". Simple as that.

 

You guys are going on and on about this belt wrap thing and pulley sizes and starting to even confuse me lol this is silly! Totally over complicating something simple.

Let me make something clear for you guys. How much belt wrap you have has nothing to do with what size pulley you choose to run, you can have just as much belt wrap around a 4.1 as you do on a 3.2 if you change the length of the belt to accommodate the difference in pulley size. Why a larger pulley is better for less slippage is becuase there is more pulley surface area on a larger pulley for the belt to grip. But your length of belt accounts for how much belt wrap you will have on that pulley, both of which have an effect on slippage/traction. This is why I had no issues with slip running a 5.45/3.2 combo and some do, becuase I ran the longest belt possible for the most "wrap" on my pulleys, without maxing the belt adjustment of course. Ive noticed some people make the automatic connotation that running the smallest/tightest belt somehow = tighter/better grip in their minds, and inadvertently creating a scenario where there is the least belt wrap possible as a result.

So to reiterate, run the largest/loosest belt possible without maxing out the adjustment for the most wrap/traction on your pulley arrangement, and if you try that and still doesn't work, then you can up size the crank and sc pulley size, and go up on the belt size again to accommodate obviously.

The other thing you can do before changing out to a whole new larger pulley system, is up the stock idle pulley size from the 3" to 4", this I found out inadvertently puts even more wrap on the sc pulley. When the bearing let go on my 3"idle pulley I replaced it with a 4". Several manufactures make them. Some auto parts stores carry these on their shelf even.

 

Yeah I'm all confused now too.

 

I never really considered it part of the equation until you guys made me re analyze the whole system at play here in this thread. The larger idle pulley shortens the distance the belt runs to the sc pulley increasing the angle towards more sc pulley wrap, much the same net effect that a larger belt would and having to compensate by increasing the tentioner position. In any event, having the longer belt is key in being able to take advantage of either.

 

Is there room to run a bigger idler on the sos system? I may have the largest belt possible for my 3.2 setup, but I am really close to hitting a bolt with the pulley if I go any tighter on the adjuster. I guess running a larger pulley would wrap the belt more though and essentially keep me in the same spot as far as adjustment goes. Given that I increase the belt size too.

 

I just responded to Dagou PM about this too, I know there are slight variance in the Comptech and SOS tentioner arrangement but since I ran Comptech I don’t know for sure how much room you will have to accommodate a 4" idle pulley. But what you describe is correct, that it’s the overall pulley dimension itself that has to clear, in particular the mounting plate in my case, not the tentioner bolt, so just examine your clearances and then run the larger belt for the larger idle pulley if you need. Keep in mind you can pull that idle pulley off and change the back or front spacing on it a few mm with an assortment of washers or bushings without compromising the belt ride position, IF you need to help clearing the backing plate etc. The pulley is typically a few mm wider then the belt on either side so it gives you a little wiggle room if needed to play with this.

At one time I actually removed both press fit bushing on the idle pulley and cut one side down a few millimeters to create an offset in either direction, simply by flipping the pulley front to back. I could then add a washer or two if needed to the cut side to re center. Doing this gave me the ability to adjust my tentioner pulley fore and aft to always keep in alignment and clear the plate with the larger 4" when the adjuster was near maxed. Compressed old mounting plate bushings will start to misalign the idle pulley as well, and so it’s good for that too for a quick fix.

 

It's not just belt wrap, with larger pulleys you have more leverage so the transmitted torque is lower (although, that does mean higher surface speeds, which can lead to higher belt temperatures depending on what the slip was like before.

 

I just responded to Dagou PM about this too, I know there are slight variance in the Comptech and SOS tentioner arrangement but since I ran Comptech I don’t know for sure how much room you will have to accommodate a 4" idle pulley. But what you describe is correct, that it’s the overall pulley dimension itself that has to clear, in particular the mounting plate in my case, not the tentioner bolt, so just examine your clearances and then run the larger belt for the larger idle pulley if you need. Keep in mind you can pull that idle pulley off and change the back or front spacing on it a few mm with an assortment of washers or bushings without compromising the belt ride position, IF you need to help clearing the backing plate etc. The pulley is typically a few mm wider then the belt on either side so it gives you a little wiggle room if needed to play with this.

At one time I actually removed both press fit bushing on the idle pulley and cut one side down a few millimeters to create an offset in either direction, simply by flipping the pulley front to back. I could then add a washer or two if needed to the cut side to re center. Doing this gave me the ability to adjust my tentioner pulley fore and aft to always keep in alignment and clear the plate with the larger 4" when the adjuster was near maxed. Compressed old mounting plate bushings will start to misalign the idle pulley as well, and so it’s good for that too for a quick fix.
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Makes sense. I'll play with it if I experience any slip. Fingers crossed that I don't.