What's the magic/science behind the 15% reduction in pulley size? It seems like a factory-approved system (JCW) would have to stay well within the tested limits of the supercharger/drivetrain, yet they only use a 15% reduction pulley as well. In other words, I would think the tuners would be less conservative than the factory (factory endorsed JCW) Wondering why we haven't seen more aggressive aftermarket pulleys (in the 16-20% range, maybe). Any thoughts or direct knowledge on how 15 became the norm? JR

 

GREAT Question :smile:

I've been thinking the same thing.


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His, Chile RED all over, Madness intake, MINI-Motorsport Airbox Mod, Magnaflow Exhaust, BMP spark plug wires, RDR rear sway bar and Helix P/D 15% reduction pulley.
Hers, EB UJ on top and BONE stock

 

till we get a "real" answer...can we guess?

my guess is that it's a combination of 1) can only spin the Supercharger so fast before it breakes 2) can only make the diameter so small before screwing up the belt as it bends around it and 3) it's a nice number divisible by 5 and it just sounds right....

Am I close?

 

I thought it was due to blower RPMs being near max with the 15% and a lack of software to take advantage of the 16-20% reduction boost?

X2

 

The JCW kit changes the innards of the supercharger, i am pretty sure in another thread it says its a coating on the impeller blades to make higher boost, the pulley just spins it faster, exsisting supercharger innards. pulley is much cheaper, but JCW kit would be more reliable.

 

I agree that the software probably would have to be tweaked, but it already should be to get the full benefit of the 15% pulley. The supercharger definitely has a limit, but my hunch is that MINI wouldn't let JCW get anywhere near that limit, unless the unit was modified to handle it (and I don't think that's the case).

 

From a previous thread, I believe it might have been Randy who mentioned that a 15% reduction pulley spinning at an engine speed of 7200 RPMs (what the rev limiter is moved to with the Evotech pulley software) will be at the end of the limit of what Eaton is comforatble with for their supercharger. A smaller pulley would spin at too high an RPM for the supercharger and would not be reliable. I believe the size of the 15% reduction is the optimal size for performance and reliability. MINI just chose to go with a smaller one for an even safer track record (afterall, if MINIs kept blowing engines, the MINI line would fail) AND b/c they wanted to make it underpowered a bit so they could come out with a more moreful S (JCW) to sell for a larger profit.

 

My understanding, at least with regards to the 15% P&D pulley is that jlm tried 10% that didn't make much difference so 15% was tried and it seemed to work well. More of a guess than anything really scientific. Granted I'm sure the maximum operating RPM of the supercharger was in the back of his mind. :smile:

Paul

 

The supercharger pulley size also controls how fast the water pump operates at and by reducing the pulley by 15% you are increasing the water pump as well. 15% keeps these other components within normal ranges and going beyond that will move these other items outside their operating ranges...anything smaller and your going to have to start changing more then just the pulley. Additionally I believe the coating on the blades is there in stock form as well and that JCW was just stressing the fact that they were coated for marketing reasons...Do a search and i'm sure that you will find something to that extent...i know there was threads dicussing that...i believe the only thing that JCW is doing with the SuperCharger is the pulley reduction...they just make you swap pulleys as they swap them out at the factory in england and just pop in someone elses supercharger into your car with the pulley reduced.

 

>>
>>My understanding, at least with regards to the 15% P&D pulley is that jlm tried 10% that didn't make much difference so 15% was tried and it seemed to work well. More of a guess than anything really scientific. Granted I'm sure the maximum operating RPM of the supercharger was in the back of his mind. :smile:
>>
>>Paul

If it's 10%, wouldn't it create more boost at a much lower RPM and get you going quicker? But i guess you gotta avoid high rpms. Hey is there a way to make it so that the SC doesn't keep spinning faster at some set point, for example at 7000rpms something internal to not allow the SC to turn anymore, like a clutch! haha

 

Combine Greatgro's answer with Casey's and you get the winning result!

To summarize, the Eaton supercharger can only spin so fast. With a 15% smaller pulley going up to 7200rpm with the EVOtech, it's spinning darn near max allowable speed per Eaton's own specs. Also, with the water pump attached, more than 15% could case cavitation unless you went to a stand alone electric pump.

R

 

The refered to 10% means 10% smaller then stock...thus the 15% pulley is smaller thus spins the supercharger faster and creates more boost lower in the power band.

 

Casey: Can you explain how the smaller supercharger make the water pump spin faster. I was trying to figure it out and it seems like the water pump speed would be unchanged unless it's own pulley were smaller. I've never inspected it too closely. Does the serpentine belt encircle the s/c, water pump and crank pulleys? If so, it seems like the crank pulley would be the determining factor in how fast the water pump spins. Probably a brain fart on my behalf, I know.

 

I'm wingin' this from what Randy told me:
Some tuners have tried 17%, and I think even 20%, but that is pushing it from what Eaton says (maker of the M45 supercharger that is used in the MCS).
And actually the supercharger doesn't spin at engine speed (like 7200), it's something like 15000 RPM at redline.

Jayar: Somebody correct me if I'm wrong, but I think the waterpump is attached to and drives directly from the SC, it does not have it's own pulley.

_________________
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Einstein said E=MC2, I say S = MC2

 

>>>>
>>>>My understanding, at least with regards to the 15% P&D pulley is that jlm tried 10% that didn't make much difference so 15% was tried and it seemed to work well. More of a guess than anything really scientific. Granted I'm sure the maximum operating RPM of the supercharger was in the back of his mind. :smile:
>>>>
>>>>Paul
>>
>>If it's 10%, wouldn't it create more boost at a much lower RPM and get you going quicker? But i guess you gotta avoid high rpms. Hey is there a way to make it so that the SC doesn't keep spinning faster at some set point, for example at 7000rpms something internal to not allow the SC to turn anymore, like a clutch! haha :smile: Set the computer / ECU to have speed limiter of 100mph. (Really who is going to take their MINI to 135 for any period of time.





His, Chile RED all over, Madness intake, MINI-Motorsport Airbox Mod, Magnaflow Exhaust, BMP spark plug wires, RDR rear sway bar and Helix P/D 15% reduction pulley.
Hers, EB UJ on top and BONE stock.

 

orbhot is correct about how the waterpump is driven.

 

thanks for the explanation!

 

>>Combine Greatgro's answer with Casey's and you get the winning result!
>>
>>To summarize, the Eaton supercharger can only spin so fast. With a 15% smaller pulley going up to 7200rpm with the EVOtech, it's spinning darn near max allowable speed per Eaton's own specs. Also, with the water pump attached, more than 15% could case cavitation unless you went to a stand alone electric pump.
>>

Right! I forgot to mention the water pump which is supercharger belt driven!

 

>>Great idea. :smile: Set the computer / ECU to have speed limiter of 100mph. (Really who is going to take their MINI to 135 for any period of time. :smile:

I think I'm really starting to get some of this stuff....

 

The pulley reduction is limited by real estate. You can't get much smaller than 15% without sacrificing the integrity of the pulley (depending on the design).

 

so far I have made 10%, 15% and 19%; 15% is what we settled due to cheese' real estate response, the lesser performance of the 10%, and the tight wrap of the 20%

I would be happier with 20% and a suitably lower redline to make better use of the <6k rpm ability of the mill, but the belt wrap is too tight to really last, I speculate. would probably help those short term sprinters at the drags, though.

no-one has reported any sign of water cavitation, so that seems to be a non-issue. since the water pump is reduction gear driven from the blower, it will speed up by the same %, but the cavitation limit seems to be out of reach.

 

If anything, maybe us pulley mod-ees now have higher engine cooling ability. Now if we can just keep from heat-soaking the intercoolers.

 

There are several reasons for the choice, all based on different aspects. Here are the main points:

Supercharger Redline
After discussions with several different engineers at several different manufacturers, the redline of the supercharger is in the neighbor hood of 17,200 RPM - depending on which one you talk with. More than 15% at a redline of 7200 RPM on the motor will put you right at that. Using 15% keeps it at roughly 17,100 RPM (again depending on who you talk with). This could be avoided by limiting the motor RPM, but because the horspower curve does not peak below redline, this would also limit your overall output.

Efficiency
It takes horsepower to make horsepower. What I mean by that is that there is a break-even point to where you are creating a lot of drag to make the boost. The more boost you are making using a supercharger gives more drag. Up to a point, the trade off is acceptable - you make 25 horsepower but it costs 10 to get the supercharger to push that much air, so the net gain is 15. Eventually, it will cost you more to make the power than you get back. This is only a minor reason not to go smaller, but it is part of the big picture. Heat can also fall into this category because of the way the air is moved. To make the boost, the Eaton creates quite a bit of heat - and the more boost is made, the more heat is generated. The heat reduces the density of the intake charge, and therefore costs power. This again is a balancing act - the boost makes power, but the heat caused by genereating the boost costs power. That is one of the reasons the intercooler is such an important option on this car - either cooling it externally (CO2 for instance) or making it more efficient.

Belt Life
When going smaller on the pulley diameter, you have more belt rap on the pulley. This may cause some belt interval issues, but I haven't seen that, and from the other guys I've talked with, they haven't either. It could be a significant enough reason though.

Water Pump
The water pump is driven by the supercharger, so if you reduce the pulley on the supercharger, you are spinning the water pump and the supercharger faster. Unless you change the angle of the blades in the water pump, at some point, the pump will be spinning fast enough to stall the blades - causing cavitation. This has yet to be an issue even at the 17% reduction some of the Euro guys are trying out.

The JCW pulley is 2.21 " in diameter, and the 15% reduction pulley is 2.17", so it is basically a wash - they use a pulley just like we do. The rotors are coated with graphite (not Teflon) instead of the epoxy coating on the S M45. The efficiency is very close to being the same on both types of coatings.

I hope that helps answer some of the questions - great question by the way.

Randy
720-841-1002

 

Randy: you need to send me something, and then 8 more something elses,

 

>>Randy: you need to send me something, and then 8 more something elses,

Oh, code. How spy like.


Paul