Does anyone have a pulley that is smaller than a 15% reduction? Helix's pulley allows for the changeout of smaller pulleys, and they also advertise that they can special order smaller sizes. My question is has anyone taken advantage of this? I have heard about some 20% reduction, but have not heard a review on this. I know that there is an upper limit rpm speed on the supercharger, but if we limit the redline of the Mini, I would think that a smaller pulley would be very beneficial for mid range torque. I also know that there is a limit for the other accessories attached to the belt. So how far has anyone gone? how do they like it? what is the difference in horsepower?

 

I used the search function. Here is a good thread that should answer most of your questions.

 

I was gonna say that, but since he's already asked, it has to do with the fact that if you go much smaller than 15% you run the risk of going over the superchargers redline or something like that, I believe it's like 18000rpm and you run at 16 or 17000 rpm with the 15%.

 

>>I used the search function. Here is a good thread that should answer most of your questions.
>>
Thanks it is a good thread and it boils down to two comments.
The first by Davbret:
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.

And the second longer answer by RandyBMC:
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

Excellent question and good to know.

 

Thanks for the responses, but I think you are all missing the points. I am aware of the issues surrounding going smaller. I wanted to know has anyone tried it, not what the issues are around going to a smaller pulley. That has been well documented already. I know that there is an issue with the max speed of the supercharger, but if you lowered the redline by 10%, then you would not hit that max speed, while at the same time fattening the torque and increasing hp in the mid range, which is more important. Most of us spend our time driving between 3500-5500 rpm, not cruising down the highway at 7200 rpm. I think the power would be more useful and more fun in the mid range of the engine and using it during passing and entrance ramps. I am also aware of the fact that at some point it will be robbing to much of the engine horsepower to drive the supercharger and heat may play a role as well, but it has all been theoretical discussions and no calculations showing the exact point of diminishing returns. That is why I wanted to see what other people have felt using the smaller pulleys, does it continue to increase horsepower, does it continue to increase midrange.....

 

Hmm, that's an interesting idea. To answer the correct question, I have never heard of anyone using a smaller pulley than 15%, I've only heard stuff about why people wont go any lower than 15%.

 

>>>This has yet to be an issue even at the 17% reduction some of the Euro guys are trying out.


Hopefully we can find out who these euro guys are and perhaps they will post their results. More torque at lower RPMs and a shorter red line would different, but interesting.



_________________
'03 MC Pure Silver/Black #1,#3 Stock 16"
Swapping very soon for:
'04 MC Dark Silver/black All three HK

 

I plan to do some testing with smaller than 15% pulleys to see how the power curves look. Personally, I would not like to reduce the rev limit in the interest of making more boost at lower RPM because I value wide power bands.

 

I was not thinking of reducing it by much. The new ECU's have upped the rev limit to @7200rpm. I was thinking of bringing it back down to 6500 rpm. This might allow a few more pecentage in reduction to the pulley. I personally don't like driving the car much above 6000 rpm all the time. If someone can remember the calculation (I think Randy posted it at one time) between pulley size and rpm limits of the supercharge (which I thought was somewhere areoun 17,200 rpm), we could see what the theoretical limits are between pulley size and redline. This would give us a more realistic numbers to work with. Also if not much extra horsepower is acheived then it may not be worth it. I am not sure if you can take a linear calculation of what is currently achieved with the 15% pulley. If 15% reduction gives us 18whp, then would that mean that 20% reduction would give us 24whp? or almost 27hp at the crank. That could be pretty interesting then. That would bring cars to 190hp at the crank and some decent torque with only a pulley.

 

as has been written many times, max HP is misleading as a performance measurement.

dgsz..is on the right track. make more torque and make it sooner in the rpm band and your track times will improve, even though your dyno runs may show less power (using a lower peak rpm.) save the raised rpm range for a valve train and lower end that can really make use of it.

 

mmmmmmm Torque :smile:

 

Mmm ... open differential.