I've been reading a lot about the new 19% pulley and I was just wondering what Randy's (randyBMC) take on this? Is he doing pulley parties with the 19% pulley or is it still the 15% (or 17% now) pulleys? anyone now when there's going to be another pulley party here in So. Cal.? Thanks.

 

This is Rand'y take:
Here are the issues:

Water pump
Supercharger redline
Supercharger efficiency limit
Thermal loading
Gearbox limitation

To address these concerns, at Webb Motorsports, we did dyno testing with A/F ratios and CHT readings, we did track testing with temp readings, we did high ambient heat endurance testing, and we had several engineering discussions with multiple disciplines.

It is the nature of the aftermarket to push limits, but the mods I do here are safe, and proven as such. We came up with the 15% pulley being safe through exhaustive testing, and found that higher percentage reductions weren't worth the potential cost.

Another area that isn't being addressed all that much is the effect of getting that additional boost. The Roots type supercharger is not designed for high boost applications because of the poor adiabatic efficiency at those levels. You get to the point of diminishing returns - the boost goes up, but the heat goes up, and they are not linear. Once you spin the supercharger past a certain point, the heat generated is so great that it would not be worth any potential gains you may see in boost.

The other thing not mentioned all that often is the gearbox. Sure, you could make lower RPM torque sooner, at the sacrifice of your overall numbers due to the discussion above, but will your gearbox be happy with it? The Getrag guys have told me no (well, actually they were Ford guys familiar with the Getrag because of their involvement with the SVT Focus project). The pinion would not be in a warm and fuzzy place with the increase in torque from say a 19% reduction.

I'd say BMW agrees with all of our research here as well based on the JCW kit for the S. You see, we had the 15% reduction pulley a few months before anyone had their hands on the JCW kit. The 15% pulley is a 2.18" diameter. Once the JCW kit came, we found that it was a 2.21" diameter pulley - again reaffirming that we had done the correct homework.

You can do a 17 or even 19% pulley for sure, and they may even make a bit more power in some areas, but you will also be past the design criteria of the components you are working with. Again that is the nature of the aftermarket, but if you haven't yet driven a car with a 15% pulley, I'd suggest that before you take the risk of a higher reduction - I haven't had a dissatisfied customer yet!

Hope that helps.

Randy

 

...and followed by:

I have done testing and am not happy with the 17% option. I highly doubt the claims of 15 more wheel hp for a 19% over a 15% - based on the increased thermal load on the system, the gain is just not linear. The more boost you make, the more heat you make and it is exponential. I think we may be talking about an additional 5 hp with the 19% and another 2 or 3 with a 17%. For the love of all that's good - you only make 15hp or so with the 15% over stock, so it makes no sense at all that you would see another 15hp from 4% more reduction especially with the decrease in adiabatic efficiency.

The rev limiter really depends on the track and how comfortable you are with the car. I have over 7000 track miles on the MCS and bounce off the limiter at certain points on the track to save the time of shifting for a split second more of acceleration. I bounce off it at every autocross just about. Some tracks, the shift points are just so that you want to short shift to keep the car from becoming unsettled at another point - such must be Kevin's case.

Hope that helps!

Randy

Read the whole thread here: http://www.webbmotorsports.com/forum...er=asc&start=0


That being said, I love my 19%.

 

I'm not really sure then what the answer is... it seems that the 2 posts from Randy are different wherein the 1st says that he wouldn't recommend it and then the 2nd says that it all depends.... I guess my question now is, does Randy have a 19% pulley installed now? if I was to install a reduction pulley at one of his parties, without any other mods on my car, what would he recommend? 15, 17 or 19%..... sorry if I'm dragging this one... just want to make sure before I take the plunge so I won't have to repeat the process....

 

I do believe that Randy will only recommend the 15%. 'Til his dying breath "15%".

 

Well when I read it, it sure sounded like he was poo-pooing reductions other than the 15% in BOTH posts.....

That said, I too, love my 19%. And I had the 15% for 15k miles before I switched. Night and day. But hey, the 15% is cool too.

 

If in doubt just call him.

I've talked to him about this issue and options on pulleys.
My feeling is that he chooses to be conservative and recommend that combination which at this time appears the most tested and reliable which is a 15% reduction pulley with ECU upgrade chip or similar if you want to be complete.

It's an investment (about $1000-1100) but will perform very well indeed and you don't have to worry about it.
If you requested other pulley sizes at one of his pulley parties I think he would install it on request.

 

Love my 19%. I too bump the rev limiter under several circumstances, but overall spend so much more time at lower rpm's that I'm putting my money (literally) on that side effect of the 19% offsetting some of the aforementioned disadvantages such as increased supercharger revs, increased heat, water pump concerns, etc.

Yes I do think you reach a point of dimininshing returns and the 19% is at that point IMO. The gains at high rpm are relatively less than the gains at lower rpm. In effect my power band is 2200 thru 6000. The gearing allows me to use this powerband effectively for the types of driving I do, for the most part.

Just waiting for him to blow!

Oh yeah, if you're in my area I'll install a pulley for $200 any day. Just kidding, I'm not a mechanic nor do I want to do this. Just saying there's really not much to it.:smile: