Does changing the pulley, either 15% or 17% put more stress on the engine?

 

I'm not sure that I am quite the guy to answer this but here goes (please correct me if I'm wrong). Getting a 15,17, or 19% pulley will put stress on the supercharger in turn wearing it out quicker. But by the looks of it everyone seems to have at least a 15% so it cant be that harmful. If it helps any I plan on putting a 15% on mine, maybe a 17% if 'm feeling lucky.

 

I went for 17% because it gives you more boost so if you are already wearing the SC down then why not give it even more boost while being just as safe.

 

not only are you spinning the supercharger faster you are also spinning all your accessories fast as well. so in turn yes your are putting more stress on the engine.

 

I'm having my 17% installed on Wednesday. I cant wait.

 

I've had my 16% installed for a while now...probably over 20k miles. No issues, the pulleys are safe to put on, but yes I'm sure there is added stress...but probably not enough to justify anything major.

-Cody

 

This is incorrect. It is also the reason underdrive crank pulleys are bad, because they DO spin all the accessories faster.

OP, a reduced pulley will DEFINITELY put more stress on the engine: the smaller radius on the pulley leads to faster rotation which builds more pressure in the intake manifold and therefore gives the engine more oxygen to make more power.

The question you should be asking is whether this added stress is anything to worry about, to which the answer is debatable. From an engineering standpoint, the increased rpm may or may not be out of it's design range (I would say it isn't), but I would say the service life of the SC would be diminished by a very small amount. How much is debatable, it could be anywhere from absolutely insignificant to something far more measurable as it depends on how the SC was designed. I would say if you're gonna go for it just get a 17%... you're going to be replacing the SC eventually anyways (again, I have no idea how long these last) and it's only 2% different from a 15% pulley.

This is not meant to scare you in any way, it's just supposed to make you become familiar with the costs associated with making your SC work harder. I would slap on a 17%, say a prayer to the MINI god that my SC doesn't blow significantly earlier than a normally equipped unit, and go out and have fun.

 

... er, that should be "function of engine RPM," not "factor".

Anywho, I'm totally not a tech person, but wouldn't engine RPM determine how "bad" a reduced-size SC pulley is on the supercharger and other engine components? In other words, is there going to be a lesser likelihood of doing damage to my engine if I generally operate in the 2500-3000 rpm range rather than higher up the RPM curve? So a smaller pulley reduction (e.g., 15%) would be safer for someone consistently running high RPM (e.g., on track), while 19% would be okay for someone who's car is "strictly street". (17% being something of a compromise between the two -- which is where I'm personally thinking of heading for my "strictly street" MINI, basically trying to eek out some additional low- to mid-RPM torque and hp.)

All this is just conjecture on my part, so I'd love confirmation or refutation by someone who actually knows what they're talking about.

 

The FACTORY JCW pulley is basically 15%...(ok some say 14.45% or so). Since the JCW is fully warrantied, the 15% should be VERY SAFE!!
The 17%...I have one...a bit more controversial, but a better around town/highway pulley....if you don't track it, and keep the rpm's below red-line, and don't run it on red-line for periods of time..., and change the belt once per year, I think after all my research, all should be ok. If you are worried, do the 15%, and then you can do like many people do, and get the 17% next year!!
The under-drive crank pulley with no harmonic damper (harmonic vibrations can destroy engine bearings, effectively destroying a motor worst case!!) IMO is asking for trouble.....Webb Motorsports has a great page under FAQ's on Pulleys...did testing on 15% and 19%, with some interesting results. He did not test under-drive crank pulleys, but there is much debate on the forums.