To Under-drive or Not to Under-drive... that is the question

[Rabid Panda]

So what do you think? Is an under-drive system an option for some track but mostly street driving?

[ZStig]

the gains are so minor, I wouldn't even bother

[Z4Murder]

i've been debating this for so long. it seems it does very little for our cars. maybe different for your car, im not sure. i know on a m, the consensus is that it is not worth it

[Rabid Panda]

I think that if you have an M...the gain would be negligible. The M is already a Tier-1 engine. I mean...BMW did everything the could to make that engine awesome. the 3.0 and the 2.5...are "dumbed down" a bit. So there is some room for improvement. Perhaps under-diving the lesser of the 3 engines would reveal more of a gain.
Coincidently, the only Dino i have seen, showed an 11hp gain and about the same in torque on the 3.0 engine. Not huge, but its something.

[mousitch]

I have a set and honestly it's probably not worth the gain.

[Z4-VILLA]

i say pointless between parts and labor on an M like 600 bucks....pointless...
with 6 bills i can get a JBR flywheel

[The HACK]

There. Is. No. Magical. Power.

Repeat after me.

I've taken the liberty to "bold" certain parts of my dissertation to make it easier for those of you with short attention spans to follow the discussion. And if you REALLY have no interest in reading what I have to say, skip to the part with the big words and start there. Not saying that most of you are simply too ignorant to understand the principles. Just saying that you only need to understand what you need to understand, and if you don't understand the rest, well, it doesn't matter anyway. But if you chose to read through the entire post there are probably quite a few nuggets to discover.

Under-drive pulleys' work by making the ratio between the main crank pulley and the accessories it's trying to drive smaller therefore freeing up power that's used to drive the accessories. Depending on the pulley set, you can either just under-drive one accessory, like power steering, or you can under-drive ALL the accessories like water pump, alternator, and AC (if you under-drive AC you need 2 sets of pulley's and belts).

I've only been working in the automotive aftermarket industry for a couple of years. What I can tell you is this. OEM equipment are engineered for durability, comfort, and compromise. While it is entirely true that OEM engineering resources are immense compared to what aftermarket companies can provide, a lot of that engineering resources goes into making sure the parts last past the warranty period, provides a certain level of "comfort" in less NVH levels, and COMPROMISES are often made in performance in order to achieve said goal.

So in the area of accessory pulleys, the power steering pump pulley is designed to provide MORE cycles to pump more steering hydraulic fluid to make it easier to turn the steering wheel. Or the alternator pulleys are designed to pump more often than needed to make sure the alternator is providing enough juice to charge battery AND provide power for the slew of electronics in the car, and drive powerful speakers, and power-up high wattage head-lamps and "angel" lights. And the water pump must be designed to pump at higher than nominal rate in order to make sure that there's a decent safety margin for coolant flow.

By "under-driving" each one of these pulleys you in essence give up certain compromises to gain more power. On the power steering pump pulley you may be giving up some additional power-steering boost at low speeds (since at higher speeds there's relatively little steering boost anyway) for a couple of HP. Or you're willing to give up alternator charging current/voltage since you don't listen to music super loud and never have all your lights, like head-lights, DRL, angel lights, fog lights, rear fog lights all on at the same time, while charging your iPhone with your passenger plugged into her laptop to download pr0n, then you can also under-drive the alternator pulley to get back a few more horses. And since BMW engineers make the mechanical water pump flow at (number grabbed out of my @ss) 20% over nominal rate, by under-driving that pump you can again gain back a few more horses*.

*Case in point, BMW figured out that by using the alternator's out-put ONLY to drive the water pump and power steering pump they can free up 7-8 hp on the naturally aspirated 3.0 liter engine...N52s on Z4s only come equipped with electronic water pumps and power steering pumps.

"So...Great! Let's all go out and get under-drive pulleys!" you say.

Not so fast.

There are certain draw-backs with under-driving a set of pulleys. First and foremost, you have to figure out which one of the accessories CAN be under-driven. Take water pump for example. Sure, you can under-drive the water pump to get some power back, but at the risk of lowering the flow rate your engine may suffer some issues with heat. Or the alternator. Fewer turns per engine RPM means less charge/current being generated and may result in the battery not charging properly. Second, you have to have a longer belt of a specific size, which means every 60,000 miles you'll have to go hunt down that specific belt size, rather than buy OEM belts which are typically very high in quality and durable, and readily available. A small inconvenience but trust me, if and when you need to replace the belt it's a real hassle**.

**At 45,000 miles a friend's aftermarket pulley had a set of frayed belt and snapped at the track. Fortunately he shut the car down almost immediately and was able to be towed back into the paddock. Unfortunately NO ONE has a belt in the right length since it's aftermarket so he was "stranded" for the day, and had to basically find a belt that is a few mm too short to get him home, and hunt down the right belt in the next few days. Sure he found the right belt, but it was still one of those freaky incidences where everything that could go wrong, had gone wrong that resulted in him losing nearly two whole days of track.

Lastly, while the inline 6 engines is one of the smoothest and most balanced crank design, SOME inline 6es from BMW does use the main crank pulley as a "harmonic balancer." Under-driving certain accessories means the main pulley will be turning at a different load and would result in a vibration in the main crank, which over time could be a huge issue.

So, if you're still reading at this point, and still pondering whether or not to get a set of under-drive pulley, you probably should have skipped everything above and start reading here instead. You have to keep in mind if the HP gain is worth SOME of the potential minor downfalls. For example, take a guy in club racing, if rules allow, it's a no brainer. A club racer would have upgraded cooling capacity, no need for additional alternator power, and little need for a lot of power steering assist. So a lower flow rate can be compensated with a Steward water pump and a racing thermostat, the alternator is a no-brainer, and most likely power steering isn't going to be a big issue and AC compressor probably would have been removed anyway. Then again if you're a club racer I probably don't have to explain ANY of this to you. If you're an adventurous, experienced DE guy, an alternator pulley and a water pump pulley will probably get you that few more horses to get by the Z06 with the lead foot and no skills on the straight (although SKILL is still the biggest determining factor), provided you upgrade your pump to a Steward race water pump that is capable of churning at a higher rate at a lower speed. If you're just driving your car from point A to point B during the week and enjoys a few canyon runs? The benefits are marginal at best.

But either way, if you're going to get the pulley set, make sure you talk to the people who make the pulleys, talk to their engineers (oh they'll love me for this. Sorry guys) and make sure the belts are a common length AND they've tested the system and made sure the pulley set does not interfere with the harmonic dampener's operation on the main crank pulley.

[The HACK]

Originally Posted by Z4-Villalona

i say pointless between parts and labor on an M like 600 bucks....pointless...
with 6 bills i can get a JBR flywheel

Holy crap $600 for parts and labor?

[mousitch]

Originally Posted by Z4-Villalona

i say pointless between parts and labor on an M like 600 bucks....pointless...
with 6 bills i can get a JBR flywheel

$250 for TMS pulleys and about an hour labor doing it yourself. Easy mod if you wish to go with them. I've done it twice and it takes about as much skill as putting in a set of plugs. Not saying it's worth it but only $250. This price are for the TMS pulleys not the AA ones. I stayed away from that set cause you change the crank pulley too. I didn't want to mess with that one. Looks like that could defiantly cause problems. To each their own. If I grow weary I can always sell the set and put the stock back on. They do use a longer belt that could be a issue as The Hack mentioned above.

[ZStig]

Originally Posted by The HACK

There. Is. No. Magical. Power.

Repeat after me.

I've taken the liberty to "bold" certain parts of my dissertation to make it easier for those of you with short attention spans to follow the discussion. And if you REALLY have no interest in reading what I have to say, skip to the part with the big words and start there. Not saying that most of you are simply too ignorant to understand the principles. Just saying that you only need to understand what you need to understand, and if you don't understand the rest, well, it doesn't matter anyway. But if you chose to read through the entire post there are probably quite a few nuggets to discover.

Under-drive pulleys' work by making the ratio between the main crank pulley and the accessories it's trying to drive smaller therefore freeing up power that's used to drive the accessories. Depending on the pulley set, you can either just under-drive one accessory, like power steering, or you can under-drive ALL the accessories like water pump, alternator, and AC (if you under-drive AC you need 2 sets of pulley's and belts).

I've only been working in the automotive aftermarket industry for a couple of years. What I can tell you is this. OEM equipment are engineered for durability, comfort, and compromise. While it is entirely true that OEM engineering resources are immense compared to what aftermarket companies can provide, a lot of that engineering resources goes into making sure the parts last past the warranty period, provides a certain level of "comfort" in less NVH levels, and COMPROMISES are often made in performance in order to achieve said goal.

So in the area of accessory pulleys, the power steering pump pulley is designed to provide MORE cycles to pump more steering hydraulic fluid to make it easier to turn the steering wheel. Or the alternator pulleys are designed to pump more often than needed to make sure the alternator is providing enough juice to charge battery AND provide power for the slew of electronics in the car, and drive powerful speakers, and power-up high wattage head-lamps and "angel" lights. And the water pump must be designed to pump at higher than nominal rate in order to make sure that there's a decent safety margin for coolant flow.

By "under-driving" each one of these pulleys you in essence give up certain compromises to gain more power. On the power steering pump pulley you may be giving up some additional power-steering boost at low speeds (since at higher speeds there's relatively little steering boost anyway) for a couple of HP. Or you're willing to give up alternator charging current/voltage since you don't listen to music super loud and never have all your lights, like head-lights, DRL, angel lights, fog lights, rear fog lights all on at the same time, while charging your iPhone with your passenger plugged into her laptop to download pr0n, then you can also under-drive the alternator pulley to get back a few more horses. And since BMW engineers make the mechanical water pump flow at (number grabbed out of my @ss) 20% over nominal rate, by under-driving that pump you can again gain back a few more horses*.

*Case in point, BMW figured out that by using the alternator's out-put ONLY to drive the water pump and power steering pump they can free up 7-8 hp on the naturally aspirated 3.0 liter engine...N52s on Z4s only come equipped with electronic water pumps and power steering pumps.

"So...Great! Let's all go out and get under-drive pulleys!" you say.

Not so fast.

There are certain draw-backs with under-driving a set of pulleys. First and foremost, you have to figure out which one of the accessories CAN be under-driven. Take water pump for example. Sure, you can under-drive the water pump to get some power back, but at the risk of lowering the flow rate your engine may suffer some issues with heat. Or the alternator. Fewer turns per engine RPM means less charge/current being generated and may result in the battery not charging properly. Second, you have to have a longer belt of a specific size, which means every 60,000 miles you'll have to go hunt down that specific belt size, rather than buy OEM belts which are typically very high in quality and durable, and readily available. A small inconvenience but trust me, if and when you need to replace the belt it's a real hassle**.

**At 45,000 miles a friend's aftermarket pulley had a set of frayed belt and snapped at the track. Fortunately he shut the car down almost immediately and was able to be towed back into the paddock. Unfortunately NO ONE has a belt in the right length since it's aftermarket so he was "stranded" for the day, and had to basically find a belt that is a few mm too short to get him home, and hunt down the right belt in the next few days. Sure he found the right belt, but it was still one of those freaky incidences where everything that could go wrong, had gone wrong that resulted in him losing nearly two whole days of track.

Lastly, while the inline 6 engines is one of the smoothest and most balanced crank design, SOME inline 6es from BMW does use the main crank pulley as a "harmonic balancer." Under-driving certain accessories means the main pulley will be turning at a different load and would result in a vibration in the main crank, which over time could be a huge issue.

So, if you're still reading at this point, and still pondering whether or not to get a set of under-drive pulley, you probably should have skipped everything above and start reading here instead. You have to keep in mind if the HP gain is worth SOME of the potential minor downfalls. For example, take a guy in club racing, if rules allow, it's a no brainer. A club racer would have upgraded cooling capacity, no need for additional alternator power, and little need for a lot of power steering assist. So a lower flow rate can be compensated with a Steward water pump and a racing thermostat, the alternator is a no-brainer, and most likely power steering isn't going to be a big issue and AC compressor probably would have been removed anyway. Then again if you're a club racer I probably don't have to explain ANY of this to you. If you're an adventurous, experienced DE guy, an alternator pulley and a water pump pulley will probably get you that few more horses to get by the Z06 with the lead foot and no skills on the straight (although SKILL is still the biggest determining factor), provided you upgrade your pump to a Steward race water pump that is capable of churning at a higher rate at a lower speed. If you're just driving your car from point A to point B during the week and enjoys a few canyon runs? The benefits are marginal at best.

But either way, if you're going to get the pulley set, make sure you talk to the people who make the pulleys, talk to their engineers (oh they'll love me for this. Sorry guys) and make sure the belts are a common length AND they've tested the system and made sure the pulley set does not interfere with the harmonic dampener's operation on the main crank pulley.

I'm gonna go get my bookmark so I can finish reading this later. lol

[dazed^]

Another insightful post from The Hack-- thanks for that!

[roo97ss]

as everyone has said it's just a few hp. yes on a 100 hp car, 5 hp is 5% but on a 300 hp those 5 hp are only 1.5%, very minimal gain to say the least.

And not sure how this is in the s54, but my s52 m roadster with u/d at an idle, the a/c wouldn't be cold, so on those hot days your sitting at a light revving to 900-1000 rpm just to keep the cold air blowing. I doubt it's that big a compromise in the z4, but still it's just a few HP. remember, the biggest drag is the water pump, removing gained 7, so by decreasing 10-20% you're getting 1-1.5 there, and another 1 or so from each accessory.

For 90 bucks which is i think what it cost me a few years ago on the s52 maybe worth it; here at 250-300 hardly.

[pokeybritches]

I think the big takeaway is this: make sure you get one that's been properly engineered.

I wouldn't do this unless I HAD to do it. A poorly engineered set can have dramatic consequences, unlike some other mods out there.

[Rabid Panda]

Yep.. Nixxing the UD pulleys. Not worth for me... But I would like to squeeze a few more ponies out of her. Thanks for all the input!!

[NickyC]

To me, pulleys have always been the ricer mod that comes after intake/exhaust/plugs/and nology hot wires lol.

I remember back in the day when I had my single turbo Supra, and guys would run down their mod list including Nology hotwires and pulleys, saying it gained them "at least 25hp".



Oh, and thanks for the great info as always Hack.