Underdrive crank pulley

[Captain]

Has anyone fitted an underdrive crank pulley to there car, if so was it worth it, in terms of performance gains achieved. I've been thinking about doing it for a while, thought I would rather get some feedback from someone that's done it, just to be sure

[Southernboy]

If you change the crank pulley, it will require a change to all the pulleys to some extent, otherwise the drive belt won't fit. ie. you will need a set of pulleys...Have a look on Turner Sport... $250.00 for a complete set + drive belt.

[Captain]

That's why I'm trying to find out if its worth doing.

Sent from my HTC Desire

[Southernboy]

There seems to be really great benefits with added HP of up to 30% in some cases. The only downside seems to be that the ammount of power (electrical) you use while driving. eg. If you have headlights, Aircon, radio, fan, lights, spots, and any other appliances running at the same time, you may find that the battery is drained quicker than the alternator can charge it. The other concern is overheating, due to the slower rotational speed of the waterpump. Other than that, there are no downsides.
Because the rotational speed of things like alternator, aircon pulley, & waterpump are slower, you get a longer life from bearings, drivebelts shafts etc. so the general life of that type of thing is extended.
My point about Turner motor Sport, is that they have provided a "balance" between added power and operational stability. You could go to Cosmo Racing and buy a crank pulley for $75, but that's all you're getting. The other pulleys on the Waterpump, aircon, and alternator remain as is, or you will need to consider them at some point.
Must say, the Cosmo racing billet ally pully is very cool, lightweight etc. I would email them and ask a few questions regarding the other pulleys, belt length requirements etc. I would hazard that Turner supplies a slightly smaller waterpump pulley and also a slightly smaller alternator pulley to compensate the loss of rotational speed.
Alternative is to go for the Cosmo unit, and be aware of the electrical power consumption at any one time. Also, it might be advisable to get an upgraded electric fan switch fro Bavarian Auto. This switch fits to the radiator, but has a reduced temp resistor. It will switch the electric fan on at a lower temp than the OEM one. Again, it will be consuming electrical power when the fan kicks in, so you would need to drive by the light of the moon or on low beam if it's a really hot night.
Or, toss the roof off, and let the girlfeind get really sweaty....
If you go for it, you can also fit an upgraded thermostat - also from Bavauto - which opens at a lower temp, this will ensure a cooler running temp before the electric fan is called for.... I think all in all it might be a very cool upgrade to achieve another 20HP at the wheels.

[Captain]

I've done a lot of research regarding the above, but always better to get feedback from someone that's done it. As far as overheating and electrical problems are concerned, I don't think there would be a problem, unless you were driving in traffic all the time and at night, which I don't as the zed is my weekend car.

[markrnorton]

I dont think you'll see much of a gain, maybe 1-2hp. you would notice improved pick-up off the throttle.
Main reason for using underdrive is in a race/high rpm situation so you dont cook the alternator.

not sure where the 30% figure come from in this thread.

[Brian H]

Well every day is a school day, never knew these existed, a quick google and now I know what you are all talking about

[Deano1712]

Agree with Mark you will get a handfull of extra hp max. Its just a change to the crank pulley and a shorter belt.
I considered it on my LS engine but it was expensive for the power gained and some bad reports around of unreliability of aftermarket pulleys. They usually have a damper inbuilt so have to be well engineered and manufactured.

Sent from my GT-I9100 using Tapatalk 2

[Captain]

Thanks to all for the replies, it seems that it's not worth doing for a few HP

[Southernboy]

From what I read on the web, it seems one can achieve as much as 15HP at the wheels - with a "kit". I would assume from that they mean all the pulleys require changing.

[markrnorton]

From what I read on the web, it seems one can achieve as much as 15HP at the wheels - with a "kit". I would assume from that they mean all the pulleys require changing.

I dont believe you'd get anywhere near that figure. Anyhow, if you change all the pulleys you're not underdriving, your just changing the mass of the pulleys for the lighter ones.

2 different things, underdriving and lighter pulleys, however most underdrive crank pulleys are billet alloy and so lighter.

[Gazza]

Underdrive pulleys come into play when you add Superchargers, enabling the SC to spin up quicker.

[Southernboy]

I'm more confused now. From my understanding, the underdrive crank pulley will result in a slower rotating alternator, AC, power steering pump and waterpump. Therefore the ammount of energy saved is what you gain. If you have a look at the Turner Motorsport website, they sell a "kit" of 3 pulleys specific for the Z3. So, it means they're changing the crank and 2 other pulleys. I don't think the weight of the OEM pulleys vs a billet pulley will make any significant HP gains, but certainly the fact that less power is used to rotate the accessories will show a power gain to the wheels. Whether it's 15HP or 3HP is not the issue, any power to the wheels is better than turning an AC pump etc. Also fuel economy will be slightly enhanced etc. If one could absolutely say that 15HP would be the gain, it would be well worth the mod, but for 2 or 3 HP it doesn't warrant the cost of the parts. I guess if one were a racer, and you wanted to cut 1/1000th second of your time, it would be worth the expense, but Turner's price of $250 for the kit is a tad silly to maybe gain 2 or 3 HP.

[Gazza]

On further examination I am totally wrong

You can use different size pullys on SC's to give more or less boost.

[Southernboy]

The way I understand it, if you replace the OEM pulley with a slightly smaller diameter, the result will be that all the other pulleys on that drivebelt will turn slower. If it were a larger pulley, it would increase the speed of the other pulleys.
Consider the gears on a bicycle, the faster you wish to go, the smaller the sprocket on the rear axle. If this was reversed, the front cog would need to increase in size to drive the rear wheel faster. On a bicycle it makes weight sense to reduce the cogs on the rear axle. On an underdrive pulley, it will need to be smaller than OEM to reduce the rotations of all the accessories on the drive belt circuit. I would say it would be considerate to reduce the crank pulley, and proportionately increase the alternator pulley, the other pulleys can remain the same without any deficit in performance. To compensate the waterpump, simply fit a lower temp thrmostat and radiator sensor switch for the electric fan, or even fit an electric fan which will will assume total responsibility for engine temp maintenance, and dump the viscous fan which is a huge drag on engine power. At the moment, in 34C. daytime temp, I'm driving without a viscous fan on my car. I switch the AC fan on everytime I stop at a light, or find myself in very slow moving traffic. The guage remains steady in the top / middle position. But the free revving motor really loves the extra power. I deally I would like a full electric cooling system, and it's something I will look into with the good advice of my mechanic.
The other item which is a huge drag is the air conditioner pump. Unfortunately at 34.C I'm not likely to surrender that blessing.
I have just replaced my waterpump for one with metal impellers, however while I was looking at the options, I was linked to Turner Motorsport, and they have a "super-pump" to fit the Z3 / (and other BMW's). It seems from what I read there, that this pump has the ability to pump greater volumes than the standard impeller type which I fitted. It's also considerably more expensive too. But if one were fitting an underdrive pulley, a slower rotational speed on the waterpump could be compensated by fitting such a waterpump and still achieve OEM or better water volumes shifted in the cooling system.
Looking at the "kit" offered by Turner Motorsport, and one sees 3 pulleys in the photograph, I can only surmise that the third pulley is required for the power steering pump. It would certainly need a minimum pressure for the steering pump to operate efficiently, and allow the driver fair control over the vehicle.
Personally, I reckon the gain in power from a fully electric cooling fan and the removal of the viscous clutch is less costly, and more easily achieved by the average car owner with common sense and a spanner or two. I would hazard that one of the newer cars which already operate on that basis will have a fan size which will fit into a Z3. It may require some new mounting adaptation, and possibly some wiring as well as the respective temp sensor for the radiator to activate the switch on the fan motor. I have seen these on some of the French cars and some of the newer Mercs. The motor has an integrated sensor which receives info from the temp sensor, and instructs the electric motor to spin at differing speeds and durations as required to achieve a steady operating temperature.
I digress....Underpulleys are cool! But expensive too. If you want power....get a turbocharger or supercharger fitted. Electric cooling will free up horses easily and cheaply.

[markrnorton]

The way I understand it, if you replace the OEM pulley with a slightly smaller diameter, the result will be that all the other pulleys on that drivebelt will turn slower. If it were a larger pulley, it would increase the speed of the other pulleys.
Consider the gears on a bicycle, the faster you wish to go, the smaller the sprocket on the rear axle. If this was reversed, the front cog would need to increase in size to drive the rear wheel faster. On a bicycle it makes weight sense to reduce the cogs on the rear axle. On an underdrive pulley, it will need to be smaller than OEM to reduce the rotations of all the accessories on the drive belt circuit. I would say it would be considerate to reduce the crank pulley, and proportionately increase the alternator pulley, the other pulleys can remain the same without any deficit in performance. To compensate the waterpump, simply fit a lower temp thrmostat and radiator sensor switch for the electric fan, or even fit an electric fan which will will assume total responsibility for engine temp maintenance, and dump the viscous fan which is a huge drag on engine power. At the moment, in 34C. daytime temp, I'm driving without a viscous fan on my car. I switch the AC fan on everytime I stop at a light, or find myself in very slow moving traffic. The guage remains steady in the top / middle position. But the free revving motor really loves the extra power. I deally I would like a full electric cooling system, and it's something I will look into with the good advice of my mechanic.
The other item which is a huge drag is the air conditioner pump. Unfortunately at 34.C I'm not likely to surrender that blessing.
I have just replaced my waterpump for one with metal impellers, however while I was looking at the options, I was linked to Turner Motorsport, and they have a "super-pump" to fit the Z3 / (and other BMW's). It seems from what I read there, that this pump has the ability to pump greater volumes than the standard impeller type which I fitted. It's also considerably more expensive too. But if one were fitting an underdrive pulley, a slower rotational speed on the waterpump could be compensated by fitting such a waterpump and still achieve OEM or better water volumes shifted in the cooling system.
Looking at the "kit" offered by Turner Motorsport, and one sees 3 pulleys in the photograph, I can only surmise that the third pulley is required for the power steering pump. It would certainly need a minimum pressure for the steering pump to operate efficiently, and allow the driver fair control over the vehicle.
Personally, I reckon the gain in power from a fully electric cooling fan and the removal of the viscous clutch is less costly, and more easily achieved by the average car owner with common sense and a spanner or two. I would hazard that one of the newer cars which already operate on that basis will have a fan size which will fit into a Z3. It may require some new mounting adaptation, and possibly some wiring as well as the respective temp sensor for the radiator to activate the switch on the fan motor. I have seen these on some of the French cars and some of the newer Mercs. The motor has an integrated sensor which receives info from the temp sensor, and instructs the electric motor to spin at differing speeds and durations as required to achieve a steady operating temperature.
I digress....Underpulleys are cool! But expensive too. If you want power....get a turbocharger or supercharger fitted. Electric cooling will free up horses easily and cheaply.

You are missing the point of underdriving , its in the title, underdrive. you do it to underdrive the alternator in high RPM situations so it doesnt cook, the bi-product is a lighter crank pulley which aids engine pick-up

Nuff said