Zetetic

The underdrive pulley was all the rage about 10 years ago, is there any detriment to applying this concept in the current 2.0 TFSI engines? What might be the real HP gain?

April

I am not so sure there would be much to gain - typically it's 5 to 15 hp, right?

Remember, the steering is already run electrically, and not directly off the accessory drive. Which brings up the question of how and when you drive and the size of the replacement pulley with regards to charging. Short trips, lots of a/c, night running, all might mean the car cannot keep a charge.

If you live in a warm climate, you may need to find a cooler thermostat to keep the engine in the optimal range - especially if driving in stop and go.

The a/c in the car is already marginal on hot days, so expect it to take longer to cool down the car, and it may not get as cool no matter how long it runs.

Finally, the largest question is whether the A3 uses a harmonic damper, and if the underdrive pulley would be correctly designed to mimic those frequencies so as to reduce the chance of engine damage. I know the engine has internal balancers, but not so sure if there is more. See the link for more info.<ul><li><a href="http://www.dinancars.com/bmw/technial-info/the-dangers-of-power-pulleys-and-understanding-the-harmonic-damper">http://www.dinancars.com/bmw/technial-info/the-dangers-of-power-pulleys-and-understanding-the-harmonic-damper</a</li></ul>

Zetetic

Thanks... This may have fallen out of favor with newer engine designs.

MShields

In the 1950's British sports car engines would 1) spin the guts out of their generators, and 2) suffer from cavitation of the water being pulled past the impeller (very not good) once stiffer valve springs and bigger cams were installed. The solution was to slow these parts back down to their design speeds, while raising the engine rev limit 1500 or 2000 rpm. This was the sole engineering intent of the original pulley kits.

Usually such modifications are not harmful, nor they provide meaningful power gains.

There are a few horror stories like Honda Civic's breaking crankshafts with the replacement of their harmonic balancer front pulley by solid pulley kits or some other engines tendency to jump camshaft timing.

Nice way to sell parts for some rather unscrupulous aftermarket companies, or at the minimum, plenty ignorant. But, the long view is sort of a case somewhere between "if it ain't broke don't fix it" and "leave a sleeping dog lay" IMHO.

Zetetic

"leave a sleeping dog lay" ? Do you also apply that to ECU chip tuning?

MShields

I stated there are often some real world engineering reasons to keep the stock harmonic balancer pulley found on almost all engines. Second, and perhaps more significant to my understanding, is that exponential increases in power give roughly linear reductions acceleration times. So, let's take a look at chip tuning with this fact in mind.

A simple 4-10hp increase in power, usually much less in the normal engine operation speeds, provides an insignificant improvement in performance. In the same vein, chipping modern, normally aspirated engines might yield 2-4% gains in power, and usually just peak power, unless the factory setup was completely wrong. Try chipping or changing cams on a Honda V-tech motor. Usually there will be a power loss in torque for gain in HP or the other way around. So, yes, when it comes to normally aspirated engines I would have to apply my remarks to chip tuning as well. These are pretty lightweight solutions that imply an incomplete appreciation of the laws of physics. Desire without focus tends to dissipation.

It is with turbo motors that chipping comes into its limelight. This is because the motor is basically just a hot gas generator for the turbocharger. We are not chip tuning the engine; we are chip tuning the turbocharger. Because of the dynamics involved a qualified tuner is able to make significant, and that is the operative word, improvements to acceleration.

I am a big fan of tuning turbo motors by chipping them (from a known professional or expert source, please). Besides chipping them, we can also increase the injector size, the turbo size, install larger intercoolers and other internal modifications in pursuit of the exponential power requirements of improved acceleration. Real improvement is possible all because we are tuning a turbine, not a stand-alone internal combustion engine.

A second aspect of turbo motors besides the high level of horsepower possible for very little work, is the far better midrange torque. In effect there is power everywhere in each gear. It is the area under the power curve that represents the power, so a 30-degree upward slope showing increasing power to a 300hp peak of a typical normally aspirated engine is not the power of a 20 degree or even flat power curve show by a turbo motor. These turbo motors are much more "driveble". This better torque is particularly attractive result of chip tuning because we often are peak power limited by the size of the turbo and/or fuel injectors.

That is why it is a no brainer that the 2.0TFSI engine goes to ~250hp and ~300ft-lbs torque when chipped. My 1995 Subaru WRX-RA from the factory is 275hp and 260 ft-lbs torque. The Audi has a longer stroke, a much higher compression ratio and a smaller turbo, so it makes more torque down low, but less horsepower on top. Change to the S3 with larger turbo and injectors, thus the horsepower climbs to 280-300+. So, chip tuning is a matter of finding the practical, safe limit for each engine/turbo combination or making modifications to limitations of an `as delivered' motor such as going to larger turbochargers, intercoolers and injectors, while keeping all modifications in balance with each other and still maintain long term reliability. However, long term reliability is as much a function of rpm and the percentage of full power used as it is the actual full power potential any motor. An easy driver will get far greater life from either setup.

As a final thought, we will possibly see a pulley kit offered for the new 3.0tfsi engine for the A6 and S4, so as to increase the supercharger pressure, but this will a touchy business for the average tuner due to the limited injector pulse width of the stock injectors. Unless Audi offers larger flow injectors, again only modest gains are possible. This is because the turbocharger is a highly non-linear pressure device that allows us to make extra power down low and taper power off up high if necessary. The supercharger is pretty much a linear pressure unit, making for some limitations on what you can and cannot do in this regard. But, that is a discussion for another time.