Home > Cars, Jeff > The new turbos, part 1

The new turbos, part 1

I got new turbos for the S4.  The B5 S4 (2000-2002) came stock with a 2.7L bi-turbo (that’s German for twin turbo) V6 engine rated at 250 hp and 258 ft-lbs of torque.  In typical European fashion, the “peak” torque is available over a wide RPM range, like from 1850 rpm to 5500 rpm.  This is accomplished by truncating the boost curve so that the manifold pressure stays constant over a wide RPM range.  Lost yet?  An engine makes torque by exerting a force on a rod that turns a crank.  The “force” is the expansion of combustion air, heating from the burning fuel, acting on the piston, pushing on the connecting rod, and turning the crank.  It’s pretty much what you see on an old-fashioned steam locomotive, except the on the train the crank is integral to the drive wheel.  If you want more torque, you need a bigger engine so more air is acting on the piston(s).  The airflow into the engine is directly proportional to manifold pressure on a forced-induction engine, and torque is directly proportional to airflow, so a flat boost curve makes a flat torque curve.  One of the easiest (and often first) modifications to an engine with this behavior is to remove the “artificial” limit on the manifold pressure by reprogramming the ECU (or removing the ECU control of waste gate duty cycle and putting a manual boost controller in its place if you are a caveman – Hi Alex!)

Technically not fair; Alex’s hair is spikey.

This procedure allows the turbo to provide a higher peak manifold pressure, with a natural increase and decrease on either side.  The ramp-up in manifold pressure comes from the time it takes for the compressor to spin up to a useful rpm (between 100,000 and 200,000 rpm – hey, these things take time!), which is usually mitigated as much as possible by reducing intake restriction and optimizing turbo sizing and construction, and the taper at higher RPMs is the result of the engine sucking in more air than the turbo can compress (the manifold pressure or boost is a pressure, which corresponds to a volume in our fixed-volume intake systems, and at higher rpms the engine is consuming more air than can be supplied by the turbo at that manifold pressure, thus pulling the manifold pressure down).  The spool-up and taper down usually come from the physical limitations of the turbo-engine combination, although some ECU programs control the taper to protect different components.

Here’s a before-and-after dyno (at the wheels) of my chip tuning.  Note that wheel HP on a stock S4 is registering at 171 on this dyno, which is 250 hp crank????

Are there any downsides to raising the manifold pressure on an otherwise-stock engine?  Potentially, yes.  Like just about any other “performance” modification you can do to a car, modifying only one component in a system stresses out the other components in the system.  Removing the artificial cap on boost allows/commands the turbo to spin faster.  A good ECU program will prevent the turbo from rotating too fast and killing itself (and a manual boost controller may or may not, depending on how ham-fisted the guy setting it is – Hi again Alex!), but increasing the maximum speed of the turbo may cause it to wear out faster.  This brings me to the title of this article.  No, I didn’t blow my stock turbos, but I was concerned that I could, so when a cheap replacement came along, I picked it up.

K03 replacements: K03-16 and K03-17

These K03s are the stock size and reportedly have about 60,000 miles on them.  New they’re about $2000.00 for the pair, and I paid $300 shipped.  They have a little shaft play (heh heh) in the lateral and axial directions (heh heh…huh?)  Unfortunately, replacing turbos on the B5 S4 requires removing the engine.  This is nuts and bolts work, but it’s more than most owners will do in their garage.  So when the stock turbos die either through owner neglect, mis-use, foreign object damage (FOD), or otherwise, you start calling the shops and checking the forums.

Naturally, the forums are full of enthusiasts and they say “It’s going to cost you $1000-1800 to do the work, so why put the same-size turbos back in, and worse, why put used ones in?  The clock’s ticking on those things and you’ll just be pulling the motor in a year; don’t save a few bucks now just to have to go through the exercise again in a year or two.”  Point taken.

Like I said, I haven’t blown my stock turbos yet, so I set my new used K03s on my desk and didn’t think much about it, until I saw a post from this guy  www.nogaroblue.com.  He was selling something.  Tune in next time to find out what.

– Jeff, overlapping data with inuendo since 1932.

Categories: Cars, Jeff
  1. Jeff
    March 30, 2011 at 7:02 am

    BTW, I’m selling those K03s if anyone wants them. Make offer. Lowball offers will be laughed-at and ignored.

  2. alexg57
    April 2, 2011 at 9:56 pm

    In my defense (twice), the stock ECU in my 1992 is the caveman here, not me. It’s not my fault it’s too old and busted (read: OBD-I) to be reprogrammed like the newer ones can. Also, I did my homework. Plenty of people have run my set up with no negative effects and I don’t anticipate I will, either.

  1. No trackbacks yet.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s

%d bloggers like this: