Weapon of Choice

With naturally aspirated motors, intake and exhaust systems are designed to flow as much air as possible. More air means more fuel can be burned, hence more power. But what about forced induction (turbo/supercharged) motors? We can get (to a point) all the air we want, just by changing the boost map in the chip. More boost= more air. However, more boost also equals higher charge tempratures, and that is what, in my opinion, is usually the limiting factor.

If an intake can reduce the intake charge temprature over the stock intake, then it's going to add more HP. Denser, cooler air also allows for less fuel to be used, and therefore by adding an intake, you're not going to *add* any additional strain to your fuel injectors either. Makes you wonder about the FMIC too doesn't it? How much "free" HP can be unlocked, just by cooling down your intake charge a few more degrees? Furthermore, how much more can you get out of your stock (K03) turbo? I'll bet we can get quite a bit more than we think. Especially if you run the car in fairly hot weather (90*+). I live in Las Vegas, so this is a prime consideration for me. I do wonder if the ECU could take into account pressure loss though, and make the turbo put out more boost to compensate. I mean, as long as the turbo isn't completely maxed out (and it isn't), I'm sure the ECU could 'see' the loss in pressure and increase the boost accordingly, wouldn't it?

As for exhausts- less back pressure means lower exhaust gas tempratures. This means less heat in the turbo and therefore cooler intake temps... back to the main point- cooler denser air! More HP! Magic! Hmm... exhaust system= cooler intake temps, that means even less fuel is needed, and even more HP is produced. Doesn't make sense does it? Unless you consider engine effeciency. More HP with less fuel is possible if the total efficiency of the entire system (the engine, in this case) increases.

Why not run 3" exhaust then? For super low EGT levels? Not enough backpressure means not enough low end torque. No free lunch!

I wonder if it's possible to be able to fit a FMIC that is actually *too* big for the stock turbo to the B6 A4, given the dimensional contraints of an aftermarket front airdam/spoiler? I doubt it.

What do you guys (and girl, April ) think?

mthill77

I don't believe a boost map works the way you think. Its not like you can simply turn up the boost with a switch in the software. More boost is made my transfeering more energy to the turbine impeller/shaft. This can be done by have the exhaust flow faster, of flow more mass. Both occur natually as rpms rise. Boost can be limited by the a computer controlled wastegate, but I doubt it is used in that manner. So, what is the limiting factor on boost? Well, there is EGT (exhaust gas temp), turbine shaft speed (maybe 150k-200k rpms), cylinder pressure (too much causes all kinds of problems), etc. Its never just one thing.

So if you could determine that the maximum safe boost for a given engine was say 30psi, you'd want to do what you could to get it there. Lowering EGTs with a free flowing exhaust, lower intake temps with large intercooler, change ignition timing to prevent detonation, etc can all help. Engine tuning has many variables and is NOT a simple task to explain...nevermind actually executing it.

Intercoolers: The intake air (ambient temp) is compressed by the turbine compressor...heating it significantly. The compressed charge passes through an intercooler, cooling it significantly before it enters the combustion chamber. The theoretical maximum for cooling the air for a 100% efficient intercooler would be to cool it back to ambient. Bigger intercoolers will generally cool more because the compressed air charge is in it longer. An example of a typical system would be 60F ambient compressed to 300F and then cooled to 110F in an intercooler. This is not data from an Audi, but that is the general idea.

Your statement about no free lunch is right on...everything you change has tradeoffs and limitations. Otherwise we'd all be driving around in 500hp A4s with 100% reliability.

Weapon of Choice

I didn't mean to say that you could just magically add/subtract boost at will, anywhere along the performance envelope, but what I am saying is that the stock K03 turbo isn't operating at maxium capacity in stock configuration. Truth be told, it's far from it. The motors can handle upwards of 21+ psi of boost, and stock they are recieving around 7-8 psi (.5 bar?)?

With chips producing 1 bar of boost with the stock turbo, adding an exhaust will drop the EGTs further, allowing for more boost to be added, without increasing the EGT beyond what it was without the aftermarket exhaust there. This means more power across the board, at safe levels, with the stock turbo, with the *stock injectors*.

This brings up the question: why arn't chips available with programs specifically designed to be used with aftermarket exhausts, intakes, intercoolers, etc.? I think that just because boot *could* be increased, due to the reduced EGTs, it isn't increased any more than with the stock exhaust, by an aftermarket chip. The boost maps remain static, and the true performance allowed by the aftermarket exhaust isn't seen because the chip isn't designed to take advantage of the additional performance available due to the reduced EGTs. Same thing goes for the intercooler or intake.

mthill77

Agreed. Perhaps they run into trouble with something other than EGT as the limiting factor.

Turbine speed?

Weapon of Choice

As far as the turbo running out of RPM range, I don't think that is the limiting factor. You can get 1.5 to almost 2 bar out of the K03, but the problem is that you severely heat up the charge air, so it quickly becomes self defeating to run it up that high. That's where a larger turbo comes in: higher boost at lower temps. Anyway, all I'm trying to say is that if you take an objective look at everything, it's suspicious as to how tuners are getting the power gains they are, and why they take the approaches they do.

GünterT

In addition to all the good points made so far...a higher level of performance will also require significant changes to pistons, rods, and/or crankshaft in order to keep your motor running as advertised.
It is one thing to run a 1.8T at 230+ hp but quite another to make the engine truly reliable without spending major $$$.
One of the common fantasies perpetuated by "performance" tuners is that a $500 chip plus a $700 exhaust will get you bags of power with no consequences.
There's more than a few clapped out S4's around to show how this ends up.
This doesn't even get into discussions about driveability...

Weapon of Choice

The 2002 A4's 1.8T has all forged internals, and because of things like the 5 valve heads, the motor can be made to perform at levels almost doubling stock horsepower, with no problems regarding the reliability and/or strength of the components. The transmission and clutch are a different story however.

GünterT

I did not realize that the 1.8T was built to such a high standard. My impression was that 230 - 240HP was about the limit for the stock internal components before problems would appear.
That's good to know. My wish list for my 2003 1.8T Avant is about 220HP with stock reliability. This seems to be easily attainable as long as the clutch and transmission can deal with the increase.

Yippers®

link but will hold for a while.
FWIW there are well over 100 longitudinal Stg III APR 1.8ts, with hundreds of thousands of miles collectively, running over 300lbft of torque as daily drivers. I have ~80k miles on my 99.5, over 50k of that with the Stage III kit.
FWIW .. the K03 starts to go outside of it's efficiency envelope well before 280 hp. The K04 probably around 260hp. You'll need to go bigger than that for reliable HP above 270.

Yippers®

changes if you add an aftermarket exhaust.
Part of the chips decision making is to analize IATs, knock, throttle position, boost pressure(for you DBW types), mass air flow, and O2 content in the exhaust stream . .among other items. Depending on the sensors inputs the ECU varies timing, throttle plate, N75 duty cycle, injector pulse width etc...
Anything that keeps knock down will result in keeping more of the timing the tuner has plugged into his maps. Lower EGTs will result in less fuel used to drop EGTs, etc.