[Doug007]

Originally Posted by shiv@vishnu

Correct, a good FMIC will keep both turbo set-ups (stock and upgraded) within the same ballpark in terms of IAT. So the big power gains, all other things equal (boost level, operating conditions, other mods) come from the upgraded turbos allowing exhaust gas to exit the turbine housing more readily. Think of an undersized/overspun turbo as a plug/restriction in the exahaust. There comes a point at which more boost in the intake manifold wont result in more power because flow through the engine is restricted on the outlet side.

So 20psi on the stock turbos may make 440whp while the same 20psi on upgraded turbos may be worth 525whp.

On the other hand, the STOCK turbos may make the same (or even more) power at lower boost/airflow levels where they operate very efficiently (say 7-12psi). Whereas upgraded turbos may have a slight (but measurable disadvantage) since their turbine side may not be operating within its peak efficiency range.

Thanks for the breakdown Shiv.

Makes sense that eventually the stock turbos are limiting on the exhaust side. The car has to be able to breathe in AND exhale!

While boost increases are proportional to compressor speed squared (and speed proportional to sqrt of boost), the flow is directly proportional to speed. So with a 50% increase in turbo speed you get 2.25x the boost but only 1.5x the flow. Eventually this catches up to you. (for those of you who want to know about the engineering check here...http://en.wikipedia.org/wiki/Affinity_laws)

[OpenFlash]

Originally Posted by themyst

you need a new start button too

very true. Now it just reads S T.

[R1000K3]

Originally Posted by shiv@vishnu

72k miles on my original turbos and around 200 passes at 20psi of boost. Of all the things that have needed to be replaced on my car (2 fuel pumps, 1 steering column, 1 water pump and a steering rack), the turbo and and engine have proven to be the most reliable.

Upgraded turbos, IMHO, should be considered as a performance upgrade. Not necessarily a reliability upgrade. In terms of failure rate, I'd my money on the stockers.

Good to hear and that is also my impression. STD turbos will take a lot of hard punishment before failing, which is proven on thousands of 335i, and the cost to replace them is very low compared to the total costs for the car during a couple of years.

[themyst]

Originally Posted by shiv@vishnu

Correct, a good FMIC will keep both turbo set-ups (stock and upgraded) within the same ballpark in terms of IAT. So the big power gains, all other things equal (boost level, operating conditions, other mods) come from the upgraded turbos allowing exhaust gas to exit the turbine housing more readily. Think of an undersized/overspun turbo as a plug/restriction in the exahaust. There comes a point at which more boost in the intake manifold wont result in more power because flow through the engine is restricted on the outlet side.

So 20psi on the stock turbos may make 440whp while the same 20psi on upgraded turbos may be worth 525whp.

On the other hand, the STOCK turbos may make the same (or even more) power at lower boost/airflow levels where they operate very efficiently (say 7-12psi). Whereas upgraded turbos may have a slight (but measurable disadvantage) since their turbine side may not be operating within its peak efficiency range.

So you are saying we will need to upgrade the exhaust side (midpipes, muffler) in order to reap the full benefits of increased boost? I am not worried about the cost associated with doing so, more along the lines of the increased noise driving me to the brink of insanity. I figured the gains associated with doing so are not worth the increased noise.

[jpsimon]

Originally Posted by themyst

So you are saying we will need to upgrade the exhaust side (midpipes, muffler) in order to reap the full benefits of increased boost? I am not worried about the cost associated with doing so, more along the lines of the increased noise driving me to the brink of insanity. I figured the gains associated with doing so are not worth the increased noise.

my car fully catless is pretty quiet plus you could always throw on a couple resonators to keep things even more at-bay.

[Doug007]

Originally Posted by themyst

So you are saying we will need to upgrade the exhaust side (midpipes, muffler) in order to reap the full benefits of increased boost? I am not worried about the cost associated with doing so, more along the lines of the increased noise driving me to the brink of insanity. I figured the gains associated with doing so are not worth the increased noise.

No, he's saying the stock turbos themselves become an exhaust restriction eventually.

At that point, an upgraded turbo becomes desirable to reduce the exhaust flow restriction through the turbine.

Catless (or high flow cat) downpipes are needed for these very high boost/power levels but stock midpipes/mufflers should be fine.