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Okay, bear with me here...I'm more of an IT tech than a mechanic, so it's quite possible my understanding is a bit wrong here...only just started looking at how my car works!...that said, here goes:-

Does changing the exhaust to a freer flowing exhaust have any effect (good or bad) on the turbos?

My logic here is that if the exhaust gases are used to spin up the turbos, then reducing the back pressure by changing the exhaust could cause the turbos to spin up faster? Maybe this could help reduce lag, but couldn't this potentially cause the turbos to spin up to rpms over their rating ?

Or maybe I should stick to IT...

Think I might be able to answer my own question....the wastegate controls the amount of pressure allowed, not allowing the turbines to spin up too highly?

doesn't the wastegate only limit the boost pressure, not the rpm of the turbine? - so you'll limit excess boost but not the rpm your turbo will get to. That said though, I've heard goo reports regarding reduced lag & no adverse effects if a decent exhaust is chosen.

A

with a freer flowing exhaust the turbo does spool up better. Ive had a 3" turbo back system fitted on my S2 and it certainly spools up alot better.

As for the wastegate it bleeds of exhaust gas so the correct pressure can be maintained and the turbo doesnt overspin. This doesnt mean to say if it has been remapped for higher boost than it should have that i wont over spin. If you see what i mean.

Greg

Hrm...

So if your car has been re-mapped with a standard exhaust, and that map is close to the limits in terms of pushing the turbos for boost, then by changing the exhaust, the same map could over-spin the turbos because you now have reduced back pressure ? - I'd have thought that the pressure sensor would simply open the wastegate earlier to maintain requested pressure?

Or do you simply mean that it is technically possible to over-spin the turbos by controlling the mapping? (ie, you can override the wastegate).

Hope that makes sense!

Yes, reducing back-pressure on the turbos does allow the turbines to accelerate more and therefore lag is reduced. Additionally, airflow can be increased at the top end, allowing boost to be kept higher, higher up the rev range.

However as you have correctly indentified, the ECU controls the wastegates in a closed control loop with the boost sensor. It opens them when the demanded boost is reached, which occurs at a given rate of airflow at a given engine rev. Unless you also improve airflow through the intake system, simply increasing exhaust gas flow will not increase overall airflow, so no, the turbos will not 'overspin'.

As James points out, by remapping the ECU the wastegates can be held closed for longer, allowing a higher boost at a given rev, which means increased airflow and 'overspin'. However the definition of 'overspin' is subjective (the manufacturer will specify a maximum envelope, and most tuners will be within this by a reasonable margin).

The other main risk of overspin is because of air-leaks, a common occurrence in S4s because of the TBB. It can also occur in both the S4 and RS4 if the DVs leak.

Cool, thanks guys!