As I understand it, turbos are all about managing pressure. In simple terms, you want higher pressure before the turbine and lower pressure after it. This would seem to militate in favor of a free-flowing exhaust (e.g., cat-less sports-cat downpipes), which has the effect of lowering the pressure behind the turbine.

The question is: Is there a point at which this can lead to too much spool and over boosting in a higher-boost tune?

Here are some explanations I found on the web:

In other words, the theory is that an exhaust can be so efficient that it leads the turbine to spin too fast and create a situation in which boost is more or less uncontrolled, forcing the ECU to take drastic action with a boost cut/limp mode. If so, this would militate in favor of retaining the stock exhaust or at least not going too crazy with the exhaust mods.

I ask because a tuner has recently suggested that their tune works better with the factory downpipe, otherwise you are liable to get an overboost situation and go into limp mode. Thus, they apparently discourage anyone from going catless or with a high-flow DP.

This is the first I'd heard of it. I'll admit, I was skeptical. From what I can remember from Maximum Boost, I've always understood that the less backpressure in the exhaust (i.e., the bigger and less catted), the better. I don't remember anything about overboost from an exhaust that was too free flowing, though it's be awhile. And I'm surprised I haven't heard of this before if it is a real issue.

Then again, there seems to be some support for the idea from the Interwebs and, obviously, from the tuner himself. So I thought I'd toss it out to the masses, as I know there are lots of experts on here who can set this issue straight.

 

Ok, I'm feeling pretty lazy right so I don't want to type a lot. Basically no. It has to do with air velocity, volume, and other stuff. I'm really not going to explain but it sounds like your tuner is crap.

 

Typically forced induction vehicle benefits from a free-er flowing exhaust than a naturally aspirated vehicle. I tune turbo Subaru's and there certainly is room to improve on factory tunes with the stock exhaust, in general....usually by creeping the boost up a little and adjusting timing and fueling target.

With a higher flowing downpipe it will prevent back pressure from building after the turbine(which would slow the turbine down), therefore boost can build quicker, and higher, while allowing exhaust to exit the hot housing more freely (and not cooking the turbo to death at higher boost). This is assuming the tune is calling for this, if the MAP exceeds what the boost target is, you will overboost and likely limp mode. It's never a good idea to do engine and turbo mods without a proper tune.

Remember how wastegates work and how boost control works, the more closed the wastegate is the more exhaust is routed to the turbine thus the faster it will spin, and the faster the compressor wheel will be sucking air and forcing into the intake manifold.

Tuning for higher boost with a stock downpipe is rather limited, but possible. It really depends on the efficiency range of the particular turbo and if it will pump out higher CFM without roasting itself to death(because wastegate is closed and all exhaust gas is now in the turbine housing and CHRA).

IMHO, a free flowing downpipe is a great benefit to tuning for more power in a turbo vehicle. It really depends how bad the OEM one is, though. I'm not too familiar with the stock downpipe and if there is a poorly shaped flange on it, and if the cat is really restrictive. Some testing with and without it would have to be done on the dyno, monitoring post turbo temps and power gains at X psi boost with and without stock downpipe. (Which I'm sure has been done before....)

 

You've answered your own question quite nicely.

 

Thanks for the replies fellas. It sounds like everyone is virtually on the same page here.

I'm with you so far . . .

This raises the question of whether overboosting might be a concern with a canned tune on a high-flow downpipe. If the tune is not tailored to the car, does overboosting from a high-flow DP become a real possibility there, or is it still unlikely?

The OEM downpipe is at least a 400 cell cat. It looks pretty dense down there. I swapped it out for an Akrapovic. Just hoping I didn't set myself up for some serious overboosting problems since I'm only looking at canned tunes.

 

From what I understand canned tunes are usually pretty conservative to avoid such issues which is why a tune custom designed for your specific car will almost always make more power than a canned solution. And for what it's worth, I've never heard of an exhaust or dp, even catless, causing overboosting issues, but then I'm relatively new to turbocharging and am still learning.

 

As it was explained to me overboost can occur due to how the Mini does boost based on engine load and that the blow-off/diverter valve is electronically activated. There are tenths of seconds that the engine can in theory be over boost at WOT.

Thats why you see some of us go with the mechanical diverter from Alta or a Forge BOV.

 

Would that explain the "fluttering" noise I hear from either the OEM diverter valve or wastegate when I'm at WOT with about 15psi+?

 

Turbo lag aka bad news

 

At WOT yes...at partial throttle lift fluttering sound is 'normal'.

 

Interesting. If that's true, wouldn't Alta's all-mech valve only compound the problem by waiting for vacuum (which will never come at WOT) before closing the diverter and dumping boost to intake tract? At least with an electronic control, the ECU can close the diverter if it senses an over boost situation. What am I missing Taz?

In any event, this sounds like we've just replaced overboost with a boost cut, which is perhaps a step in the right direction but still not awesome. The goal is to achieve and hold max (or near max) boost at WOT, without it creeping or spiking beyond that point by an exhaust wheel spinning faster and faster from the much lower pressure gradient created by the high-flow exhaust.

Naturally, it's not a true positive feedback loop: Even a Charles's, straight, mandrel bent, 3" exhaust is going to have some back pressure. So at some point the exhaust wheel will be slowed and held in check by the gasses building up and creating back pressure. But if that point is not until 25 psi and the stock sensor on an "S" will shut things down at, say, 20 psi, you're still going to have issues, I'd think.

I guess the question is: When the exhaust system is no longer a restriction on achieving max boost and actually could, if left unchecked, take boost into dangerous territory, what mechanisms are in place to hold boost at or near the max set point you've targeted with your tune?

 

Here's Jeff Perrin's blog post on their diverter design:
http://blog.perrinperformance.com/al...-recirc-valve/

I think its all on the BOV/DV, ECU, and MAP sensor to keep things in line. This is one of the biggest sticking points of tuning the R56. Targeted boost is a suggestion at best...
Couple that with the ECU trying to keep the car as lean as possible at all times...I now see why many prefer tuning the R53.

 

Have you done anything to try to fix this? Like buy the ALTA CRV or the Forge BOV? I would imagine that noise isn't good for the turbo even though it's considered 'normal' as you said. I drive the car hard for maybe 10mins tops every 1-2 weeks so it's not like this is an everyday issue.

 

Disregard my last question. I posted it just after TazMinianDevil posted his info.