Just purchased a turbo and was wondering if an exhaust cut out would help with the stock system. I was already tuned with the stock exhaust. So if I were to add this, Im assuming i would need to get retuned. Not sure if its worth it. Anyone know the HP gain with this mod? Currently boosting 9 pounds and hit 345 to the wheels.

 

by exhaust cutout I assume you mean something that you can control to do an open dump to atmosphere. If so then it would not affect your tune that much, it will just help more with the spool of the turbo. It will spool faster since there is less restriction, as for gains it depends on your manifold set up. If it is highly effecient then not much I'd say less than 15hp. If the manifold is restrictive then more than 15hp, just don't set up the cutout upstream of your wideband if you have one and your ecm is using it to adjust fuel.

 

Actually heard i can gain 30-50hp since the stock exhaust is 2.36 inch compared to upgrading to a 3" or a dump since the stock exhaust restricts the turbo on top.. But wasnt sure..

 

depends on what turbo you have. The best exhaust for a turbo is the least amount of exhaust. Changing out the exhaust with a 3 inch open down pipe or just a full 3 inch will help A LOT.

 

This is very bad advise! You will most definitely need a tune after such a drastic change in exhaust flow. If the tune is not setup properly, you run the chance of a lean condition. Now if this were a car with a mass air flow sensor instead of a map sensor you might not have as much trouble. What I would recommend is to pick up a nice 70mm, or even better a full 3" exhaust system and then just have the car re-tuned.

 

Could you please explain what causes the tune to be affected. I thought that if your fuel tables and trim were based on the MAP the ECM would simply adjust for the turbo spooling earlier.

 

The turbo spooling up faster does not have as much to do with the tune as you may think. You are right in the aspect of when the turbo spools up faster, the boost pressure increases faster and the map sensor will recognize this and adjust the fuel accordingly (based on your current tune/fuel map). However, this is only a small part of the equation (and the RPM range).

The exhaust is not only causing a restriction on the turbo spooling, it's causing a restriction (back pressure) on the engine. When you reduce this back pressure, the engine will be able to take on more air into the intake, which will require more fuel, which will create more power. Since your fuel is directly proportional to the map voltage (among other things), you will need to adjust your tune to allow more fuel into the engine at any given time because you engine is now taking in more air. If you don't get more fuel, but you do get more air you will get a lean condition. Even though your pressure/boost will stay the same (because of the waste gate), the engine is only going to be able to ingest so much air. So the map sensor will see the same pressure at the manifold, and the waste gate will keep that pressure at the right spot, but your engine will actually be ingesting more air. Since your computer is putting in a certain amount of fuel based on the map voltage, if that does not change you are not going to get the additional fuel you need with that additional air you have in the engine.

Things change when you use a MAF (Mass Air Flow) sensor as opposed to a MAP (Manifold Absolute Pressure) sensor. Things also change when you are not at WOT (Wide Open Throttle), and your oxygen (02) sensor gets involved.

Hope this makes sense, I tried to describe it in layman's terms.

 

Hmm never thought about the back pressure created by the exhaust, but I see the point. As I have read that the best designed exhausts are not just big in diameter, they actually match the back pressure generated to be harmonic and help pull exhaust gases out of the cylinder (unless of course this is just mumbo jumbo). Thanks for explaining that, as I have only considered the one aspect of faster spools (I do understand that there are other things affected by the larger exhaust, but seems like the main desire for the larger exhaust is faster spools for more psi and more power).

I stand corrected and retract my earlier statement of not needing a tune.

One other question since the tune is affected how do those who run a butterfly valve that allows for all exhaust to just be dumped at the flip of switch do it? I mean how does their tune work as sometimes it is through the exhaust and other times it isn't.

Thanks for the info.

 

from what i read, you have your base tune and then your dump tune. If you run a boost activated control, anytime you hit 5psi lets say, that valve will open up. I would rather have this method than an electrical one since you would need to hit a switch anytime you want to use it.

 

So how is it they switch tunes while it is driving around? On the K-Pro to switch tunes requires a restart of the ECM and this shuts off the vehicle, not to mention that you would be driving around with your laptop hooked up all the time.

 

Why would you need two tunes for that?

 

I guess according to white smoke's reply that my advice of not needing a new tune being incorrect, has lead me to believe there is a need for two seprate tunes.

Since changing the back pressure of the exhuast requires a new tune. Wouldn't someone who ran a cut out sometimes ie when they want max power, vs just driving it around and running it through the exhuast, need two separate tunes?