biff420

Then you've heard wrong. It's like the old saying "it's not the size that counts, its how you use it." A larger exhaust can have severely detrimental effects on a turbo system depending on how its implemented.

biff420

Again, you've been told wrong. How do you suppose turbos spool up? Its not about just flow, there needs to be pressure behind it in order to perform work. In this case, that is getting the turbo to start turning and sending, wait for it, pressurized air back into the engine.

Lowering the back pressure also increases turbo lag. So in low rpm, high torque applications like was mentioned (towing a trailer) more back pressure is a good thing.

jcain

You're sadly misinformed. Back pressure past the exhaust outlet of the turbo ISN'T needed. The back pressure is determined by the turbine size and/or design, ie VGT. Smaller the turbine, the higher the BP gets at higher RPM, but spools quicker. Larger turbines have less BP but are a little more laggy. The use of VGT tries to please both anomalies by changing the pitch of the vanes, but they suffer from higher BP too. A way to fix that is a wastegate. You can use the wastegate to help BP issues or from overspeeding smaller turbos.

Stu Cazzo

For those guys getting 11mpg

papa tiger

Look at the tiny exhaust manifold exit size.

biff420

So wait, back pressure isn't needed, until its needed. Did I get that right?

phantomblackgto

I think you need to understand turbo systems a little better before you go spouting off. Pre turbo, back pressure is needed. Post turbo (i.e., exhaust), back pressure is counter productive to the turbos. And, I have no idea what you mean by it depends on how a larger exhaust is implemented.

jcain

No. But seeing as you don't have even a clue as to the fundamentals of turbos, I'm the least bit surprised that you're confused.

biff420

And I think you need to understand how the pressure before and after a turbo doesn't change much. How could it (unless you have a wastegate and in that case the turbo isn't lowering pressure, a valve is ahead of the turbo). The exhaust gases pass right through the turbo, as long as the exhaust is the same size as the tube exit there is no change in back pressure. Make the exhaust too large, and you lose back pressure. Too small, you get increased back pressure.

In either case, reductions in power can occur. One from the turbo not spinning as fast, the other from the engine having to work harder to expel exhaust.

A turbo isn't much different than a person blowing on a pinwheel. Blow with your mouth wide open (low pressure) and the wheel moves a little. Blow with your lips closer together (high pressure) and the wheel moves a lot faster.

biff420

Sure thing keyboard commando.