The biggest straightest exhaust is best?

[CoHPhasor]

You bet your a** it is.
Laugh at the imports with 3" exhausts all you want, but if they're straight, made with good material, have good welds, and proper merges, the bigger the better.
(Especially with turbos)

Just in case you need educated:

 

[TMac]

Bigger is NOT better. Although nothing is said in that video is false, they are trying to sell systems. However, using giant pipes gives up inertial flow which is important. For every flow rate/temperature, there is a size of pipe that would be optimum. He's right in that an engine (other than a 2 stroke) doesn't need backpressure. I did a thread a few years back on this. You're all welcome to read the entire thread, but I think this post sums it up nicely.

 

[2020FordRaptor]

Agree with @TMac bigger is not always better. The stock exhaust has been proven to handle a lot and 3” will not net worthwhile, or maybe any gains until you reach built levels of power.

 

[I Bleed Ford Blue]

Agree with the last two posts, a 3" exhaust is a waste of time if you can't generate the exhaust flow that needs it. Think of it this way, if you filled your pool with a garden hose or if you filled the same pool with a fire hose hooked up to your outdoor faucet it would take the same amount of time because the 1/2" pipes in your house can only flow so much. Just because the hose on the outside is huge means nothing. A lightly modded engine can not support the exhaust flow that needs a 3" exhaust.

 

[TMac]

If you watch near the end of the video, the presenter also talks about the "patent pending" tailpipe which according to him actually creates a "vacuum". Notice that he doesn't present any evidence of this, nor is there any mention of how they measured this. The only thing "patent" about it is that it's patently ridiculous.

 

[CoHPhasor]

I think some points are missed here.
1. Pressure differential is key to turbo operation - this is a fact, not a question

2. Although you can absolutely, incorrectly setup a larger exhaust, the statement I made is based on the assumption that a system is properly engineered

3. I have heard all this counter-argument before, and seen the slack jaws when a 3.0L, twin-turbo engine gets a 3" exhaust, and has dyno slips to back it up.

4. Yes, depending on how hard your engine is maxed, the gains will be substantially different, but it still does not change fact 1. Your engine will be more responsive, and make more efficient horsepower with a larger, PROPERLY DESIGNED exhaust.
(Yes, emissions gear interferes, there is no way it wouldnt, but just because the big toe is bad, you dont chop off the whole foot)

 

[CoHPhasor]

As a note about the sales pitch on the exhaust tip - so what? You act like the whole time they did nothing but sling it.

Did I say we should get them for the ST?
I would however note that if I spent all night digging through that thread to hear how ambient temps and other factors cause differences, Im sure the claim they make about the tip falls into the same category.

 

[UNBROKEN]

As a note about the sales pitch on the exhaust tip - so what? You act like the whole time they did nothing but sling it.

Did I say we should get them for the ST?
I would however note that if I spent all night digging through that thread to hear how ambient temps and other factors cause differences, Im sure the claim they make about the tip falls into the same category.

Honestly…there’s virtually zero benefit to a 3” exhaust on this car with stock chargers. It’s been proven time and again. Hell the stock turbo record for a while was on a stock exhaust minus high-flow downpipes. Considering the turbine wheel is about as big as a quarter it doesn’t take much pipe size to accommodate it. Many have dynoed before and after and you’re talking low single digit differences after downpipes which will generally net about 20hp give or take a few. The cat-back part is just picking the sound you like and I agree the 3” sounds deeper and better. I built my own dual 3” a long time before there were any available to buy…just for the sound

 

[TMac]

You bet your a** it is.
Laugh at the imports with 3" exhausts all you want, but if they're straight, made with good material, have good welds, and proper merges, the bigger the better.
(Especially with turbos)

Just in case you need educated:

You wrote this- and even emphasized it in bold. Everyone who says the same is missing the points I seem incapable of making: (or no one bothered to read the post!)
1. Once exhaust gas runs through the turbine, it loses energy having imparted it to the turbine wheel. Meaning a larger exhaust will lose inertial energy.
2. The turbine wheel takes the laminar flow exhaust gasses and converts them into a turbulent, spinning flow around the inside of the post turbine exhaust pipe.
3. A dyno chart does not include/account for shifts or throttle modulation.

A properly designed exhaust post turbine will not just be a sewer pipe, it will account for these factors and incorporate a diffuser and transition to reintroduce laminar flow to the exhaust stream- especially before the catalytic converter. The exhaust diameter post cat will also be designed such that the exhaust flow rate will be somewhere around 100 meters/second at peak HP. That will give you the best HP and the best inertial gas energy when shifting. Those who constantly state that a larger exhaust makes more peak HP aren't wrong, but they also aren't addressing the actual problem which is the turbulent flow and if you aren't designing for this, then obviously a larger exhaust will post a higher peak dyno number, but it will be lacking the inertial energy which is critical in actual racing.

That my friend is a fact.

 

[scoobysportD]

A properly designed exhaust post turbine will not just be a sewer pipe, it will account for these factors and incorporate a diffuser and transition to reintroduce laminar flow to the exhaust stream- especially before the catalytic converter. The exhaust diameter post cat will also be designed such that the exhaust flow rate will be somewhere around 100 meters/second at peak HP. That will give you the best HP and the best inertial gas energy when shifting. Those who constantly state that a larger exhaust makes more peak HP aren't wrong, but they also aren't addressing the actual problem which is the turbulent flow and if you aren't designing for this, then obviously a larger exhaust will post a higher peak dyno number, but it will be lacking the inertial energy which is critical in actual racing.

That my friend is a fact.

Just out of curiosity, could you show a comparison pic with a "sewer pipe" vs a properly designed exhaust, for the same car/engine? What would such a diffuser look like?

Or is it as simple as the divider i circled in the pic?

 

[TMac]

No, that is a splitter or divider. A diffuser is a conical section (think ice cream cone being fed from the bottom) that extends from the turbine exit. For example, if the turbine outlet is 3", a diffuser would be a transition from 3" (turbine exit diameter) to say 5" (cat entry diameter) over a 6" or so length. Then a sharp 90 degree bend into the catalytic converter. From the cat, it would neck down to a 3" single exhaust. I designed a single turbo system for a forum member that used those exact dimensions. Because the exhaust exits the turbine tangentially (swirling or helix- direction depends on whether the turbine is speeding up or slowing down), the gradual change in diameter causes the velocity to be lowered and the pressure to be raised along with lessening of the angle of the helix. The sharp transition to 90 degrees directs the slower exhaust gasses back into a laminar flow as they enter the cat. I understand that this 90 degree transition seems counter-intuitive, but that's because we are not dealing with laminar flow out of the turbine.

That is the ideal way to engineer an exhaust post-turbine.

Without this mechanism, the swirling exhaust gasses are need to "screw" their way into the catalytic converter cells which increases backpressure. The cat itself will end up returning the flow to laminar- but the least backpressure occurs when the flow is laminar PRIOR to the cat.

 

[scoobysportD]

Ok, so its kind of like the part i circled here? (Not completely the right dimensions/shape without the 6" section to taper it down before the cat but i guess its the same idea?)

Dont think i've ever seen one, in the way you've described it.

Would it be improved further if rifling/spiral grooves were used in the section before the cat? To help shape the airflow? (If i understood it correctly, before the cat the air is moving in circles and after the cat it basically is straightened out?)

 

[TMac]

You have it backward- perhaps you are not understanding. The taper is from a smaller diameter to a larger diameter.
And no, the exhaust post-turbine does not need any sort of "rifling".
The exhaust you picture seems to be one that divorces the wastegate exit from the turbine exit. A completely different animal.

 

[scoobysportD]

A

You have it backward- perhaps you are not understanding. The taper is from a smaller diameter to a larger diameter.
And no, the exhaust post-turbine does not need any sort of "rifling".
The exhaust you picture seems to be one that divorces the wastegate exit from the turbine exit. A completely different animal.

Ah yeah, read it a couple of times but now again and realized indeed i had the 1st step backwards.

If you know of a product that is setup as you described, id love a link/pic to see it.

Thanks for the explanations!