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Just get the 4" it doesnt hurt performance and then youll never need an upgrade. So says the guy who got a deal on 80mm (3") and now wants the extra Hp from a 4"
 

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What are the pros and cons of each diameter? And when is it appropriate to use each diameter ?

Thanks for helping a newbie out!
Well you got the vendor and "go big" answers so let me be a bit more scientific. There is absolutely no reason for you to run a 4" over a 3.5" system until you probably hit a range around 1000AHWP - 1200AWHP and yes that is dependent on how your engine and turbos are setup but it is a good ballpark number. Where your exhaust gas is the hottest and thus has the highest volume, along with speed, is where it is closest to the engine. It also depends on how efficient your 3.5 system is. So mufflers, no mufflers, bends in the piping, etc etc. So making sure that your downpipes are appropriately sized and your mid-pipe design is geared towards scavenging the dual outlet from the engine into a single pipe in an efficient manner is far more impactful than the sole act of stepping up those 0/5 inches. As you probably know as gas cools its density changes and it slows down (when it has been accelerated). So unless you push a very high volume of heated gas through an exhaust setup the volumetric increase in changing pipe diameter from 3.5 to 4.0 on an engine with our displacement really does not make that much of a difference.
However as soon as we go into super big build power territory that equation changes a little. 4.0 will be louder and most likely produce a little more drone, so pick what you want, but do you need it? Not unless you go super big power.
 

· 7.9 @ 175mph (Gen 3 SBD1400R)
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We saw 25whp @ 925whp on my personal car, I'd be happy to upload the overlay.

3.5" starts to become a restriction over 900whp roughly.

Absolutely you can make more power, but you will see gain by increasing diameter above that level.

Yes, somebody will pipe in and tell us how they made (1xxx) through a 3.5"

We all get that, but I guarantee you at that level a 4" system will pick up tq/hp.
 

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Discussion Starter · #11 ·
Well you got the vendor and "go big" answers so let me be a bit more scientific. There is absolutely no reason for you to run a 4" over a 3.5" system until you probably hit a range around 1000AHWP - 1200AWHP and yes that is dependent on how your engine and turbos are setup but it is a good ballpark number. Where your exhaust gas is the hottest and thus has the highest volume, along with speed, is where it is closest to the engine. It also depends on how efficient your 3.5 system is. So mufflers, no mufflers, bends in the piping, etc etc. So making sure that your downpipes are appropriately sized and your mid-pipe design is geared towards scavenging the dual outlet from the engine into a single pipe in an efficient manner is far more impactful than the sole act of stepping up those 0/5 inches. As you probably know as gas cools its density changes and it slows down (when it has been accelerated). So unless you push a very high volume of heated gas through an exhaust setup the volumetric increase in changing pipe diameter from 3.5 to 4.0 on an engine with our displacement really does not make that much of a difference.
However as soon as we go into super big build power territory that equation changes a little. 4.0 will be louder and most likely produce a little more drone, so pick what you want, but do you need it? Not unless you go super big power.
Wow. Amazing info. Thanks.
 

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I would probably start by asking what kind of power you ultimately want to make. If you are never going to make over 900whp I wouldn't want a 4 inch exhaust on my car. I would much rather have something with no drone,sounds good, and isn't too loud.

If going over 900whp is even in the back of your head, then yes, you need to plan for it and should probably run a 4 inch.
 

· Will trade patents for car parts
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I would probably start by asking what kind of power you ultimately want to make. If you are never going to make over 900whp I wouldn't want a 4 inch exhaust on my car. I would much rather have something with no drone,sounds good, and isn't too loud.

If going over 900whp is even in the back of your head, then yes, you need to plan for it and should probably run a 4 inch.
Examples of such systems? (I'm curious). I'm currently on Cobb catless DP and MP and I get terrible drone above 80 MPH.
 

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The Akrapovic, although more expensive, offers the least amount of drone. You're next best would be a resonated or catted mid pipe with something like an HKS system.
Yeah, I have heard the Akrapovic in person and it's nice, but the cost is completely outrageous in my opinion. Nearly $6K for just the rear section? I hear good things about HKS Legamax. My incredibly not good reason to dislike that setup is that I just don't like the spaghetti noodle design of the exhaust. I may try it anyway.

I have been mostly focused on 4", but many of these comments are concerning me about drone on 4" vs smaller diameter exhausts. My ultimate power goals are sub 1000 WHP ... for now at least.
 

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Yeah, I have heard the Akrapovic in person and it's nice, but the cost is completely outrageous in my opinion. Nearly $6K for just the rear section? I hear good things about HKS Legamax. My incredibly not good reason to dislike that setup is that I just don't like the spaghetti noodle design of the exhaust. I may try it anyway.

I have been mostly focused on 4", but many of these comments are concerning me about drone on 4" vs smaller diameter exhausts. My ultimate power goals are sub 1000 WHP ... for now at least.
This is from facebook

Jason JimenezBattalion30five Classified

1 hr ·

Akrapovic Titanium Slip On Exhaust w/ carbon tips
$3,250 - Austin, TX
Used Akrapovic Titanium Slip On Exhaust w/ carbon tips ($1450 option new) . Used for 3k miles $2950 plus shipping
 

· Will trade patents for car parts
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507 Posts
This is from facebook

Jason JimenezBattalion30five Classified

1 hr ·

Akrapovic Titanium Slip On Exhaust w/ carbon tips
$3,250 - Austin, TX
Used Akrapovic Titanium Slip On Exhaust w/ carbon tips ($1450 option new) . Used for 3k miles $2950 plus shipping
Tried to follow link, but no good. Any way to put me in touch would be appreciated.
 
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