Trying to understand the cumulative effect of breathing mods

jcosta79 · 01-25-2013, 03:31 PM

You don't want back pressure. What you want (and what most people mistake back pressure for) is velocity. In broad, general terms, the smaller the piping diameter, the faster the exhaust gasses will flow.

Think of blowing air out of your mouth with your mouth wide open, versus blowing out of a straw. Not much velocity to the air coming out of a wide open mouth, but from the straw it's quite different.

So what happens when you slap on a larger diameter exhaust system is that you lose velocity (and with it, low to mid-range torque) until the engine is moving enough air to properly use the larger diameter piping.

Tuning an exhaust system properly is quite difficult. Just look at how many updates F1 teams make to their exhausts. Bigger is usually NOT better, especially if you are not adding some sort of forced induction.

What you typically want to look for in an aftermarket exhaust is smooth bends and transitions, as well as smooth welds on the inside of the piping. Anything on the inside of the piping that can cause turbulence will hurt performance because air running through a tube travels along the walls, not the "empty space" on the inside.

As a (very) general rule of thumb, smaller diameter piping (and long header primaries) will help improve low to mid range torque, while larger diameter piping (and short header primaries) will help improve top end power. Unless you are drag racing, you want a system that favors low to mid-range torque because that's where you are spending the majority of your time. As much as we would like to think that we are speed demons that drive everywhere at WOT, very little time is actually spent at redline.

Hope that helps somewhat.

Coon-azz · 01-25-2013, 05:31 PM

Quote:

Originally Posted by jcosta79

You don't want back pressure. What you want (and what most people mistake back pressure for) is velocity. In broad, general terms, the smaller the piping diameter, the faster the exhaust gasses will flow.

Think of blowing air out of your mouth with your mouth wide open, versus blowing out of a straw. Not much velocity to the air coming out of a wide open mouth, but from the straw it's quite different.

So what happens when you slap on a larger diameter exhaust system is that you lose velocity (and with it, low to mid-range torque) until the engine is moving enough air to properly use the larger diameter piping.

Tuning an exhaust system properly is quite difficult. Just look at how many updates F1 teams make to their exhausts. Bigger is usually NOT better, especially if you are not adding some sort of forced induction.

What you typically want to look for in an aftermarket exhaust is smooth bends and transitions, as well as smooth welds on the inside of the piping. Anything on the inside of the piping that can cause turbulence will hurt performance because air running through a tube travels along the walls, not the "empty space" on the inside.

As a (very) general rule of thumb, smaller diameter piping (and long header primaries) will help improve low to mid range torque, while larger diameter piping (and short header primaries) will help improve top end power. Unless you are drag racing, you want a system that favors low to mid-range torque because that's where you are spending the majority of your time. As much as we would like to think that we are speed demons that drive everywhere at WOT, very little time is actually spent at redline.

Hope that helps somewhat.

I agree here.

tylerk · 01-27-2013, 12:52 PM

Quote:

Originally Posted by jcosta79

You don't want back pressure. What you want (and what most people mistake back pressure for) is velocity. In broad, general terms, the smaller the piping diameter, the faster the exhaust gasses will flow.

Think of blowing air out of your mouth with your mouth wide open, versus blowing out of a straw. Not much velocity to the air coming out of a wide open mouth, but from the straw it's quite different.

So what happens when you slap on a larger diameter exhaust system is that you lose velocity (and with it, low to mid-range torque) until the engine is moving enough air to properly use the larger diameter piping.

Tuning an exhaust system properly is quite difficult. Just look at how many updates F1 teams make to their exhausts. Bigger is usually NOT better, especially if you are not adding some sort of forced induction.

What you typically want to look for in an aftermarket exhaust is smooth bends and transitions, as well as smooth welds on the inside of the piping. Anything on the inside of the piping that can cause turbulence will hurt performance because air running through a tube travels along the walls, not the "empty space" on the inside.

As a (very) general rule of thumb, smaller diameter piping (and long header primaries) will help improve low to mid range torque, while larger diameter piping (and short header primaries) will help improve top end power. Unless you are drag racing, you want a system that favors low to mid-range torque because that's where you are spending the majority of your time. As much as we would like to think that we are speed demons that drive everywhere at WOT, very little time is actually spent at redline.

Hope that helps somewhat.

So are you saying that for a DD, you would recommend keeping the stock exhaust (non-nismo touring sport) as apposed to upgrading to say a Fast Intentions CBE?

I'm not disagreeing with your post, just curious why everyone raves on performance gains from this particular product. Maybe it's the aesthetics that everyone likes with this mod?

SS_Firehawk · 01-27-2013, 01:16 PM

Quote:

Originally Posted by tylerk

So are you saying that for a DD, you would recommend keeping the stock exhaust (non-nismo touring sport) as apposed to upgrading to say a Fast Intentions CBE?

I'm not disagreeing with your post, just curious why everyone raves on performance gains from this particular product. Maybe it's the aesthetics that everyone likes with this mod?

What Jcosta was eluding to was being naturally aspirated, 2.5" is the best size for the car. 3" is best for forced induction applications making 650+ bhp.

Example: Our car's stock exhaust is a coffee straw. 2.5" exhaust is a regular straw, and a 3" exhaust is a water hose. Blow through the coffee straw and sure you get velocity, but there is a lot of back pressure. Blow through the regular straw and we still have very good velocity, but we also have much less back pressure. Blow through a water hose, the size of the hose is so large, there is very little velocity and no back pressure. It's too big to be efficient for the application.

jcosta79 · 01-28-2013, 10:17 AM

Quote:

Originally Posted by tylerk

So are you saying that for a DD, you would recommend keeping the stock exhaust (non-nismo touring sport) as apposed to upgrading to say a Fast Intentions CBE?

I'm not disagreeing with your post, just curious why everyone raves on performance gains from this particular product. Maybe it's the aesthetics that everyone likes with this mod?

Quote:

Originally Posted by SS_Firehawk

What Jcosta was eluding to was being naturally aspirated, 2.5" is the best size for the car. 3" is best for forced induction applications making 650+ bhp.

Example: Our car's stock exhaust is a coffee straw. 2.5" exhaust is a regular straw, and a 3" exhaust is a water hose. Blow through the coffee straw and sure you get velocity, but there is a lot of back pressure. Blow through the regular straw and we still have very good velocity, but we also have much less back pressure. Blow through a water hose, the size of the hose is so large, there is very little velocity and no back pressure. It's too big to be efficient for the application.

I'm not sure what diameter piping FI uses for their CBE, but if it's 3" it's likely too large for N/A applications. I had 3" exhaust piping on my 517 RWHP Corvette, so it's a good bet that on a 370Z it's way overkill.

The power gain you are going to get by replacing the stock exhaust is in the muffler section. OE mufflers tend to be very restrictive because they have multiple baffles inside slowing down and quieting the exhaust. Aftermarket mufflers are typically less restrictive and offer a more "straight through" exhaust flow.

Hope that helps.

01-25-2013, 03:31 PM	#1 (permalink)
jcosta79 A True Z Fanatic Join Date: Jul 2012 Location: N/A Posts: 1,430 Drives: N/A Rep Power: 16	You don't want back pressure. What you want (and what most people mistake back pressure for) is velocity. In broad, general terms, the smaller the piping diameter, the faster the exhaust gasses will flow. Think of blowing air out of your mouth with your mouth wide open, versus blowing out of a straw. Not much velocity to the air coming out of a wide open mouth, but from the straw it's quite different. So what happens when you slap on a larger diameter exhaust system is that you lose velocity (and with it, low to mid-range torque) until the engine is moving enough air to properly use the larger diameter piping. Tuning an exhaust system properly is quite difficult. Just look at how many updates F1 teams make to their exhausts. Bigger is usually NOT better, especially if you are not adding some sort of forced induction. What you typically want to look for in an aftermarket exhaust is smooth bends and transitions, as well as smooth welds on the inside of the piping. Anything on the inside of the piping that can cause turbulence will hurt performance because air running through a tube travels along the walls, not the "empty space" on the inside. As a (very) general rule of thumb, smaller diameter piping (and long header primaries) will help improve low to mid range torque, while larger diameter piping (and short header primaries) will help improve top end power. Unless you are drag racing, you want a system that favors low to mid-range torque because that's where you are spending the majority of your time. As much as we would like to think that we are speed demons that drive everywhere at WOT, very little time is actually spent at redline. Hope that helps somewhat. DEpointfive0, Hotrodz and Sorrovv like this. __________________ 2012 PW NISMO #0559 - GT Spec Brace Kit - Swift Springs - ARK TP & Invidia CBE K&N Intake - UpRev Tune - 15mm Z1 Spacers & Studs - StopTech Slotted Rotors & SS Lines - ZSpeed CMAK with Stage 2 Clutch and Lightweight Flywheel

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