I've noticed my exhaust has been loud for a while had several people inspect it, everybody said it was fine and stock, while having my radiator replaced it was up on a lift and I was able to look at the rear end from underneath, I noticed that my tailpipe has come disconnected from the flange that connects it to the muffler at the very rear, leaving about a quarter inch gap all the way around... Obviously this reduces back pressure and possibly gives me more power at the top end although I'm not sure how much considering it's all stock and untuned, My question is if I fixed the exhaust so that it runs through the muffler and increases the back pressure Will I get more torque at 1200 to 1700 RPM? Or am I better off just leaving it slightly open and enjoying the wide open top end?

 

You’ll get exhaust gasses in your trunk - therefore into your cabin area with a leak there.

maybe this is a health mod?

get a new flange welded in, or weld the exhaust pieces together lol.

 

You're not losing or gaining power, the catalytic converter is the most restrictive point on the coupe.

A coupe without a muffler sounds pretty harsh from outside the vehicle while it's under power though, I would definitely hook it back up just for the sound aesthetic.

Now to get into the weeds and some of the technical garbo about torque/backpressure/scavenging. Most of what you used to hear is either myth or a misunderstanding of why an "exhaust upgrade" caused a loss of low end torque, and how/why it happens.

A smaller diameter tube pushing the same volume of air will have a higher air velocity (speed), a larger diameter tube pushing the same volume of air will have a lower air velocity.

Exhaust comes out in short PULSES, it's not a steady stream of air like from an air compressor. The pulses alternate bank1, bank2, bank1, bank2, etc. Each pulse has a certain amount of weight to it (yes air does actually weigh something). Each pulse will actually help SUCK the next pulse forward because the exhaust system is a sealed pipe. This is called SCAVENGING.

So a smaller diameter pipe with fast moving air will have more scavenging, this increases overall efficiency of the motor because the next exhaust stroke works a little less due to the previous exhaust stroke helping to suck the air down the pipe. And the length of the pipe determines how many of these individual pulses will help pull (each pulse adds up to a combined weight inside the pipe). Overall this becomes exhaust VOLUME inside the exhaust pipe system.

This scavenging increases torque, HOWEVER if you go too small in pipe diameter you are restricting the overall VOLUME of air that the engine can move especially at high rpm. The engine is just an air pump, it moves 10-13 times as much air as it does fuel, for horsepower you need to move a high volume of air so you can consume more fuel, fuel makes power, but moving the AIR is always the restricting factor.

So the engineers have to decide exactly what tube diameter and length works best for the VERY SPECIFIC application they're designing. What is the engine displacement, max operating rpm, economy vs. performance, etc.

Typically you will see a smaller diameter tube that increases in overall diameter as you get nearer to the tailpipe. This is the balance most engineers aim for to create scavenging velocity closer to the exhaust valve but still allowing enough overall volume throughout the system, and on sports cars (or anything generating high rpm power) the tube diameter will be sufficiently large enough to allow enough total air volume to pass through the engine without being overly restrictive.

All this math factors into the final aspect of exhaust design, TURBULENCE and for the most part happens in the muffler and catalytic converter. As air encounters turbulence it loses it's velocity, all stock vehicles sold in nearly every market in the world are mandated to meet certain decibel requirements. In order for the muffler to do this the sound has to REFLECT off enough solid objects that it lowers the overall decibel within the acceptable levels.

Imagine you have a bucket of water, hit the side of the bucket, it makes waves, now watch as the waves crash off the opposite wall and off each other. Each time the waves REFLECT their energy off something they become less intense, eventually the bucket will be perfectly calm as all the energy is reflected. Sound does almost the exact same thing inside the muffler, however there is eventually an exit in the muffler and the bucket doesn't have one.

So a muffler needs to be large enough in volume to not impede the scavenging of the exhaust system, with an internal design to maximize reflecting sound energy to reduce decibel noise. However you wouldn't want a GIGANTIC muffler because it adds total weight to the vehicle which reduces the overall performance.

So it's this great triangular juggling act, performance vs. aesthetic vs. efficiency.

This is why when people put a giant 4 inch exhaust on their car too close to the header they lose torque, they have lost all their scavenging. This is why race cars typically use a very large diameter short length exhaust, they need to move a massive volume of air and are only concerned with horsepower at high rpm.

Design the exhaust YOU need for YOUR specific application. The muffler on both G35 cars (coupe and sedan) are basically just to final tune decibel and sound quality, the stock catalytic converters are very restrictive and create a significant amount of turbulence as well.

 

Thank you for this great explanation! The cats stink, I'll probably replace them with hfc and get the flange fixed.