Back Pressure Myth

[eagle eye]

Re: Back Pressure Myth

ok, I have read it all and I've heard it all. I went to meineke and asked if a cat back system would get more power for my v6 3.8 and he told me it's just for sound not for gains. Now if what you guys say is true, if I get a slp cai, would a cat back help in gains? Because if not, then why buy it just for sound?

[flamingchicken]

Re: Back Pressure Myth

cat backs help a little, but there is still the 2.25" S pipe that few people replace.

most people that drive F-body cars are men, a couple awesome women, but majority just want their car to sound nice, or different.

[jeep350]

Re: Back Pressure Myth

well cat back is only for sound is from the cat forward u get ur power

[crysmatic]

Re: Back Pressure Myth

The original analogies are strained, if not incorrect or misleading.

First, we have to make a distinction between flow and pressure. A hose will flow the same amount whether you put your finger on the end or not - your finger will increase *pressure*. That's why the water shoots farther. the OP implies that blocking the pipe increases flow! :/ Flow is determined at the spigot...hose size is inconsequential on flow - only pressure.

Wave velocity is dependent on air DENSITY, and is roughly 514 m/s in hot exhaust - regardless of tube size. GAS velocity is dependent on tube diameter. A tube that can flow enough isn't necessarily optimum for pressure pulses. The OP explained this somewhat well. However, it's not a cylinder's pressure wave that scavenges...it's the cylinder that fires 360 degrees BEFORE it! This is why tri-Y headers, equal length, etc are used...to time each cylinder's pressure wave with other cylinder's exhaust valve opening.

The exhaust sees the cylinder pressure at the time which the exhaust valve opens (the exhaust valve is like the spigot). Higher cylinder pressure at valve opening (either from increased power, or opening the exhaust valve earlier) both increase exhaust wave pressure. This is why bigger cams make more noise, and larger diameter exhaust lets more noise out. Between pressure waves, there is a low pressure wave (vacuum if you prefer). The larger the diameter tube, the WEAKER the pressure wave, the weaker the scavenging. Highest cylinder pressure is at peak torque, and trails off at higher rpm. This is why slightly smaller tube won't necessarily hurt power above peak torque.

Generally, larger tubes push power curves to higher rpm - not necessarily the power under the curve where the engine operates the most (this is key). Ten bhp from a larger exhaust system may not make a car faster, depending on where the power is gained. For a naturally aspirated 275 bhp V6, l calculate a 1-5/8" primary pipe with a 3" collector for best wave tuning. A 2-1/2" exhaust may make almost as much hp of the 3" system, with much less noise. tbh, a 3800 isn't a highly optimised beast that makes tuning very rewarding.

An OE exhaust is small for several reasons: larger tubes are more expensive, harder to package, and excessively loud. A dual mode system makes virtually NO backpressure at low rpm - it reduces back pressure at higher rpm. Besides, we don't want to over scavenge at low rpm - it may pull raw fuel out the tailpipe and hurt fuel economy. And low speed/part load operation will suffer, as well and tip in (it hurts driveability).

A turbo is a completely different beast. Any scavenging occurs BEFORE the turbo. Back pressure after the turbo doesn't benefit anything whatsoever. Note that a turbo blends individual pulses and turns them into a steady stream...this is why they quiet the exhaust. Instead of the high/low pressure pulses, a turbo exhaust sees roughly half the highest pressure. Lower pressure, less noise.