HFC vs decat?

[jamzee500]

Hi guys,

 

got decats on at the moment with a nismo cat back and love the sound but getting annoyed with swapping around every year for MOT's, will it sound the same with the berk HFC's? or will that quiet it down a lot?

 

Cheers

 

JM

[Tarmac@TarmacSportz]

It will be pretty similar to be honest bud, and like you say a whole lot simpler at MOT time,

 

I happen to have some stock due Monday too

[RobPhoboS]

Find somewhere else to get MOT done

[rtbiscuit]

With the change in mot regs I would go HFC. Always fitted them. Swap would probably lose the rasp of the exhaust, which in my mind is a good thing.

[Dblock]

With the change in mot regs I would go HFC. Always fitted them. Swap would probably lose the rasp of the exhaust, which in my mind is a good thing.

 

HFC's rasp aswell. less though but its still there. Rasp sounds awesome IMO.

Edited April 13, 2013 by Dblock

[rtbiscuit]

With the change in mot regs I would go HFC. Always fitted them. Swap would probably lose the rasp of the exhaust, which in my mind is a good thing.

 

HFC's rasp aswell. less though but its still there. Rasp sounds awesome IMO.

Exhaust note is such a personal preference, to me it screams chav, and poorly designed exhaust. As rasp is a by product of a badly designed system.

[Dblock]

With the change in mot regs I would go HFC. Always fitted them. Swap would probably lose the rasp of the exhaust, which in my mind is a good thing.

 

HFC's rasp aswell. less though but its still there. Rasp sounds awesome IMO.

Exhaust note is such a personal preference, to me it screams chav, and poorly designed exhaust. As rasp is a by product of a badly designed system.

 

I disagree personally. Most saxos and corsas always have a boomy system. Rasp on a N/a engine sounds the nips like on a e46 m3 CSL. Ferraris and lambos rasp too.

[rtbiscuit]

Like I said exhaust note is a very personal preference I come at it from an engineering perspective and after having spent 6 months designing the exhaust that was put on my xkr I came to realise what works for me. What is one mans Chopin is another mans noise.

[jamzee500]

cheers for the advice guys.

[Ian]

Like I said exhaust note is a very personal preference I come at it from an engineering perspective and after having spent 6 months designing the exhaust that was put on my xkr I came to realise what works for me. What is one mans Chopin is another mans noise.

 

We both know how wrong you are when it comes to exhaust systems

[rtbiscuit]

Like I said exhaust note is a very personal preference I come at it from an engineering perspective and after having spent 6 months designing the exhaust that was put on my xkr I came to realise what works for me. What is one mans Chopin is another mans noise.

 

We both know how wrong you are when it comes to exhaust systems

from the man that proved arse is not just a word, but now a sound as well Edited April 15, 2013 by rtbiscuit

[Ian]

[Parsnip]

I have got a Nismo and HFCs on mine, No complaints about it being too quiet

 

Didn't sail through on emissions at the last MOT, but did pass with some coaxing.

 

They do rasp a bit - proper growl low down, through to a bit of raspyness getting up the rev range - to me it gives the impression that there is a lot of gas (and unburnt v-power ) going through, rather than poor design.

 

I have got a (crap) video if you want to compare, but it a phone one, done at night - could probably manage something better if you want a true comparison.

[rtbiscuit]

 

They do rasp a bit - proper growl low down, through to a bit of raspyness getting up the rev range - to me it gives the impression that there is a lot of gas (and unburnt v-power ) going through, rather than poor design.

 

 

Its not an impression its what causes the rasp, a well design exhaust can flow lots of gas and not rasp. a badly designed exhaust...rasps....fact

[Dblock]

They do rasp a bit - proper growl low down, through to a bit of raspyness getting up the rev range - to me it gives the impression that there is a lot of gas (and unburnt v-power ) going through, rather than poor design.

 

 

Its not an impression its what causes the rasp, a well design exhaust can flow lots of gas and not rasp. a badly designed exhaust...rasps....fact

 

Why?

[CrumbMC]

They do rasp a bit - proper growl low down, through to a bit of raspyness getting up the rev range - to me it gives the impression that there is a lot of gas (and unburnt v-power ) going through, rather than poor design.

 

 

Its not an impression its what causes the rasp, a well design exhaust can flow lots of gas and not rasp. a badly designed exhaust...rasps....fact

 

Why?

 

A 458 rasps. As does an LFA, 599 GTO.... the list goes on and on. i very much doubt they are considered as poorly designed. rasp with a crap exhast note will never sound good, but with a nice exhaust note, it can sound savage... which can sound there are nice exhasts that dont rasp, such as a masser GT. it's not for everyone.

 

I hate blue cheese and for ME, the mould ruins it. but try telling your average Frenchman that a bit of mould is a sign of a bad cheese.

[rtbiscuit]

Save typing my own answer here is someone elses who can put in words better than i can

 

Exhaust harmonics are dependent on a couple of factors including intake pulses and valve sizes. However the main factor involved is the pulse timing. An air pulse exiting the cylinder head creates a shock-wave that travels down the exhaust path and is follwed by a mometary vacuum pulse.

 

V8 and 4-cylinders have pistons that rise up in perfect pairs at the exact same time. That explanes the deep note that you get from since they perfectly sinc exhaust pulses together. V6s on the other hand, usually have offset or split crank-pins to even out the firing sequence. So pistons are slightly off-sinc which in tern causes the exhaust pulses to slightly off. This leads to more raspyness because of gaps in the air-pulses.

 

Exhaust design utilizes one of three designs to control exhaust sound. The first type (and most common) is absorbsion. These mufflers use sound absorbstion material and internal channels to smother out noise. Although quiet, they are restrictive and they are the least desirable of the three/

 

The second type is deflection style mufflers. Mufflers of these type use internal baffles to bounce sound waves back at the incomming pulses and cause them to nullify each other. Flow-master (AKA choke-master uses this design)

 

The last, and best of the bunch are the "resonator" type. Resonators fall into one of two catagories. The first is "helmholtz" and the second is mesh type.

 

The mesh type mufflers are what you usuallt hear on the rice burners. They utilize a flow-through design with a baffled pipe that passes through a cavity that is either filled with fiber-glass or stainless mesh to act as a silencer. "Cherry bomb" mufflers fall into this catagory.

 

The "helholtz" resonator uses a unique bulb shaped chamber which causes incomming air to suddenly expand over its inner surface area. As it enters the larger part of the bulb the shock wave slows down a bit. As the next shock-wave enters the chamber, the first wave exits through the other end, and speeds up as it leaves the bottle-neck. Both shock-waves mesh and causes the waves to mesh into one-another. A resonator gives a rich deep tone to your exhaust note while getting rid of unwanted "raspy" harmonics. That unique sound that you hear from a G35 can be attributed to this type of exhast design.

 

There are a few other factors like exhaust legnth, temperature and thermal expansion rates, but thats really the jist of it.

 

though tit easier to give examples and this thread just so happens to explain it http://www.350z-tech.com/forums/153-intake-exhaust/39837-raspy-exhaust-myths-facts-theories.html

 

to take extracts from said thread

 

Stock headers --- Non-Resonated Test Pipes ---Stock Y-Pipe --- Rasp = Yes

 

Stock headers --- Resonated Test Pipes --- Stock Y-Pipe --- Rasp = No

 

Stock headers --- Resonated Test Pipes --- Aftermarket Y-Pipe --- Rasp = Yes

 

Aftermarket headers welded to test pipes --- Non-Resonated Test Pipes --- Aftermarket Y-Pipe --- Rasp = No

 

(exception is this setup with the Borla dual)

Stock headers --- Resonated Test Pipes --- Borla Dual --- Rasp = No

 

If you look over this list carefully it seems that the test pipes are not necessarily the cause of the raspiness. I believe it is a mis-match between pipes bolted together. When i had my exhaust taken apart, I noted that all the pipe inside diameters were of different sizes. I put the resonated test pipes on and had zero rasp. I then added an aftermarket y-pipe and the rasp was there just as bad as if I had non-resonated pipes..

 

by playing with different parts shows that it is exhaust desing that causes rasp, get the design wrong, you get rasp, get it right, you don;t

 

hollowing out a catalytic converter will cause resonance problems. The symptom is most often seen as a hesitation, popping, or backfire at or about 3,200-3,500 RPM. Upon hard acceleration, the engine will appear to cut out for a split second as RPMs drop and a split second later recover and continue past 3,200-3,500 RPM. Additional symptoms are a loud and continual popping sound when the vehicle is allowed to decelerate under load and a whooshing sound that is delayed from or following the engine speed. This is due to the open chamber within the hollowed out catalytic converter case. It loads up and releases pressure in the exhaust system that acts almost as a capacitor. The open chamber allows the formation of standing waves within the chamber which reflects back toward the engine and causes the hesitation and miss in the RPM range.

The symptoms become more pronounced as the exhaust system is made freer flowing, meaning that hollowing out a cat with the stock exhaust may cause mild annoyance, while it becomes a serious problem when a header and performance exhaust system are installed, mimicking a ignition miss or backfire.

 

again another example of what causes noise in the exhaust.