I searched the forums and the web for a scientific reason for rasp and couldn't find any conclusive answer. If any of you have one for me please let me know.

We have all come to realize that opening up the exhaust (usually after losing the cats or adding after market headers) introduces rasp.

Theory:
The stock exhaust / cats/ headers are built to the engine size and exhaust flow. Throughout the RPMs the pipes are filled with pressure (because it can't escape fast enough) that neutralizes most all vibrations.

When you replace exhaust components, and free up air flow this pressure is lost. Sound waves bounce around because the engine can't fill the exhaust and rasp occurs. It is more apparent at specific RPMs because...

Low RPM 750-1800: I don't normally notice rasp here, not enough exhaust to create pressure but not enough to cause annoying rasp.

Mid RPM 1800-3000: usually the only place I have noticed rasp, there is a mild amount of exhaust to begin to fill the pipes but not enough to neutralize the vibrations in the metal.

High RPM 3000 - Max: I never notice rasp here either because the exhaust sound drowns it out or its not there. The pipes are now letting the engine breathe and its filling them up sufficiently to eliminate the vibrations caused by the engine pulse

I realized these categories are not properly distributed, I just made them to break them up into my own personally realized rasp areas. They will vary by exhaust components chosen, materials used and other supporting mods**

**I came to this conclusion when I added my pop charger after already having Strup headers/ AAM h-flow cats/ Bassani exhaust (x-pipe) and having a small amount of rasp before the system heated up. The rasp seemed to be introduced after the headers were installed. The rasp disappeared once the engine heated up, I think the metal expanding and being near the same temperature as the exhaust caused the vibrations to diffuse. The very interesting thing that led me to these conclusions is that 90% of my startup rasp disappeared after installing the Pop charger. It makes sense to me that if you are flowing more air in you can flow more air out.... hence the pipes will generally be more full with air at all times throughout the RPMs and rasp will be less noticeable.

I'm sorry If I rambled on so long and this is common sense to everyone already, honestly I did do a bit of research before posting.

I'm just wondering do you guys agree? Do you have a better explanation? Did I completely miss it and if I did can you please inform me on what exactly causes rasp instead of just saying TEST PIPES!

 

"Scientifical" ...?

 

1 - at first I thought this thread said 'Rape: a not so scientifical analysus'

2 - this thread would have been much better if that was true.

3 - there is already understood reason for rasp, you are pretty late to this party.

 

I never made it past "Scientifical".

 

 

please explain 3 so I may be enlightened.

I guess it wasn't funny but I was making fun of the fact that I was no scientist and thus this observation of mine was not to be taken seriously. I was merely trying to confirm my thoughts.

 

yes you are right, no one should take your observations seriously.

 

17,000 + posts and you have nothing useful to say?

 

I have been very useful in this thread I think.

 

I'm only here to help. so shoot: what is your question of the almighty motor?

 

thanks for your input, now I'd appreciate someone informing me on what I missed out on being "late to the party"

 

too slow. the answer is 42

 

but seriously, now ask me a question and I'll try to answer it. be quick though.

 

what, 49 + posts and you have nothing useful to ask?

 

It's hard to define it. It's very much an anomaly as far as exhausts are concerned, at least from a non-engineering stand point. I'm sure theres an acoustical reason that it turns up after removal of specific parts or installation of others.

For example, I've run some self R&D, and removal of the mid resonator causes instant rasp at anything above 50% throttle. Noticeable when free revving, and no hiding it under load. However leaving the exhaust stock, and removing just the muffler, causes none.

Based on that, and what we know about test pipes causing rasp, I'm going to venture a guess and say it's highly location specific. I doubt its a pipe vibration, as your theory suggests. If my suspicions of it being a local phenomena are correct, then if it were a vibration, one could simply dampen that part of the pipe, and it would get rid of the rasp.

 

T K... are you an engineer. I surely hope not with this paltry analysis leading you and since you decided to repeat it outloud - at least textually, if not sexually - and anyone who believes you, to the wrong conclusion.

 

Ok. Why don't you live up to your name and tell me "what causes exhaust rasp in our exhaust systems in particular?"
Is that a clear enough question?

 

the question has been asked multiple times if you read the thread. It would be nice if you could provide this thread with some substance instead of snarky responses

 

I wasn't talking to you.

 

oh, that question.

yeah, you were right on with what causes it.