What IS rasp?
Tech Resident
Joined: Jan 2006
Posts: 5,117
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From: Philly
as far as i know, rasp is caused mainly by two things:
the engine: our engines have a weird firing order. it's like 1-8-7-...something....the point is, the exhaust gases are leaving the engine unevenly. this is causing fluxuation in the amount of gases in the pipes at any given time. that means, when we rev it high and there is more gas in the passanger side of the exhaust than the driver's side of the exhaust, it shoots out more gas from the passanger's side. this big variation in pressure causes the BLAT BLAT.
the exhaust piping: exhaust piping simply adds to the rasp the engine already creates. if you use a y-pipe setup, you want to have the best merge possible. a bad merge will only make the differentiation of pressure in each section of the y pipe even greater. this just adds to the amount of rasp the engine is putting out.
the purpose of the muffler is to dampen the rasp. it does it in a few ways. the ways i understand is by quieting the exhaust note so the rasp is less noticable. the other way i can see it dampening is it collects the gas from each side into the center of the muffler creating a closer equalization of the exhaust pressure on each side...though this equalization is not perfect.
this is why true duals have a low occurrance of rasp. since they each flow out of an individual pipe and there is no merge, the only rasp that can be made is from the engine's uneven firing order which generally isn't much at all since you're only taking in one side of the engine per tube. rasp is the most evident when you take the entire engine and compress it all into one, uneven tube since each cylinder doesn't fire evenly per side.
the engine: our engines have a weird firing order. it's like 1-8-7-...something....the point is, the exhaust gases are leaving the engine unevenly. this is causing fluxuation in the amount of gases in the pipes at any given time. that means, when we rev it high and there is more gas in the passanger side of the exhaust than the driver's side of the exhaust, it shoots out more gas from the passanger's side. this big variation in pressure causes the BLAT BLAT.
the exhaust piping: exhaust piping simply adds to the rasp the engine already creates. if you use a y-pipe setup, you want to have the best merge possible. a bad merge will only make the differentiation of pressure in each section of the y pipe even greater. this just adds to the amount of rasp the engine is putting out.
the purpose of the muffler is to dampen the rasp. it does it in a few ways. the ways i understand is by quieting the exhaust note so the rasp is less noticable. the other way i can see it dampening is it collects the gas from each side into the center of the muffler creating a closer equalization of the exhaust pressure on each side...though this equalization is not perfect.
this is why true duals have a low occurrance of rasp. since they each flow out of an individual pipe and there is no merge, the only rasp that can be made is from the engine's uneven firing order which generally isn't much at all since you're only taking in one side of the engine per tube. rasp is the most evident when you take the entire engine and compress it all into one, uneven tube since each cylinder doesn't fire evenly per side.
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Verdad Gallardo Tech Resident
Joined: Jan 2006
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From: Philly
Originally Posted by STRIPSTAR
Hes pretty close to being right. Aluminum blocks add to it also for some reason.
the "aluminum factor" of the LS1 doesn't produce the rasp, it just hides it less since it's so thin and light. if the LS1 were the same motor, just iron, it would blanket that rasp by absorbing the sound waves better.
Originally Posted by staringback05
he pulled the "hence" factor...hmm
Originally Posted by ChocoTaco369
it's not just from unequal y pipe lengths. it's from the uneven firing order of the engines too.
Tech Resident
Joined: Jan 2006
Posts: 5,117
Likes: 3
From: Philly
Originally Posted by prince xizor
What about the 4 cyl ecotec engines being an uneven firing order? Most of the little cavaliers I hear around here are raspy. Or is that just a function of the fart cannon?
Originally Posted by BuBbABFP
hmmmhhmm a choco taco sounds good right now!
TECH Addict
Joined: Jun 2005
Posts: 2,660
Likes: 14
From: England
Originally Posted by orangeapeel
He stole it from 300/bhp probably. 
I think some of the factors where covered, but it's not an uneven firing order that that causes rasp. Most V8's including all previous SBC's have an uneven firing order. Meaning 2 cylinders on the same bank fire one after the other. The only V8's I know that have "even" firing orders are the single plain crank V8's from Ferrari, TVR and the now defunct Lotus V8 turbo.
The LS1 is unique because it has a different firing order to other SBC's (most American V8's in fact) but is still the same type of V8. Essentially two V4 engines.
I also don't believe aluminium affects rasp either as there have been many engines all the way back to the 1950's that don't suffer rasp the same.
The alternate view is LQx blocked engines are cast iron variants of LSx engines and they still suffer rasp.
Other factors would be heads, combustion chamber size/shape, cam, LSA, overlap, inlet/exhaust valve sizes, head flow and exhuast design.
As to what you are hearing? Well there was a good thread a while back in the Adv Tech forum on it.
But essentially an engine is a gioant air pump, suck in air - compress & ignite = explosion - expel exhaust gas. It does this rapidly from each cylinder.
Last edited by 300bhp/ton; Jul 7, 2006 at 10:04 AM.
TECH Addict
Joined: Jun 2005
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From: England
Originally Posted by Z2K_MrJ
its due to fireing order / Ironblock/Aluminblock.. EVer flick a Frying pan?
Honestly I really don't know. Assuming you are trying to say aluminium = rasp where cast iron doesn't as it seems to be the popular belief.
The Rover 215 OHV V8 also known as the Buick small block originally designed in the 50's and used in vehicles such as Land Rovers right up until a year or so ago never suffered rasp, it had an aluminium block.
The Jaguar V12 SOHC also had an aluminium block and never suffer rasp.
Same goes for the Aston Martin DOHC V8.
BMW/Mercedes Bends/Audi V8's are all aluminium and don't suffer rasp.
You can even buy aluminium variants of the Mopar big block yet they are totally raps free.
The Jaguar AJV8 found in todays Jaguars is aluminium and no rasp.
I can't remember but wasn't the LT5 V8 from the ZR1 aluminium?
TECH Fanatic
Joined: Aug 2005
Posts: 1,691
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From: Sunny London, UK
If you guys are talking about the noise when you get on and off the throttle, then its likely DFCO. Basically it ramps the spark in and out of optimal.
A retarded spark wont have enough time to burn all the charge, so it continues to combust down the exhaust.. and guess what.. it makes a noise. You can turn dfco off, but then you get no engine braking and end up burning fuel for no reason.
Maybe you can ramp spark in and out faster. A restrictive exhaust will muffle the sound but it still happens.
A retarded spark wont have enough time to burn all the charge, so it continues to combust down the exhaust.. and guess what.. it makes a noise. You can turn dfco off, but then you get no engine braking and end up burning fuel for no reason.
Maybe you can ramp spark in and out faster. A restrictive exhaust will muffle the sound but it still happens.
