Exhaust theory
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Joined: Jun 2000
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From: Youngstown, Ohio, USA
Exhaust theory
I was talking to another member about the modification of the Y-pipe while attempting to install the hooker headers I had been surpised to find in the storage cover in the back of the new Trans Am. The quote basically was, "The stock Y-pipe will kill the benefits of the headers". This is so, but few people know why, and this is to help explain exhaust system, design, theory and excecution.
We'll begin at the very start, the headers, the primaries. Primaries common come in basic sizes from 1 5/8" to around 2". The primary size is determined by engine displacment, rpm range, and it's power. A stock 305 will not breathe the same as a modified 427. Primary sizes need to be determined by your specific engine, most people start off with 1 5/8's and may move up to 1 3/4 and some high RPM 350's do support 1 7/8. If your primary tubes are too big you will kill low-end torque and since most people reading this will have a street machine rather than a dragstrip terror, smaller is usually better. The idea is to have primaries being the same length, however, this is hard to accomplish with most car designs having clearence problems. Long tube headers have longer primary tubes and do a better job of eliminating excess gas from the cylinders, however, these are for more advanced engines.
The primaries then join at the collector. The popular collector sizes go from 2 1/2" to 3 1/2". The major misconception here is that bigger is better. Once again, larger is not better. A 3 1/2" collector will be most likely to use a reducer to go into the Y-pipe (explained in the next paragraph). A 3 1/2" collector left open will kill low end torque for the majority of us. Many people use 3" unnessicarly. 2 1/2" is plenty!.
The Y-pipe joins the headers at the collectors and then combines them into one exhaust (dual exhaust explained further down). Now, the transition from collector to Y-pipe is usually accomplished through a device called a reducer. The thing about reducers is this, once gas hits a larger area it loses velocity and expands, which makes the transistion into a smaller pipe harder, requiring time for the gas to recirculate and build up pressure inside the "dead" area. The best transition to a Y-pipe is for the Y-pipe to be the same size as the header. Following this idea, of expansion and velocity, to keep the velocity in the Y-pipe constant, you need to keep the area of the pipe combined pipe either the same or larger. With 2.5" collectors transitioning into a 2.5" Y-pipe, the needed area would be sqrt(2(2.5/2 * pi)/pi)*2 means you would need the pipe they join into to be roughly larger than 3.5". Now, most of you will agree that 3.5" may even be too much for a car (contractdicting yourself with the math!) so if you feel 3.5" is too large, than you need smaller collectors (if there such a thing). Note: This is actually a volumetric measurement, however, I left out the length (l) because it would be a constant and thus divide out. Now, this means either step up the exhaust here slightly to encourage it to flow for pressure equilibrium or leave it the same.
Now we move onto the catalytic converter, I will not deal with this here, most cars looking for the minute performance which I list above will probably not be running a cat. However, a point of reference is you want your cat to have the same flow rate as your exhaust (if they even make one that good).
After the cat, you have the cat-back. This should be the same size or step up slightly as the last piece (getting the theory of stepped headers yet?). The muffler is an important choice, a muffler is not just to quite down the sound, but it also helps in exhaust scavaging (also dampened by the cat because sound has to go through an object, i.e. why there is no sound in space).
The crossover in a dual exhaust system is best accomplished by the H-pattern instead of the X. The less bends in the exhaust system the better, in the X it is forced to bend, in the H if the pressure variance is there it will have the option to flow over, not be forced to.
Sorry this got so long, happy reading.
------------------
1984 WS6 Trans Am Hartop
Former L69 Car under restoration
1984 WS6 Trans Am T-top car
4-bolt main 350, headers, Holley 650, T-5 and 3.23's.
Daily driver and restoration
We'll begin at the very start, the headers, the primaries. Primaries common come in basic sizes from 1 5/8" to around 2". The primary size is determined by engine displacment, rpm range, and it's power. A stock 305 will not breathe the same as a modified 427. Primary sizes need to be determined by your specific engine, most people start off with 1 5/8's and may move up to 1 3/4 and some high RPM 350's do support 1 7/8. If your primary tubes are too big you will kill low-end torque and since most people reading this will have a street machine rather than a dragstrip terror, smaller is usually better. The idea is to have primaries being the same length, however, this is hard to accomplish with most car designs having clearence problems. Long tube headers have longer primary tubes and do a better job of eliminating excess gas from the cylinders, however, these are for more advanced engines.
The primaries then join at the collector. The popular collector sizes go from 2 1/2" to 3 1/2". The major misconception here is that bigger is better. Once again, larger is not better. A 3 1/2" collector will be most likely to use a reducer to go into the Y-pipe (explained in the next paragraph). A 3 1/2" collector left open will kill low end torque for the majority of us. Many people use 3" unnessicarly. 2 1/2" is plenty!.
The Y-pipe joins the headers at the collectors and then combines them into one exhaust (dual exhaust explained further down). Now, the transition from collector to Y-pipe is usually accomplished through a device called a reducer. The thing about reducers is this, once gas hits a larger area it loses velocity and expands, which makes the transistion into a smaller pipe harder, requiring time for the gas to recirculate and build up pressure inside the "dead" area. The best transition to a Y-pipe is for the Y-pipe to be the same size as the header. Following this idea, of expansion and velocity, to keep the velocity in the Y-pipe constant, you need to keep the area of the pipe combined pipe either the same or larger. With 2.5" collectors transitioning into a 2.5" Y-pipe, the needed area would be sqrt(2(2.5/2 * pi)/pi)*2 means you would need the pipe they join into to be roughly larger than 3.5". Now, most of you will agree that 3.5" may even be too much for a car (contractdicting yourself with the math!) so if you feel 3.5" is too large, than you need smaller collectors (if there such a thing). Note: This is actually a volumetric measurement, however, I left out the length (l) because it would be a constant and thus divide out. Now, this means either step up the exhaust here slightly to encourage it to flow for pressure equilibrium or leave it the same.
Now we move onto the catalytic converter, I will not deal with this here, most cars looking for the minute performance which I list above will probably not be running a cat. However, a point of reference is you want your cat to have the same flow rate as your exhaust (if they even make one that good).
After the cat, you have the cat-back. This should be the same size or step up slightly as the last piece (getting the theory of stepped headers yet?). The muffler is an important choice, a muffler is not just to quite down the sound, but it also helps in exhaust scavaging (also dampened by the cat because sound has to go through an object, i.e. why there is no sound in space).
The crossover in a dual exhaust system is best accomplished by the H-pattern instead of the X. The less bends in the exhaust system the better, in the X it is forced to bend, in the H if the pressure variance is there it will have the option to flow over, not be forced to.
Sorry this got so long, happy reading.
------------------
1984 WS6 Trans Am Hartop
Former L69 Car under restoration
1984 WS6 Trans Am T-top car
4-bolt main 350, headers, Holley 650, T-5 and 3.23's.
Daily driver and restoration
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