Is Backpressure Critical?
01-17-2003, 10:48 PM
#1
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Join Date: Dec 2002
Posts: 28
I have heard from several people that backpressure is very critical on rotaries. I have done a search on here and read anything associated with backpressure, but no one really deals with the 12a specifically. My factory muffler is shot. It has a large hole in the front and the car is fairly loud, but oddly enough, it doesn't sound too obnoxious. I actually have had friends tell me they think it sounds pretty good. I don't like it. I have a nice Flowmaster 40 Series original style, non-Delta Flow, that I would love to see how it would work on there, but I don't want to fry my motor. If anyone could help me with any advice from experience, or another source, then please let me know. 
NoPistons Rocks!
01-18-2003, 01:18 AM
#2
to infinity & beyond, me
Join Date: Aug 2001
Location: santurce
Posts: 5,124
all cars need some sorta back pressure from what i understand. back pressure is significant on most 13Bs, cause the six port variations, use the backpressure to ope the 5 and 6 ports on their engines. the rotary, however, will benefit greatly from a frrer flowing exhaust.
01-18-2003, 01:42 AM
#3
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Location: Vanderhoof,BC(Canada)
Posts: 5,542
Treceb is right.Rotaries really don't gain from backpressure unless you have a 6-port engine and need it to open the ports(My sleeves are pulled for a reason).On my 12a I had this exhaust on it which is a full straight thru system with minimal pressure and it runs and screams better than it ever did.Make sure to leave the main cat with the air hose there so your car doesn't backfire as bad.Mine shoots a good flame after its warm and you shut it off because I pulled out all the cats.
01-18-2003, 02:11 PM
#5
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Thanks for the response. I'm trying to figure out what to do until I can afford a nice full exhaust system. Thank you j9fd3s aka mike, I'll have to see about using the Flowmaster now. I'm not looking for a loud system. My neighbors would appreciate a quieter system than my leaky muffler.
01-18-2003, 02:14 PM
#6
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Also, I have had people tell me that if you get a hole in the exhaust, if not fixed quickly, then the motor will let go. I'm not looking for that to happen even though I do have a spare car with a good motor. That is not something I want to deal with.
01-18-2003, 06:03 PM
#8
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Originally Posted by j9fd3s' date='Jan 18 2003, 12:21 PM
the racing beat mufflers are about the only bolt in setup this side of japan
mike
mike
01-18-2003, 06:04 PM
#9
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Originally Posted by nathandt' date='Jan 18 2003, 12:14 PM
Also, I have had people tell me that if you get a hole in the exhaust, if not fixed quickly, then the motor will let go. I'm not looking for that to happen even though I do have a spare car with a good motor. That is not something I want to deal with.
01-19-2003, 09:25 PM
#10
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Posts: 9
Okay, here is a chance to destroy the utter idiocy of thinking a motor needs ANY backpressure at ALL!
First of all, inferior piston engines have an exhaust event that lasts, say 3/4s of a revolution of the crank. So, every two full turns, the exhaust is only flowing for 3/4 of a turn or less. This makes it so the flow from the engine is extremely PULSED. Nothing, nothing, nothing, then an exhaust pulse, then nothing, etc, until the exhaust valve opens.
NEVER MISTAKE PULSED GAS FLOW FOR STEADY-STREAM GAS FLOW!!!!!!!!!!!!!!!
If your fluid, whether that fluid is a gas or a liquid, is flowing steadily, then the lowest resistance to flow will be an exhaust pipe that is infinite in diameter, and has a perfect vacuum on the other end. Backpressure is UTTERLY AND COMPLETELY USELESS, COUNTERPRODUCTIVE, AND A WASTE OF ENERGY.
The people who claim there is such a thing as "correct backpressure" are just wrong.
What they are not getting is the fact that the exhaust header tube gets exhaust flow for 270 degrees or less. Then, for the next 450 degrees of crank rotation, the exhaust valve is slammed shut, so, NO FLOW. Those 450 degrees are plenty of time for the exhaust to stop, turn around, and head back at the exhaust valve, just like a Slinky that was falling and you caught it by one end. It stretches, then snaps back up at your hand.
The lower the RPM of your piston motor, the smaller your exhaust header tubes have to be to maintain, not STUPID BACKPRESSURE, but
VELOCITY
VELOCITY
VELOCITY
VELOCITY
VELOCITY
VELOCITY
VELOCITY
The critical thing in exhaust gas flow in a piston engine is VELOCITY. On an 8 liter V8 engine, one liter or so of exhaust gas is getting dumped into each header tube per two engine revolutions. At lower RPMs, that liter of exhaust wants to stop, turn around, and bounce back at the exhaust valve, which is closed this is known as reversion.
If the exhaust header tube is too large, it is easy to do that because the liter of exhaust gas hasn't travelled far, and is not going very fast. So it is easy for it to turn around and head back. That is why low-rpm piston motors "need" smaller diameter headers than high-RPM motors. As RPM increases, the liters of exhaust gas per minute flowing out AND the speed at which it is flowing both increase, so the larger tube can be used, but it destroys lower-RPM power.
But, on the mighty rotary engine, the exhaust flow is fairly constant, changing a bit in volume as the apex seal first crosses over the exhaust port, the pressurized gas which was just making the rotor rotate is now suddenly given an exit, so it starts to rush out. Then, as the rotor continues to rotate, closing the exhaust chamber almost completely, the last of the exhaust gas is pushed out.
Rotary engines have FAAAAAAAAAAAAAAAAAAAR less pulsation in their exhaust gas flow than piston engines, so that means that their exhaust flow behaves more like constant flow. Which means what? THE BIGGER THE PIPE YOU CAN FIT, THE BETTER.
It is ironic that these little boys with their sissy Hondas have these McDonald's drink straw size (and quality) exhaust system leading to a manhole-sized 1000 dollar muffler, for an extra .0000000000000000000000000000000000000000000000000 005 hp gain.
Rotaries, quite on the other hand, can actually USE a huge-diameter exhaust system, like 4, 5 or 6 inch, since they behave more like a constant-flow pump than a piston engine.
Once again, the rotary proves its superiority.
But, even on a rotary, the ideal exhaust pipe would end in a perfect vacuum. And, since the flow of exhaust is slightly pulsed, due to the pressure difference between the just-opening chamber and the just-closing chamber both exposed to the same port at the same time, you need something smaller than infinite sized pipe for maintaining velocity right by the motor. But, the rest of the exhaust system can be fairly large, and ACTUALLY HAVE A SOUND PHYSICS REASON FOR BEING THAT WAY. Unlike the above-mentioned Hondas.
And, the point of juggling pipe diameters and such is to maintain velocity, not "CORRECT BACKPRESSURE". That is like a doctor saying "You have just the right amount of cancer". It's incorrect.
And, the more rotors you have, the easier it is to maintain velocity in your system, and scavenge the little bit of residual exhaust even more.
I mean, think of it. Supposing you have an engine, and it is on a space station experimental dyno. and its intake is a normal intake, in normal atmosphere. And the exhaust ports go straight into full-vacuum outer space. How much exhaust gas would remain in the rotor chamber after rotating past the port? NONE. So, how much charge dilution would you have, exhaust contaminating the fresh intake charge? NONE. How much heat would be transferred from the exhaust gas directly to the intake charge? NONE! No exhaust gas left! It's on its way to Pluto, or wherever.
So, backpressure sucks. Any amount of it at all. The ideal exhaust system on a rotary would have a perfect vacuum right at the exhaust port, or something as close as possible to it.
First of all, inferior piston engines have an exhaust event that lasts, say 3/4s of a revolution of the crank. So, every two full turns, the exhaust is only flowing for 3/4 of a turn or less. This makes it so the flow from the engine is extremely PULSED. Nothing, nothing, nothing, then an exhaust pulse, then nothing, etc, until the exhaust valve opens.
NEVER MISTAKE PULSED GAS FLOW FOR STEADY-STREAM GAS FLOW!!!!!!!!!!!!!!!
If your fluid, whether that fluid is a gas or a liquid, is flowing steadily, then the lowest resistance to flow will be an exhaust pipe that is infinite in diameter, and has a perfect vacuum on the other end. Backpressure is UTTERLY AND COMPLETELY USELESS, COUNTERPRODUCTIVE, AND A WASTE OF ENERGY.
The people who claim there is such a thing as "correct backpressure" are just wrong.
What they are not getting is the fact that the exhaust header tube gets exhaust flow for 270 degrees or less. Then, for the next 450 degrees of crank rotation, the exhaust valve is slammed shut, so, NO FLOW. Those 450 degrees are plenty of time for the exhaust to stop, turn around, and head back at the exhaust valve, just like a Slinky that was falling and you caught it by one end. It stretches, then snaps back up at your hand.
The lower the RPM of your piston motor, the smaller your exhaust header tubes have to be to maintain, not STUPID BACKPRESSURE, but
VELOCITY
VELOCITY
VELOCITY
VELOCITY
VELOCITY
VELOCITY
VELOCITY
The critical thing in exhaust gas flow in a piston engine is VELOCITY. On an 8 liter V8 engine, one liter or so of exhaust gas is getting dumped into each header tube per two engine revolutions. At lower RPMs, that liter of exhaust wants to stop, turn around, and bounce back at the exhaust valve, which is closed this is known as reversion.
If the exhaust header tube is too large, it is easy to do that because the liter of exhaust gas hasn't travelled far, and is not going very fast. So it is easy for it to turn around and head back. That is why low-rpm piston motors "need" smaller diameter headers than high-RPM motors. As RPM increases, the liters of exhaust gas per minute flowing out AND the speed at which it is flowing both increase, so the larger tube can be used, but it destroys lower-RPM power.
But, on the mighty rotary engine, the exhaust flow is fairly constant, changing a bit in volume as the apex seal first crosses over the exhaust port, the pressurized gas which was just making the rotor rotate is now suddenly given an exit, so it starts to rush out. Then, as the rotor continues to rotate, closing the exhaust chamber almost completely, the last of the exhaust gas is pushed out.
Rotary engines have FAAAAAAAAAAAAAAAAAAAR less pulsation in their exhaust gas flow than piston engines, so that means that their exhaust flow behaves more like constant flow. Which means what? THE BIGGER THE PIPE YOU CAN FIT, THE BETTER.
It is ironic that these little boys with their sissy Hondas have these McDonald's drink straw size (and quality) exhaust system leading to a manhole-sized 1000 dollar muffler, for an extra .0000000000000000000000000000000000000000000000000 005 hp gain.
Rotaries, quite on the other hand, can actually USE a huge-diameter exhaust system, like 4, 5 or 6 inch, since they behave more like a constant-flow pump than a piston engine.
Once again, the rotary proves its superiority.
But, even on a rotary, the ideal exhaust pipe would end in a perfect vacuum. And, since the flow of exhaust is slightly pulsed, due to the pressure difference between the just-opening chamber and the just-closing chamber both exposed to the same port at the same time, you need something smaller than infinite sized pipe for maintaining velocity right by the motor. But, the rest of the exhaust system can be fairly large, and ACTUALLY HAVE A SOUND PHYSICS REASON FOR BEING THAT WAY. Unlike the above-mentioned Hondas.
And, the point of juggling pipe diameters and such is to maintain velocity, not "CORRECT BACKPRESSURE". That is like a doctor saying "You have just the right amount of cancer". It's incorrect.
And, the more rotors you have, the easier it is to maintain velocity in your system, and scavenge the little bit of residual exhaust even more.
I mean, think of it. Supposing you have an engine, and it is on a space station experimental dyno.
So, backpressure sucks. Any amount of it at all. The ideal exhaust system on a rotary would have a perfect vacuum right at the exhaust port, or something as close as possible to it.