Back Pressure Necessary?
03-14-07, 12:18 AM
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Back Pressure Necessary?
I'm currently building my 7 and I had a quick question pass through my head, but no one can tell me if i'm thinking right. Back pressure on a piston engine helps with compression because of the recipericating motion and the delay in cams hitting the valves (I think that's a correct understanding but I might be well off base). In which case back pressure on a rotary does nill if not robs the engine of potential horse power. If you have a stock exhaust system there is a certain amount of back pressure that occurs there so if you open up the exhaust you get more power from the engine (is this thinking right?).
My question comes down to this, would it prove detremental to the engine if I put in an electric sylinoid valve prior to the cats to relieve excess back pressure? IE, flip the switch when I want an extra few HP and flip the switch when I want to pass emissions?
My question comes down to this, would it prove detremental to the engine if I put in an electric sylinoid valve prior to the cats to relieve excess back pressure? IE, flip the switch when I want an extra few HP and flip the switch when I want to pass emissions?
03-14-07, 12:20 PM
#5
Originally Posted by jgrewe
No engine runs better with or needs back pressure. Sometimes stuff outside the engine needs it to work though.
Well, not really that easy. Back pressure doesn't necassarily give or take horsepower. It's all about the exhaust dynamics. ON NA cars with high flow heads and large exhaust valves(anything over 1.75 " or so) back pressure helps keep the exhaust near the port longer and allows it to heat more evenly. When this occurs you can maximize the exhaust gas velocity coming out and it helps scavenge the gasses out of the head. When you maximize this scavenging effect the exhaust flow maintains a nice flow that allows the exhaust valves to move more freely(less residual exhaust gasses in the port following combustion) There by allowing for better torque production. In a fairly large and free flowing head you will gain power by lowering the back pressure and allowing the exhaust to exit quickly. It really depends on the whole combination when it comes to this topic. In a fairly well modified na V8 reduction of backpressure will result in a gain in hp and not suffer much if any negative effect.
03-14-07, 12:48 PM
#6
Backpressure isn't important. It's the enemy of performance. Show me a race car with backpressure and I'll show you a losing race car.
Backpressure as many think of it on an exhaust system is a bit misunderstood. What people get confused about is assuming that a smaller pipe gives more backpressure which gives more low end and then draw a conclusion that it must have been the backpressure that gave the low end gain. Then this way of thinking gets passed on to others who also believe it and so forth and so on. What really makes power is exhaust gas velocity.
Remember that an engine has different airflow needs based on both rpm and load. A system that makes low backpressure at redline will definitely make none at 3000 rpm. Since exhaust velocity is low at this rpm, power suffers. You just don't get a good suction from high velocity exhaust scavenging. The only tuning you are getting is through resonance tuning. Remember there are 2 different types of tuning waves to take advantage of. If we make this pipe smaller so that power is good at 3000 rpm, it is because exhaust velocity is higher and it is getting a better scavenging through this. However a pipe that is sized for 3000 rpm is grosly undersized for redline. This shows up as backpressure at that point. This confuses people.
Faster exhaust gasses mean that they are lower in pressure. A lower pressure exhaust coupled to a higher pressure exhaust port means gasses will try to flow through that port. Everything moves from high pressure to low pressure. There does come a point though in trying to go too small that you start hitting the negative effects of backpressure. We see them as pumping losses as suddenly it is taking effort to forced the exhaust through those pipes. The key is in designing a system that is focused right where you want it. An exhaust system is a compromise. If you want good low end, top end will suffer and vice versa. This of course is based on a comparison to optimal efficiency and not what the car may have come with. We need to make our frame of reference clear. The perfect exhaust system would be a shape shifter in which would get shorter and increase in area as the rpm's rise. When you find a pipe that can do that, then you'll have the perfect setup.
Backpressure never helps at any rpm that you are fighting against it. You don't want it. What you do want is exhaust gas velocity. If you have a bypass valve that goes around a cat rather than dumps out into the atmosphere, then you may get some benefit. A cat is a restriction. It slows down flow and raises pressure in the system. An exhaust still tunes even after the header. It's due to velocity. You should try to keep all of your exhaust flowing out the back unless of course you have designed a new system that is intended to be short. It's very easy to just set up a cat where it can be installed one day a year. I used to do that. 4 bolts and about 10 minutes of time was all it took.
Backpressure as many think of it on an exhaust system is a bit misunderstood. What people get confused about is assuming that a smaller pipe gives more backpressure which gives more low end and then draw a conclusion that it must have been the backpressure that gave the low end gain. Then this way of thinking gets passed on to others who also believe it and so forth and so on. What really makes power is exhaust gas velocity.
Remember that an engine has different airflow needs based on both rpm and load. A system that makes low backpressure at redline will definitely make none at 3000 rpm. Since exhaust velocity is low at this rpm, power suffers. You just don't get a good suction from high velocity exhaust scavenging. The only tuning you are getting is through resonance tuning. Remember there are 2 different types of tuning waves to take advantage of. If we make this pipe smaller so that power is good at 3000 rpm, it is because exhaust velocity is higher and it is getting a better scavenging through this. However a pipe that is sized for 3000 rpm is grosly undersized for redline. This shows up as backpressure at that point. This confuses people.
Faster exhaust gasses mean that they are lower in pressure. A lower pressure exhaust coupled to a higher pressure exhaust port means gasses will try to flow through that port. Everything moves from high pressure to low pressure. There does come a point though in trying to go too small that you start hitting the negative effects of backpressure. We see them as pumping losses as suddenly it is taking effort to forced the exhaust through those pipes. The key is in designing a system that is focused right where you want it. An exhaust system is a compromise. If you want good low end, top end will suffer and vice versa. This of course is based on a comparison to optimal efficiency and not what the car may have come with. We need to make our frame of reference clear. The perfect exhaust system would be a shape shifter in which would get shorter and increase in area as the rpm's rise. When you find a pipe that can do that, then you'll have the perfect setup.
Backpressure never helps at any rpm that you are fighting against it. You don't want it. What you do want is exhaust gas velocity. If you have a bypass valve that goes around a cat rather than dumps out into the atmosphere, then you may get some benefit. A cat is a restriction. It slows down flow and raises pressure in the system. An exhaust still tunes even after the header. It's due to velocity. You should try to keep all of your exhaust flowing out the back unless of course you have designed a new system that is intended to be short. It's very easy to just set up a cat where it can be installed one day a year. I used to do that. 4 bolts and about 10 minutes of time was all it took.
03-14-07, 05:56 PM
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Originally Posted by rotarygod
Backpressure isn't important. It's the enemy of performance. Show me a race car with backpressure and I'll show you a losing race car.
Backpressure as many think of it on an exhaust system is a bit misunderstood. What people get confused about is assuming that a smaller pipe gives more backpressure which gives more low end and then draw a conclusion that it must have been the backpressure that gave the low end gain. Then this way of thinking gets passed on to others who also believe it and so forth and so on. What really makes power is exhaust gas velocity.
Remember that an engine has different airflow needs based on both rpm and load. A system that makes low backpressure at redline will definitely make none at 3000 rpm. Since exhaust velocity is low at this rpm, power suffers. You just don't get a good suction from high velocity exhaust scavenging. The only tuning you are getting is through resonance tuning. Remember there are 2 different types of tuning waves to take advantage of. If we make this pipe smaller so that power is good at 3000 rpm, it is because exhaust velocity is higher and it is getting a better scavenging through this. However a pipe that is sized for 3000 rpm is grosly undersized for redline. This shows up as backpressure at that point. This confuses people.
Faster exhaust gasses mean that they are lower in pressure. A lower pressure exhaust coupled to a higher pressure exhaust port means gasses will try to flow through that port. Everything moves from high pressure to low pressure. There does come a point though in trying to go too small that you start hitting the negative effects of backpressure. We see them as pumping losses as suddenly it is taking effort to forced the exhaust through those pipes. The key is in designing a system that is focused right where you want it. An exhaust system is a compromise. If you want good low end, top end will suffer and vice versa. This of course is based on a comparison to optimal efficiency and not what the car may have come with. We need to make our frame of reference clear. The perfect exhaust system would be a shape shifter in which would get shorter and increase in area as the rpm's rise. When you find a pipe that can do that, then you'll have the perfect setup.
Backpressure never helps at any rpm that you are fighting against it. You don't want it. What you do want is exhaust gas velocity. If you have a bypass valve that goes around a cat rather than dumps out into the atmosphere, then you may get some benefit. A cat is a restriction. It slows down flow and raises pressure in the system. An exhaust still tunes even after the header. It's due to velocity. You should try to keep all of your exhaust flowing out the back unless of course you have designed a new system that is intended to be short. It's very easy to just set up a cat where it can be installed one day a year. I used to do that. 4 bolts and about 10 minutes of time was all it took.
Backpressure as many think of it on an exhaust system is a bit misunderstood. What people get confused about is assuming that a smaller pipe gives more backpressure which gives more low end and then draw a conclusion that it must have been the backpressure that gave the low end gain. Then this way of thinking gets passed on to others who also believe it and so forth and so on. What really makes power is exhaust gas velocity.
Remember that an engine has different airflow needs based on both rpm and load. A system that makes low backpressure at redline will definitely make none at 3000 rpm. Since exhaust velocity is low at this rpm, power suffers. You just don't get a good suction from high velocity exhaust scavenging. The only tuning you are getting is through resonance tuning. Remember there are 2 different types of tuning waves to take advantage of. If we make this pipe smaller so that power is good at 3000 rpm, it is because exhaust velocity is higher and it is getting a better scavenging through this. However a pipe that is sized for 3000 rpm is grosly undersized for redline. This shows up as backpressure at that point. This confuses people.
Faster exhaust gasses mean that they are lower in pressure. A lower pressure exhaust coupled to a higher pressure exhaust port means gasses will try to flow through that port. Everything moves from high pressure to low pressure. There does come a point though in trying to go too small that you start hitting the negative effects of backpressure. We see them as pumping losses as suddenly it is taking effort to forced the exhaust through those pipes. The key is in designing a system that is focused right where you want it. An exhaust system is a compromise. If you want good low end, top end will suffer and vice versa. This of course is based on a comparison to optimal efficiency and not what the car may have come with. We need to make our frame of reference clear. The perfect exhaust system would be a shape shifter in which would get shorter and increase in area as the rpm's rise. When you find a pipe that can do that, then you'll have the perfect setup.
Backpressure never helps at any rpm that you are fighting against it. You don't want it. What you do want is exhaust gas velocity. If you have a bypass valve that goes around a cat rather than dumps out into the atmosphere, then you may get some benefit. A cat is a restriction. It slows down flow and raises pressure in the system. An exhaust still tunes even after the header. It's due to velocity. You should try to keep all of your exhaust flowing out the back unless of course you have designed a new system that is intended to be short. It's very easy to just set up a cat where it can be installed one day a year. I used to do that. 4 bolts and about 10 minutes of time was all it took.
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03-14-07, 06:37 PM
#8
It sounds like you are descrbing a pipe that expands as it gets longer. That's a megaphone. What I was referring to was that in a perfect world, our pipe would physically change shape. At low rpms it would be long and have a smaller diameter. At high rpms it would be short and have a larger diameter. Technology isn't there yet.
You can get a benefit throughout the powerband with a good exhaust design. Remember we don't live in a perfect world. No exhaust is perfect. You are looking for the MOST benefit at the point in the powerband where you want your power to be. Keep in mind that although this system may suffer at other rpms compared to a system that was designed to work at those spots, it will still probably be better than stock. Every exhaust system is a compromise.
What type of porting do you have? What rpm are you going to shift at? How quiet does it need to be? I see you need to worry about emissions so I'm assuming that you've probably got stock or streetports. Do you have a turbo? A few answers can really narrow down what type of system would work nicely for you.
You can get a benefit throughout the powerband with a good exhaust design. Remember we don't live in a perfect world. No exhaust is perfect. You are looking for the MOST benefit at the point in the powerband where you want your power to be. Keep in mind that although this system may suffer at other rpms compared to a system that was designed to work at those spots, it will still probably be better than stock. Every exhaust system is a compromise.
What type of porting do you have? What rpm are you going to shift at? How quiet does it need to be? I see you need to worry about emissions so I'm assuming that you've probably got stock or streetports. Do you have a turbo? A few answers can really narrow down what type of system would work nicely for you.
03-14-07, 11:32 PM
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i recently got a fix-it ticket in my car and i had to change my exhaust system back to "stock." the muffler shop put a 3 foot resignator and an after market exhaust with bolt-on flanges so that i can swap them back and forth... and i wanted to tell you that i hated it... my gsl-se full exhaust set-up from racing beat is A LOT FASTER... but being that i notice a HUGE difference with different exhaust set-ups. i'm trying to build an exhaust set-up that will show FULL potential of my engine. 13B monster street port with 48IDA Webers. Does anyone have any suggestions? I heard 3-inch? what muffler?
03-15-07, 09:47 AM
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Originally Posted by rotarygod
What type of porting do you have? What rpm are you going to shift at? How quiet does it need to be? I see you need to worry about emissions so I'm assuming that you've probably got stock or streetports. Do you have a turbo? A few answers can really narrow down what type of system would work nicely for you.
What I'm a little confused on is if the exhaust gases are traveling faster and have less pressure associated with them wouldn't a short pipe work just as well as a long one? Or do long pipes help with "pulling" the extra gas through the pipe at a higher velocity?
03-15-07, 01:08 PM
#11
The length of the pipe has to do with how many exhaust pulses are in the system. Remember we've got 2 different pressure waves in the system. The first is sound waves and they obviously travel at the speed of sound. The next is the actual pressure wave from the moving exhaust gasses. The speed of these waves depends on engine rpm. On a short system, there is only a very narrow range in which you can get any benefit from as the wavelengths can not be very long in the system. A longer system will give you a wider rpm range of tunability. It doesn't necessarily mean that you'll make less top end power though so don't think that. Every system tunes at multiple rpms and not just one.
For your engine, there is really no point in going with a pipe larger than 2.5" after the header collects. If you are only running a single exhaust then this should be the pipe size you run. If you still have a Y-pipe and intend to have a new one made, use dual 2" or at the most 2.25" pipes here. Keep in mind that the 2.5" single pipe ahead of this is the determining factor in tuning. You don't need to go too large. Don't be tempted to use a 3" pipe. Unless you are getting into high rpm bridgeport or peripheral ports, you don't need it. You'd lose usable power on a stock engine. The nice thing about this setup is that it would also work great with the stock TII turbo. No problems there.
Mufflers don't have to be loud to make power. The biggest misconception that people make is in assuming a muffler must be better flowing if it is straight through. This isn't always true. It also isn't always true that a non straight through muffler will hurt power. People like to buy loud mufflers because they think they flow more and hence make more power. There are actualy straight through mufflers that flow terrible! A louvered core muffler is a good example of this. if you want a muffler, look for one that uses a performated core. Stay away from the louvers at all costs! Mufflers that have inserts in the back that reduce noise are also terrible. With thos inserts in place, you'd be lucky to get more power than stock mufflers. They are that bad. Peopel think they still have power because they've still got much of the noise and rattles but the power just isn't there.
If you don't want loud, a great muffler or pair of mufflers to use would be the Dynomax superturbo mufflers. They flow plenty and can quiet even the loudest of systems. They are also relatively cheap. Don't buy into hype that just because they are fiberglass that they will fall apart. I've used them for years. on ther reverse side I've seen stainless mufflers such as Edelbrocks fall apart fairly quickly. You don't want to use a fiberglass muffler as a presilencer near the engine but at the back of the car they are fine. My old Brullen mufflers were fiberglass packed and they too never had any issues. When a muffler is at the back of the car, how it's made is just as important as what it's made out of.
The reason why this type of muffler may not hurt power at all is easy. We see people throw flow numbers around and make guesses but if they aren't testing they are guessing. One test result is worth more than a thousand expert opinions. Most people here guess and then pass on their theories as if they were known facts. From flow testing let me tell you some things I've seen. I'm not going to find any actual numbers but let me throw out some scenarios here so you get the idea.
What if we have 2 mufflers, 1 straight through and one chambered. Most people will say that the straight through flows more since it is a direct shot through it. Let's say it is a good design and not a louvered core. Now we go do our testing to see which one is better. Through testing it's found that muffler A which is the straight through flows 405 cfm at our reference pressure. Then we flow muffler B which is chambered and it flows 340 cfm. Logically people will say muffler A must be better than muffler B since it flow more. Is it? What if our engine's flow at max rpm (adjusted to fit with the reference muffler test pressure) revealed that we only flowed 320 cfm? That's all we need. Both mufflers flow more than that. Is one really going to make anymore power over the other one? More is not always better or necessary. See how this works? You can't just go solely off of looks and assumptions. There are mufflers out there that don't flow enough, no one is arguing that. Just don't judge a muffler's potential on your car by it's configuration. It is very possible to have a quiet exhaust system that makes power. Of course it is also very possible to have a loud one too!
For your engine, there is really no point in going with a pipe larger than 2.5" after the header collects. If you are only running a single exhaust then this should be the pipe size you run. If you still have a Y-pipe and intend to have a new one made, use dual 2" or at the most 2.25" pipes here. Keep in mind that the 2.5" single pipe ahead of this is the determining factor in tuning. You don't need to go too large. Don't be tempted to use a 3" pipe. Unless you are getting into high rpm bridgeport or peripheral ports, you don't need it. You'd lose usable power on a stock engine. The nice thing about this setup is that it would also work great with the stock TII turbo. No problems there.
Mufflers don't have to be loud to make power. The biggest misconception that people make is in assuming a muffler must be better flowing if it is straight through. This isn't always true. It also isn't always true that a non straight through muffler will hurt power. People like to buy loud mufflers because they think they flow more and hence make more power. There are actualy straight through mufflers that flow terrible! A louvered core muffler is a good example of this. if you want a muffler, look for one that uses a performated core. Stay away from the louvers at all costs! Mufflers that have inserts in the back that reduce noise are also terrible. With thos inserts in place, you'd be lucky to get more power than stock mufflers. They are that bad. Peopel think they still have power because they've still got much of the noise and rattles but the power just isn't there.
If you don't want loud, a great muffler or pair of mufflers to use would be the Dynomax superturbo mufflers. They flow plenty and can quiet even the loudest of systems. They are also relatively cheap. Don't buy into hype that just because they are fiberglass that they will fall apart. I've used them for years. on ther reverse side I've seen stainless mufflers such as Edelbrocks fall apart fairly quickly. You don't want to use a fiberglass muffler as a presilencer near the engine but at the back of the car they are fine. My old Brullen mufflers were fiberglass packed and they too never had any issues. When a muffler is at the back of the car, how it's made is just as important as what it's made out of.
The reason why this type of muffler may not hurt power at all is easy. We see people throw flow numbers around and make guesses but if they aren't testing they are guessing. One test result is worth more than a thousand expert opinions. Most people here guess and then pass on their theories as if they were known facts. From flow testing let me tell you some things I've seen. I'm not going to find any actual numbers but let me throw out some scenarios here so you get the idea.
What if we have 2 mufflers, 1 straight through and one chambered. Most people will say that the straight through flows more since it is a direct shot through it. Let's say it is a good design and not a louvered core. Now we go do our testing to see which one is better. Through testing it's found that muffler A which is the straight through flows 405 cfm at our reference pressure. Then we flow muffler B which is chambered and it flows 340 cfm. Logically people will say muffler A must be better than muffler B since it flow more. Is it? What if our engine's flow at max rpm (adjusted to fit with the reference muffler test pressure) revealed that we only flowed 320 cfm? That's all we need. Both mufflers flow more than that. Is one really going to make anymore power over the other one? More is not always better or necessary. See how this works? You can't just go solely off of looks and assumptions. There are mufflers out there that don't flow enough, no one is arguing that. Just don't judge a muffler's potential on your car by it's configuration. It is very possible to have a quiet exhaust system that makes power. Of course it is also very possible to have a loud one too!
03-15-07, 03:29 PM
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okay so the pressure waves are what the lengths of the pipes are all about. I like the idea of dual exhaust and not having the Y-pipe (I may be doing dual turbo, but it's still up in the air which means the dual pipe might not be necessary, in which case the single would work fine for me). My personal preference would be that of a quite car that sounds maybe just a tad louder than stock, but deffinatly gives me some horse power. Which leads me to my next question with a properly tuned exhaust would there be an increase of torque as well?
03-15-07, 05:49 PM
#13
The General RE
The goal of any good exhaust system is to have as little backpressure as possible in the RPM range that the engine is supposed to be tuned for. Basic rarefactive wave tuned headers work pretty well to achieve this if properly designed and constructed.
The system that worked well for my 12A SP was a short primary system made from a RB header cut up and welded back together so it actually met the specs they list in their catalog. Megaphoned it to 3.5" ran a decent length with no flow disturbances to a presilencer and split to dual 2.5" mufflers at the back.
When people talk about running a 2.5" exhaust and loosing torque to me that means they don't have a proper header. The throat diameter of the header should be the limiting orifice not the main piping. The pipe down stream should be as big as you can stand it being loud.
The system that worked well for my 12A SP was a short primary system made from a RB header cut up and welded back together so it actually met the specs they list in their catalog. Megaphoned it to 3.5" ran a decent length with no flow disturbances to a presilencer and split to dual 2.5" mufflers at the back.
When people talk about running a 2.5" exhaust and loosing torque to me that means they don't have a proper header. The throat diameter of the header should be the limiting orifice not the main piping. The pipe down stream should be as big as you can stand it being loud.
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