Engine school - presented by sdhotwn
Dec 3, 2003, 10:30 AM
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Engine school - presented by sdhotwn
This question comes up a lot from new and old members alike. So I thought a thread dedicated to it would be a good idea.
Here is some explanation I borrowed on 12/3/03 from http://modernmusclecars.net/articles...tdynamics.html
by: ragtop
Contrary to common popular belief, a larger exhaust does not always result in better engine performance. There are situations where an engine could loose low-end torque from "too little" backpressure. I know it seems counter-intuitive to think that an engine actually needs a certain amount of backpressure but here's why. Most engines are set up from the factory for a certain level of backpressure. Changing the exhaust can create a situation where the cam has too much overlap for the RPM range it's being driven in. In that case, the incoming fuel/air will come in the intake valve, only to have part of it sucked straight out the exhaust valve without getting burned! Obviously, engine power will suffer if it doesn't get a full fuel charge to burn. Proper backpressure will prevent this. So will choosing a more appropriate cam for your RPM range, however.
There is another, more complex reason why 5" pipes on a normal sized naturally aspirated engine won't work. It isn't that the engine needs more backpressure, it's that static pressure is only half the equation. The equation for the "equivalent" pressure at the exhaust port is P-pv^2, the static pressure minus the exhaust density times the square of the exhaust velocity at the port. A 5" pipe may see a slight reduction in static pressure but will kill off the velocity making it harder to push the exhaust out. Some tuned header systems can make the exhaust velocity high enough that the engine effectively has a lower amount of backpressure than the atmospheric pressure! This is known as exhaust scavenging and is what separates good headers from bad ones.
In the case of a turbocharged car, everything is different. They can run a huge exhaust pipe, like the HKS 5" pipes and see a performance gain rather than loss. The reason is the turbine. The exhaust coming out of the cylinders only sees the velocity going into the turbine. The velocity drop across the turbine doesn’t effect flow. On a turbocharged engine, there is no need to worry about the exhaust velocity downstream of the turbine. The size of the header primary tubes and collector, or the exhaust manifold design, still plays a larger role in determining exhaust velocity, but the pipes downstream of the turbo are a lot less important.
As for the turbo itself, you want to maximize the pressure (and temperature) difference across the turbine for the highest efficiency. A low velocity of the gasses exiting the turbine won't make it any less efficient; in fact, it can theoretically improve the efficiency of the turbine. So the exhaust on a turbocharged car can be designed to minimize static pressure, without concern for the exhaust velocity.
So what does all of this mean? It means that while a 5" exhaust would be desirable for a 350 hp turbocharged Supra, it would be a very bad idea on a 350 hp naturally aspirated Camaro and an awful idea on a 1.8 liter naturally aspirated Honda.
-ragtop
Not my words... so ragtop at the address above are where the credit goes.
EDIT:
Some of what he wrote is a bit convoluted to understand thorougly. What he is really arguing for is that you need a high velocity of your exiting gases, that's the entire purpose of the exhaust system... to evacuate gases at a high velocity. Without that you lose power and low end torque. Static pressure is what he is referring to as back pressure, but that is kind of a mislable. What he is referring to is the labeling of pressures as they would be on a pitot tube when using the pitot tube for measuring velocity. In that case you have one pressure point that is considered to be "static" and that is what he is calling both the static pressure and the backpressure... but that pressure is entirely velocity dependent. The higher your velocity the higher static pressure you have on a pitot tube.
See also - http://www.automotiveforums.com/vbulletin/t13199.html
Discuss this here.
Here is some explanation I borrowed on 12/3/03 from http://modernmusclecars.net/articles...tdynamics.html
by: ragtop
Contrary to common popular belief, a larger exhaust does not always result in better engine performance. There are situations where an engine could loose low-end torque from "too little" backpressure. I know it seems counter-intuitive to think that an engine actually needs a certain amount of backpressure but here's why. Most engines are set up from the factory for a certain level of backpressure. Changing the exhaust can create a situation where the cam has too much overlap for the RPM range it's being driven in. In that case, the incoming fuel/air will come in the intake valve, only to have part of it sucked straight out the exhaust valve without getting burned! Obviously, engine power will suffer if it doesn't get a full fuel charge to burn. Proper backpressure will prevent this. So will choosing a more appropriate cam for your RPM range, however.
There is another, more complex reason why 5" pipes on a normal sized naturally aspirated engine won't work. It isn't that the engine needs more backpressure, it's that static pressure is only half the equation. The equation for the "equivalent" pressure at the exhaust port is P-pv^2, the static pressure minus the exhaust density times the square of the exhaust velocity at the port. A 5" pipe may see a slight reduction in static pressure but will kill off the velocity making it harder to push the exhaust out. Some tuned header systems can make the exhaust velocity high enough that the engine effectively has a lower amount of backpressure than the atmospheric pressure! This is known as exhaust scavenging and is what separates good headers from bad ones.
In the case of a turbocharged car, everything is different. They can run a huge exhaust pipe, like the HKS 5" pipes and see a performance gain rather than loss. The reason is the turbine. The exhaust coming out of the cylinders only sees the velocity going into the turbine. The velocity drop across the turbine doesn’t effect flow. On a turbocharged engine, there is no need to worry about the exhaust velocity downstream of the turbine. The size of the header primary tubes and collector, or the exhaust manifold design, still plays a larger role in determining exhaust velocity, but the pipes downstream of the turbo are a lot less important.
As for the turbo itself, you want to maximize the pressure (and temperature) difference across the turbine for the highest efficiency. A low velocity of the gasses exiting the turbine won't make it any less efficient; in fact, it can theoretically improve the efficiency of the turbine. So the exhaust on a turbocharged car can be designed to minimize static pressure, without concern for the exhaust velocity.
So what does all of this mean? It means that while a 5" exhaust would be desirable for a 350 hp turbocharged Supra, it would be a very bad idea on a 350 hp naturally aspirated Camaro and an awful idea on a 1.8 liter naturally aspirated Honda.
-ragtop
Not my words... so ragtop at the address above are where the credit goes.
EDIT:
Some of what he wrote is a bit convoluted to understand thorougly. What he is really arguing for is that you need a high velocity of your exiting gases, that's the entire purpose of the exhaust system... to evacuate gases at a high velocity. Without that you lose power and low end torque. Static pressure is what he is referring to as back pressure, but that is kind of a mislable. What he is referring to is the labeling of pressures as they would be on a pitot tube when using the pitot tube for measuring velocity. In that case you have one pressure point that is considered to be "static" and that is what he is calling both the static pressure and the backpressure... but that pressure is entirely velocity dependent. The higher your velocity the higher static pressure you have on a pitot tube.
See also - http://www.automotiveforums.com/vbulletin/t13199.html
Discuss this here.
Last edited by Blacksheepdj; May 13, 2005 at 11:02 AM.
Dec 3, 2003, 10:31 AM
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This is some additional information from one of my posts in the thread:
https://www.evolutionm.net/forums/sh...5&pagenumber=3
First of all understand a couple fun things.
Pipe flow doesn't behave like normal air flow... so there is more to be concerned with.
Turbulent flow actually results in a greater volumetric throughput than laminar flow even though it's coefficient of resistance from friction with the walls is higher. It has a greater "average" velocity profile. So... in theory you can actually make an argument that mandrel bent is bad. How so?? Because mandrel bent piping is smooth enough that laminar flow can result. This will theoretically reduce your overall flow rate compared to the exhaust shop "crinkle" bend approach. So why do all companies make everything mandrel bent. Can we say marketing?!? Also, there are some cases when the unit is properly tuned with the right diameter and so forth that yes indeed mandrel bent is better.
The key to power with engines is allowing for high rates of exhaust transfer while maintaining backpressures that work with the original tune of the engine. Therefore the "funnel" idea helps this in some ways. It makes it possible to have a high flow rate of exhaust while maintaining enough backpressure to keep the engine running well. A high torque engine is one with some seriously bottled exhaust, a high horsepower engine is a very high flowing exhaust. What we need is in between. Our engine's are pretty torquey for their size, so we can afford to dump torque in favor of hp, and still come out with more torque than most uh.. civic engines.
If you want to understand the funnel effect look up the concept of a C-D Nozzle (converging diverging nozzle). Essentially what happens is that the gases are compressed by the geometry of the "funnel" making for a greater density of air. The same amount of mass has to flow through the nozzle regardless, so velocities increase. Now when the nozzle expands again the speeds go right back to where they were before. Except in one special case, when mach 1 is achieved at the throat. then the gases actually accelerate further and exit the nozzle at very high speeds until the pressures collapse back to atmospheric creating a sonic boom. (But the boom isn't really a factor... as your air compressor nozzle does this every single time you use it to blow dust off the car... no broken windows though )
So although you can increase your immediate tip exit velocity of your exhaust gases, you still have not increased your mass flow. So that is where back pressure comes in. If you have no back pressure you achieve your highest mass flow possible. The air "falls" right out of the engine pretty much with nothing to stop it. This result in lower torque... not sure why at this point.. I'll poke around and see if I can figure it out.
So although "funnelling" your exhaust is all great and dandy for higher exit velocities, you've done nothing to improve your mass flow rate, the mass flow rate is going to be determined by the engine and the restrictions present. You have to always take things back to mass flow as volumetric flow is misleading. But also in this case if you take your higher exit velocity and multiply it by the exit area, you aren't going to be any better off than at your slower but larger area'd flow.
Make sense? Shoot with questions as need be.
Hope this helps people a little bit in their exhaust and header decisions!
EDIT:
Backpressure is a misnomer and incorrect. It is merely a termed used to help people "visualize" the concept of what is needed in an exhaust. To avoid the syphoning off of the incoming mixture, or leaving too many leftovers of the spent gases, you need to provide the proper high velocity flow of exhaust gas. This is accomplished with the proper size piping. If your piping is too big you actually reduce your exit velocity of your exhaust gases. Basically it works out to the fact that based on the CFM output of your engine, the pressure waves generated, etc you need to size your exhaust properly to create scavenging. If your exhaust is too big this effect does not take place, and you will lose low end torque. Hope that clears up some discrepancies.
https://www.evolutionm.net/forums/sh...5&pagenumber=3
First of all understand a couple fun things.
Pipe flow doesn't behave like normal air flow... so there is more to be concerned with.
Turbulent flow actually results in a greater volumetric throughput than laminar flow even though it's coefficient of resistance from friction with the walls is higher. It has a greater "average" velocity profile. So... in theory you can actually make an argument that mandrel bent is bad. How so?? Because mandrel bent piping is smooth enough that laminar flow can result. This will theoretically reduce your overall flow rate compared to the exhaust shop "crinkle" bend approach. So why do all companies make everything mandrel bent. Can we say marketing?!? Also, there are some cases when the unit is properly tuned with the right diameter and so forth that yes indeed mandrel bent is better.
The key to power with engines is allowing for high rates of exhaust transfer while maintaining backpressures that work with the original tune of the engine. Therefore the "funnel" idea helps this in some ways. It makes it possible to have a high flow rate of exhaust while maintaining enough backpressure to keep the engine running well. A high torque engine is one with some seriously bottled exhaust, a high horsepower engine is a very high flowing exhaust. What we need is in between. Our engine's are pretty torquey for their size, so we can afford to dump torque in favor of hp, and still come out with more torque than most uh.. civic engines.
If you want to understand the funnel effect look up the concept of a C-D Nozzle (converging diverging nozzle). Essentially what happens is that the gases are compressed by the geometry of the "funnel" making for a greater density of air. The same amount of mass has to flow through the nozzle regardless, so velocities increase. Now when the nozzle expands again the speeds go right back to where they were before. Except in one special case, when mach 1 is achieved at the throat. then the gases actually accelerate further and exit the nozzle at very high speeds until the pressures collapse back to atmospheric creating a sonic boom. (But the boom isn't really a factor... as your air compressor nozzle does this every single time you use it to blow dust off the car... no broken windows though )
So although you can increase your immediate tip exit velocity of your exhaust gases, you still have not increased your mass flow. So that is where back pressure comes in. If you have no back pressure you achieve your highest mass flow possible. The air "falls" right out of the engine pretty much with nothing to stop it. This result in lower torque... not sure why at this point.. I'll poke around and see if I can figure it out.
So although "funnelling" your exhaust is all great and dandy for higher exit velocities, you've done nothing to improve your mass flow rate, the mass flow rate is going to be determined by the engine and the restrictions present. You have to always take things back to mass flow as volumetric flow is misleading. But also in this case if you take your higher exit velocity and multiply it by the exit area, you aren't going to be any better off than at your slower but larger area'd flow.
Make sense? Shoot with questions as need be.
Hope this helps people a little bit in their exhaust and header decisions!
EDIT:
Backpressure is a misnomer and incorrect. It is merely a termed used to help people "visualize" the concept of what is needed in an exhaust. To avoid the syphoning off of the incoming mixture, or leaving too many leftovers of the spent gases, you need to provide the proper high velocity flow of exhaust gas. This is accomplished with the proper size piping. If your piping is too big you actually reduce your exit velocity of your exhaust gases. Basically it works out to the fact that based on the CFM output of your engine, the pressure waves generated, etc you need to size your exhaust properly to create scavenging. If your exhaust is too big this effect does not take place, and you will lose low end torque. Hope that clears up some discrepancies.
Last edited by sdhotwn; Dec 17, 2003 at 07:12 PM.
Dec 10, 2003, 07:37 PM
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Understanding Engine Timing, AFR, and tuning
I had the unique opportunity to design my own experiment for my last couple of weeks of an engineering lab as I'm graduating.
Knowing how much there is to try and learn and understand about internal combustion engines and how to tune them properly and understand timing and so forth, my group and I worked hard to develop an experiment and record data and develop a lab report that would be beneficial to a lot of people.
So here it is! First I'm posting a link to the actual lab report document. And then I'm going to post the lab report piece by piece in this thread.
Link:
http://www.cae.wisc.edu/~nackers/car...t%20Report.doc
I recommend everyone to read it, you'll probably learn at least something, or it'll reinforce your knowledge. I'm happy to answer questions about anything as well. So enjoy the long read, and I hope it helps you in your quest to understand engines and engine tuning!
Later!
Steve
More discussion here.
Knowing how much there is to try and learn and understand about internal combustion engines and how to tune them properly and understand timing and so forth, my group and I worked hard to develop an experiment and record data and develop a lab report that would be beneficial to a lot of people.
So here it is! First I'm posting a link to the actual lab report document. And then I'm going to post the lab report piece by piece in this thread.
Link:
http://www.cae.wisc.edu/~nackers/car...t%20Report.doc
I recommend everyone to read it, you'll probably learn at least something, or it'll reinforce your knowledge. I'm happy to answer questions about anything as well. So enjoy the long read, and I hope it helps you in your quest to understand engines and engine tuning!
Later!
Steve
More discussion here.
Last edited by Blacksheepdj; May 13, 2005 at 11:05 AM.
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Feb 18, 2004, 04:11 PM
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The Torque Vs. Horsepower Difference
Horsepower is in raw units form 550 ft-lbf/s
If you go back F=m*a or lbf = lbf/32.2ft/s^2 * ft/s^2 you will see that the units in the system cancel out. Therefore FORCE which is units of lbf relative to your mass is what generates your acceleration. So you get your applied force by compensating for rim radius (your lever arm) and using your vehicle mass... which then gives you your acceleration rate theoretically.
Now that's all fine and dandy but then you need to be able to have the horsepower available to balance out forces in a system... So in other words if the vehicle is seeing 50 lbf of resistance over 1 ft for 1 second you'll need ~.1 hp to achieve that. But to accelerate at a given rate you need to have the torque necessary to accelerate the mass/overcome a resistance force. Horsepower put simply is what is necessary to overcome a certain amount of force over a certain distance in a certain amount of time. lbf-ft/s Hence top speed is limited by horsepower as you have to be able to provide force at a great enough rate to overcome the drag in the system.
So torque is what gets you moving and allows you to accelerate, but horsepower is what keeps you moving/allows you to STAY fast.
Hope that helps! I apologize in advance if I made any mathematical/reasoning errors, but I believe I should have it spelled out ok.. but feel free to correct!.
Discuss this thread here.
If you go back F=m*a or lbf = lbf/32.2ft/s^2 * ft/s^2 you will see that the units in the system cancel out. Therefore FORCE which is units of lbf relative to your mass is what generates your acceleration. So you get your applied force by compensating for rim radius (your lever arm) and using your vehicle mass... which then gives you your acceleration rate theoretically.
Now that's all fine and dandy but then you need to be able to have the horsepower available to balance out forces in a system... So in other words if the vehicle is seeing 50 lbf of resistance over 1 ft for 1 second you'll need ~.1 hp to achieve that. But to accelerate at a given rate you need to have the torque necessary to accelerate the mass/overcome a resistance force. Horsepower put simply is what is necessary to overcome a certain amount of force over a certain distance in a certain amount of time. lbf-ft/s Hence top speed is limited by horsepower as you have to be able to provide force at a great enough rate to overcome the drag in the system.
So torque is what gets you moving and allows you to accelerate, but horsepower is what keeps you moving/allows you to STAY fast.
Hope that helps! I apologize in advance if I made any mathematical/reasoning errors, but I believe I should have it spelled out ok.. but feel free to correct!.
Discuss this thread here.
Last edited by Blacksheepdj; May 13, 2005 at 10:59 AM.