gotdatfiyah

i hear a lot of talk about hks exhaust...below is the exhaust i have, whats the difference?

gotdatfiyah

btw, its dynomax, www.dynomax.com, can be ordered through advance auto parts

Digi7ech

iTrader: (1)

Well, Some people buy it due to the brand name.

I don't know about any special characteristics about it.

I have Corksports Single exhaust.

The type of catback should be dependant on how much you want to spend and what type of application.

If you have an N/A then you don't really want huge diameter due to not a whole lot of back pressure.

A True Dual(uncollected) setup is great for midrange HP and Torque. The brand names True Duals are over 1K.

I went with Corksport cause it was cheapest and it is single but 2 1/2 inch to 3 inch outlet. That gives a little more topend HP but still retains some backpressure due to it being single and not dual.

If you are a TII then I believe bigger diameter == better.
You would need to back up the increase of boost with more fuel though.

My understanding would be bigger diameter exhaust flows better meaning the turbo is able to exspell the gas quicker meaing it spools quicker and better.

The A-PEXi Dual and singles are nice for this.


But make sure if you piece together a system yourself that you look into steel padded mufflers instead of glass packs cause our exhaust is hotter than piston engine so it burns out glass packs quick.

Pretty much the only reason to buy a brand name exhaust is to have something that will just mate up with your current confirguration without havinng to cut/weld/etc.. or for names sake.

The one thing I like about my exhaust is that I can remove it from my car in 15 minutes and bolt it back up without problems.

Hopefully this blabbing helped you out.

gotdatfiyah

yea thanks for the reply.....i have to get a small piece of mine welded...but after putting it on, i noticed a difference in power...and the sound....is great

Agent_D

iTrader: (2)

contrary to popular saying, back pressure is a bad thing. just to let you know

gotdatfiyah

ive never seen the actual science of back pressure explained...if anyone cares to do this.

Agent_D

iTrader: (2)

someone can explain this better than i can, although i know how it works and why, i cant really explain, someone??

Digi7ech

iTrader: (1)

Well I understand on piston engines you need some backpressure. Don't exactly know for rotary but guess you need little to none.

I heard that backpressure on a piston adds a bit of torque. Don't know how though(scavenging or something).

Also if you run open headers I heard you can sometimes suck in cold air and **** up the hot valves by distorting them.

With rotary I guess we don't need it since we are a constant revolution engine which just pushes instead of push/pull like a piston.

The intake portion of our combustion can kind of be considered pull but it is still pushing.

Digi7ech

iTrader: (1)

Please correct anything I said.

This is all logical deduction on my part and here say.

gotdatfiyah

thanks...so backpressure is pretty much, if your pipes are too big, it sucks air back in from the pulling of the engine?

Tofuball

Lots of people get velocity and backpressure confused.

Agent_D

iTrader: (2)

they get air in the open headers because their short, an exhaust that runs the full length of the car is best, and if you tune it right will make much better power.

no back pressure on pistons either back pressure in general is a bad thing, it cuts fuel mileage down, and reduces the possibility to make max power and torque. if i can find the article on the write up ill post it, its much easier to understand

Edit: rotaries cant suck air back into the chamber because they spin, nothing to suck back in, only pistons can do this

drago86

backpressure= bad
velocity=good
tune exaust pipe diameter to have a velocity of .6 mach wherever you want your torque peak to be. The motor will then be most effecient at expelling gasses at that rpm, so smaller diameter= lower rpm, larger= higher, if the gasses are flowing to fast, or too slow, you loose effeciency, and thus, power.

That backpressure creates torque is a stupid myth that refuses to die

Icemark

On a N/A some back pressure is required, as part of the way a rotary works is that the some of the unburned gas (and some of the burned) gets pulled back into the next combustion cycle in that rotor face.

If there is insufficent back pressure, emissions go way up, and HP goes down as all that unburned mixture is forced out, instead of helping the next combustion cycle (the unburned mixture obvously helping richen the next burn, and the burned mixture, helping lower the combustion temp in the next burn so that it burns longer- a sort of built in first stage EGR). The new Reni motor uses this same principle to a much further extent, where the exhaust is actually routed through the side to feedback into the intake stream.

So the big key is to get a system that is not too big and has a little back pressure, but not as much as stock.

For example if you were to use the HKS 60MM system for a turbo on a N/A you would have less HP and a lower torque curve than if you used the 50MM system.

Why??? because the bigger 60mm system flows better and has reduced back pressure.

So biggest fastest and no back pressure only applies to Turbo cars... on N/A cars it is manditory to have some back pressure for increased torque and HP. Anybody that says that zero back pressure is a good thing doesn't understand even the basics on Internal Combustion motors.

Agent_D

iTrader: (2)

backpressure is a myth
think about it
you have something in the way of you exhaust and not letting it move free in turn it makes your motor work harder to push the exhaust out which causes pumping loss at ALL rpms, and since your motor is going to have to work to push the exhaust out that means the exhaust has a lower velocity which in turn is not going to create a vacuums in the exhaust system to help with scavanging so you are not going to pull out as much of the exhaust with a free flowing system


what you are confusing is backpressure with velocity
the reason why ppl might go to a smaller pip is not to make backpressure higher but to make the velocity of your exhaust faster.

backpressure will do nothing more then slow the stuff down
kinda like me standing in your way when you are trying to walk.
just doesn't work well
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you can NOT have too little amount of backpressure

the less the better
low backpressure will create low end torque as well as high rpm power

if you have something that will prove me wrong I would please like to see so. again just think of the meaning of word. from just the way the word is alone it is stating that you are putting pressure against the flow of the exhaust. how is putting pressure that your motor has to overcome to give you more low end torque
ideal would be a vac inside your exhaust ( that would have no pressure at all ) and with that it would help reduce pumping loss from the motor trying to force all your exhaust gas out. and also would help suck out a lot more of your exhaust which would create a high volumetric efficiency.
when you induce backpressure into the exhaust you are doing nothing more then making everything fight.
problems that come from backpressure is lower VE, pumping loss, high exhaust temps, high motor temps, more stress on your motor (small as it might be) and I am sure there are other things
just remember backpressure does not create a high velocity in the exhaust, and there is a difference between backpressure and velocity.
though by putting huge pipes on your car that is not really going to help you out either, it is not due to lack of backpressure though. what will go on if you put a large pipe on the car without the flow to back it up then the exhaust gas doesn't really have the walls of the pipe holding it into a controlled flow pattern instead the gas will flow around and have eddies in the system and try to turn back itself. if you get the right size pipes for your car you will reduce as much backpressure as low as you can but also keep the exhaust in check by not letting it just wander around but keep it all flowing in the right direction
and if backpressure was something that helped performance wouldn't ppl try to put something to create backpressure inside there intake also? instead ppl try to open up the intake, port it out, polish it make everything as smooth as they can to reduce the force the motor has to give to pull the air in. same thing on the exhaust side
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No, you absolutely cannot have too little backpressure. That is one of the biggest and dumbest fallacies that exist in relation to 4 stroke IC engines. Why? I'll tell you.
Pumping Losses The concept of backpressure means that there will be a high pressure area at the exhaust port. When the valve opens, the escaping exhaust gases have to push against that high pressure area. How can a parasitic loss be good for your engine? The only engines that NEED backpressure are various small engine designs, mostly 2 stroke. The problem with them is they have the tendency to be too effective at driving the exhaust out, loosing compression. I assure you that on a big 4 stroke engine such as an SBC you want the exhaust to have the free-est path possible. In fact, it would be ideal to have a low pressure zone at teh exhaust port. A low pressure area would help to pull the exhaust gases out. Freeing up more power that your engine would have wasted pushing the gases out...which brings up the next topic beautifully.

Scavenging We've heard this before, but what does it mean in relation to our discussion of exhaust. Easier for me to use an example. So your engine fires and now it's on the way up on the exhaust stroke. Exhaust doesn't come out at a consistent rate, it comes out in pulses. Each pulse is a high pressure area, and as it moves, it leaves alow pressure area behind it. Aha, there's our scavenging. You want that low pressure area to be at it's peak when the exhaust port opens on the next exhaust stroke. Thats another reason why headers make better power than manifolds. besides just flowing better, instead of all the pulses being dumped in a log fighting with each other, the tubular runners allow the exhaust pulses to stay seperate and create a nice low pressure area behind it. This is also where tuned and equal length headers come into play. Tuned headers are sized such that the length of the tube corresponds the speed of the exhaust pulses so that the low pressure area is maximized at certain rpms. No surprise that short headers are better for high rpms than longtubes.
Problems can surface if you use too large of a primary diameter, loss of torque. The morons are quick to spout 'you lost backpressure and thus torque.' Next time you hear that you will smile and know that that person failed physics in high school. The problem with using too large of a primary is this. The exhaust pulse only has so much gas and energy in it. If the tube is too large, the pulse expands to much, losing energy and thus velocity. When it loses velocity, it can potentially stall and stop moving in the tube, or at least slow down. aha! Too large of a header actually CAUSES backpressure, and thus lost power. We feel this power loss as a loss of torque because usually this effect is much more pronounced at low rpms as much less gas is moving.

The same principles apply to the entire exhaust system, from primaries to collectors to pipes to mufflers. I am too tired to explain it all, books have been written on these topics. I have just scratched the surface, but hopefully you all understand a little better why their is no such thing as good backpressure. I know some of this has been a little oversimplified, but it think it gets the message across

Agent_D

iTrader: (2)

This myth of "backpressure" and good street throttle response is completely backwards. The correlation is spurious at best. "Inertia" is the guilty property where headers are concerned.
The "backpressure" that is seen in a small diameter header tube is caused by the column of gas not wanting to move RIGHT AWAY when the slug of exhaust gas comes out of the port.

In other words, the slug comes out but the gas in the tube is at rest and will compress before it moves. Once the column of gas in the tube starts moving, it will want to keep moving. THIS property of inertia (a mass of gas in motion) is used later in the exhaust cycle to create a relative vacuum and scavenge the combustion chamber. There is a certain RPM range at which the compression and rarefaction waves are ideally suited for scavenging. This is when the header tube is "resonating". Header tubes that are too large will not have sufficient velocity to prevent the rarefaction wave from pulling the exhaust back into the cylinder. Hence, sucky performance on the street.

A properly designed exhaust manifold works just the opposite way. It is designed to offer no reaction at idle and low RPMs. A good exhaust manifold has almost no runner and a large open space for expansion. The slug comes out with no column of gas to push out of the way and lots of room to spread out. The drawback comes in the form of turbulence and lack of a column of moving gas to scavenge as RPMs go up. Somehow, people look at the backpressure problem and jump to a false conclusion about what makes low RPM torque.

The presence of backpressure reveals the deficiencies of either design. Loss of LOW RPM performance, after installing headers, is NOT caused by lack of backpressure! Loss of street performance is caused by the inertia of the gas in the tube and the carb not being jetted properly for headers. Most stock carbs are not calibrated with headers in mind. Again, loss of performance and jumping to a wrong conclusion about backpressure.

Bottom line: The myth of performance and backpressure is completely backwards.



as well as Glenn91L98GTA another one of our great moderator

Backpressure is bad...it is a complete myth that should be exterminated like the plague.
What people are confusing is VELOCITY with backpressure. You need to maintain the right velocity to obtain maximum torque. This is the argument behind too large of headers or too large of an exhaust system. On smaller engines they don't maintain enough velocity to obtain maximum torque at lower engine speeds.

Backpressure is when you have restrictions that do not necessarily maintain optimum velocities. Similar to shoving a potato with holes in your exhaust system. Yes, you have backpressure, but you have no velocity. An extreme example, but old pellet type catalytic convertors or certain mufflers do just that. They increase backpressure, but do nothing for velocity.

1 5/8" headers vs 1 3/4" headers introduce very little backpressure, but the 1 5/8" headers maintain higher velocities. At lower rpms this promotes torque. At higher rpm, the 1 5/8" headers become saturated with exhaust gas due to the volumes they can hold from the cylinder, and produce less power at top rpm. At this point they introduce backpressure and kill performance.

Conclusion, backpressure is BAD!
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Imagine for a moment you’re in the engine cylinder. The piston moves up and pushes the air out of the exhaust valve. This creates a high pressure region at the exhaust valve. Air will flow to a low pressure region. This happens to be the tail pipe. Any restrictions in the exhaust system will impede the flow of air from the exhaust valve to the tail pipe, effectively creating “backpressure” in the exhaust system. This backpressure works against the air flow. ANYTHING YOU DO TO INCREASE BACKPRESSURE WILL DECREASE FLOW RATE and decreased flow rate will reduce your power output. Additionally, the interaction of the air streams from each header tube as they merge into the exhaust pipes has an effect on the air flow.

The purpose of an exhaust system is to purge air from the cylinders. The idea is to maximize air flow from the engine to the tail pipe. Different engines move different quantities of air at different engine speeds. The quantity of air you need to move determines the capacity of the exhaust system (pipe diameter). The time you have available to purge air from the cylinder determines the design of the header tubes. Keep in mind that the time available to purge air from the cylinders is dependent upon the camshaft profile and the engine speed. The smaller the cam the less time the valve is open. The higher the engine speed the less time the valve is open.

A street engine doesn’t move much air. In order to keep the air flow rate high enough to purge the cylinder in time, a small diameter header tube is employed (1-5/8). The rest of the exhaust system is sized appropriately for the volume rate of air that the engine flows. Again, anything you do to increase backpressure will decrease flow rate and decreasing flow rate will reduce your power output. Putting 3” duals on an ordinary engine is complete overkill because the engine doesn’t need that must flow capacity. In fact, due to the dynamics of air flow it’s actually hurting performance. But that’s too complex to describe here. A race engine moves a lot of air and it does it at high rpm. The shear volume rate of air needed to be moved increases the size requirement for the exhaust system. The header tubes will have to be increased in size but it’s still important to keep them small enough to efficiently purge the engine cylinder AT THE RPM THAT THE ENGINE WILL BE USED. Everything has to be sized appropriately. Bigger is not better. Small is not better. Getting it right is better.
First of all, drop the term backpressure. I don’t want to hear it any more. That in and of itself is your main point of confusion. Anyway, we need to back up for a second and understand torque. Torque is the work that the engine can deliver. As the engine goes from idle to redline it will produce different amounts of torque at various rpms. As you change the configuration of the exhaust system you will change the state of tune of the engine and effect how much torque it makes throughout the rpms. By restricting the exhaust system you take away the ability for the engine to develop torque at higher rpms and will increase torque development at lower rpms. Why? Because the exhaust won’t flow well at higher rpms. Note that if you restrict the exhaust too far you’ll simply reduce torque across the board and have a dog of an engine. Likewise, if you use a larger exhaust system you will encourage torque development at higher rpms and will make less torque at lower rpms. Why? Because the exhaust is tuned to flow well at higher rpms but is inefficient at lower rpms. Same warning applies though. Go too big with the exhaust and you will reduce torque across the board and have a dog of an engine.

What you need to realize is that Hp is a function of torque and rpm, where

Hp = (Torque)(rpm) / 5250

What you see here is that the more torque you can make at higher rpm, the more Hp you’ll develop. So, which is faster… a car with a dinky exhaust system making tons of torque down low or a car with a healthy exhaust system making tons of torque up high? I’ll take the car that makes more Hp.

The other thing you should realize is that for a street setup, a car making a ton of Hp will make a lot more torque down low anyway. The rpm limit on a street engine is so low that whatever you do to make Hp at 5000 rpm is also going to bump up your low end torque. It’s a complete waste of time to purposely build low end torque.
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So there you have it, i found the texts so i copy pasted for ya to read, i know its long, sorry bout that

Icemark

#1 too bad you didn't put in the majority of the sources to confirm it... otherwise it is no different than any other person spouting off on the internet about he or she believes. However as noted in #3 there are a few of the views that are applicable.

#2
The majority of your arguments are using piston and carbed 4 stroke engine analogies which don't hold up in a fuel injected rotary because of very basic design concepts.

#3 your post of this:
is much more appropriate in the case of a rotary, since its inherent design is basicly a two stroke.

Perhaps you should read the texts youself, unless again you were wanting to prove my point on the operation of a rotary motor (not a piston engine running in a 4 stroke design).

Agent_D

iTrader: (2)

well from what is stated, the rotary is in no case a 2 stroke engine, but some say neither is it 4 stroke, the above text is what i got from various sources, and whether its piston or not, it all pertains to exhaust, one thing that wasnt mentioned was also sound wave tuning, which can be found in several exhaust write ups, you can read about exhaust from Paul Yaw, which i hear is supposed to be the utmost knowledgable guy about rotaries, he has an exhaust write up about rotaries in specific.

not trying to start a flame war, just posting what i have read.

Agent_D

iTrader: (2)

Here are the tech articles by paul yaw, sorry for the length


Last months article described the basic internal workings of the rotary engine. The next several articles will break this down into separate cycles, and descirbe them in detail. I will begin with the exhaust cycle because it has the greatest effect on the power output of the engine. If the engine cannot exhaust itself completely, further modifications will result in very little improvement. This is true of naturally aspirated, and turbocharged engines. This first article will explain a few basic terms and concepts. Next months article will present some more new information, and then describe how all of this comes together to affect the complete exhaust cycle.
When attempting to increase the power output of the rotary engine, there are three basic aspects that can be improved upon. Volumetric efficiency, combustion efficiency, and reduction of pumping losses. As most of you know, the rotary engine has four separate cycles. Intake, compression, expansion, and exhaust. Of the four, only the expansion cycle contributes to the power output of the engine by exerting force on the output shaft. The other three cycles actually reduce horsepower by resisting the rotating force. This reduction in power is referred to as pumping loss. Pumping losses occur in both the intake and exhaust cycles. This article, and the next will deal with the importance of reducing pumping losses during the exhaust cycle.
Blowdown Period


Early internal combustion engines opened the exhaust valve at BDC of the expansion cycle. This required the piston to pump, or physically force the exhaust gasses from the cylinder during the period from BDC to TDC. The force required to pump the gasses from the cylinder considerably reduced the power output of the engine. As performance, and rpm requirements increased, it was discovered that by opening the exhaust valve before BDC the residual combustion pressure could be used to help evacuate the cylinder at the beginning of the exhaust cycle. This is referred to as the blowdown period, and is responsible for approximately half of the exhaust flow. In theory, this will reduce thermal efficiency by releasing pressure that is still applying force to the crankshaft. In practice however it was determined that the reduction in pumping losses far outweighed the loss of pressure at the end of the expansion cycle. Since most of the useful work is done in the first third of the expansion cycle, the pressure loss caused by early exhaust valve opening is minimal. This also applies to the rotary engine. Referring to last months article you can see that the exhaust port of a stock engine opens approximately 75 degrees before BDC.
Pressure Wave Tuning


Pressure wave phenomena is probably the least understood aspect of exhaust tuning. Right now I am thinking that it is also the hardest to explain! Entire books have been written on this subject, but I will try to boil it down to a few paragraphs.
Any time there is a pressure change in an elastic meduim (like air for instance) a series of resonances or vibrations will occur. Any time you hear a sound, it is the result of a pressure disturbance in the air. For instance, if someone across the room claps their hands together, the air pressure between their hands will increase. This rise in pressure will be transferred from one group of molecules to the next (at the speed of sound of course) until it finally reaches your ear. While this energy transfer is invisible, you can easily picture it by dropping a stone into an undisturbed pool of water. Pressure waves radiate outward from the center of the disturbance. This same thing happens in the exhaust system, but because of the higher pressures involved it is more like an elephant doing a belly flop in your swimming pool.
The main difference between the swimming pool analogy, and the exhaust system is that the pressure waves cannot travel outward in all directions from the source of the pressure disturbance, beacause they are enclosed by the tubing itself. In the case of the exhaust system, the initial pressure wave, or pulse caused by the exhaust port opening will travel towards the open end of the tube.
So far I have only referred to pressure waves as being positive, or caused by an increase in pressure. In fact, pressure waves can be negative, or caused by a decrease in pressure. Picture a wave in the ocean with the highest point of the wave being positive, or above sea level, and the trough between two waves being negative, or below sea level. This is analogous to the pressure waves in the exhaust system. These waves can also be referred to as high pressure, and low pressure.
These pressure waves can be used to our advantage because they have the effect of moving gas particles along with them. A positive, or high pressure wave will propel gasses in the same direction that it is travelling. A negative, or low pressure wave will propel gasses in the opposite direction that it is travelling. Take a moment to let this sink in, because this simple fact is at the heart of exhaust system tuning. Although the pressure wave is moving at the speed of sound, it will propel the gasses at a much slower speed. An example of this is a boat that catches a wave from another boat that is motoring by. As the wave passes it will propel the boat in the same direction the wave is travelling, but at a much slower speed, and the wave will eventually pass the boat completely. This is the same thing that happens to the gas molecules in the exhaust system as a pressure wave passes through them.
These pressure waves respond in an interesting manner when they reach a sudden area change in the pipe. An example of a sudden area change is the collector, where the two pipes empty into a larger diameter pipe, a megaphone, or the end of the exhaust where the pipe empties into the atmosphere. When a pressure wave reaches a larger cross sectional area, it will reverse its sign (positive becomes negative, and negative becomes positive) and its direction. For instance, when the exhaust port first opens, a strong positive wave will travel to the end of the pipe, change to a negative wave, and travel back to the exhaust port. This is called a reflection. Both the positive wave travelling towards the end of the pipe, and the negative wave travelling towards the exhaust port will propel exhaust gasses towards the end of the exhaust system which is exactly where we want them to go. The amount of time that this cycle takes is dependant on the total distance that the wave has to travel.
By changing the length of the header pipes, you can time the cycle so that the negative return wave arrives at the exhaust port at the end of the exhaust cycle where it is most beneficial. Assuming that the negative return wave is timed correctly for a given engine at 6000 rpm, lengthening the headers will further delay the return wave so that it is timed appropriately for a lower rpm, and shortening the headers will time the return wave so that it is timed appropriately for a higher rpm. The key to header length tuning is simply timing the low pressure return wave to give the greatest benefit for a given rpm.
This is a VERY basic description of pressure waves, and how they affect the exhaust system of an internal combustion engine. For a more detailed analysis, I would suggest researching two stroke exhaust system design. There is a great deal of information in print, and much of it can be found at public, or university libraries.
Velocity


Velocity refers to the speed at which the exhaust gasses are travelling. The exact speed is not important to this discussion, but an uderstanding of how velocity affects exhaust flow is. There are two ways that velocity can be increased. One, by decreasing the cross sectional area of the orifice that the gasses are flowing through. (Making the headers or exhaust ports smaller) Two, by increasing the volume of air that is flowing through the orifice. (Increasing engine rpm) Velocity will increase proportionally with an increase in rpm. In other words, if you double the rpm, the velocity will also double. Velocity is inversely proportional to an increase in cross sectional area. Doubling the cross sectional are will halve the velocity, and halving the cross sectional area will double the velocity.
Velocity is important for one simple reason. Inertia. Websters dictionary describes inertia as "The property of matter by which it retains its state of rest or velocity so long as it is not acted upon by an external force." In other words, once it is moving, it will continue to move until some external force stops it. If you apply this theory to the gasses in the exhaust system you can see that once they have been accelerated by the pressure in the combustion chamber, It will take a given amount of energy to stop them, and even more to cause them to reverse direction. Since energy equals mass times velocity squared, you can see that doubling the velocity of the gasses will quadruple the amount of energy required to stop them. This is important because the flow of exhaust gasses is not steady. During each exhaust cycle, the gasses are accelerated, and decellerated rapidly. Often in the forward and reverse direction.
Next months article will take all of these concepts, and describe how they affect the exhaust cycle. Like everything else that seems complex, it is just a combination of many very basic theories. If you take the time to fully understand this months article, next months article will leave you with a thorough understanding of the exhaust cycle.
My goal in writing these articles is to inform you, the reader so that you can go faster, and make appropriate decisions when modifying the rotary engine, or buying performance parts. Until next month, have fun, and thank you for reading.
Hopefully all of you have digested last month's information, and are ready for more. Last month I described the importance of velocity, pressure wave tuning, and opening of the exhaust port before BDC. Now it is time to take this information and see how it can be used to optimize the exhaust cycle. Let's start by looking at the effects of a less than optimum exhaust cycle.
A motor has fully exhausted itself (When it is really tired?) when the pressure in the chamber is equal to, or below atmospheric at the end of the exhaust cycle. Several things happen when the motor cannot fully exhaust itself. If the pressure is above atmospheric at the end of the cycle, the result is lowered volumetric efficiency, increased pumping losses, and reduced combustion efficiency as compared to an optimized exhaust cycle.
Swept Volume, Clearance Volume, and Compression Ratio.


In December's port timing article, I stated that top dead center, or TDC refers to the point at which the chamber is at its smallest possible volume. The space in the chamber at TDC is referred to as the clearance volume, and this in part determines the compression ratio. The compression ratio is specified as (Volume at BDC/Volume at TDC) Using an '87 13B as an example, the chamber volume at BDC is 9.4 times greater than the volume at TDC, for a compression ratio of 9.4 to 1. The difference between the volume at TDC, and BDC is referred to as the swept volume, or displacement. This is the volume of gasses that will be displaced in one complete cycle assuming 100% volumetric efficiency. A little bit of high school algebra shows that the volume at BDC is 44.66 cubic inches, and the volume at TDC is 4.75 cubic inches, or 10.6% of the total volume.
Volumetric Efficiency


The exhaust gasses that occupy the clearance volume will be carried around into the following intake stroke. As you can see, even at 100% volumetric efficiency the mixture will still only be 89.6% fresh intake charge. If the chamber pressure does not reach atmospheric by the end of the cycle, this 10.6 %, or 4.66 cubic inches of exhaust gasses will be pressurized, and will take up even more space once they are allowed to expand as the chamber volume increases during the intake stroke. This will reduce volumetric efficiency considerably, as the exhaust gasses will occupy space that could be used for fresh mixture. These exhaust gasses effectively "take away" from the swept volume, or displacement of the motor. The goal of the exhaust system then, should be to evacuate as much of the spent gasses as possible.
Inertial Scavenging


Inertial scavenging is easiest to understand if you think of the gasses in the exhaust system as a big piece of elastic. While they are not directly connected, a change at one end of the system will have an effect on the gasses at the other end of the system. For instance, towards the end of the cycle, the flow through the exhaust port slows down, but the high velocity gasses from earlier in the cycle are still travelling through the system. (Note: A system made up of 100" long, 1/34" inside diameter header tubes, as you might see on a race car, will contain about six complete cycles worth of exhaust gasses per pipe.) These high velocity gasses will "pull" on the slower moving gasses near the exhaust port, helping to evacuate the chamber. This is inertial scavenging. Just imagine two cars rolling down the road, connected to each other by a bungee cord. If the car at the back slows, it will not immediately be jerked back to speed, but rather gently pulled back up to speed by the car in front. As some of you may have guessed, a series of resonances will then occur, with each car alternately pulling at the other. This is very much like what happens to the gasses in the exhaust system.
Pumping Losses


Well, here we are at pumping losses again! Luckily this is quite easy to explain and understand. It all comes down to exhaust flow. Not just the airflow capability of the exhaust port, but of the entire system from the port to the end of the exhaust pipe. Quite simply, if the exhaust flow is insufficient, the blowdown period will only account for a small amount of the total exhaust gasses, and the remainder will have to be squeezed out by the rotor itself. Physically forcing the gasses from the chamber through a restrictive exhaust system requires a substantial amount of horsepower. So much in fact that many diesel truck engines have a mechanism which blocks the flow of exhaust gasses to slow the vehicle down, thus saving wear on the brakes. Just think about slowing an 18 wheeler with nothing but exhaust pressure, and you get an idea how much this can affect your engine.
Combustion Efficiency


We have already discussed how insufficient exhaust flow reduces volumetric efficiency, but the presence of exhaust gasses in the intake charge (Exhaust gas dilution) causes other problems as well. The rotary engine is known for its poor combustion characteristics. Due to the shape of the chamber, and the location of the spark lugs, a large percentage of the intake charge does not burn in the chamber. The end result is a fair amount of unburned gasses, or hydrocarbons being passed into the exhaust system. This reduces power output, because a portion of the mixture that we tried so hard to put into the engine did not burn. This also reduces fuel economy, and increases emissions. Another effect that is not often realized is excessive exhaust gas temperatures. These hydrocarbons will then burn in the exhaust system raising the exhaust gas temperatures.
The addition of exhaust gasses to the intake charge will reduce the already poor combustion quality. The end result is that the mixture is harder to ignite, and when it finally does light up it will burn at a slower rate further reducing power output. In a turbocharged engine excessive exhaust gas dilution will cause its own unique set of problems.
Detonation


We tend to think of combustion inside of the engine as a series of explosions, but in fact the combustion occurs at a very slow rate, at least compared to an explosion. In the absence of detonation, the mixture in the vicinity of the spark plugs is ignited first, and the "flame front" travels from that point, through the rest of the mixture in a fairly controlled manner. Detonation occurs after the combustion has initiated, and the pressure, and temperature in the chamber rises to the point that the remaining mixture literally explodes. Anyone who has ever experienced detonation understands that it certainly is an explosion! Detonation is caused by a combination of heat, and pressure, and so it stands to reason that excessive exhaust gas dilution, (remember these are hot gasses) will increase the likelyhood of detonation. As most of you know, detonation will destroy a turbocharged engine in a big hurry.
A "Perfect" Exhaust Cycle


Now that we have all of the pieces, it is time to put the puzzle together. I personally have a hard time understanding anything unless I can see it in front of me. For that reason I will refer once again to the illustration of the engine during its different phases.
As the exhaust port opens, (#13 in the illustration) the high pressure in the combustion chamber will force the gasses through the port and down the exhaust system at a high rate of speed. This, as you remember, is the blowdown period, and a large portion of the gasses will exit the chamber at this time. At the same time that the flow is initiated, a high pressure wave will travel towards the end of the exhaust system at the speed of sound. (Note that this high pressure wave will help to propel the slower moving exhaust gasses with it.)
Further into the cycle (#15) as the pressure differential between the chamber and the exhaust system has decreased, (ie., the chamber has "blown down") the velocity through the exhaust port will also decrease, and the remaining flow will be the result of the decreasing chamber volume. At this point, approximately half of the exhaust gasses will have exited the chamber.
At 135 degrees after bottom dead center, (between #15, and #16) the chamber will be at its maximum rate of decrease of volume. In other words, it is at this point in the cycle that the rotor will be travelling at maximum velocity. If the exhaust flow is insufficient, it will require a great deal of force to expel the gasses from the chamber. This is where the pumping losses during the exhaust stroke will be the greatest. Keep in mind that these losses cannot be eliminated, but they can certainly be lessened by providing sufficient exhaust flow.
Moving on to #17, and #18, the chamber volume is decreasing at a very slow rate, and the motor is doing very little to mechanically expel the gasses from the chamber. It is at this point in the cycle that pressure wave tuning comes into play. The high pressure wave that originated when the exhaust port first opened will have travelled to the collector, and been reflected back as a low pressure wave. (Remember last months section on pressure wave tuning?) If timed correctly, the wave will arrive at this point, just before the intake port opens. This low pressure wave, in conjunction with the "pull" created by the high speed gasses still in the exhaust system will lower the pressure in the chamber to sub atmospheric. When the intake port opens, this vacuum will help to initiate the flow of fresh mixture into the chamber, which will increase volumetric efficiency.
Looking back to December's port timing article, you can see that the intake port does not open until approximately 30 degrees after top dead center. That means that for the first 30 degrees after TDC, (The distance between #18, and #1 in the illustration) the chamber volume is increasing, but because only the exhaust port is open, the chamber will be filling with exhaust gasses by pulling them back out of the exhaust system. This is called exhaust gas reversion. If the exhaust gas velocity is low, (Such as at low rpm) the vacuum created by the increasing chamber volume can easily reverse the flow and pull the gasses back into the chamber. If, on the other hand, the exhaust gas velocity is high, it will take a great deal more energy to reverse their flow, and the result will be less exhaust gas dilution. This is why large exhaust ports, and large diameter exhaust tubing reduce low speed power.
Low RPM Operation


The above paragraphs describe a "perfect" exhaust stroke, and unfortunately this can only happen over a very narrow rpm range. Let's look at what happens when we halve the rpm. We will assume that the above example is at 8000 rpm. Now let's look at the same cycle at 4000 rpm. Since the exhaust cycle lasts twice as long at 4000 rpm, the chamber will have reached sub atmospheric pressure approximately half way through the cycle, assuming of course that we have sufficient exhaust flow. This sub atmospheric condition will send a low pressure wave travelling towards the end of the exhaust system at the speed of sound. (Remember that a pressure wave is intiated anytime pressure deviates from atmospheric.) This wave will reach the collector, and reflect back as a high pressure wave. Since we have halved the rpm, it is likely that this wave will arrive near the end of the exhaust stroke, (#18) and so the chamber pressure will be above atmospheric when the intake port opens. This will result in excessive exhaust gas dilution as compared to the 8000 rpm example. In addition to this, the exhaust gas velocity will be low, and during the period from TDC, to intake valve opening, the exhaust gas flow will reverse momentarily. This will also add to the amount of exhaust gas dilution.
If we wanted the exhaust stroke to be optimized for this lower rpm, several changes would be necessary.
1. Later exhaust port opening. Since we have more time to exhaust the chamber, the total exhaust duration can be lessend. The result of this will be that we can hold pressure in the chamber for a greater period of time. This will increase the amount of time that torque will be applied to the eccentric shaft.
2. Smaller cross sectional areas. Decreasing the cross sectional area of the port, and the exhaust tubing will increase the velocity of the exhaust gasses. This will result in less reverse flow, or exhaust gas reversion after top dead center, and will make the inertial scavenging towards the end of the cycle more effective.
3. Longer tuned lengths. Since the exhaust cycle occurs over a greater period of time at low rpm, the pressure wave must be further delayed if it is going to arrive at the appropriate time. In the case of optimizing the system for 4000, rather than 8000 rpm, the header lengths would need to be approximately twice as long. This is easiest to understand if you think of the headers as a delay source. What we are trying to do is delay the wave from the time it initiates to the end of the exhaust cycle. The further that the wave travels, later it will arrive at the exhaust port.
As you can see, we can only optimize the exhaust cycle over a fairly narrow rpm range. If at first this seems discouraging, it is important to consider that an optimized cycle over a narrow range is much better than a less than optimum cycle throughout the operating range. A "perfect" intake stroke can also only occur over a fairly narrow rpm range, and so it is important to consider the trade-offs when contemplating performace upgrades. If for instance you wish to "street port" your engine, you must understand that the increase in top end power will be accompanied by a decrease in low speed power.
The intent of these articles is not to make specific reccomendations, but to give you the knowledge to make informed decisions, and sort through the hype. For all of you racers, using the lessons learned from the exhaust system articles will allow you to make sense of exhaust tuning. If you apply these theories, and do some trial and error testing, you will likely unleash some hidden power. Now that you have the facts, you will understand why one system affects the engine differently than another, and this will make it much easier to arrive at the "correct" setup.
Until next month, thanks for reading, and stay tuned. There is much more to come. As for the topic of next months article...well, you'll just have to wait and see.

KiyoKix

Man I love technical stuff . I'm happy as a kid at christmas time! Keep it going guys, my brain has been started again...

Icemark

I am glad you posted pauls listing as it 100% agrees with what I am saying:

This is the whole concept of what I am refering too

Too big pipe= not enough back pressure to gain sufficent velocity

Too small pipe= too much back pressure and loss of HP and compression

Just right tuned for the engine pipe= best power gains.

Agent_D

iTrader: (2)

ahh, i see, thanks for pointing that out mark

gotdatfiyah

is the 2" i have fine on my gxl then?...would i benefit from getting 3"?

Psychoblue23

Ice,

After all this has been said, being that i drive a 100% stock NA I would be better of to stay with the smaller duels? But whats to say if I added the RB header and presilencer? Would I then need to change a bigger exh?

- james

Icemark

The RB header is tuned to the proper length that you will see a noticable power increase, with either the stock or an aftermarket system.

The idea that I am trying to instill in this thread, (like what some of AgentDs posts also suggest) is that huge open super free flowing pipes generally don't work well on two stroke type vehicles like rotaries.

There needs to be some restriction, be it a more compact pipe or even as small as the stock system. This is suggested in several of agentDs posts where velocity is brought up (and I should have really said to start with instead of the mis-nomers I was using), as it relates to pipe sizing.

But don't get me wrong most aftermarket exhaust systems are a radical improvment over the stock system. It's just using the right size rather than the biggest most free flowing system that can be found. Don't use that cat back designed for a Turbo on a N/A, use the one designed for a N/A.

Its been my experience that for a single pipe on a N/A; 3" is about the absolute maximum, while in a dual set up 2" to 2 1/2" are about all you want to do for either a true dual or y-pipe/catback. Any bigger than either of those on a N/A and you are just loosing power at under 5K.