Originally Posted by sLiVeRwOrM

well lowering your backpressure to much can be very bad!

think of it like a surenge with the little black rubber stopper inside.. if you try to foce the air out it wont go out very fast.. but if you open the hole alittle more then more air goes out.. if you rip off the end of the surenge then the black thing just falls though!

Originally Posted by Tuan @ HondaLife.com

Backpressure and Cross-sectional Area by: Tuan
Backpressure: Friend or Foe?

Forum Thread
Backpressure can influence in 2 places along the engine cycle: Just at the start of when the exhaust valve opens and at cam overlap.


Figure 1. Pressure measurements at the exhaust valve during the start of the exhaust stroke at BDC to cam overlap at the end of the exhaust stroke/beginning of the intake stroke at TDC.
Notice the positive (backpressure) spike at the far left as the exhaust valve just opens at BDC. The exhaust gases must now push against this POSITIVE (back)pressure before it can leave the combustion chamber. The pressure tracing is upwards and positive. Energy must be used up in order to overcome the initial positive (back)pressure in the exhaust system before the exhaust gas is pushed out of the combustion chamber.
After we are able to overcome the positive backpressure, you see that the exhaust gas begins to travel faster and creates a NEGATIVE pressure. The pressure tracing in the diagram is downwards or has a negative value. The more negative a pressure becomes means that you are creating more suction or a vacuum in the system. The system is literally sucking or pulling out exhaust gas from the combustion chamber or cylinder. This sucking or SCAVENGING effect not only helps remove more exhaust gas from the cylinder, it also helps suck in more intake air & fuel mix at cam overlap. The faster the exhaust gas travels the more vacuum it creates. We want to get as much as negative pressure created before cam overlap.


Figure 2. Pressure at the intake port, in the combustion chamber, and in the exhaust port at cam overlap and afterwards. Everything is interconnected. The pressure in one section affects the pressure inside another section.
At cam overlap, if you look at Figure 1., there is a reflected pressure wave travelling backwards towards the engine. This reflected wave or REVERSION is what contaminates the intake charge at cam overlap and reduces or dilutes the oxygen content coming into the cylinder. Less oxygen going in means less power. Notice the pressure at the exhaust valve is still negative but less negative than before. This reflected exhaust wave or pulse is the second BACKpressure we experience and again reduces exhaust flow speed or energy because the exhaust pulse must now push against this pressure to move forward. A loss in flow speed means less negative pressure, or vacuum, or suck.


Figure 3. Electronically Controlled Exhaust Throttle Valves: Honda's H-VIX System in the Honda Fireblade Motorcycles.

Some very smart people in motorcycle racing at Yamaha developed an ingenious device called an exhaust throttle valve (called the EXUP valve). These valves have are placed at the merge points of the header primaries. They are kept open and are continuous with the header. At cam overlap, the valve partially closes. This prevents both the intake air-fuel mix from shooting into the header (called overscavenging) and blocks any reflected exhaust wave from arriving back to the combustion chamber. When cam overlap is over, the valve re-opens. So there is a brief increase in backpressure at cam overlap only with the exhaust throttle valve and nowhere else along the engine cycle. The valve is activated (closed) by a potentiometer and then disabled (opened) by the ECU which measures ignition timing to determine when cam overlap occurs and potentiometer to determine the position or angle of the throttle valve itself.


Figure 4. Honda Fireblade dyno using their H-VIX exhaust throttle valve. The blue hp/torque graphs labelled STOCK are with the exhaust throttle valve partially closed only at cam overlap and fully opened at all other times. The red graphs are with the the exhaust throttle valve open all the time...essentially like having no valve at all. The green graphs are with the exhaust throttle valve partially closed all the time: giving more backpressure all the time. Notice that adding backpressure all the time kills power at the upper rpm powerband location. Having no throtttle valve weakens the lower rpm powerband location: The stock blue graph has much more power in the lower rpms than the red graph. You may these applied to cars in the future.


The bottomline to remember is that more backpressure means adding it at 2 places along the engine cycle and that it slows down flow speed. Slowing down flow speed reduces scavenging and efficient removal of as much exhaust gas out of the cylinder before we start filling the cylinder back up again with fresh air (oxygen) and fuel for the next engine cycle (next set of intake, compression, combustion, and exhaust strokes). The upper rpm power suffers as a result.
If you think that leaving some exhaust gas behind in the cylinder before the next intake stroke is not important, look again at Figure 5 below. This is , once again, Jim McFarland's classic graph comparing the volumetric efficiency curve versus the torque curve. As I stated in the cylinder head article where you first saw this, notice that these 2 curves have the same shape but are not exactly identical or overlaid on top of each other. You would think that once you have maxed out on the engine's breathing ability (volumetric efficiency), the torque or power curve and volumetric efficiency would be identical. They are not. Why? Flow quality on the intake side and inefficient removal of exhaust gases out of the cylinder are what separates a winner from the car placing second in a race. The people that make that extra winning power are the ones that pay attention to ensuring these 2 other factors (intake flow quality and cylinder exhaust gas removal) are optimised as well as working on cylinder filling (flow volume or bulk flow).
Figure 5. Volumetric Efficiency Curve Compared to Torque Curve. The VE Curve shows how much power you would make if you maxed out and improved engine breathing (flow volume), flow quality, and exhaust removal. The torque curve shows you the power if you don't pay attention to flow quality (in the low to mid rpms) and cylinder exhaust gas removal (in the upper rpms).


Comments from Some Experts on Exhaust Backpressure:
1. Larry Widmer of Endyn on Exhaust Backpressure:

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"from 21st Century Performance Book
Few tests have been done that clearly show the effect of changing back pressure. Most muffler and exhaust comparison tests change more than one parameter simultaneously, making the identification of exhaust back pressure as a culprit difficult.
However, Wollongong (Australia) mechanic Kevin Davis has done extensive testing of varying back-pressure on a number of performance engines.
These range from turbocharged Subaru Legacy RS flat fours to full-house traditional pushrod V8s. In not one case has he found any improvement in any engine performance parameter with increased exhaust back pressure.
The tests came about because Kevin has developed a patented
variable-flow exhaust that uses a butterfly within the exhaust pipe. He initially expected to use the system to cause some back pressure at low loads 'to help torque.'
However, he soon changed his mind when any increase in back pressure proved to decrease torque on a properly tuned engine. What increasing the back pressure does do is dramatically quieten the exhaust.
One of the engine dyno tests carried out by Kevin was on a modified 351 4V Cleveland V8. Following the extractors he fitted a huge exhaust that gave a measured zero back pressure. Torque peaked at 573Nm (423 ft-lbs) at 4700 rpm, with power a rousing 329 kW (441 hp) at 6300 rpm. He then dialed-in 1.5 psi (10.4 kpa) back pressure.
As you'll see later, very few exhausts are capable of delivering such a low back pressure on a road car. Even with this small amount of back pressure, peak torque dropped by 4 per cent and peak power by 5 per cent. He then changed the exhaust to give 2.5 psi back pressure. Torque and power decreased again, both dropping by 7 per cent over having zero back pressure. These results were achieved on a large engine with a large overlap cam - one of the type some people suggest is 'supposed' to like back pressure.

If, in fact, power does increase with increased exhaust back pressure, it is most likely the air/fuel ratio and/or ignition timing that are no longer optimal for the altered state of engine tune."
Larry Widmer comments on the above textbook quote:
At less than WOT and peak power rpm, the diameter of the tubing should change in ID. Just as with intake ports (unless we're just running off port volume), cross sectional area should be only sufficient to supply the flow rate necessary to feed the engine.
High velocities, that don't incur pumping losses are the rule.
The exhaust system is much the same. Just changing back pressure is a bogus way of trying to create the "ideal" pressure in the system. The exhaust system should work like a correctly conceived header. It should extract the exhaust from the header, to minimize pumping pressures.
The only way to create a system that will serve as an extractor is to properly size the tubing to allow the flow velocity to create a sort of "vacuum" behind it.
Just as with headers, creating a system that will provide the best of all worlds at all throttle positions and rpm ranges is impossible. It's all going to be a trade-off. You can tune for the throttle positions and rpm ranges where you desire the greatest performance, but you'll sacrifice performance at the other end of the rpm range.

Building a system to divert the flow into a smaller system can help bolster lower rpm power, just as with today dual runner intake manifolds, but you'll never find a dual runner intake on any engine that's targeting the greatest performance potential possible. I should also add that such systems are inefficient from a standpoint of weight and surface area.
For mid-performance applications, these type systems will be as popular as their costs will allow.

In our quest for "more", we seldom work to achieve mid-level (mid rpm range) performance, so just as the gentleman who wrote the book in the post from above, we prefer to tune with the least amount of backpressure possible. We do have to observe rules and regulations (noise levels and EPA regulated emissions) and the systems must fit the vehicle in question without dragging the ground, so there will always be compromises.
I suppose that I should mention that cost is another consideration. If it wasn't, a lot of our street systems would have greater area and they wouldn't necessarily be circular in configuration either.
In the stock ITR, backpressure becomes a power "liability" by the time the engine's making 210 flywheel HP. Relative to wheel HP, if you're making more than about 11 HP more than "stock", the system's costing you....and yes, detonation can be caused by excessive back pressure.
The other problem you face with excessive back pressure is one of reversion. The higher the back pressure, the more inert exhaust components re-enter the cylinder. A few of these bad-guys can really steal big hunks of power in a hurry. If you don't believe me, just run a pipe from your exhaust tip up near the air cleaner on your next trip to the dyno. A little sniff of the exhaust will absolutely kill your power.

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2. Calculations and Comments by Dave Stadulis of SMSP Exhausts Relating Flywheel HP to Exhaust Cross-Sectional Area (Diameter):

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Here are the numbers for 16g tubing:
OD (in.)....ID (in.)...Area....%Increase......HP.......HP/in^2
2.25........2.120.......3.53.....0%............... 200.......56.66
2.50........2.370.......4.41.....25%.............. 275.......62.34
2.75........2.620.......5.39.....22%.............. 318.......59.00
3.00........2.870.......6.47.....20%.............. 400.......61.83
OD is exhaust outer diameter, ID is inner diameter, Area is tube cross-sectional area, % Increase is increase from the prior OD, HP is Flywheel hp, and HP/in^2 is hp per square inch cross-sectional area.

For the 2.75 in. tube, I assumed 59 HP per square inch of flow area, I used Larry's numbers for the others....you are talking HP at the crank :
2-1/4" for up to 200HP @ the crank, 2-1/2" for 275HP, 2-3/4 for 320HP...
or 60HP (at the crank) per square inch of (cross-sectional) flow area.
This 60HP/in^2 is to get you in the general vicinity. It also is based on the inside diameter of the tubing not the OD (ie. 2" in your example). The ID for 2' 16g tubing is 1.87" and this will yield a limit of 165 crank HP. 2-1/4" 16g (212 HP), 2-1/2" (265 HP). Now you can get different sized tubing such as 2-1/8" and 2-3/8" to fine tune a vehicle but you can't get cats and mufflers in those sizes so you should go up a size when building an exhaust in those cases.
The stock ITR exhaust is 2-1/4" but not everywhere. At the B pipe flange, the tubing actually necks down to 1.9" OD or 1.75" ID and then opens up to 2-1/4". It again necks down (not as bad) around the flex joint prior to the axle.
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Question: Do you have an opinion concerning the best choice for harnessing exhaust pressure waves when a catalytic converter is present at the manifold? ... I'm just curious at a theoretical level. : Here are the possible choices:
1. Gut the manifold mounted converter, so it acts like a pressure wave plenum, and replace the smaller converter downstream with a larger converter (coating the exhaust to encourage light off). As with the intake manifold, there should be some pressure wave tuning occuring somewhere in the rpm band.

2. Use a manifold/header which does not have a converter mounted to it, and install a pressure wave plenum immediately in front of a larger manifold type converter downstream (once again coating the exhaust to encourage light off). Same benefit as above, but allows some tuning flexibility since the plenum location can vary.

3. Ignore exhaust pressure wave tuning; it's not a significant issue with a stock valvetrain.
SMSP replies:
I believe you can effect the overall peformance of your system by the placement of the cat, since you have a volume change when the exhaust gases enter the cat. But then temperature does become an issue for the emissions performance of the cat.
Same goes for running an open header versus a header with a short tuned tail pipe. Tail pipe length is very important, it just takes a lot of testing to determine what is right. I know of a guy who played with different length tail pipes and picked up 5-6 HP on a 190+ WHP engine.
To take full advantage of the system you have test and tune, many many times. I believe a tuned length tailpipe will get you the most power, versus running the header open (nothing after the collector)
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3. John Grudinsky at HyTech Exhausts comments on the usefulness of backpressure:

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I have seen where a little back pressure, helped out the very low end of a 4 cycle engine because it had a lot of valve timing and it stopped some of the scavenging of the cylinder and (therefore) helped the power. But as the motor revved up, the gains were diminished and it lost power on top (in the upper rpms). There have been exhaust systems designed to actually reverse feed the cylinder through the exhaust port, before the valve closes on overlap. It actually has worked, but it didn't seem to work over a large rpm band but in a short (rpm) one, it worked quite well.
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Originally Posted by Scott_Tucker

The exhaust valve opens quite a few degrees (BBDC) before bottom dead center (lets say 65 degrees for sake of argument). This is while the piston is coming down on it's power stroke and there is quite a bit of pressure that is still in the cylinder acting on the piston when the exhaust valve opens. The period from the time the exhaust valve opens until BDC is called the blowdown period. Because there is still pressure in the cylinder when the exhaust valve opens, all that cylinder pressure starts blowing out the exhaust port(s). You might wonder why the hell you would waste all that pressure that could have been pushing down the piston and just throw it out the exhaust. First of all, the exhaust valve needs to get open far enough so it can flow enough exhaust gases by the time the piston gets to BDC (the valve(s) need to be open to the point where opening them any further won't produce any increase in flow, on a flow bench, by the time the piston reached maximum velocity - when the crank is 90 degrees to the rod. Piston velocity is a whole other thread though, I'm not going to get into that.). Second, if you did not start relieving the pressure that early the piston would have to push all the exhaust gas out the valves on what people traditionally call the 'exhaust stroke', BDC to TDC. Because the valves are a restriction, the pressure in the cylinder would rise and it would take a lot of energy to push that piston up and get those gases out. Where does that energy get 'sucked' from? Whatever cylinder is now on its power stroke. This is called a pumping loss. That energy could have gone into making more power in that cylinder that is on it's power stroke. With a proper cam by the time the piston reaches BDC the cylinder pressure is nearly atmospheric. This makes it easier for the piston to push the exhaust out the valves.
Any increase in exhaust pressure is going to be in the cylinder or exhaust port at this point. The header can flow a hell of a lot more exhaust gas than the port can. As the piston moves up it pushes this 'slug' of exhaust gas out the port and increases the velocity and inertia of this 'slug'. The velocity of the gas in the header tube is proportional to its diameter. A skinny (2 1/4") header primary will flow at a higher velocity that a fat (2 1/2") header primary. The velocity of the gas is important as to whether the engine makes it's torque low in the rpm band or higher in the rpm band. If you put a 10 lb. weight in your hand and put it against your chest and then threw it out as fast as you could, while still holding the weight, you run the risk of popping your arm out of its socket. This is due to the mass of the weight and the the velocity at which you throw the weight out. Now if you do the same thing except you do it very slowly it's not going to hurt at all. This is because is has very little velocity, although the same mass. So as the piston gets up toward TDC we start the valve overlap period. The intake valve opens a certain number of degrees before TDC - while the exhaust valve is still open, although quickly on its way closed. The slug of exhaust gas that just went out the header pipe is going to keep going the same direction even though the intake valve has opened and created another place for exhaust to leak out - if the exhaust slug has enough velocity. The slug keeps on moving down the primary and creates a low pressure area behind it. This drags most of the residual exhaust gas out of the cylinder and also helps pull the fresh air/fuel mixture from the intake into the cylinder.
There are several things that can go wrong here. If the header primary is too large the velocity will be low and it will be unable to properly scavenge the cylinder. When the cylinder is not properly scavenged it will leave behind a portion of residual exhaust gas. There are a couple of problems with this. First, the residual exhaust gas takes up space where air/fuel mixture could be instead. Second, the exhaust gas is smokin' hot because it was just burned. It's about as hot as exhaust gas gets. This adds tremendous heat to the intake charge which makes it expand and therefore less dense. This is called reducing the 'charge density'. This less dense mixture has far less oxygen in it and therefore the engine will make less power. To add insult to injury, the exhaust gas has heated up the air/fuel charge and may increase its temperature on the compression stroke to the point where the engine detonates in really bad cases.
But . . . at higher rpm the exhaust gases will have higher velocity and things should work better.
If this same engine had a smaller header primary it would not have these problems and would make torque lower in the rpm band.
ACCOUSTICAL TUNING
As the exhaust valve opens the gases are (or should be) expelled at a velocity of 200 - 300 (fps) feet per second. The sound wave 'pulse' that is emmited when the exhaust valve opens travels at 1500 - 1700 fps (the speed of sound changes depending on the density of the medium it is traveling through). This sound wave shoots down the header primary and when it gets to the collector (I'm thinking 4 into 1 header here) something called a rarefaction is reflected back up the pipe. A rarefaction is the region of a sound wave which causes the pressure to drop when traveling through the exhaust. Think of it as a low pressure doughnut traveling back up the exhaust. If the header is tuned just right you can get this low pressure doughnut reflected back up the pipe and have it arrive at the exhaust port right as the intake valve is opening. A high pressure will always go to a low pressure so the air/fuel mixture will be pushed into the cylinder by the relatively high pressure in the intake manifold and hopefully as much nasty residual exhaust gases will go out the exhaust port. This only happens perfectly at one rpm range so you want to tune the header where you have tuned the cam, etc. to produce peak torque. It happens at other rpms also although the pressure wave may miss the first time and bounce off the exhaust valve come back down the pipe as a pressure wave and then back up as a rarefaction but it will have lost energy in the process and will not be as effective.

Quote, originally posted by LudeyKrus »
At cam overlap, if you look at Figure 1., there is a reflected pressure wave travelling backwards towards the engine. This reflected wave or REVERSION is what contaminates the intake charge at cam overlap and reduces or dilutes the oxygen content coming into the cylinder.

I think you mean RAREFACTION not REVERSION. If you do have a restriction like a plugged cat. the exhaust gas may actually do this and travel back up another cylinder on its exhaust stroke and blow right into the cylinder though.
So really, there is no need for 'backpressure' and it won't gain you any power by hammering on your downpipe to make it skinnier. I guess the only time it would be benificial is if you had a car that was absolutely torque-less at low rpm and you wanted to drive it on the street. If you put some smaller diameter headers on it it would help low rpm torque but velocity would be so high at high rpm that you would start building backpressure. This would just mean you built the engine wrong for your application in the first place.

Originally Posted by Texas Coyote

I have thought since the first time I crawled under my car and looked at the exhaust that it looked very adequate and was well designed.