........


Modified by SiB16A at 11:42 PM 1/22/2008

 

leave it bolted up . you will lose back pressure open header. you will run better times. you gonna run at sac?

 

yes leave it bolted up.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by NitrousDreamz &raquo;</TD></TR><TR><TD CLASS="quote">leave it bolted up . you will lose back pressure open header. you will run better times. you gonna run at sac?</TD></TR></TABLE>

you from marina?


Modified by SiB16A at 11:42 PM 1/22/2008

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by NitrousDreamz &raquo;</TD></TR><TR><TD CLASS="quote">leave it bolted up . you will lose back pressure open header. you will run better times. you gonna run at sac?</TD></TR></TABLE>


No you won't run better times with the exhaust bolted up. The word "back pressure" is so overrated, you will lose scavenging effects of the exhaust gases.


Run open header at the track and you will see what I am talking about.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by hybrid honda &raquo;</TD></TR><TR><TD CLASS="quote">
Run open header at the track and you will see what I am talking about.</TD></TR></TABLE>

With earplugs.

-travis, who's car is still too loud with earplugs, a head sock, a helmet, and a resonator...

 

racecars run a header with a dump pipe.

run open header at the track. backpressure is stupid. who cares if its loud.

 

you dont want BACK PRESSURE, you want FLOW VELOCITY, there is a HUGE DIFFERENCE, w/ an OPEN stock style header (EG one that is not designed to be run w/o an exhaust system) your flow velocity goes to ****

"Backpressure: The myth and why it's wrong.

I. Introduction

One of the most misunderstood concepts in exhaust theory is backpressure. People love to talk about backpressure on message boards with no real understanding of what it is and what it's consequences are. I'm sure many of you have heard or read the phrase "Hondas need backpressure" when discussing exhaust upgrades. That phrase is in fact completely inaccurate and a wholly misguided notion.

II. Some basic exhaust theory

Your exhaust system is designed to evacuate gases from the combustion chamber quickly and efficently. Exhaust gases are not produced in a smooth stream; exhaust gases originate in pulses. A 4 cylinder motor will have 4 distinct pulses per complete engine cycle, a 6 cylinder has 6 pules and so on. The more pulses that are produced, the more continuous the exhaust flow. Backpressure can be loosely defined as the resistance to positive flow - in this case, the resistance to positive flow of the exhaust stream.

III. Backpressure and velocity

Some people operate under the misguided notion that wider pipes are more effective at clearing the combustion chamber than narrower pipes. It's not hard to see how this misconception is appealing - wider pipes have the capability to flow more than narrower pipes. So if they have the ability to flow more, why isn't "wider is better" a good rule of thumb for exhaust upgrading? In a word - VELOCITY. I'm sure that all of you have at one time used a garden hose w/o a spray nozzle on it. If you let the water just run unrestricted out of the house it flows at a rather slow rate. However, if you take your finger and cover part of the opening, the water will flow out at a much much faster rate.

The astute exhaust designer knows that you must balance flow capacity with velocity. You want the exhaust gases to exit the chamber and speed along at the highest velocity possible - you want a FAST exhaust stream. If you have two exhaust pulses of equal volume, one in a 2" pipe and one in a 3" pipe, the pulse in the 2" pipe will be traveling considerably FASTER than the pulse in the 3" pipe. While it is true that the narrower the pipe, the higher the velocity of the exiting gases, you want make sure the pipe is wide enough so that there is as little backpressure as possible while maintaining suitable exhaust gas velocity. Backpressure in it's most extreme form can lead to reversion of the exhaust stream - that is to say the exhaust flows backwards, which is not good. The trick is to have a pipe that that is as narrow as possible while having as close to zero backpressure as possible at the RPM range you want your power band to be located at. Exhaust pipe diameters are best suited to a particular RPM range. A smaller pipe diameter will produce higher exhaust velocities at a lower RPM but create unacceptably high amounts of backpressure at high rpm. Thus if your powerband is located 2-3000 RPM you'd want a narrower pipe than if your powerband is located at 8-9000RPM.

Many engineers try to work around the RPM specific nature of pipe diameters by using setups that are capable of creating a similar effect as a change in pipe diameter on the fly. The most advanced is Ferrari's which consists of two exhaust paths after the header - at low RPM only one path is open to maintain exhaust velocity, but as RPM climbs and exhaust volume increases, the second path is opened to curb backpressure - since there is greater exhaust volume there is no loss in flow velocity. BMW and Nissan use a simpler and less effective method - there is a single exhaust path to the muffler; the muffler has two paths; one path is closed at low RPM but both are open at high RPM.

IV. So how did this myth come to be?

I often wonder how the myth "Hondas need backpressure" came to be. Mostly I believe it is a misunderstanding of what is going on with the exhaust stream as pipe diameters change. For instance, someone with a civic decides he's going to uprade his exhaust with a 3" diameter piping. Once it's installed the owner notices that he seems to have lost a good bit of power throughout the powerband. He makes the connections in the following manner: "My wider exhaust eliminated all backpressure but I lost power, therefore the motor must need some backpressure in order to make power." What he did not realize is that he killed off all his flow velocity by using such a ridiculously wide pipe. It would have been possible for him to achieve close to zero backpressure with a much narrower pipe - in that way he would not have lost all his flow velocity.

V. So why is exhaust velocity so important?

The faster an exhaust pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The guiding principles of exhaust pulse scavenging are a bit beyond the scope of this doc but the general idea is a fast moving pulse creates a low pressure area behind it. This low pressure area acts as a vacuum and draws along the air behind it. A similar example would be a vehicle traveling at a high rate of speed on a dusty road. There is a low pressure area immediately behind the moving vehicle - dust particles get sucked into this low pressure area causing it to collect on the back of the vehicle. This effect is most noticeable on vans and hatchbacks which tend to create large trailing low pressure areas - giving rise to the numerous "wash me please" messages written in the thickly collected dust on the rear door(s).

VI. Conclusion.

SO it turns out that Hondas don't need backpressure, they need as high a flow velocity as possible with as little backpressure as possible."

as posted by user "Chunky" on automotivetech.org

http://www.automotivetech.org/...=6283

 

TEMP That is probably one of the best discussions of exhaust design that I have seen in a long time. Simple yet concise. The myth of bigger is "always" better is based on the misconception that louder means its breathing better. Best to try and balance the usable range that is most frequenly needed. On the street we rarely keep the rpm above 3-4K. If you want to make noise bigger is better but if you want true performance increases in a usable range for a 1.6 liter engine about 2 inches is a good compromise.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by tpr &raquo;</TD></TR><TR><TD CLASS="quote">TEMP That is probably one of the best discussions of exhaust design that I have seen in a long time. Simple yet concise. The myth of bigger is "always" better is based on the misconception that louder means its breathing better. Best to try and balance the usable range that is most frequenly needed. On the street we rarely keep the rpm above 3-4K. If you want to make noise bigger is better but if you want true performance increases in a usable range for a 1.6 liter engine about 2 inches is a good compromise.</TD></TR></TABLE>


2" depending on the engine setup, 2" is what i usually recommend for a mild N/A setup, there is another backpressure discussion on that board as well, some real informative ****

 

Some tracks won't let you run if the car is too loud so leave it bolted...

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by temp &raquo;</TD></TR><TR><TD CLASS="quote">


2" depending on the engine setup, 2" is what i usually recommend for a mild N/A setup, there is another backpressure discussion on that board as well, some real informative ****</TD></TR></TABLE>


IMO 2" is too small for a b16 or b18 with bolt ons. Screaming at 8K+ RPM I want a little more than 2" pipe diameter.


I know first hand that putting too big of an exhaust pipe will kill the power of the engine. I had a old prelude that had a 2.5" pipe and it killed my power. ( not like it had any to begin with)


But I run 2.5 on my b16 now and it puggin rips on the top end, bottom end power was hardly sacrificed too.

 

i belive thats about the 450,384 time that "backpressure myth" has been copied from another forum.

exhaust scavenging really comes into play in the lower rpm range.

 

then how about this one

"Backpressure and Cross-sectional Area
Backpressure: Friend or Foe?

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:

quote:
--------------------------------------------------------------------------------


"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.


--------------------------------------------------------------------------------


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


quote:
--------------------------------------------------------------------------------


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.

--------------------

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)
--------------------------------------------------------------------------------


3. John Grudinsky at HyTech Exhausts comments on the usefulness of backpressure:


quote:
--------------------------------------------------------------------------------


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.
--------------------------------------------------------------------------------"

as posted by user "Tuan" on automotivetech.org

http://www.automotivetech.org/...13719

EDIT: pictures were omitted from this post because all of the pix were from dead links

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by hybrid honda &raquo;</TD></TR><TR><TD CLASS="quote">


IMO 2" is too small for a b16 or b18 with bolt ons. Screaming at 8K+ RPM I want a little more than 2" pipe diameter.


I know first hand that putting too big of an exhaust pipe will kill the power of the engine. I had a old prelude that had a 2.5" pipe and it killed my power. ( not like it had any to begin with)


But I run 2.5 on my b16 now and it puggin rips on the top end, bottom end power was hardly sacrificed too.</TD></TR></TABLE>


this is where i am going to have to agree to disagree with you, depending on setup the pipe size will have to be adjusted, IMO 2" is perfect for a mild N/A setup, hell even a little aggressive setup would be great w/ 2" diameter

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by temp &raquo;</TD></TR><TR><TD CLASS="quote">


this is where i am going to have to agree to disagree with you, depending on setup the pipe size will have to be adjusted, IMO 2" is perfect for a mild N/A setup, hell even a little aggressive setup would be great w/ 2" diameter</TD></TR></TABLE>

Do you have dyno results provng that 2" is adequate for a mild NA setup. By "mild", I will give my car as an example. b16 with JDM ITR header, high flow cat and a 2.5" mandrel bent exhaust. The reason why using a 2" pipe would kill air flow is simple regarding most people's setup.


Most headers come with at least a 2.25" collector. Using my car as an example agian, the collector on my header is 2.5". So it would kill the airflow going from a 2.5" collector down to a 2" pipe. Subsequently, I run 2.5 all the way from the header through the high flow cat through the 2.5" pipe. Because running a 2" pipe coming from a 2.5" collector would slow down the air flow causing a bottleneck and kill my power.


I know you have a lot of theories posted but show me a dyno where using 2" pipe yielded more HP than a 2.5" pipe and only then will I agree with you.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by hybrid honda &raquo;</TD></TR><TR><TD CLASS="quote">

Do you have dyno results provng that 2" is adequate for a mild NA setup. By "mild", I will give my car as an example. b16 with JDM ITR header, high flow cat and a 2.5" mandrel bent exhaust. The reason why using a 2" pipe would kill air flow is simple regarding most people's setup.


Most headers come with at least a 2.25" collector. Using my car as an example agian, the collector on my header is 2.5". So it would kill the airflow going from a 2.5" collector down to a 2" pipe. Subsequently, I run 2.5 all the way from the header through the high flow cat through the 2.5" pipe. Because running a 2" pipe coming from a 2.5" collector would slow down the air flow causing a bottleneck and kill my power.


I know you have a lot of theories posted but show me a dyno where using 2" pipe yielded more HP than a 2.5" pipe and only then will I agree with you.</TD></TR></TABLE>


whoa whoa whoa, like i said, agree to disagree, your header has a 2.5" collector BUT the cat does not, you adapt the cat to it, and you could reduce down the cat size, the stock down pipes on most hondas do not have such a large collector, matching them to the exact size would produce the ideal flow, and my theory is that 2" for most street applications is perfect, and if you notice most of the time i said it depends on the setup, also where you want the powerband to be at, 2" would, IMO, produce most ideal gains for most mild to slightly modded setups for N/A, but again this depends on the setup and items used.....here is a way for you to think of it, which takes more effort to blow air thru, a straw w/ a .5" opening on both ends or a straw w/ a 1.0" inch opening on both ends? you are going to have to blow harder and more air thru the 1.0" straw then the .5" straw because you have less flow velocity and you have more area for the air particles to float around in

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by temp &raquo;</TD></TR><TR><TD CLASS="quote">


whoa whoa whoa, like i said, agree to disagree, your header has a 2.5" collector BUT the cat does not, you adapt the cat to it, and you could reduce down the cat size</TD></TR></TABLE>


I was just disagreeing, no hard feelings.


My high flow cat does have a 2.5" collector on both ends with the flange kit designed for the 2.5" collector. But in most cases the setup isn't 2.5" all the way through, I happen to know what I am doing and made sure everything from the header to the muffler is 2.5" for ideal air flow.


You brought up a lot of good info though

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by hybrid honda &raquo;</TD></TR><TR><TD CLASS="quote">


I was just disagreeing, no hard feelings.


My high flow cat does have a 2.5" collector on both ends with the flange kit designed for the 2.5" collector. But in most cases the setup isn't 2.5" all the way through, I happen to know what I am doing and made sure everything from the header to the muffler is 2.5" for ideal air flow.


You brought up a lot of good info though </TD></TR></TABLE>


good stuff, thats the problem, a lot of people do not know what they are doing....LOL.....

 

thanks for all the replys. it was very helpfull and a whole lot of reading

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by EL Vap133 &raquo;</TD></TR><TR><TD CLASS="quote">Some tracks won't let you run if the car is too loud so leave it bolted... </TD></TR></TABLE>
Thoes must be in Cali?

 

i tend to belive the numbers.

My brother had a 91 STD hatch, D16Z6, DC 4-1, 3" shortram, stage 2 zex 59300 cam, and 2" exhaust. in the 1/8 mile, first run with this setup, he ran a 10.4. second time, with a better launch, he ran a 10.2, then 10.2, 10.3, 10.2 consecutively.

he removed the cat from the exhaust, in effect making an open header (the cat was not "clogged" by teh way, you could easily see through the baffle)

the very next run, nothing else changed, 9.7

at the track, run that bitch open

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by hybrid honda &raquo;</TD></TR><TR><TD CLASS="quote">


No you won't run better times with the exhaust bolted up. The word "back pressure" is so overrated, you will lose scavenging effects of the exhaust gases.


Run open header at the track and you will see what I am talking about.</TD></TR></TABLE>

correct

you going to kill your self if you drive like me exhaust gases no joke I am driving open header currently for about a month that **** is crazy I feel tired all the time I need to get it bolted asap or I will die

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by bossman032 &raquo;</TD></TR><TR><TD CLASS="quote">

correct

you going to kill your self if you drive like me exhaust gases no joke I am driving open header currently for about a month that **** is crazy I feel tired all the time I need to get it bolted asap or I will die </TD></TR></TABLE>

i know what your talkin about. I been/drove in a h22a Cx hatch daily driven open headers from a member on board here (h22aeg)....it's hella loud and smelly and you cant even hear music...no low end torque what so ever lol

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by PhatOptimo &raquo;</TD></TR><TR><TD CLASS="quote">
Thoes must be in Cali? </TD></TR></TABLE>


you see no gains anyway so there is no point