Back pressure - exhaust designers would love to see it go!!
You end up with it because of physics (gas and particle flow) and in a lot of cases the need to meet noise levels by the manufacturers.
The reason reducing is a problem sometimes is due to the fact that the car will have it's cam timing (overlap and scavenging) etc based on the factory exhaust size and it's psi back pressure so changing any of this will effect something.
De-catting may help breathing in the high rpm range but can effect the target rpm for scavenging and cam settings and could make the engine a bit quicker to response also.
If you decat then depending on where in the exhaust system the cat is you can end up with a wee bit too much valve overlap because of the lower back pressure and then air/fuel mixture gets sucked out at the end of exhaust valve opening period so reducing power. If the engine is being limited by breathing at high rpm then you may get an extra couple of 100 rpm before the engine gets strangled.
De-catting helps some cars and others makes a small difference unless you can change the cam timing to match - either way on a NA engine it is doubtful if you even notice the change apart from the noise (although it does make them sound good ).

 

I see that you upgraded your intake to a CAI, replaced the header for bigger pipping and removed the Catalyser for a downpipe with a resonator and sensor prong... oh! and a free flowing muffler. So all your exhaust is missing is the 50mn mid-pipe, am I right?

Now, from your comments:
- I can't figure out if removing the Cat helps at mid or low RPM.
- Is it noisier, do you get smell from unburned sulfer and other nocive fumes?
- Will it cause heat, will it consume more fuel, will it shorten the engine life?

I seriously need to know, because I have a similar set-up: Intake,header,mid-pipe & axle back. My Megan downpipe is not installed yet and I wonder if I should (for consideration other than environment) if I choose not to go for forced induction.

Thanks for bringing up the subject.

 

thank u!!! some logic in this room haha. when taking backpressure away from a small n/a motor ur not gonna make as much power as most ppl think...

 

just a little tid bit that I randomly picked up somewhere...(i dont really know its truth, but it made some sense)
isnt some backpressure good for mpg?

 

Hey Bro,
Yes all I am missing is the Mid Pipe!!
I am going to going with the Megan Mid Pipe....it's a 57MM pipe instead of the 50MM. My downpipe is 57MM as well as my Axle Back so I want to stay wit da 57MM the whole way!!!
IMO the car makes far more power with the 57MM, just preference though
My car is mainly built for Mid to High range RPM's...just depends on what you want to do with your FIT. I myself am going quarter mile....so I would much rather have the Mid to Higher RPM's....but to answer your question about the Cat removal helpn'....YES (but in conjunction with my other upgrades). You should see a SWEET burst up above 4 grand throughout the power band.....it pulls!!
Yes it is louder....nice low rumble at idle and throughout the lower powerband but once you slam it watch out . I guess what you are trying to get at is....staying low key from the COPS? Shouldn't be an issue....I drive past them all day long.
The smell is only really bad if your rev the engine......got used to it in about a week. To be honest...I like the smell of GAS .
It will consume more fuel if your keep flooring it. If I drive normal...I can get close to 375 miles per tank w/ all my upgrades. Just keep it under control and you will have NO issues with loss of MPG.
Actually removing your cat will help your engine breath better then before...it will NOT shorten your engine life!!! As far as heat.....haven't noticed anything different n I live in the DESERT....NO issues there .
If you choose to go FI on the FIT most companies offering turbo kits have a downpipe w/o a cat....so you may as well get used to it NOW.....hope I have been helpful.

Peace tonyd3773

 

Is this the Cat Delete you installed?

Except for muffler, the catalyser is probably the most restrictive portion of the exhaust. Is it also the most "noise reduction" section as well?

I upgraded the "breathing" of my Fit step by step. First I did the intake with a TR1 Box: Now, that's noisy! Then the exhaust with a Cat Back TR1 50S: That's relatively quiet if you keep the silencer on. Then the 4-1 Megan Header: It added power around 4k and hardly any additional noise.

The next and last logical step should be the downpipe, but I still feel unclear about an extra dose of noise and undesirable loss of back pressure. (And the parfume of unburned exhaust isn't appealing to me).

Any more comments about your exhaust mods?
Aren't there any benefit to back pressure?

P.S.: Is there a way to change this tread's title to "Exhaust Mod vs Back Pressure" or something in this line, just so that other knowledgeable members may be attacted to chime in

 

That's the downpipe from Megan Racing....it has a Hi Flo Resonator on it to help reduce noise. Reason I went with Megan exhaust is because it sounds the least RICEY and it is competitively priced!
The sound can be controlled....that part is up too u! Hey you can look at it like this....buy the pipe (relatively inexpensive)...if you DO NOT like it for your DD then you can use it for TRACK TIME!!! It is pretty easy to install and uninstall...let me know what u think??

tonyd3773

 

removing your cat only helps at the top end like a race header verus a street header, and with no cat you may loose some tq

 

first thing i notice when i run a low backpressure (high flow) exhaust
is i significanly loose lower-end torque.

primary reason why i dont plan to run any more than the megan
axle back i have on this car.

 

IMO Honda's are notoriously low in torque. True you may lose low end but most races finish in the high end (were it counts).
If I am not mistaken most videos (Honda car racing)...they start out losing but usually pull it out because of this higher end all Honda's have.

Peace,
tonyd3773

 

So, letting go of too much back pressure translate into loss of torque at low end? That is NOT what I am looking for

I might end up selling my brand new downpipe without ever installing it... unless I go all out and get KW SuperCharge, then there would be added back pressure to let go of (and I don't think I care to spend another 4k$ on top of my actual mods)!

 

dont get rid of your down pipe if its a megan then its a test pipe and it does loss some back pressure but not all, should be fine

 

i got an ssi & tanabe touring axle-back and am still debating whether to (1) get a tr1 header and some kind of mid-pipe or (2) just getting the tr1 header or (3) just getting the mid-pipe. dont wanna loose low-end tq. my fit is a dd.

found the following posted in a couple of places when doing some research....googled 'backpressue honda'. fyi...

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.

 

Wow! Simple words for complex physics! Thanks for the enlightment.
I have yet to make a decision about my downpipe though!!!!!!

 

You will NOT regret it!! I wouldn't change my Megan Downpipe fo shat...love my downpipe.
GOOD LUCK...keep us posted!!

tonyd3773

 

Raton that was a good read. Thanks for posting that up

 

wow, really good read man, learned quite a bit.

wander if there s some kinda way of calculating the best piping size....there needs to be a car sim thats made for tuning cars...

 

OUTSTANDING POST the Honda must have backpressure gang will be on your case big time for HAVING FACTS TO BALANCE THEIR BS.

Great really great to see the facts.

BUT you really really need to credit the real author of this information. I know some people say well it's the internet to try and cover their plagiarism but hey we at FITFREAK are better than that aren't we?

 

Just wanted to post something that helped me.

Here's the actual link where the write-up came from []Exhuast back pressure MYTH??????? [Archive] - Preludepower.com.

The link actually has an additional write-up that's pretty interesting.

Googling "backpressure" turned up may links w/ some pretty heated arguments on backpressure. The write-up I originally posted kind of clarified it best for me.

Cheers!

 

I intend to have my Weapon R Street header AND my Megan Cat Delete (downpipe) installed this week end. How do I re-install both O2 sessor to avoid CEL?

I do not have any bung spacer of any kind but I read in another tread (that I can't locate anymore) that a fellow member simply screwed the sensor a little bit away from it's original position and that cured it.
Oh well, I am still debating about installing that pipe... it seems a bit trouble some.

Please help