Most people I have seen tend to run 2.5 inch collector or upgrade to them. Has anyone done a 3" ? If so, what were the results ? Or would a 3 inch cat back with a 2.5 inch collector be sufficient ?

 

2.5 all the way back less your turbo, so i've heard. honda's like the back pressure.

 

NA or FI, 3" exhaust gains power through the entire band. It's been tested on the dyno.

 

I wouldn't run anything bigger than 2.5 on a N/A car. Turbo, you can go 3. I have 2.5 on my 88 Prelude. from the header back to my fart can. I'm gonna redo my exhaust one of these days when i get one of those universal mandrel bend exhaust kits that you can buy from Summit. I got a mig welder, and i've been itching to try it out.

 

Blake has already proven a 3" exhaust on an NA prelude gains more than a 2.5 and well obviously on a turbo.

 

You know a guy running 3 in. on an N/A Prelude? What's his setup? Is it stock or has he done some serious modding to it? Reason why i ask, is because when i had my exhaust redone at the muffler shop i used to work at, the owner told me that i didn't need to go bigger than 2.5 for backpressure reasons. He's been in the muffler and exhaust business for over 20 years. I would tend to believe him before i believe anyone else. Not that i'm doubting you or your friend. It's just that i find it a little hard to believe 3in. exhaust on an N/A prelude is getting better gains than a 2.5 in. exhaust.

 

Search google for "Hondas & backpressure"

 

Will do.

 

Here's your reason why 3in. is not better than 2.5in. for "back pressure".


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.

 

Great.

Here's one 2.5-3" comparison.
https://honda-tech.com/forums/all-motor-naturally-aspirated-44/3%22-catback-exhaust-stock-jdm-h22a-dyno-2197695/

 

you believe someone at a muffler shop? The people at the local muffler shops at my home town i could run a ****ing train on with theory. does that mean they will listen even if i give them dyno charts they wont understand how to read?

 

right on blake and vinu.. here, i'll finish him off.
boostedmonkey, your claim is for 2.5" throughout correct?
I have a couple questions for you:
1) where did you come up with this size?
2) how much exhaust gas is evacuated from a stock naturally aspirated H22?
3) who's to say that the 3" is too big and doesn't allow for the best volume/velocity combo?

In conclusion, you're the same as the people you tried to make a mockery of. The misguided "Honda's need Back pressure" group and you share the same thing, ignorance of the topic at hand. You didn't do the tests, you brought up a theory and created an arbitrary mark (2.5" exhaust) and explained volume vs. velocity and the balance. The flaw there is that you don't actually know where the balance is.

 

I said collector size, obviously it has been proven that the rest of the exhaust being 3 inch is ideal.