|Today 08:15 PM|
|Jburks||that was my whole point in resurrecting this thread lol. I wanted to know if it was going from 2.5 inch to 3.0 that gave him that extra 8whp. also, would a three exhaust be beneficial without the header. I know Focusboy has the f2 header to go along with the 3in exhaust but by keeping the stock exhaust manifold would a 3in exhaust see the same gains or would a 2.5 be better?|
|Today 07:57 PM|
Primaries and secondaries (aka header[s]) absolutely can be too large and absolutely will change the powerband. That's basically the point of having them. A vehicle with no tuning or cam changes will see an improvement in one area of the tach with a reduction in another.
Beyond that, the sole intent is the evacuation of those spent gases. Stop the exhaust right there and you'll have the maximum power available. This is why most race cars have a dump or a short run to a side-exit. If it's a dump then is most likely a drag car because having those gases beneath the passenger compartment is no bueno.
Most of us can't/won't hack up our car to get those gases out in a faster fashion. The only alternative is to open the tubing diameter going out the rear of the car. Because the velocity increase that's necessary occurs within the primaries/secondaries, you're only limited by what physically fits or that you can afford.
If velocity in the CBE was so important, you would see people ceramic-coating them or wrapping them with header wrap. It's also worth noting that Gabe's 183 whp had come from a 3" exhaust after getting 175 whp on 2.5". But in equal fashion, I will also gladly change my stance if someone ever proves otherwise on the Duratec. If 2.5" was enough (or, at worst, too much) then there would have been no gains with the increase.
|Today 07:34 PM|
I love my exhaust, whether I've lost power or not. lol
|Today 07:30 PM|
I'm not racing anyone anyway, lol. My great pleasure comes from having a car that gets 40+ mpg but can still be called "spritely". The car has enough oomph to pass someone down a backroad, and that's all I need out of it. I just want it to sound good doing it, haha!
|Today 07:12 PM|
|Today 07:07 PM|
|RonMaiden||I've seen a couple dynoed Hondas "upgrade" their exhaust to a 3" system and did see a gain in peak hp but peak torque was lower at the same rpm.|
|Today 07:02 PM|
You can indeed have too large a diameter exhaust piping. Once you go too big you lose exhaust velocity, thus reducing correct flow. Once this happens you lose power, most noticeably low end torque.
I won't say anything about our little 2.0's since I've yet to play with the exhaust. However I do find it hard to believe a 3'' pipe isn't too large for the 2.0. I'll happily admit I'm wrong should someone pay to have my car's exhaust done in 3'' catback. I think the biggest thing with our cars is all the restrictions. One to two cats, two resonators, and a muffler the size of Texas. I'm curious what gains our cars would see exactly with all of that deleted while maintaining a 2'' or at most a 2.25'' pipe diameter.
This response is not hostile at all, and I am just saying my personal thoughts and opinions, which are worth what you all paid for them. My experience comes from modding a 300's exhaust system, a lot. I have also modded (to a much lesser degree two 3.0 Ford Escape's exhaust.
I'm not saying a 2.0 doesn't see horsepower and torque improvements by opening up the exhaust to 3'', but I feel that the increase in power is done so primarily by deleting all of the restrictions, not so much from increasing pipe diameter.
I want you all to know my response is done so with humble intentions. I have not had the pleasure of modding my Focus' exhaust yet. I had an appointment tomorrow morning to swap out my muffler, but that's it. I'm doing so in full knowledge I will not see any kind of meaningful horsepower increase, just noise.
I'm just a Focus newb happily learning away here!
|Today 04:38 PM|
|Today 04:37 PM|
The issue for me is not if there are gains, there will be..but how the powerband is moved,
Ive seen people do a 3" exhaust on even a larger displacement na motor and they lost
A.lot of low.end...very annoying not to have it in stop and go city driving...now this was
without any tuning done.
|Today 04:35 PM|
Here's a good read from Honda-tech regarding back pressure and exhaust velocity.
Backpressure: The myth and why it's wrong.
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).
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.
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