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Cat Back - 2 3/4 or 3in?????

Old 27-Apr-2005, 08:03 PM
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Cat Back - 2 3/4 or 3in?????

What is the best way to go for a stainless custom catback size???? I can go either way but am wondering if there is any real significant difference between the sizes(power gain, noise)??????
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Old 27-Apr-2005, 08:09 PM
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I've read on various other sites that unless you plan on going turbo, then 2.5" is the MAX you should go.
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Old 27-Apr-2005, 08:12 PM
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i have 2.5 all the way back and its fine
like DumbasSi said unless u are going boost stay with the 2.5
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Old 27-Apr-2005, 08:18 PM
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even 2 1/4 is fine
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Old 27-Apr-2005, 09:06 PM
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exhaust flow is good........but u gotta remember if there isn't enough back pressure produced then u can run into engine trouble like engine oil burning etc.....
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Old 27-Apr-2005, 09:23 PM
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if u'r running stock then most people say 2" is enough..if your goin the custom route
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Old 27-Apr-2005, 10:14 PM
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I have 2.5, and if i could go back i would go 2, if anything 2.25

remember, we dont' have 5L's
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Old 27-Apr-2005, 10:47 PM
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but make sure to plan ahead. I had to redo my exhaust to get 2.5" after building my head....but at the time i first changed the exhaust i wasn't even thinking about that, so i got a smaller diameter.
Thats one peice of advice that i offer, is to mod with your future mods in mind. I could have saved a bunch of money if i didn't have to learn this the hard way.
Like don't buy a dc 4-1 with 2" collector like i did, only to change it to a different 4-2-1 w/2.5" collector after a bunch of headwork and a visit to the dyno. Then replace the 1 year old custom exhaust i had, with larger pipe.
2.5" works well with built engines, i was skeptical at first but then i tried it and it made quite a difference, but for an engine with just bolt on's i'd say 2" or 2.25" is fine.
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Old 27-Apr-2005, 10:47 PM
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Originally posted by BWK
exhaust flow is good........but u gotta remember if there isn't enough back pressure produced then u can run into engine trouble like engine oil burning etc.....
That sounds like one of legendary myths that have been around forever.
I'm quite sure it's NOT true.
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Old 27-Apr-2005, 11:37 PM
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Originally posted by BWK
exhaust flow is good........but u gotta remember if there isn't enough back pressure produced then u can run into engine trouble like engine oil burning etc.....
Total myth. Time to debunk it. Here's some info I got from a poster over at ephatch.com, who in turn borrowed it from a Miata forum. Enjoy.

*****
The purpose of the carís exhaust system is to evacuate gases from the combustion chamber quickly and efficiently. The exhaust gasses do not flow in a smooth stream. Because the gasses are vented at each opening of the exhaust valves there is a pulse of gasses from each cylinder. Just put you hand near the exhaust tip and you will feel the pulses. In a honda engine there are four pulses per cycle.

The exhaust gasses produce a positive flow in the exhaust pipe. Backpressure can be likened to resistance to the positive flow of the exhaust stream. Taken to its extreme backpressure can lead to a reversal (albeit momentarily) of the exhaust stream.

Is Bigger Better or is Faster Best?

When contemplating a modified exhaust system there are those who want the biggest diameter pipe that can be had. Their idea must be that fatter pipes are more effective at venting than narrower pipes. This sounds reasonable but it is not quite correct. Sure wider pipes have greater volume and higher flow capacity, but that is just half of the story. Capacity is one consideration but gas velocity is the other factor.

An experienced exhaust designer knows that the best exhaust is one that balances flow capacity with velocity. A given volume/time of gasses will travel faster through a 2" pipe than the same volume of gas passing through a 3" pipe. So when taken to its extremes we can see that a too narrow pipe will create backpressure (restrictions to positive flow) problems and a too wide pipe will cause a very slow flow with no backpressure.

The optimum is where the fastest velocity is achieved with the least constriction possible.

This situation will arise when the pipe is wide enough so that there is the least level of positive backpressure possible whilst achieving the highest exhaust gas velocity.

The faster the exhaust gas pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The scavenge effect can be visualised by imagining the high-pressure pulse with a trailing low-pressure area behind. The faster the high-pressure pulse moves the stronger the draw on the low-pressure gasses and the gasses behind that. The scavenge action is like (but not exactly) suction on the gasses behind.

The greater the clearance burned fuel from the combustion chamber the less diluted the incoming air/fuel mix is. Scavenging can also aid intake on overlapping valves (where the exhaust and inlet valves are open at the same time) by drawing in the intake. These are good things to happen.

So instead of going for the widest pipe possible we should be looking for the combination of the narrowest pipe that produces the least backpressure possible. In this scenario we achieve the least restriction on positive flow and the highest gas travel speed.

Exhaust pipe diameters are best suited to a particular RPM range. If we used a constant RPM engine this would be easy to specify. But a variable RPM engine will mean that not one size suits all. It is possible to vary the size of exhaust volumes according to rpm but it is very expensive (Ferrari has done it). The optimum gas flows (volume and speed) are required at the RPM range that you want your power band to be located. For a given engine configuration a small pipe diameter will produce higher exhaust velocities at a low RPM (good) but create unacceptably high amounts (bad) of backpressure at high rpm. If you had a car with a low RPM power band (2,000-3,000 RPM) you would want a narrower pipe than if your power band is located at 5,000-7,000 RPM.

Urban Myth: "Hondas need backpressure"

It is easy to see how this misunderstanding arises. Letsí say that Max puts a 3-inch system on his normally aspirated car. He soon realises that he has lost power right through the power band. The connection is made in his throbbing brainÖ. put on 3" pipe = loss of backpressure = loss of power. Max erroneously concludes that you need backpressure to retain performance. He has ignored the need for exhaust gas velocity to get that scavenge effect.

If Max had chosen a 2 1/4" pipe he would have achieved better performance in the mid- to high-RPM power band. You need the combination of the least positive (close to zero) backpressure possible with the highest gas velocity achievable to create performance. The diameter of the pipe (and smoothness of internal finish and bends) will strongly influence if your exhaust change is going to create performance or lose power.

As a general rule, a normally aspirated Honda will get better high RPM performance with a 2 1/4" - 2 1/2" exhaust system (anything above is just too wide to retain exhaust gas velocity for street driving). The general consensus is that a 2 1/4" - 2 1/2" system is for mid to high RPM heads. Your mechanic should be able to advise you what exhaust system will best suit you driving style and needs.

Forced induction (turbo or supercharged) Hondas perform better with the high volume pipes (2 1/2" to 3"), but thatís another story. The choice of a 4 into 2 into1 or a 4 into 1 header to exhaust set is yet another story.

Hope that helps, this was adjusted from miata forum but the theory still applies.
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Old 28-Apr-2005, 12:18 AM
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i vote for 2.25"....ever wonder why all these reputable companies like HKS, DC, GReddy, etc all make a 2.25" civic exhaust for normally aspirated trim as opposed to 2.5" or 2.75". these companies dyno test like mad when making their product so it lives up to the claim hp figures.
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Old 28-Apr-2005, 11:19 AM
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2.25
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Old 30-Apr-2005, 08:09 AM
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3" on an n/a vehicle is way over killer. it will probably kill the torque on your engine too.

3" exhaust's are basically for turbo'd hondas and thats it. plus 3" pipe is expensive
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Old 30-Apr-2005, 04:11 PM
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I say 2.5. But a 2.25 mandrel bent flow's as good as a 2.5 non mandrel bent. You wont' lose any torque going to a 2.5 because your keeping the stock cat and midpipe, If you plan on a cat back like you mentioned. The stock midpipe and cat will be giving you enough backpressure as it is. So go with a 2.5 if you can but a 2.25 is still better than stock and will be a touch more quiet. Id go 2.5 myself.
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Old 30-Apr-2005, 06:04 PM
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had 2.25" on my ef and it was great

car pulled better even on the stock motor
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Old 30-Apr-2005, 07:09 PM
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2.25" thats what i have aawsome
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