Does the exhaust size matter that much?

Fredo · 16-02-2009, 05:32 PM

Focus Folks,
New Cat Back coming up so am wondering . . . in your experience, does 2'' - 2.25'' - 2.5'' make much of a difference?
If so, what?
I'm not after a constantly loud, flatulent, droning cop bother.
Just whant it to work more betterer . . .
Your thoughts?
Fredo

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Velociraptor82 · 16-02-2009, 07:18 PM

I've upgraded mine to a 2.5" magna flow cat-back.

performance wise I think it has made a slight improvement but hard to judge. Propably more noticable on the dyno than by sight/feel.

under heavy accell particullary in 2nd it sounds and feels awesome. At constant speed in 4th (remember mine is an auto), it can hardly detected.

Only really drones if you are climbing a steep hill and the rpm's are over 2.5k rpm.

melbzetec · 16-02-2009, 09:09 PM

Which Focus do you have?

819_FLY · 16-02-2009, 11:12 PM

Going anything over 2.25 on an atmo engine is a bit overkill, drone will get the best of you. 2" is best for performance and nice note, where 2.25 will give a nice deep note but will prob have a minimal effect on performance. Save the 2.5 and up for turbo setups.

Mullett · 16-02-2009, 11:33 PM

Quote:

Originally Posted by 819_FLY

Going anything over 2.25 on an atmo engine is a bit overkill, drone will get the best of you. 2" is best for performance and nice note, where 2.25 will give a nice deep note but will prob have a minimal effect on performance. Save the 2.5 and up for turbo setups.

2.25 will be fine, I'm going 2.5 on mine because it will probably get a turbo soon. Exhaust is basically a peice of tube, once it's past the headers, you want as little restriction as possible, so bigger is better, to an extent, but bigger also means more noise. More important than size is the design of the mufflers, you want straight-thru free flowing mufflers for the best performance, or baffled mufflers for noise suppression. Best compromise if you don't want the noise level is a well designed 'double back' style muffler, some manufacturers include a 'unrestricted HP level' in their catalog, this will be a good indication. I'll probably have 2 centre resonators and a large rear muffler, possibly with twin tips.

-RM.

More revs · 17-02-2009, 12:35 PM

Mine is 2.25" mild steel with twin yes count them two outlets...

Actually my goal was to simply hear the thing to avoid stalling and bunny hopping.

Power output made no diff at all but it does sound great. If you do change your system be sure to bring the original one back home so you can put it back on to sell etc...

blue_focus · 19-02-2009, 11:27 AM

I've got a 2" cat back on mine. Bigger will give a louder deeper sound but you may loose performance as back pressure is still needed through the whole exhaust system. If it hasn't got a turbo then a 2" will more than suffecient.

Fredo · 19-02-2009, 12:40 PM

Quote:

Originally Posted by blue_focus

I've got a 2" cat back on mine. Bigger will give a louder deeper sound but you may loose performance as back pressure is still needed through the whole exhaust system. If it hasn't got a turbo then a 2" will more than suffecient.

Um . . at the risk of sounding thick, why is back-pressure needed in the exhaust system?

Kinda doesn't make sense to me to restrict exhaust.

Fredo

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Mullett · 20-02-2009, 12:14 AM

Quote:

Originally Posted by Fredo

Um . . at the risk of sounding thick, why is back-pressure needed in the exhaust system?

Kinda doesn't make sense to me to restrict exhaust.

Fredo

:

It's not. In (tuned length) headers, back-pressure is used to 'scavenge' the ports (almost like pipe hammer, the pulse is bounced backwards to generate a vacuum at exhaust port which has just opened and increase the exhaust flow), but after the headers you basically want as little restriction as possible. Remember the law of diminishing returns though...once your exhaust is flowing to the limit of the engine (maybe 2" straight pipe, maybe less) a bigger exhaust will make a very minimal difference in backpressure, but will make a lot more noise. Muffler design is crucial to flow, as are mandrel bends.

-RM.

HM Headers · 19-02-2009, 01:27 PM

PM Sent.

blue_focus · 21-02-2009, 10:09 AM

Ah k, i always thought the the whole exhaust still needed some sort of back pressure in it. My mistake.

BlackLS · 21-02-2009, 01:31 PM

Quote:

Originally Posted by blue_focus

Ah k, i always thought the the whole exhaust still needed some sort of back pressure in it. My mistake.

I always though this was the case as well. Case being I had a 2" straight through exhaust on my old 1.5L Pulsar and the front original part of the exhaust rusted through (where the cat would have been) so the exhaust was coming straight out just after the headers, and I felt like a loaded car. A few months after fixing that, I was defected for 103 decibels so I replaced the end section with a 1.5" pipe (I think) to get it to pass, which increased low down torque.

819_FLY · 21-02-2009, 04:49 PM

Turbo cars need as little restriction as possible, due to the flow of exhaust gasses backing up and interfering with the rotation of the turbine wheel. For many years people have said there is no point putting a 3" system onto a car with 2 litre displacement as it is pointless and the decibel rating would be illegal. Even going 2.5" on some cars isn't beneficial. 2.25" has been a solid performer for many cars running NA with improvement with both noise and performance. up to 5% would be a realistic figure when talking performance increase with a new system on a stock engine.