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Discussion Starter #1
Ok, real input only, not, well, the bigger the better, or well i heard you need this or that

In reality, is a 2.5" exhuast really necesary for a NA car that will only run to about 7100rpm and about 170whp?

I have the HKS right now, its about 2.1", not a full 2.25" but is it worth the efford to upgrade to a 2.5" or will the HKS flow enough...

I dont want advice saying, well the 2.5" wont hurt...is it really necessary though?

For example, jon is runing 260whp with a 2.5"

They sell UPGRADE exhuasts for RSX type S that are 2.25" and gain nice power, and they make 170-190whp with mods...

so should i just stay with the HKS, which isnt huge but is fairly straight and flows well, or should i upgrade to a yodude 2.5"?

Im already upgrading my raceheader to a 4-1 vs my current 4-2-1 FC 1 1/2" pipping (the 4-1 is 1 5/8") and a 2.25" flexpipe, but i really like my HKS and unless i will see real gains with a 2.5", i really dont wanna upgrade

Can someone who knows shed some real light on this????????

The HKS on a stock focus gained 3.5whp, STOCK focus otherwise, which is pretty damn good for exhuasts on stock focuses...

EDIT: Ok, the HKS on my car is 55mm...which is about 2.2", so its not a full 2 1/4" but close

[ 10-15-2002, 08:12 PM: Message edited by: nazthug ]
 

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naz, on a serious note i say 2.5...if u think u lost some power, just tune it...the bigger the better
the 2.5 has proved itself to me and others on the dyno
 

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I'd say keep that HKS is pretty decent until you're thinking about a REALLY serious build up. A 2.25 exhuast is used on a lot of N/A motors, I think your best bet is to get rid of that nasty stock flex pipe. That no doubt is creating a lot of slow down and turbulence in the exhaust gasses. You may want to wait and see how the dynos come out for the SVT header (i know i am) before you spend more on another header. I'd say get the flex pipe, then if you go to the SVT header in the future you can just fab that little 18" or so section. For real, dont get rid of that sweet sounding thing just yet.
 

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Discussion Starter #5
i think so tooo, im gonna have a magnaflow 28" muffler put in instead of the HKS resonator to quiet and smooth it out and slap on a FS flexpipe and raceheader, i think the the FS 4-1 raceheader will be just what i need, not too big but not too small...hopefully it will all work out

Now i gotta solve fuel...
 

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Originally posted by nazthug:
i think so tooo, im gonna have a magnaflow 28" muffler put in instead of the HKS resonator to quiet and smooth it out and slap on a FS flexpipe and raceheader, i think the the FS 4-1 raceheader will be just what i need, not too big but not too small...hopefully it will all work out

Now i gotta solve fuel...
<font size="2" face="Verdana, Arial">So you're going from the FC to the FS? interesting, i thought the FC was better...but i guess a 4-1 is always better for high revvin' engines.
 

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Discussion Starter #7
why would the FC be better? Its not whats better or worse, its what is appropriate to what application..

The FC race header is appropriate for all around power, torque and sound...nothing huge and not for a full out NA car...we know this, they know this, thats why they willl soon be offering a RACE race header thats 4-1 and much thicker

The FS is a tad bit larger in diameter and is also 4-1 so it should flow just a bit more for MY application...thats how i figure it
 

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I say keep your HKS.. my theory is that the HKS (with its very minimal bends) can flow as much as a 2.25 inch exhaust that routes above the rear suspension..

but that is probably BS...but thats my theory..

Maybe get rid of the center muffler?? and just have the front resonator and the rear can to quiet the sound??

I am thinking about doing that..but I wanna see how loud it will get before I hack up my HKS.
 

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Discussion Starter #9
actuallly...im adding a muffler, i cant stand it being loud, i want it as quiet as possible, its fairly loud with my raceheader and cams and headwork
 

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nazar: you might want to consider the kamakazi 4-1race header...I'm waiting for one...should be getting one in a week or two

its a 4-1 design, bigger tubes that even FS (forgot the exact size) and a 2.5 collector

check the honda boards, kamakazi is what all the bid dogs run and you can get the ceramic coated for around $320
 

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SCC did an "Exhaust sizing guide" that based exhaust size on the displacement of the engine. Their reccomendations were similar to.

1.5 - 1.6 : 2"
1.8 - 2.5 : 2.25"
2.5 + : 2.5"

After that article the owner of Bill's Isuzu Performance wrote in (with dynos) debunking their sizing and saying that their reccomendations where .25" to .5" too small.

I know I had a huge increase in lowend when I went to the 2.5" exhaust with the Crower Cams. The exhaust pulses are still very prominent coming out of the tail pipe so any reduction of the pulse velocity is probably nil.
 

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Discussion Starter #12
wait...u had an increase in the LOWEND with a BIGGER exhuast? Isnt it usually the other way around?
 

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The Borla folks noted on their web page that many thought there was a correlation between tubing size and efficient scavenging of exhaust gasses out of the cylinders.

Now Borla makes a 2.5" pipe so you have to decide if their explanation and findings are the chicken or the egg. I went with Borla.

I've seen enough bad information in SCC to suspect most of what they claim. So if somebody has documented dyno sheets where they've controlled the variables, I put some weight on that, too!
 

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Discussion Starter #14
but the mufflers on the borla are 2.25"...so go figure
 

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naz

why dont you just get a FC flex pipe and dyno the car. then add the FS 4-1 header and redyno, then get a 2.5" exhaust then redyno, so you can tell what is working for you and what isnt! thats what id do, dyno one piece at a time, with no cam tuning.

chris
 

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Naz, keep this in mind. Nader was pushing 170ish HP to the wheels, and wasn't able to move up in HP till he changed to the 2.5" inch exhaust.

The SVT is only pushing about 150 to the wheels, and so far the best gaining exhaust is the 2.5" diameter of Borla.

You're at 160 whp. Now I personally love the HKS, but 2.1 has to be somewhat of a restriction with the power your making now. You ask if it's a necessity, I say "no" it's not a necessity, but if you want more power, then yes it's a necessity. But if things are fine the way they are, you're good with the HKS.
 

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Discussion Starter #18
but i just love the HKS so damn much

Maybe i can get yodude to make me a 2.5" with all magnaflow mufflers, and then get jon to send me a HKS canister, just cause i love the damn thing so much
 

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Naz, here is the math.

Pipe ID = 2 times The sqaure root of: (per cylinder displacement x 2) divided by ((Header primary pipe length + 3) times 25)

This equation presupposes that you have a tuned header.
 

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Discussion Starter #20
yeah but does it take into account that my 2.0L now makes 190flywheel hp not 130hp????????

How does it not take HP into account? I mean, you can have a 1.8L make 75hp, and then you can have a Type R motor, tuned, make 220hp, same size...im sure both would require TOTALLY different sizes...i just emailed jon, so now i patiently await his response
 
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