Ford Focus Forum banner

1 - 20 of 24 Posts

·
Registered
Joined
·
2,668 Posts
Discussion Starter #1
why are there people running 2.5\" on a zetec?

I've always heard 2.25" is good enough, but i'm seeing more and more people running 2.5" with only cams and bolt ons.....does this help out more then a 2.25"? just wondering
 

·
O .
Joined
·
2,372 Posts
Re: why are there people running 2.5\" on a zetec?

i put it on as an incentive to put a turbo in line with it to get the most out of it
also i thought it would quiet down my header...WRONG!
 

·
Registered
Joined
·
163 Posts
Re: why are there people running 2.5\" on a zetec?

the thing is that it kind of depends on how many litters the motor is and how big it is.2.25 on just bolts would be good but if ur going with head cams,cam gears,and so on i would suggest going 2.5.hondas have 2.25 on 1.6 and 1.8 and most of them r 2.25 and most rsx that that i know have 2.2 or 2.0 and they use 2.5.my dos cents
 

·
Registered
Joined
·
2,668 Posts
Discussion Starter #5
Re: why are there people running 2.5\" on a zetec?

cool I appreciate the info......then why do guys use the 2.5" exhaust if it's just a overkill? or does it increase high rpm/mph power? and sacrifice bottom end?
 

·
Registered
Joined
·
109 Posts
Re: why are there people running 2.5\" on a zetec?

mebbe I'm wrong, but ppl. using 2.5's on a mildly modd'd car (e.g., intake, street header, basic cams, etc.) are actually hruting performance. your engine is basically an air pump. on a turno/supercharged car you need to move more air. without such forced induction, you would want to remove any major obstacles to moving air, but if you go and put a huge pipe on the back end, then your gonna lose all pressure. think of the garden hose analogy. if your pressure is x, then a tiny drinking straw sized hose is gonna impede the delivery of water. of course, if you put a firehose on, then the pressue is gonna dwindle to nothing. you gotta find the right size to match the your current pressure.

then again I could be wrong since I have no go fast stickers on my focus.

curt j.
 

·
Registered
Joined
·
1,341 Posts
Re: why are there people running 2.5\" on a zetec?

hmmm do u guys know how big a 1/4 inch is?
2.5-2.25=.25 thus 1/4inch
the size difference is so small that i believe it matters not, but 3.0 is overkill.
 

·
Registered
Joined
·
409 Posts
Re: why are there people running 2.5\" on a zetec?

well, do you know the basic formula for area of a circle? Pi X radius squared sooooooo:
2.25" is 3.14159 x (1.125 x 1.125) = 3.977
2.50" is 3.14159 x (1.250 x 1.250) = 4.909
for a difference of .932 which means the 2.5" exhaust will flow 23.4% more. That's not just a marginal difference.
 

·
Registered
Joined
·
2,668 Posts
Discussion Starter #9
Re: why are there people running 2.5\" on a zetec?

ok, well since it is such a marginal difference then wouldn't that big of a size difference be considered over kill?
 

·
Registered
Joined
·
409 Posts
Re: why are there people running 2.5\" on a zetec?

It depends on how much flow is coming out of your motor. I should have stayed with a 2.25" for my JRSC because the 3" actually makes the car slower (or so it feels). If you aren't running boost over 14 lbs, I would say 2.25 is fine but can't speak for turbo Foci yet.
 

·
Registered
Joined
·
1,341 Posts
Re: why are there people running 2.5\" on a zetec?

i dont know where the 23.4% is coming from, help me out

(3.977/4.909)*100 = 81.0145%

100-81.0145 = 18.98%

isnt that math saying the area of the 2.25 is 81% the area of the 2.5? so then to find the percentage of the area the 2.5 has over the 2.25 substract it from a hundred to get 18.xx% i cant see 18% or even 23% taking huge numbers from your power output, but then again im no auto major. im not trying to be demeaning in any way, help me out cause i'd like to know also


and whats the deal with this back pressure hear-say? last i heard it was a theory yet to be proven. what ive heard matters most in the exhuast velocity, so remove any bottle necks in the system and youve got the best u can get. i dont see how putting a slightly larger diameter pipe on would hurt your performance.
 

·
Registered
Joined
·
409 Posts
Re: why are there people running 2.5\" on a zetec?

I divided the difference into the original area: 4.909 - 3.977 = .932
.932 / 3.977 = 23.4% more area
However, if you do it backwards then the smaller diameter is 19% less than the bigger diameter area. It's a ratio that depends on which way you are comparing it. The easiest way to show this is 100 less 20% is 80, but 80 plus 25% is 100.
 

·
Registered
Joined
·
388 Posts
Re: why are there people running 2.5\" on a zetec?

dbluefocus21-ulack said that it is NOT a marginal difference! unless you are pushing the air through the system with a supercharger or turbo there is no need for man than 2.25 inches. The n/a zetec(unless completely rebuilt and modded) needs more back pressure than a 2.5 can offer.
 

·
I change diapers now
Joined
·
14,444 Posts
Re: why are there people running 2.5\" on a zetec?

Unless you're shooting for more than 170whp NA, stick with a 2.25in


- Drew
 

·
Registered
Joined
·
2,375 Posts
Re: why are there people running 2.5\" on a zetec?

This is from a Miata site, but it rings true here as well. Read the WHOLE thing, but pay special attention to the parts about exhaust pipe diameter, RPM/powerband and their relation to exhaust diameter, and the myth of backpressure.
 

·
Registered
Joined
·
1,341 Posts
Re: why are there people running 2.5\" on a zetec?

about how much power are would a 2.5 lose?
 

·
Registered
Joined
·
109 Posts
Re: why are there people running 2.5\" on a zetec?

okay, some people just won't get it. So, go run a 3 or 4 inch exhaust on your automatic foucs. actually, go for a 6 inch exhaust, I mean this way you're set for any future modifications.

meanwhile, the rest of us will run a dinky little pink panties sized 2.25 exhaust. less manly sure, but I guess we're just dumb.

by the way, did you ever wonder why the big cats (such as alms cars) that run at sebring or elkart lake or etc. don't run 3/4/5/6 inch pipes if back pressure didn't matter. I mean, these are professional race teams which have plenty of bucks and everything is custom made. they can and will make an 18" diameter exhaust pipe out of platinum if they'd get better times, but they don't - why? or did you ever walk down pit lane and notice how some v-8's normally aspirated have smaller pipes than v-6 twin turbos, or bigger than v-6 normally aspirated? well, if so then the answers provided in this thread explain it. If you didn't wonder, then go get that 3" pipe, which, by the way, can be had easily at any junkyard from the remains of any ole school bus.

curt j.
 

·
Registered
Joined
·
104 Posts
Re: why are there people running 2.5\" on a zetec?

Hahah and I think the SVT team knows something about tuning cars, hence the 2.5" on the SVTF. 2.5" is not overkill, just depends on what your running. N/A its fine, and if your not running any other mods you probably wouldn't notice the difference anyway because just a catback is not going to run you much more power. With that said, I have a 2.25" FS Sport and I love it. Also, thank you 430752. Now no more of this "myth of back pressure" [censored], its a simple and proven concept. It's like arguing against gravity.
 

·
Registered
Joined
·
1,372 Posts
Re: why are there people running 2.5\" on a zetec?

As far as I've seen, with a 2.5" exhaust, yes you'll lose power/torque down low, but flow will be heavily increased in the upper-RPM ranges. Also, (if I end up keeping my car
) I'll be getting a 2.5" exhaust because it doesn't affect low-end that much and it keeps me open for a turbo/supercharger later down the line. Am I wrong for this?
 

·
Registered
Joined
·
559 Posts
Re: why are there people running 2.5\" on a zetec?

As far as I've seen, with a 2.5" exhaust, yes you'll lose power/torque down low, but flow will be heavily increased in the upper-RPM ranges.
You may lose a few HP below 3000 RPM, but in reality, what does 4 HP at 2500 RPM really mean? Its not like you spend much time there when you floor it. If you need to go faster, downshift and floor it and the 2.5" exhaust will make the car faster than the 2.25" exhaust.

Also, (if I end up keeping my car
) I'll be getting a 2.5" exhaust because it doesn't affect low-end that much and it keeps me open for a turbo/supercharger later down the line. Am I wrong for this?
You are right. It may be better to go with a 3" if you go forced induction though.
 
1 - 20 of 24 Posts
Top