Exhaust Size - 6G Celicas Forums

Exhaust Size

Apr 7, 2004 - 8:46 AM
ShadowFX View Member Profile Enthusiast Joined Jan 22, '04 From WA, Australia Currently Offline Reputation: 1 (100%)	I know I;'ve had a few posts about this before but yeah i'm finally getting it done, just making sure of some last minute things (big things), I'm getting a 2.5" cat back and I've had the celi checked that it will all fit, but will 2.5" give me less back pressure, enough to give a power loss? i need ideas fast

Apr 7, 2004 - 9:34 AM
playr158 View Member Profile Enthusiast Joined May 22, '03 From NOVA Currently Offline Reputation: 16 (100%)	i don't think 2.5" should hurt your power, thats like the line for an N/A car so you should be good i thinks

Apr 7, 2004 - 9:35 AM
ShadowFX View Member Profile Enthusiast Joined Jan 22, '04 From WA, Australia Currently Offline Reputation: 1 (100%)	okay thanks! does anyone have 2.5" on their celi? (with pics if possible?)

Apr 7, 2004 - 11:06 AM
Supersprynt View Member Profile Enthusiast Joined Feb 10, '03 From Connecticut Currently Offline Reputation: 11 (100%)	Well yeah u'll have less back pressure, but thats cuz you have a larger exhaust. Less back pressure hurts your low end, improves your top end. But your really not going to feel anything in your car. -------------------- My Flickr My eBay _Perpetual Aperture_

Apr 7, 2004 - 11:27 AM
ghostdog View Member Profile Enthusiast Joined Aug 29, '02 From ny to philly Currently Offline Reputation: 0 (0%)	assuming your SX is the 1.8l 7afe, I wouldn't go any bigger than 2.25 inch. backpressure is one issue, but its also completely pointless unless you are going turbo in the near future.

Apr 7, 2004 - 12:15 PM
Supersprynt View Member Profile Enthusiast Joined Feb 10, '03 From Connecticut Currently Offline Reputation: 11 (100%)	I have a 3'' catback. Stock ST185 exhaust is 2.25'' to give you an idea. -------------------- My Flickr My eBay _Perpetual Aperture_

Apr 7, 2004 - 5:35 PM

SpedToe169

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2.5" should be just fine. Almost perfect for an NA celica. I've done lots and lots of research on this.

As far as backpressure goes, please read this:

QUOTE

It all started back in the day when the government started regulating vehicle emissions. The first catalytic converters were incredibly restrictive, and most cars were still runing carbs.

The cats were so restrictive that at low engine rpm, air would sometimes pass through the carb twice! Carbs were then re-jetted to compensate for this - to keep the air from getting a double-dose of fuel.

When you take one of these cars and remove the restrictive cat/exhaust to replace it with a free-flowing system, the stock carb will not work right anymore - you will not be getting enough fuel at lower rpms and will get a loss in torque. The solution - change the carb!

Modern catalytic converters are free-flowing compared to early pellet-filled versions. Modern fuel injection systems are not designed to compensate for super restrictive catlysts. Modern fuel injected cars do not need backpressure.

The truth - exhaust gas velocity

Here's the real deal about choosing an exhaust that is too big. I'm not a physicist, my understanding of fluid dynamics is very limited.... but here's my understanding.

Exhaust gases moving out of your exhaust can be used to pull other exhaust gases out of your cylinders. This is why headers are sized and the tubes matched the way they are. This effect is called scavenging. The faster the gases move, the more you benefit from scavenging.

The larger the tubing, the slower the gases move, the less scavenging occurs. (but, more ultimate flow with less restriction is possible)

The smaller the tubing, the faster the flow. (but obviously, the resistance can quickly overcome the benefit of scavenging if you start reducing your tubing size... a drinking straw simply can't flow enough for a motor)

Notice I didn't say anything about backpressure. Backpressure is just a measure of the restriction in the system. It can be caused by bends, narrow tubing, badly flowing mufflers, etc. Backpressure will be present in any system - but the less that is there the better. A large diameter system with a restrictive muffler (and therefore a high backpressure) will not gain any velocity. So now you will have to worst of both worlds - reduced scavenging and a restriction to free flow at higher rpms.

This is the reason that apexi made the N1 dual. The two pipes can support the flow of a larger pipe, but maintain better gas velocity and scavenging.

Some people claim to lose low end power when strapping on a big bore exhaust. This may be true, but it's because of a decrease in the scavenging effect, not because you don;t have enough backpressure.

Thats off of: Club240.com

And the guy is exactly right.

Apr 7, 2004 - 7:11 PM
shid View Member Profile Enthusiast Joined Jan 8, '04 From LA Currently Offline Reputation: 0 (0%)	After reading that- what you're saying is that for cat back, a 2.5" would be better. What if one went the way of the N1 dual, and split it into two smaller pipes? Would it do what the N1 Dual claims- retain scavenging power but also add on more free flow? and where is this scavenging power most needed? If it's just splitting off after the cat...

Apr 7, 2004 - 8:30 PM

SpedToe169

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You're reading a little too much into it (or taking a little too much from it

). I shouldn't have quoted the last part, its not totally correct. If you read on in the thread you'll find this guy (who is also right):

QUOTE

Fluid dynamics generally has it that flow velocity in pipes is greatest at the center. The pipe surface is considered a restriction itself. Having the dual setup means more surface area and thus more restriction. Don't get the wrong idea though, the N1 dual still supports a lot of exhaust flow, but you'd have to figure that aside from weight, there has to be a reason that all high HP 240's tend towards one fat pipe instead of two.

On a car with one cylinder bank (I4, I6) a single exhaust is always the way to go. On a car with multiple banks, packaging and cylinder firing order usualy dictate that a dual exhaust is better.

In the case of the celica the 'best; power (most torque and best driveability, aka, fastest car) will be found with a well designed 4-2-1 header into a single exhaust. An extensive dyno session would be required to find the optimal length and diameter of the exhaust but it will be somewhere in the 2.25-2.75 range for 99% of street cars (our cars).

QUOTE

and where is this scavenging power most needed? If it's just splitting off after the cat...

I think you're thinking about it wrong. Think of the exhaust flow as a single column of moving air. That column of air has inertia. That inertia can create negative pressure behind the moving column. That negative (relative) pressure creates the scavenging effect. There is no 'where' of the scavenging. It happens everywhere in the exhaust. The end goal is to get the exhaust out of the cylinders as quickly as possible (and as completely as possible, some exhaust always remains in the cylinders during each exhaust stroke, the less the better).

Its a really hard concept for me to explain. Am I making sense?

Apr 7, 2004 - 9:19 PM
shid View Member Profile Enthusiast Joined Jan 8, '04 From LA Currently Offline Reputation: 0 (0%)	You are speedtoe, thanks to my HS and college physics classes I had the base I needed, it was just putting things together. Thanks VERY much for the dual vs single exhuast tip- I didn't find it anywhere else on this site (I could just be stupid tho --Sean PS: I know that Bosal 92-95 Camry headers are supposed to fit on my 5S-FE engine.. is this a 4-1 header, a 4-2-1 header? or will it even fit at all? Is it best to have a custom 4-2-1 header made? What should I ask for/look for? THanks

Apr 8, 2004 - 4:34 AM

ShadowFX

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QUOTE (gh0st_d0g @ Apr 7, 2004 - 4:27 PM)

assuming your SX is the 1.8l 7afe, I wouldn't go any bigger than 2.25 inch. backpressure is one issue, but its also completely pointless unless you are going turbo in the near future.

i have a 5S-FE, and I;ve read all the above, just making extra sure, a 2.5" should be okay? but like you were saying, does this mean i would get less low end power?

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