Ok, i've decided against turbo'ing the accent at this point and time due to the 30,000 miles i've put on the car in the last 9 months (a trend thats likely to continue). It also helps that I found out the Philly region SCCA holds autoX's starting in January, which means racing all year long. smile.gif I'm planning on running solo2 is STS class, where the only allowable mods are to the air cleaner up to the TB, emissions legal header, and header-back exhuast.

I've found plenty of good tech info on intakes, but I've yet to find anything beyond vague pipe sizing for exhuasts. And, looking at dyno charts for various 1.6L engines, i'm just getting more and more confused/pissed.

Example: The 1.6L engine of the Geo Storm. Its been dyno proven to show gains from 2000rpm to redline with a 2.5" exhuast, and more than a 2.25" exhuast.
http://www.isuzuperformance.com/bills/dyno...ustcomparo.html

However, everyone always tells me to get a 2" exhuast. I'm confused... Does anyone have a formula or some sort of semi-hard relationship between cross sectional area of the exhuast pipe and cfms? When is too big too much, when is small too small? I'm looking for maximum power from 3500rpm up.

 

well, i don't have a formula for you. if i'm not mistaken, the larger the exhaust, the more bottom end you'll lose, but you'll gain more in the top end, when the exhaust needs to be free.

 

but that theory doesnt really count too much with a boosted car.

 

<div class='quotetop'>QUOTE </div><div class='quotemain'>xxxmonoxidechild:
but that theory doesnt really count too much with a boosted car.</div>What does that have to do with anything, i'm asking about n/a setups.

Turb-Thats just the kind of vague i already know, thanks anyways.

Red, Chuek? Opinions?

 

This page covers some of the myths/facts about exhaust systems. I am late for work but wanted to post it real fast.

 

The thing you need to caculate is FLOW, and backpressure.

I can install a 5" exhaust that will perform like crap, and I can install a 1.75" exhaust that will outperform a 2.5".

What cat you have, and it's flow charastics, and what muffler you have, and it's flow charastics matter. Piping bends, and weld quality (where there are welds) matter.

What you want is as little back pressure as possible at the engine torque Peak, under load. If your engine's Torque peak is 3500 RPM, you want to design an exhaust that gives you as little backpressure as possible before the cat (and over the whole system). Under 1 psi is ideal, but 2-3 psi is 'achievable'

If you've read Maximum Boost, you know how to test for this. Just have a passenger watch the gauges while you drive. No need for expensive Dyno testing.

 

Yes, high flow and low pressure. So what does this mean in design of an exhaust system? Well, it means we have to get high temperature (i.e. low density) air pumped out of and away from the engine and eventually into the atmosphere.

To calcualte mass air flow we need to know some things. The formula is

MAF = D x V x A

D is density (pounds per cubic foot) This is affected by temperature, of course.

V is velocity of airflow in feet per minute

A is crossectional area of the pipe (or tube or vent or whatever)

This is fairly simple relationship. In a 2" diameter pipe, if I drop air density, I must increase flow velocity to maintain the same mass of air moving thru the pipe.

Ok, on with the discussiong at hand.

We have to move the same amount of air OUT of the engine as went IN. But exhaust air is at a much higher temperature, so it needs to flow at a much higher velocity. To accomplish this, it is desirable to achieve and maintain a laminar (layered) airflow inside the exhaust pipe. Laminar flow can achieve higher velocity than turbulent flow. (this is one thing 'mandrel' bending helps accomplish)
If the air can NOT flow at the needed velocity, pressure builds until a balance is achieved and the right amount of air flows out of the car.

Skierd, is this the kind of stuff you wanted to know? If you want, I can dig thru some old papers at home and give you a practicle example of numbers tomorrow morning.

[ January 11, 2003, 07:36 AM: Message edited by: blue2000 ]

 

Practical examples are always welcome, if not for me then for the less technically inclined on the boards.

Cross section is easy to get. I dont know how to find the density of air specifically, nor how it changes with temperature (i'm assuming its gonna be covered in my Thermodynamics class next semester), so some guidance would be helpful there. Also, what do i do to determine the velocity? I'm usually not this clueless, but this is the first time i've looked at something like this.

<div class='quotetop'>QUOTE </div><div class='quotemain'>We have to move the same amount of air OUT of the engine as went IN.</div>I'm assuming this means MAF of the intake needs to equal MAF of the exhuast side for best performance.

Tony-AutoSpeed also has good articles how to test for pressures. One series, Eliminating Negative Boost, is all about killing low pressure areas in the intake and finding places with positive pressure for the air intake (which i'm going to so soon to my stock intake). This is all fine and dandy on the easily repaired intake, but I'm not in a big hurry to drill into my stock exhuast just to measure backpressure. Maybe if I get an EGT for cheap in the near future...

 

"Standard" air is 70 degrees F, .075 lb/cubic foot and pressurized to 29.95 in/hg. Temperature and pressure variables can be charted (pressure is a nearly linear relationship up to 5000 feet) Samples of these charts can be found on the link below.

Some basic stuff about air and calculating airflow can be found here including a sample on calculating actual airflow for the temperature and pressure (altitude) variables.

Measuring air velocity must be done with a meter. I couldn't figure out how to do it when I was doing these calculations for my intake I had to settle for running a vacuum differential test like in that "negative boost" article.

Acutally, I would think that MAF out would have to be higher (slightly) than MAF in because of the addition of the gasses from the burned fuel.

I'll dig out some numbers in the morning to post a practical example of airflow for a Tiburon 2.0L

[ January 10, 2003, 08:43 PM: Message edited by: blue2000 ]

 

In maximum boost, Corky Bell suggested that 1.5 times the rated hp of the engine is a good guestimate for cfm's, which puts my car around 155cfm, if it helps.