[firefish]

First - I'm sorry if I'm repeating something that has been debated here endlessly in the past. After going through every topic in "Exhausted Yet" I haven't found what I'm looking for so please bare with me.



My 968 is stock engine wise. I will do the airbox mod and I will fab an exhaust and ad a chip. The RS Barn stuff looks very trick and is undoubtely the way to go but (and that's a BIG but) I cannot at this time front the cash for their system (and shipping to Sweden is a b***h) <img src="/forum/images/smilies/968/sad.gif" class="smilie" alt="" /> . My plan is to use the stock exhaust manifold and tool up a system including a race cat that prefferably would allow me to fit the RS Barn header in the future. From browsing around here it would seem that the 968 is pretty picky when it comes to choosing what dimensions and types/numbers of mufflers to use.



My questions are:

How should I design the exhaust, ie where do I "cut in" on the stock system (without losing the option for further upgrade to RS Barn header).

What size(s) should I run with regards to the plumbing? 3"? Tapered design?

What muffler design should I use?

And finally - what chip to use? Stage 1 or 2 (seeing as the above exhaust would be somewhere in the middle between the two).



Thanks in advance,



ff/Magnus

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[flash]

definitely DO NOT go 3" - use 2.5" - muffler design is VERY picky - both pete and i have been through MANY systems and designs trying to achieve the best results - i would contact pete and see what he can do for you



stage 1 chip will work with the cat-back - stage 2 is for header - rs barn offers a trade-in program on chips when it is time to upgrade

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[firefish]

[quote name='flash' post='66769' date='Feb 9 2009, 09:26 PM']definitely DO NOT go 3" - use 2.5" - muffler design is VERY picky - both pete and i have been through MANY systems and designs trying to achieve the best results - i would contact pete and see what he can do for you



stage 1 chip will work with the cat-back - stage 2 is for header - rs barn offers a trade-in program on chips when it is time to upgrade[/quote]



Would you recommend replacing the cat now or should I run the stock one until I fit the header? How restrictive, if at all, is the stock cat?



Does anyone have a dynorun that compares a stocker to a airbox/stage 1?



/ff

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[RS Barn]

The stock cat actually flows very well. Porsche did extensive testing . See attachment for optimized difuser .

Leave it be until you have a reason to take it out.

This is the restiction in the resonator to keep backpressure in the motor. If you eliminate resonator you must use a muffler that has backpressure and still flows enough for top end power.

Pete

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[RS Barn]

This is a graph and spreadsheet from dyno runs last januaryl.

Base run is on a stock car with fres cylinder head

Stage 1 chip numbers are with chip, airbox mod and k&n filter

Pete

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[xrad]

I have an issue with the whole backpressure thing. Backpressure is bad. period. Take the fastest cars..top fuel dragsters...they run short,open pipes (I know, they have superchargers)



However, a system needs exhaust pulse control which is good, as shown in well designed headers.



We want good flow to get a fresh charge of air into the cylinder and exhust gas out of the cylinder with as little wasted fuel as possible and maintaining stoich. This new A/F charge whould be at the optimal A/F ratio.



So:

1) DECREASE exhaust back pressure equals better flow....

2) Incoming charge pressure has to be greater than exiting charge pressure..

3) flow is in the direction of decreased pressure, so that exhaust flows out of cylinder BEFORE upward stroke and at early upstroke (see 6)

4) Smaller pipe, higher velocity =lower pressure (to a point)...but inabity to flow higher volumes

5) Larger pipe, lower velocity=higher pressure(to a point)...but better volume flow

6) Valve overlap so get the gasses flowing the right way

7) Laminar flow is best

8) Exhaust pulse problems and interference changing exhaust pressure

9) Intake and exhaust design go hand in hand

10) NO way any one exhaust design is best at all rpm's





Therefore: Pick where you want your power.....in other words:



all rpm range street driving...standard exhaust and intake system very good

Racing with generally high rpm's...Bigger exhaust, less restriction, AND different intake design

Or, you need a 'variable' diameter/length exhaust which changes with volume/temp...etc

Or, an exhaust with an expansion chamber design up front and a condensing design towards the rear increasing flow at both ends of the exhaust...



Of course, with modern aftermarket electronic A/F control...you can tune the mixture to correct ratio at any air volume/ rpm range



Much to think about before cutting anything. Consider some of the proven systems already on the market...

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[firefish]

Thans for the response. RS Barn, you have PM.

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[flash]

"back pressure" is often a misnomer used to reflect the resistance of a system to flow - technically yes, back pressure is bad - however, what is commonly called back pressure is often a description of resultant gas velocity

this engine is incredibly sensitive to changes in the exhaust - whether it is back pressure or velocity, it has been overwhelmingly difficult to nail down a system that works better than stock - mods make it all that more difficult

i strongly recommend against trying this yourself - i've spent over $10k working this out, and so has pete - in other threads we have listed and described things that we have found through testing that work and things that don't - there is no sense in repeating the things we have found to be less than optimal

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