exhaust design - basic issue

[landybehr]

Hi there,
(I fear this is not the optimum place to post this. Please shift as required)

I am in doubt about backpressure of exhausts and the joining of V8-engine banks "exhaust-wise".
The last may be easier to explain but somehow both questions are linked.

Well, is it a necessity to join both banks of a small block V8 in order to get a good flow ? The original setup is like two cylinders of one bank go together in the manifold and there would be 2 downpipes for a good way towards the silencer (in case of catalysator there is just one, bigger downpipe behind the manifold). If I made my a new exhaust from scratch it might be easier to join the pipes behind the silencer or even have two different paths until the end.

I am inclined to think that an exhaust has an impact on power in that the backpressure (which is good for noise, isn´t it) makes it harder for the engine to get rid of gases and thus reduces power. On the other hand engines without any exhaust run worse.
So - is it true that an exhaust without creating backpressure ("free flow") generally is better. And that it is still better than running no exhaust at all because it makes the gases run with a certain inertia through it - therefore even possibly creating a kind of small vacuum thus helping exhaust gases escape ?? In which case I am inclined to think that the tuning of a car which gets sort of freeflow exhaust may be the same like tuning for altitude ?? (ok, now this is my lame link to this "tuning fuel and air" section of the forum ).

[re-animator]

Hey All!

Here's what I have learned. I put a 2.5 inch straight through ehxaust system on my '98 civic. It's louder than heck, but the car runs well after adjusting the fuel pressure - tuning like Lance said. There's no way that a 1.6 liter will put out enough air to overload that pipe, but it works and gets 32 mpg in mixed driving. Pipes can be as big as you can go, with the limits being chassis clearance and ground clearance.

Crossover pipes are traditionally supposed to be placed close to the header collector for performace, but are usually placed where they fit on the car. I have seen people placing them rear the rear of the car recently, but as far as performance goes, I'm not sure if the benefits are the same. I have tried both "H" and "X" on my Pontiac. The "X" is quieter, but my g-tech didn't notice any difference in acceleration. The "H" has a smoother note than straight pipes. Crossover pipes weren't common on factory perfromacne car throughout the 60's and there were some damn fast cars that came without crossovers.

Try going over to a website: www.headerdesign.com. There's a ton of good advice on exhaust system design and construction, including places to plug in HP figures, RPM, etc., to get pipe diameters, lengths and more. I used it to design my new exhaust on the Pontiac and the car is remarkably quiet and still scary fast. The exhaust is quieter at cruizing speed than the tires.

good luck!

[Mk1rocco]

From Sanderson Headers:

Smaller diameter pipes flow less volume than large ones, but the exhaust in the smaller pipe flows faster. Until you reach the RPM where the sheer volume of exhaust gases require bigger primaries, smaller tubes scavenge far more efficiently

What this means is, smaller pipes can sometimes give better performance at lower rpm where it's most noticeable and where most people drive there cars most of the time. This effect is likely the origin of the back pressure myth.

[PSIG]

From Sanderson Headers:

Smaller diameter pipes flow less volume than large ones, but the exhaust in the smaller pipe flows faster. Until you reach the RPM where the sheer volume of exhaust gases require bigger primaries, smaller tubes scavenge far more efficiently

What this means is, smaller pipes can sometimes give better performance at lower rpm where it's most noticeable and where most people drive there cars most of the time. This effect is likely the origin of the back pressure myth.

That is true. The myth is about backpressure, and backpressure is never wanted. The confusion lies with negative pressure (NP) waves, and how they can be used to best scavenge the cylinders when the wave and low pressure associated are maintained with correctly sized pipes. With an NA engine, the pipes generally tend to be smaller to maintain the negative pressure wave for best scavenging. In-fact, a well designed full exhaust can flow better and produce more power than just open exhaust manifolds; a feature proven by the fact that many wide-powerband race engines use full systems to make more power than open headers. Exhaust design at this level gets pretty sticky and very expensive.

At some point (different for every setup), the energy of the NP waves is reduced to the point it is no longer helpful, and the scheme changes to larger area to help the gasses exit the system. This is where the crossover pipes come into play on dual muffler/pipe systems. By adding the crossover, the system can self-balance the remaining pressure to allow a path of least resistance. With turbo engines, the pressure waves are garbled in the turbine housing and blades, leaving little negative pressure to work with. Therefore, turbo cars use huge pipes to allow the turbulent exhaust gasses to exit with least restriction, as there are minimal NP waves to help.

David

[landybehr]

Hi yall,

excellent info. No questions left.


seems like I really have to take all that into consideration when making an exhaust from scratch resp. with some generic parts sometime. I rarely rev the engine past 3000-3500rpm so probably will benefit more from the NP thing than a racer which never operates below that RPM. (Still, if I wasted 1-5hp by bad design I wouldn´t ever notice nor bother)

If I made two independend pipes for each of the cylinder bank then a crossover pipe would be beneficial, from what I just saved into my mind Is there any rule-of-thumb (as it´s probably very much a matter of individual compromise) about: a) where it is best placed and b) which diameter to take ?

IIUnderstandC then the NP plays a part mainly in/close to the manifold. Has the end (behind center muffler) of the exhaust any influence at all ?
AND
how does a catalysator change the flow of exhaust ?