Contour Enthusiasts Group Archives Forums Modifications Duratec Performance More on exhaust pipe size

Just some fodder for debate guys... trying get the smarter people on here to use their heads, since I've been thinking about this for a long time, and my brain is just about done for the night.

I know this has been brought up before, but I still don't understand the current arguements against larger pipe sizes (say... somewhere between 2.5 and 3.0 inches).

Many people say that you need high exhaust velocity... that narrow pipes somehow help with scavenging, effectively having the exhaust suck itself out of the combustion chamber, pulse by pulse. While I could see the effect of this in a header, where a pulse from one cylinder could be timed not to collide with the pulse from another as it enters the collector, I don't see how it comes into play after all of the gases have united in a single pipe.

Besides... isn't the potential problem with an extremely high-flow exhaust allowing the intake air charge to exit through the exhaust valves during the overlap period? If we're so, why are we trying to scavenge the cylinders more, when it's this scavenging effect that has the potential to (when taken to the extreme) hurt low-end power drastically and maybe even cause damage.

The other common argument is that gasses cool off as they expand... but what of that? They're going to eventually have to cool off and fight 1 ATM of atmospheric pressure in order to exit your tailpipe no matter what you do. I fail to see the consequences of this situation, or why it would be a problem at low-rpms, when overscavaging and resulting torque loss is what we're trying to prevent, not effect.

So which is it? Is a larger exhaust (albeit one that makes many, many bends on it's way to the rear of the car) so free-flowing that intake air and fuel are getting sucked through the exhaust valves before the cylinder fires? Or is a larger exhaust restrictive enough to ruin your low-end because those cooling exhaust gasses become so incredibly heavy. In which case, would decreasing the length by an amount proportional to the increase in cross-sectional area (keeping overall volume the same) prevent this massive torque loss? Or would you then resort to the opposite argument, claiming that overscavenging is the problem? Do you see how easy it is to talk in circles about this without even realizing it?

(BTW, I don't mean to sound like a know-it-all... I'm just starting to get a feel for this, and I want to understand it better )

Are you then suggesting a shorter true dual exhaust exiting somewhere before the rear suspension with 2.5" piping?

It was suggested to me to offset the mufflers where the cat and res was putting the muffler for the front banks closest to the engine and the muffler for the rear banks after it. this may help balance the front and rear banks such as the factory bend in the Y-pipe is designed for.

cooling of exhaust gasses is a big issue, since as a gas cools it does contract, but its density also increases and its this increase in density that restricts flow.

If a larger pipe is used then the ultimate gas temperature will decrease as the gas expands to fill the pipe, not only that but the larger pipe has more external surface area that will radiate heat away and further cool the gasses.

Idealy a pipe size should be selected that will allow the "optimum" gas velocity to occur when the engine hits the RPM at which it makes its max power. If the pipe is too large, at low RPM it will actually over scavenge and may go as far as to pull raw fuel from the combustion chamber which will decrease power and waste fuel.

I believe the SVT stock system from the main cat to the rear split is 2.25" outside diameter, but due to the wall thickness the inside diameter is about 2". Increasing to the next size larger, 2.5", which has an inside diameter of 2.25", which is acutally a big increase over the 2". (remember the equation, pi x radius squared = area of a circle)

cross sectional area of 2" ID pipe is about 3.14 sq. in.

cross sectional area of 2.25" ID pipe is about 4 sq. in.

But if having cooler, denser gasses in the exhaust is restrictive, wouldn't that restriction prevent overscavenging of the cylinders... wouldn't overscavenging then be more likely with a smaller pipe, if it was indeed less restrictive at low rpms?

Guy's, Get the book titled "Scientific Design Of Exhaust and Intake Systems" written by Philip H. Smith[FIMechE MSAE] and John C. Morrison[BSe PhD MI MechE] and all will be explained. Read the book then read it again, it is very deep. You may even be surprised to learn that many of the so called new tech. that are being touted today as the latest and greatest discoveries, are in fact very old and proven many years ago NOT to work.

I'm all ears

Now I'm confused. Pulling raw fuel from the combustion chamber? Loss of high end torque? Everything I've experienced so far contradicts all this. I know nothing of exhaust, just that I thought larger pipe results in more power. When I had the three inch pipe installed, maybe the fact that the pipe starts two inches after the cat may play a role and may have turned my entire exhaust into one big muffler.

It would be real easy for me post things from the book. However from my experiance on this board all that would do is start a big flame war! I'm not trying to be a smrta$$ but I've been there and done that! I told you where to find the info. now go get the book and educate yourself. I will say this there is a threshold on pipe diameters you can go too large and you can go too small. The book also speaks of the sonic waves in the exhaust, what they do and how they work. Yes the sonic waves do exsist and they do infact have a job to perform, they can either work for you or against you. You WILL be suprised at what you will learn from the book and you will NEVER look at the aftermarket exhaust company's the same again. Especialy when you take what you learn from the book and build your own exhaust system or a set of headers or your own sheetmetal intake manifold and see that the info in the book is correct. Plus you can take your newfound knowledge and make these parts for 1/3 to 1/2 of what the aftermarket company's charge and your parts WILL outperform the others.