Guys, im a little confused. I was always under the assumption that with turbo..means you WANT/ NEED to go with atleast a 3" exhasut setup.

One of my turbo buddys with a DET told me that in the DET you want 3" piping but you need a 2.5" DP. he told me that the loss in backpressure from the 3" DP would actually hinder the performance.....true?

I' curious about this too, because I have a 3" DP.

From Mike Kojima/ Nissanperformancemag.com, I placed the most important items in BOLD

The exhaust system is the piece of piping that directs the cars exhaust stream from the exhaust manifold (the branched collector that gets the exhaust from each individual exhaust port in the cylinder head and brings it into one pipe) to the tailpipe. To get there it must first pass through the catalytic converter (the emissions device that magically converts poisonous exhaust emissions to H20 vapor and CO2), through the muffler and out the back of the car.

The purpose of the exhaust system is to contain the noisy, hot, and toxic exhaust stream and direct it away from where it could harm the car or you while cleaning it up and quietening it down. With no exhaust system, a car would be incredibly loud while spewing lots of potentially deadly fumes and poisoning our atmosphere. To prevent these two anti-social phenomena from bugging mankind, the exhaust system must primarily quiet the engine’s noise and remove its combustion by-products before discharging the exhaust stream into the general air that is shared by us all.

Now most of us car-loving punks could care less about being quiet or polluting in our quest for speed and individualism, but please folks, remember that the world is becoming a crowded place and we must all do our part to prevent from poisoning it. We will show you that your car can be both clean and fast. Now it’s time for me to get off of my do-gooders soapbox and get back to exhausts!

The exhaust system that comes stock on your car was not designed by the clever engineers at Nissan with power production, cool looks, and sound first on the priority list. The goal of the engineers who designed your cars exhaust was to make the engine quiet; so as not to piss off your parents, be 100% durable inside the cars warranty period, and to be as cheap as possible to mass-produce. These are the attributes that the majority of the motoring public, AKA, cash paying customers deem important. The goals of the OEM engineer are not necessarily in line with yours; the performance phreak. To you, the roar of a tuned engine is music to your ears. Since your car is designed to appeal to what most customers want, your exhaust ends up getting optimized (stuffed up) on the quiet scale and compromised on the performance end of things by design.

The Muffler
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The key part of your exhaust system is the muffler. The muffler is the can at the end of your exhaust whose main purpose in life is to make the engine noise quiet. To be the whisper quiet like most car owners demand. A typical stock muffler must have an intricate and labyrinth-like internal flow path to help slow and cool the hot vibrating exhaust gas. It contains baffles that cause the exhaust flow to reverse direction and intermix. These are great for reducing noise but are not so great for having power unleashing flow. This is mostly because all the twists and turns that the exhaust must endure in a stock muffler are a restriction that causes excess backpressure. You can run in a straight line faster than you can run in a tight maze in a fun house right? The same goes for your exhaust gas.

To produce the most amount of power, an exhaust should have the least amount of restriction to the exhaust flow. Restriction hampers the burned exhaust gasses from exiting your engine, causing some charge dilution with the incoming fresh fuel air mixture. If all the exhaust gas cannot escape from you cylinders, it dilutes the flammable intake mixture that is trying to come in. The diluted mixture does not burn as well as a pure mixture. This causes a loss of power. You don’t feel so energetic at a packed club with lots of cigarette smoke, sweaty bodies and hot stuffy air right? Neither does your motor. With greater restriction, backpressure is generated making the engine work harder to pump the exhaust out of the engines cylinders. This is another pumping loss like we talked about in our last Revenge of the Nerds article about intakes. The harder it is to get the exhaust out, the more power wasted to pump the stale exhaust gas out of a restrictive exhaust system.

Some stock mufflers have up to 18 psi of power robbing backpressure. A well-designed performance exhaust typically has about 2-6 psi of backpressure. For comparison sake, an un-muffled straight pipe on a real race car usually has 1-3 psi of backpressure.

The Exhaust Pipe
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To save costs, your typical stock exhaust uses small diameter, crush bent pipe. Crush bends are easy to make in mass production. However, crush bends can reduce the flow of a pipe by up to 50%. Your typical exhaust system made by the local neighborhood muffler shop is also crush bent. The best exhaust systems, like most Japanese pre-made exhaust systems, come with mandrel bends. Mandrel bending is done by a special machine that uses a non-crushable insert or mandrel that goes into the pipe while bending to prevent it from becoming crushed.

Some self-proclaimed motor gurus state that you should not run too large of an exhaust tube on your car because engines need a certain amount of backpressure to run correctly. Although the statement about not running too large of a tube is correct, the assumption about engines needing backpressure is not. Remember this, it is one of the most common performance misconceptions out there. Read on and you will be able to argue with and break off any self-proclaimed expert on the subject!

You need to have the lowest backpressure possible to produce the maximum power by keeping pumping losses low. Too big of an exhaust pipe causes power loss, especially low-end torque. This is because a big pipe has less exhaust stream velocity than a smaller pipe. Velocity is essential to get the best scavenging (or sucking) effect from tuned headers, which we will discuss in more detail later in a future installment. To simplify things, if the exhaust gas flow is kept high with good velocity, a vacuum can develop behind the closed exhaust valve allowing even better scavenging when the exhaust valve opens on the next exhaust cycle. Good scavenging is even more critical on valve overlap, the part of the 4-stroke cycle where both the intake and exhaust valves are open, especially with longer duration performance camshafts.

If the exhaust pipe is too large, the flow will be sluggish with low velocity and the scavenging will not be good. Remember that a good exhaust has low backpressure and high velocity. The only possible exceptions to this rule are for turbocharged or nitrous motors. It is almost impossible to put too big of an exhaust past a turbocharger as a turbo depends a lot on the pressure differential across its turbine to get power recovery efficiency. A turbo engine can have an exhaust gas volume about 1.5-2 times more than an equivalent displacement naturally aspirated motor. NOS motors also have a pretty high exhaust volume and require a bigger exhaust if they are to be optimized for NOS operation.

The stock exhaust system on most modern rice rockets is often so well designed nowadays that just switching to a high performance cat back exhaust often does not allow for huge gains of power. With the exception of turbo cars, you can usually expect only about a 2-10 hp at the wheels difference on a stock car. The power gain will usually be the greatest from just below the torque peak to the tach’s redline. On cars that are modified with headers, intakes, cams, headwork, etc., you can expect bigger gains with the addition of a cat back exhaust due to the higher flow volume that these mods produce.

As turbo cars are very sensitive to back pressure, you can expect much larger gains with them, especially if the boost is turned up. A free flowing exhaust usually allows the turbo to spool faster also. A turbo car usually gains from 8-30 hp at the wheels depending on how bad the factory exhaust was and how high the boost is turned up over stock.

Next month we will get into the specifics of how to modify your exhaust for more power. So in short remember, more flow=more go!

Until next time!

Happy Motoring -Mike

Thanks Mike, it sounds to me like the 3" exhaust is the way to go..which is I was guessing.