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The following is the exhaust presentation I presented for the club on 2/8/10. I made all the text available for future reference, as there is quite a bit of good info in here for thoes who don't know. If you think something should be added, just PM me, and we'll see if we can add it do the existing info. Also, since it was a .ppt, I don't have the pictures up from the presentation, but I could post them all if people think it would help. Hope its put to good use!
-Eric
Exhaust 101:All About Exhaust and the Do’s and don’t when modifying your vehicles exhaust system!
By Eric Sundell, NMUCC
Exhaust system components:
-Exhaust Manifold (Header, collector, ect.)
-Downpipe
-Catalytic Converter
-0xygen (02) Sensors
-Mid-pipe
-Mufflers/Resonators
Exhaust basics:
An exhaust system is one of the most simple systems on an automobile, but often one of the least understood. Hopefully this presentation will shed some light on how exhausts work, and what methods, and modifications can be used to improve exhausts efficiency and quietness and power.
Lets go over how exhaust works:
On 4cycle engines, the exhaust stroke of the engine is designed to push the exhaust gasses left over from the combustion cycle. For this to happen, valves in the cylinder head must open to let the gases out as the piston comes up from the power stroke. As the gasses exit the head through the valves, it enters an exhaust header or manifold. These manifolds, typically on a stock vehicle are cast iron, and designed to dissipate and absorb heat which is also a by product of the combustion cycle, exhaust manifolds reach some of the highest temperatures of any parts on your automobile. It needs to be built to withstand these extreme temperatures which can be as high as 1000 degrees F or more depending on the engine. Air fuel mixture (A/F mixture) has the biggest affect on EGT (exhaust gas temp.). Engines running rich will have a cooler temp, and leaner A/F’s will have much higher EGT’s.
The next part in an exhaust system, is the downpipe. This part of the system collects the gasses from the manifold(s) and sends them to the Catalytic converter. On a car with multiple cylinder banks, such as a v8, this component maybe referred to as a Y pipe for example. Some downpipes have bungs for oxygen 02 sensors.
The 02 sensor is found on more modern emissions equipped vehicles. The oxygen sensor or 02 sensor does exactly what you’d expect, reads the oxygen content of exhaust gasses passing through it. The cars Electronic control unit, (ECU) reads signals from the o2 sensor and can get an accurate real time reading of how rich or lean a cars engine is running, and can adjust accordingly. These systems are found on modern and most fuel injected vehicles. On older carbureted vehicles, which do not have 02 sensors, mixture is adjusted on the carb.
Another piece of the exhaust system that’s found mostly on newer cars after the late 70’s is the catalytic converter. A cat takes and scrubs the exhaust gasses and removes a lot of harmful pollutants out of the exhaust in its scrubbers The cat is often the hottest part of an exhaust system, reaching temperatures in excess of 1000 degrees F. These extreme temperatures are a result of the chemical reactions with the hot exhaust gasses that take place in the cat.
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After the cat comes the midpipe. Not much to say about the mid pipe, except that it connects the cat with the muffler. Its simply a tube, it often routes over or under the rear axle. Some midpipes have crossovers, known as H or X pipes that allow for equal pressure in a duel exhaust setup. Some mid pipes have what is called a resonator installed on them. These expansion chambers serve to quiet the sound of the cars engine before the muffler.
The final part of the exhaust system, is the muffler. This device has chambers and baffles to quiet the sound of the engine, and routes the gasses out to the atmosphere. Often, a lot of flow restriction takes place here.
The Science of Exhaust:
What is Scavenging?
When an engine starts its exhaust stroke, the piston moves up the cylinder bore, decreasing the total chamber volume. With the exhaust valve opens, the high pressure exhaust gas escapes into the exhaust manifold or header, creating an exhaust pulse comprising three main parts: The high-pressure head is created by the large pressure difference between the exhaust in the combustion chamber and the atmospheric pressure outside of the exhaust system. As the exhaust gases equalize between the combustion chamber and the atmosphere, the difference in pressure decreases and the exhaust velocity decreases. This forms the medium-pressure body component of the exhaust pulse. The remaining exhaust gas forms the low-pressure tail component. This tail component may initially match ambient atmospheric pressure, but the force of the high- and medium- pressure components reduces the pressure in the combustion chamber to a lower-than-atmospheric level. This relatively low pressure helps to extract all the combustion products from the cylinder and induct the intake charge during the overlap period when both intake and exhaust valves are partially open. The effect is known as scavenging. Length, cross-sectional area, and shaping of the exhaust ports and piping influences the degree of scavenging effect, and the engine speed range over which scavenging occurs.
The magnitude of the exhaust scavenging effect is a direct function of the velocity of the high and medium pressure components of the exhaust pulse
What is backpressure?
During the exhaust stroke, a good way for an engine to lose power is through back pressure. The exhaust valve opens at the beginning of the exhaust stroke, and then the piston pushes the exhaust gases out of the cylinder. If there is any amount of resistance that the piston has to push against to force the exhaust gases out, power is wasted. This is back pressure. However, you do not want to completely eliminate backpressure. While not as crucial to operation in a 4 cycle engine as it is to a 2 cycle, lack of backpressure will cause a drop in power of an engine at all but the top of its rev range. This is because in order to maintain the proper flow velocity and maintain exhaust scavenging, you need back pressure. As described earlier, scavenging allows an engine to pull air through it more efficiently.
What Restricts Flow?
Many things in an exhaust system can restrict flow and cause backpressure and in-efficient scavenging. The reason main reason most cars produced have restrictive exhaust systems is for quietness, lower costs of production, and for emissions reasons. Most factory systems restrict flow throughout the whole system, so by changing out something such as the muffler won’t really give you any gains. The system as a whole must flow more efficiently in order to produce power. Think of your exhaust system as a straw, does it make sense to have one big end and one small end? No. If you blow through a straw that tapers, you won’t be able to force as much air through that straw, as one that is the same size all throughout.
How to modify your exhaust! Now that you understand the principals behind how exhaust works, lets see how you can increase flow and make power gains on a vehicle.
Where do you start?
Before you even touch your exhaust, what is it you want from your car? For example, building an exhaust on a car with headers and the like makes no sense if you plan on going with a turbo. Plan a power goal and plan your exhaust system around any future mods you plan on making. If your car is going to have little more than an intake and exhaust, no sense on getting a 3in. Exhaust on your stock 4cyl. n/a engine for example. However, if you have cams, different intake manifolds, higher compression pistons, ect. A 3in system may be right for your goals.
Check your vehicle!! A setup that works good on one model of vehicle, may not work so well on another. Also, some exhausts are well tuned right from the factory, and do not really benefit, or in fact may suffer, if you change from the OEM system. Check car forums, or car groups for your make and model of vehicle for information on different setups that have been tried in the past to see if your car would even benefit from an exhaust upgrade. In addition, MAKE SURE the exhaust you purchased, or have planned being fabricated will FIT YOUR CAR. Just because it fits a base model for example, doesn't mean it fits the next trim up ect. Make sure clearance isnt an issue either. Some systems will not fit without scraping, or rubbing parts of the car or the road!! Proper and correct fitment is a must!
Headers
-Exhaust manifolds are usually the most effective modification to the exhaust system, because stock manifolds are often not optimized for top engine performance. For starters, most stock exhaust manifolds are of un-equal primary runner length. Primary runners, are the tubes that come off of each exhaust port off the cylinder head, and run to a central collection point. On stock vehicles, they are often un-equal length which adversely affects exhaust pulse, and lessens the scavenging effect and increases backpressure.
Aftermarket headers, often have these equal length primaries in addition to being larger in diameter than stock runners, and having less severe and restrictive bends in them. This results in greater flow, less backpressure, and more scavenging for more power.
Do's and don'ts
-In general with exhaust headers, you get what you pay for. Its recommended you steer clear of cheap e-bay headers, as welds on them may crack causing failure.
-Be sure you get the type of header for the type of driving you do most. Stepped deigns for example a 4-2-1 style header offer more improved mid range power, than 4-1 designs, which offer more top end.
-If you live in an area with emissions testing, make sure your manifold is CARB legal.
Downpipes and Cat's
-Downpipes are more of an issue on forced induction vehicles, on FI cars/trucks, the more flow aft of the turbine the better, as turbo’s move a lot more gas than a comparable N/A engine. However, even on N/A setups with cam’s and intake mods, the increase in airflow may require you to increase the diameter of your downpipe. Cat’s can also be restrictive, and hi-flowing models are available that flow nearly as well as a test/stright pipe. Also, with hi-flow cats, you can still pass emissions, a test pipe will not.
Do's and don'ts
-Just like with headers, you get what you pay for. Its recommended you get a semi reputable brand to prevent cracked welds.
-Make sure your downpipe isn’t so large that it will not fit your vehicle.
-Make sure your downpipe will mate up with your header collector and the rest of your exhaust system.
-Its highly recommended you get hi-flow catalytic converters over test pipes for on road use. The 1hp more you get from a test pipe won’t get you passed emissions.
Midpipes
-Most factory exhaust tube has three problems.
1.) Its often too small in diameter
2.) Its often crush bent pipe
3.) Its often has many restrictive bends
To overcome these issues, you need to get larger than stock, mandrel bent tubing with less severe bends. Mandrel bending is the process of bending pipe, without kinking the pipe. The kinks in crush bend piping disrupt exhaust gas flow, and rob you of power.
Do's and don'ts
Bigger is NOT always better!!!! If an exhaust is too large in diameter, it will lose a good chunk of its lower rev performance. The name of the game with exhaust is flow, with too big a pipe, the gasses will lose velocity, and billow around inside the pipe, resulting in less power due to poor scavenging.
Often, local exhaust shops can fabricate systems for cheaper than that of aftermarket companies, provided they have a mandrel bender. Often spending $1000 on pipe from a company will yield the same results as a $200 locally done system.
Mufflers
-Factory mufflers are first and foremost designed to be quiet, not for performance. To make the exhaust system complete and better flowing all the way through, you have to upgrade the muffler. Modern mufflers can be of twin loop, chambered, and straight through designs. Twin loops are the most expensive, but offer the best mix of quietness and performance. Chambered designs are what OEM mufflers are, but many companies such as Flowmaster, make chambered mufflers that not only are fairly quiet, they increase exhaust flow. The last type, canister or straight through mufflers, are the best for flow and power, but often VERY loud for street use.
Do's and don'ts
-Most states have Db laws stating how loud an exhaust can be, be sure you are within the legal limits. NO one like obnoxious exhaust noise.
-Stray away from canister mufflers on the street as much as possible, no one likes fart cannons.
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Quote: you could dump like 5000 into your 1.6 SOHC vtec and it still wont be as fast as a v6 mustang. the truth hurts.
-Eric
1.) 1996 Volvo 850 R
2.) 1981 Yamaha SR250h
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