backpressure question why does everyone have it wrong?

Question

Why does everyone seem to be mislead to the fact that backpressure is good? It is never good. It is a hinderance to the scavaging properties of a good exhaust. It has to do with the fact that you want a more laminar flow to create a low pressure zone behind each exhaust pulse due to the fact that you have high velocity pulses. It's just bernoulli's principle, right? So where does everyone get the wrong info? by mouth?

maybe like a good source

overly sized exhaust is bad, but that doesn't necessarily mean backpressure is bad

Answer 1

Yes, you have it right. Every naturally aspirated 4 stroke gasoline or methanol engine will run it's best with no back pressure. Any automotive engineer or racing engine builder will confirm that.

The problem is, virtually all carburetors and fuel injection systems were designed for engines that are muffled and have some back pressure. The result is many times when back pressure is reduced or eliminated the fuel curve is disturbed and the engines often loose low end torque because of the mismatched lean fuel curve. A simple jet change many times wont correct this. It requires a completely new fuel curve that is beyond most people's capability. This results in many people believing some back pressure is necessary,

There are no mufflers or back pressure on professional racing cars like F-1, NASCAR or NHRA.

A small amount of back pressure does not hurt performance much and it is impossible to dampen the exhaust noise without a little restriction (back pressure) so any street car will have some back pressure.

Answer 2

Who is everyone?
It is a suprisingly common misconception though.
But you can't really usefully compare daily drivers to F1 cars. Do you redline your SUV or minivan all the way to work every morning? No, and neither does anyone else. In most street cars, low to mid range power is more useful and sensible. Designing an exhaust to enhance such power about always has the unfortunate side effect of having an appreciable amount of backpressure. Add mandated pollution and noise controls to the exhaust and you get even more backpressure. If you are designing exhaust systems for modern production vehicles you have to live with the fact that such systems are going to have quite a bit of backpressure. The cars don't "need" it but they generally fulfill their purpose best when fitted with such a compromised exhaust. Probably where the confusion stemmed from.
Indeed, if you have a car you want to perform regularly at or close to it's RPM limits then backpressure is your enemy and should be eliminated. If you have a low rpm driver with a very mild camshaft then an exhaust system that is not so efficient at high rpms will likely make the car more enjoyable to drive. For this reason are most small economy cars hampered by tiny exhausts. They are built that way because such exhausts happen to enhance low to mid-range torque of very small engines. And they also have some unavoidable backpressure as a side effect. Since small displacement engines often need that extra bit of torque badly for normal use, the engineers have to accept the backpressure as a compromise when building a car grandma wants to drive to church. Because in reality, most people NEVER actually race their cars, they just cruise around in them.
Personally I believe a turbo or two to be a much better solution to enhancing torque characteristics than a restrictive exhaust, but what do I know. :)

Answer 3

I gotta post because this popped up.

Backpressure is ALWAYS bad. As mentioned, unless you are an idiot, more air out means more air in. More air in means you need MORE FUEL. If you do not add fuel, your engine will run lean, and power will indeed be reduced. Properly sized exhaust for a particular RPM will ALWAYS make more power. Every other RPM will be a compromise, but companies are working on varying exhaust flow paths to change that.

Answer 4

Sorry, dude, but sometimes exhaust backpressure is necessary for an engine to perform properly. Take a two stroke motorcycle engine for example; the exhaust gases are pushed into an expansion chamber to insure the correct amount of clean air and fuel remain in the cylinder (and NOT go out the exhaust ports) on the upstroke. I also understand that the Hyabusa motorcycle engine (a four stroke unit) is designed in much the same manner though for the life of me I don't understand the logic.

Answer 5

Engine Back Pressure

Answer 6

I personally Did following with my 80 cc mini dirt bike and noticed great difference in torque.

1) with company fitted muffler having back pressure: Torque was amazing top speed was less.
2) made my own muffler and reduced the back pressure, offcourse with some increased noise: Torque increased dramatically and so was its top speed. The bike easily beat others of its class and no bike could match in race.
3) Designed another muffler with negligible back pressure: There was no torque in bike, top speed reduced.

I guess that a little back pressure in small engines is good as they are designed that way. A number of variables like carburetor, engine timing and so on comes into account in the 3rd situation and makes the bike sloppy.

Answer 7

Exhaust has to be equal pressure on all cylinders or you get uneven flow> 2 cyc have to have back pressure> diesel to big of exhaust an you louse low end power.So some run good with no back pressure an some don't.

Answer 8

NO engines (today) are designed to operate off of back pressure (of course dependin on the yr ) i mean you must have never done a dyno with wideopen exaust and then a good set of flos. or dynos , or thrush , trust me almost every time wide open is 15% behind or more when your talking 400 h.p. thats a lot (60)

Answer 9

Yeah, but a little bit seems to help torque. Just like a heavy flywheel does, and low overlap, and smaller valves and carb CFM, and backing off cam timing. And we get that by looking at dyno charts.