vx muffler hole
 

The "sound of hole" in my muffler was killing me but it was sunday.. I cut a bigger hole, removed everthing what was left inside and put a steel sheet patch (stupid idea I know, sounds ****ty and I have to wear :cool: while driving) but I noticed higher fuel consumption and light hp increse. I'm aware the muffler design is to slow down the fumes, keep some pressure etc. but hypotheticaly if I put a device (with a switch), which could change the diameter of the exhaust pipe would it be possible to have extra, let say, 2hp for overtaking and -2hp for highway mpg? I know it's not worth it but am I right or it's just wrong way thinking? :confused:

With performance bein what Im used to messin with, and MPG bein somethin new to me I still feel like I can safely say that your overthinkin this. I think you could actually spend less time and money and put together a well thought out, performance AND MPG based exhaust system that allowed more power and efficiency. Headers, high flow cat, decent muffler (NOT A FLOW THRU FART CANNON) all on the right sized pipe should open up more than a couple HP and allow you a bit more efficiency (if you can keep your foot out of it lol).
Matt

Thanks for reply! Looks like I have to read more.. Sure it's not worth it to build device for this - I totally agree. There are better ways. It was just hypothesis. I was just curious what has happened exactly.

Part of what has happened is the sound has changed how the car feels, just like putting a death metal CD in the stereo will make the car feel faster.
Of course the rest of the exhaust is tuned more or less for the engine, so the muffler is really the only restriction, read up a bit on exhaust design and tuning and you will see that back pressure is a myth used by teenagers that like loud cars and people who are not good at math but that a well tuned exhaust will give you more power at a given RPM then a "free flowing" exhaust, mostly because exhaust moves in pulses and the piping has to be tuned for those pulses or they slow down and bounce off each other, to large and they bounce off each other!

Yup, backpressure is good to a degree for a natural motor. You can open flow up a bit but getting rid of the backpressure entirely doesnt benefit u at all. Turbo setups are different, more flow and less pressure is best. I would like to free up some flow a bit on my escort but keep it quiet and not loose too much pressure.
Matt

I'm a bit confused.. backpressure is a myth or not? I'm a rookie in this matter but I on your side 66sprint6 :thumbup: I've seen turboed car with 1m pipe instead exhaust line and I don't think it was "constructor" mistake. I'm thinking about exhaust pulses - waves.. It coud be similar or the same to the tricks in audio transmission lines. If I'm right, briefly there is a need to avoid stationary waves. Once I'll have consistent data about my mpg I'd like to make practical test - put simple cone cup (with let say 10mm hole) on the exhaust pipe. Then change hole diameter and again make a test.

That is very much how exhaust works, if you have a back pressure gauge the gauge should be reading in the 0-2psi range depending on how clean the bends in your exhaust are and how much soot and so on is causing roughness, going with larger pipe will not lower that 0-2psi number without causing distortion (fart can rumble) and stationary waves.
If you were to take the volume of exhaust gasses and push those as a steady stream without pulses then yes larger would be better, the vanes of a turbo tend to smooth those pulses out a bit, absorbing a fair amount of sound as well.

Another theoretical question. Some time ago I put new muffler (instead of an old empty fart can) I did around 1800 miles recently and I got better MPG than before. Most of 1800 miles were made on highways, tire pressure was increased but I drove a bit faster than usual (arround 2800rpm) and the car had to carry much more weight than usual. Tire pressure and highways are obvious things which increased MPG but theoretically the exhaust fumes were slowed down a bit by new muffler so the O2 sensor reading was more accurate - ECU could better control the engine lean burn. Is it correct?