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Topic Review (Newest First)
05-12-2013 02:36 AM
amc49 It's actually the law in most places................
05-11-2013 07:26 PM
cheap sleeper concerning the smell of exhaust, it is from the exhaust pipe no longer being long enough to vent AFTER the rear bumper. The exhaust is billowing under the car and creeping up into the cabin. If you MUST not have a muffler, then you need to extend the pipe to its original exit length just past the rear bumper
05-01-2013 02:05 AM
amc49 The back pressure thing is pretty much a myth. Under certain circumstances you can burn valve (warp it) by cold air aspiration, meaning pipe would have to be a couple feet long only or shorter. A big crack at the head flange can do it. The negative wave sucks back cold air, which the hot valves don't like at all. As far as back pressure, I used to race plenty of stuff and never ran across one single engine type that didn't run better with shorter exhaust provided it was properly designed. A properly designed exhaust header just screams to be open, the difference in power can be very marked. Wave extraction tuning, and all that...............the exhaust when running best will actually be in vacuum conditions part of the time. That fact spits 'necessary back pressure' out on the floor. I've built headered street cars that ran around at 15 inches of mercury at long cruise, measured in the collector. No back pressure there, rather, the opposite.

Or, as iminhell says, that works for me too. Especially as far as young kid romping on the gas to lower MPG. Loud pipes and the lower MPG are drenched in testosterone, and inseparable. I remember doing the same on my Bridgestone (yes, the tire people, they used to make motorcycles) bike in '66, everybody yanked baffles back then to 'hear it talk'. I absolutely guarantee I used way more gas than I should've too.

Much back pressure increase is done to quiet the exhaust note, or the reason why exhaust is constantly rammed against itself in mufflers that reverse flow. All that crashing breaks up sound waves. One big reason why single exhaust is quieter than duals, the waves interfere with each other more.
04-29-2013 11:25 PM
kyle9563 Where did you get that pipe, I'm thinking about doing the resonator delete myself and I really like the look of yours, much more that the write up with the flexible piping. Also I see that you still have the bracket that used to hold the resonator in place, how did you get the old pipe off of that bracket? Thanks
10-29-2012 10:59 PM
6SPD_soul Cutting out a section of exhaust shouldn't really change back pressure as long as the remaining pipe diameter doesn't change, and in my mind shorter pipes could only decrease back pressure since there's a shorter restricted environment it has to travel through before getting to atmosphere. Which kind of explains classic side pipes by itself.

I kind of cheated and took a refresher course in back pressure by googling it, but backpressure is only a function of overall flow vs. velocity, and I'll credit Iminhell for making a similar argument against larger throttle bodies. Smaller diameter pipes will move exhaust at faster velocities with less CFMs of air movement, and larger diameter pipes can only move air at fast velocities with substantially more CFM exhaust flow. So it basically comes down to where in the powerband an engine is making peak HP and producing exhaust flow.

There's a ton of physics behind exhaust flow and I don't pretend to know much of it, but I think what little amount of backpressure is designed into exhaust flow has more to do with synchronising exhaust gas pulses and making a smoother running engine.
10-29-2012 10:48 PM
iminhell Back pressure is a myth. Or at least in that it causes harm by itself.

What actually happens is the exhaust gas speed increases. That creates more pull on the intake air during the overlap period. That sucks more air and fuel out the exhaust. That creates a situation where combustion can keep going during the exhaust phase. This prolonged period of heat then very rapid cooling of the exhaust valve makes them brittle. Over time it can eat away at the valve, which is what a 'burnt valve' is.

It has nothing to do with reflected waves or anything like that that you may find.
Also the situation where a V-8 may burn a valve is different than situations where a V-6 or I-4 could burn a valve.

Far as the MPG drop,
Why would a kid cut a muffler off? To sound cool, right. Well, does it sound cool when just putting around? Not really. So it stands to reason that the kid would be romping on the car, lowering MPG.
Almost all of us did it as kids.
Just make him pay for fuel. He'll figure it out real quick that his driving is costing too much money.
10-29-2012 09:27 PM
Magus2727 I believe it has something to do with fresh air making it up to the valve having more oxygen than more normal exhaust air. This causes more carbon deposits. the theory that back pressure or long exhaust tubes will prevent "fresh" air from making contact with the valves. There is some discussion if this is actually true or not.
10-29-2012 08:54 PM
Originally Posted by Magus2727 View Post
Generally the CAT is able to provide enough back pressure removal of the rest of the exhaust will not remove enough back pressure to burn valves.
I don't mean to get off topic, but how does the valves get burnt without proper back pressure? I has a curious.
10-29-2012 09:08 AM
Originally Posted by Magus2727 View Post
Generally the CAT is able to provide enough back pressure removal of the rest of the exhaust will not remove enough back pressure to burn valves.
Understood. Thank you.
10-29-2012 06:59 AM
Originally Posted by markwb View Post
Doesn't the engine require the back pressure from the muffler to run properly. I've thought that the exhaust valves will get burnt w/o some type of back pressure. Is this true or not?
Generally the CAT is able to provide enough back pressure removal of the rest of the exhaust will not remove enough back pressure to burn valves.
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