what about dual exhausts...ever since I started paying attention to the BMW M Series I liked that dual exhaust. Now even some Ford's have it.

 

If you actually crawled under a Ford or Chevy and almost every other car and looked at the "dual" exhaust, it is just dual from the muffler to the rear, not a actual dual exhaust.
This is factory dual exhaust I am talking about not one installed and custom made by a local muffler shop.

Most cars do not have room to run 2 sets of exhaust pipe from the engine back.

 

Generally I don't revive threads but I hate incorrect info so I'll correct this here for future folks who look into the subject to not be misled. I realize the guy I'm quoting is probably just unaware of this info and it's common misinformation, he did say to wait for the pros to chime in on the subject.

-It may be a little louder, the mufflers are still in the system so I wouldn't call it "obnoxiously" loud but that's for individuals to decide.

-ALL combustion engines respond well to straight pipes. Mufflers do NOT serve more purpose than muffling, hence their name.

-BACK-PRESSURE is BAD. Back pressure is NOT a good thing, you do not want ANY back-pressure in an exhaust system, ever. This is an old myth. What you do want is delta pressure timed in waves to minimize back pressure at particular intervals. Because the exhaust must flow, it must have back pressure, that's a given. So to make the best of it, engineers have figured out that they can time the exhaust pressure wave pulses to occur when the cylinder is not exposed to them. Instead they time the minimal pressure timings to coincide with the engine exhaust valves being open. This in turn is effectively "sucking" the exhaust air from the combustion chamber which results in more volumetric efficiency which means more hp and better mpg. So the length and diameter will effect the hp/tq numbers of the engine and they can be modified to modify the power band of the engine. Turbocharged engines benefit VERY little because the turbo itself blocks out the pressure waves thus, turbo exhausts are typically short and large in diameter to help with a pressure differential seen at the turbo so count turbo's out of this whole law. Everything else applies here. Removing CATs from your car WILL absolutely add hp and increase mpg so long as the 02 sensors if equipped are tricked properly. Sometimes the "help me" brand spark plug defoulers found at autozone/oriellys will solve this, google those along with "cat delete" in the same search and you'll find plenty on it.

Removing cats will cause your vehicle to smoke more and smell worse, it's not something I recommend to a regular street vehicle honestly. If you're turbo or you're needing every last hp on a track, I get it, get yourself a "test pipe" and trick the sensors or flash the ECU etc as needed to keep it otherwise running correctly, otherwise your hp and mpg savings won't be had in many cases.

I hope this is helpful to other folks.
-Greg

 

Complete misinformation on this one in your first paragraph.

 

Spark plug defoulers, same thing get em at autozone for 5 bucks.

They work by spacing the 02 sensor out of the exhaust stream a little ways, effectively tricking the sensor into sending the right signal.

 

Well, you're getting incorrect info so it's no wonder you're not computing the data. Can't blame ya. Above I explained the misinformation. You're exactly correct about the normal world and it still applies here.

Actually, removing back-pressure will LOWER exhaust temps so long as it's done while maintaining exhaust note timing.

Running less back pressure is good for 2-strokes for the same reason, in fact, they count even more on delta pressure tuning than 4-strokes. You're on track with your logic, just need to be fed proper info. The dirt trackers knew this, I know a few of the old famous ones. The monster truck motor builders know this very well too but they've other things in mind.

 

It's the same on all applications, they all like open exhaust. Show me a purpose built race car running a CAT. This isn't some kind of mythical magic crystal theory about which engines like what, it's all the same. They have everything in common with our engines for the most part. Race specific powerplants is just a name. Honestly pretty much everything in this paragraph above is dead wrong.

Trust me, my daily driver street car is making 786whp from a Honda 2 liter 4cyl and I could easily make a few hundred more if I cared to.

 

It'll net a few hp actually in most vehicles. The ECU does have SOME adjustment built in but it's limited of course, still you can expect SOME hp and mpg increase.

 

Yes and it'll do so, thus it's an effective upgrade in this respect. HOWEVER, the ECU has it's fuel offset limits built in. It will do it's best to maintain stoich at all times but in certain ambient conditions it may lack the ability to supply fuel needed to avoid a lean condition, though I expect it would still be within safe limits in most all vehicles because we're talking just a few hp.

 

Sorry in advance for mashing everyone on this thread but honestly, it's intended to correct the information for future people searching these threads.