Dual Exaust Can Reduce Power?

[redhotchillipepper]

I was taking to my local Saturn dealer service rep about putting a cat back dual exaust on my 2.4L Sky and he told me that if I increase the flow of the exaust on a normally asperated 2.4 it may not create enough back pressure and reduce HP. Does that make sense? I am about to order a DM supercharger anyway but I thought more flow through the exaust meant more air being pumped through the engine thus more HP.

[Rob the Elder]

First, I dont know about the 2.4 enough to give you a definitive answer. However, some motors do not like to have an open exhaust. For example, the V6 in my Ranger looses power if you put duals on it.

I would ask some other knowledgeable people or even talk with Magna flo or a reputable manufacturer and they will tell you what they know.

[marlboromike]

without doing anything to your car...it was reduced 4 HP from 177 to 173...don't worry about a couple of horses getting away...it's a paper trail only.

[gadget]

Originally Posted by redhotchillipepper

I was taking to my local Saturn dealer service rep about putting a cat back dual exaust on my 2.4L Sky and he told me that if I increase the flow of the exaust on a normally asperated 2.4 it may not create enough back pressure and reduce HP. Does that make sense? I am about to order a DM supercharger anyway but I thought more flow through the exaust meant more air being pumped through the engine thus more HP.

Dealers install dual outlet catbacks on 2.4's
All catback Exhaust systems with the exception of the Magnaflow 3" system have the same size pipe back to the Muffler and then split into two outlets there will be no effect on back pressure.
Note: This also means that there will be no effective horse power gain or loss just a lot more noise

From what I have read the 3" system should only be used on a turbo or supercharged vehicle like the redline

[Gerry]

No car needs back pressure in the exhaust. You gain horsepower by reducing or eliminating back pressure. What you can do by using too large a pipe is to reduce gas velocity. You need the velocity to scavange exhaust gasses out of the combustion chamber. Also, the larger the pipe, the louder the exhaust will be.

[redhotchillipepper]

Originally Posted by Gerry

No car needs back pressure in the exhaust. You gain horsepower by reducing or eliminating back pressure. What you can do by using too large a pipe is to reduce gas velocity. You need the velocity to scavange exhaust gasses out of the combustion chamber. Also, the larger the pipe, the louder the exhaust will be.

Doesn't this defeat the EGR system that recirculates exaust gasses with unburnt fuel back through the combustion chamber thus improving fuel economy? I thought that is what we needed back pressure for but I could be totally wrong.

[Gerry]

Originally Posted by redhotchillipepper

...but I could be totally wrong.

Indeed, you are.

EGR is only active under higher throttle loads, but not at full throttle. The purpose of EGR is to dilute the intake charge with an inert gas. With more inert gas in the combustion chamber, there is less combustable mixture to burn, which lowers combustion temperature. The lower combustion temperature reduces oxides of nitrogen, a pollutant that causes smog. EGR, as you can see, does not improve performance or enhance fuel economy. There's no way it could as it adds nothing to increasing cylinder pressure, which is the fundamental characteristic of raising thermal efficiency.

As far as needing backpressure for EGR to work, that's partially true. It works better with backpressure, but it will still work with very little. There is also internal EGR. Here, the overlap (the period where both intake and exhaust valve are open) is extended and some exhaust gas is pulled back into the cylinder on the intake stroke.

[redhotchillipepper]

I agree that the egr system does not increase compression, or increase thermal efficency. This is a moot point now that I have researched the factory service manual and determined that the ECOTEC engine has no traditional EGR system, or at leas I couldn't find one in the book.

[redhotchillipepper]

I am going to go ahead and put the cat back system on. I have heard enough from all of your posts to put my mind at ease about the potential for power loss. I have heard nothing put positive things from people who have installed the magnaflo exaust systems and feel it can't hurt, plus it just looks cool! Thanks all for your input on this!!

[WishfulThinking]

I thought I read somewhere that the VVT in the ecotechs is used to overlap the opening of the intake and exhaust valves as an alternative to egr. (Suck some of the exhaust back in, instead of looping it around.)

The "bigger exhaust is bad" idea didn't make any sense to me at first either, and the generalization of the concept to "the engine needs back-pressure" is extremely unfortunate. What helps me, is to remember air does have mass, and that a highly organized column of moving air has inertia. If the exhaust system is designed correctly, that inertia can act something like a slide-hammer trying to pull more exhaust out of the next cylinder. I believe that one of the key tricks to making this work in your favor is to size the tube(s) to the flow at RPM that you want it to have the most effect at. Maybe moving up to a larger tube, just means the benefit happens at a higher RPM. If you have other mods that are similarly packing more volume into the engine, then it might balance out.

[Bruce]

Originally Posted by redhotchillipepper

I was taking to my local Saturn dealer service rep about putting a cat back dual exaust on my 2.4L Sky and he told me that if I increase the flow of the exaust on a normally asperated 2.4 it may not create enough back pressure and reduce HP. Does that make sense? I am about to order a DM supercharger anyway but I thought more flow through the exaust meant more air being pumped through the engine thus more HP.

I put the RL exhaust, big bore throttle body, and CAI on my 2.4 (before trading it in for my RL) and the performance seemed enhanced to me both in off the line and at speed. I'm not so sure about the calibration of my seat dyno though

[JohnD]

Originally Posted by redhotchillipepper

...plus it just looks cool!

...sounds cool, too.

[Rob the Elder]

Im sorry but back pressure is an important component of tuning the intake / exhaust.

If you want to test the physics involved, take your Sportster and put drag pipes on it. Garaunteed to loost torque and horsepower. The scavaging depends on the negative pressure waves being reflected back to the exhaust valve at the right time in the carnot cycle. If you have zero back pressure, you cant tune the pipe.

[JRinKY]

Rob the Elder : Im sorry but back pressure is an important component of tuning the intake / exhaust.

If you want to test the physics involved, take your Sportster and put drag pipes on it. Garaunteed to loost torque and horsepower. The scavaging depends on the negative pressure waves being reflected back to the exhaust valve at the right time in the carnot cycle. If you have zero back pressure, you cant tune the pipe.

I don't think so.

Exhaust scavenging does rely on a negative pressure wave that is reflected from the exit end of the pipe. The length of the pipe determines the timing of the wave, which is why pipes have to be tuned to take advantage of the effect. Making all of the pipes the same length is the reason for the "bundle of snakes" look of high performance headers, and equalizing the spacing of the pressure waves is the reason for the "4-into-2-into-1" and the "180 degree" header designs. The pressure wave timing is influenced by frequency as well as tube length, so a tuned pipe is most effective at a specific RPM range. Generally, you can tune a pipe for high or low RPM, but not both.

Back pressure in the exhaust is not necessary for the creation of the negative pressure wave. The proper gas velocity is critical, however. If the velocity is too low, as it would be from too large a pipe, the exhaust gas will not have enough energy to scavenge the cylinder. The velocity cannot be too high, but the only way to increase the velocity is to reduce the pipe diameter, and that will cause increased backpressure which would counter any gain from the higher velocity.

Proper tune in a pipe, whether exhaust or intake, is a delicate balance between pipe length, diameter, and configuration.

[cerberus]

A little over a year ago, we tested a 3" exhaust on my N/A 2.4. We didn't expect it to help a 2.4 and there really wasn't much whp increase till you got to high rpm, but the torque increase was a surprise.

The original thread.

[RPIpower]

Backpressure is a term used as a cop out for an exhaust system that is not designed correctly. Its all about volume and velocity. We had big discussion about this on the other board.

The Unofficial BMW M5 Messageboard (m5board.com) - View Single Post - Confused about Headers and Cats

Our GT system necks down to 2.5" when it splits allowing us to keep the exhaust temps up, helping with velocity. I can see if there is a dual 3" system, where performance would be lost. The exhaust gas would be so dense that the velocity would be really slow, creating more of a restriction than anything.