Good points. I guess since there appears to be enough room down there for duals and it is not being used may back up your theory.

 

Many true duals use an X-pipe or they merge the pipes in the same area where the 2 into 1 pipe is located on the G.
They do this to increase scavenging and to adjust the sound.
The G's exhaust system is pretty much a true dual system. The 2 into 1 pipe is not much of a bottle neck and looks like a factory attempt at scavenging.

 

I think a worthwhile upgrade would be high-flow cats. HFC's regardless give better flow, better sound.

 

I would want proof showing that after market cats flow better.
When I had my vettes, the after-market cats showed no real improvement in flow over the factory ones. The sound is subjective.

 

As someone who works hard to dispense myths among auto enthusiasts, this article needs to be shared. The term we should be concerned with is "Exhaust Velocity." not backpressure! This article was written for Honda, but the same exact principles still apply.

Interesting reading for those interested.

-Paul

Backpressure: The myth and why it's wrong.

I. Introduction

One of the most misunderstood concepts in exhaust theory is backpressure. People love to talk about backpressure on message boards with no real understanding of what it is and what it's consequences are. I'm sure many of you have heard or read the phrase "Hondas need backpressure" when discussing exhaust upgrades. That phrase is in fact completely inaccurate and a wholly misguided notion.

II. Some basic exhaust theory

Your exhaust system is designed to evacuate gases from the combustion chamber quickly and efficently. Exhaust gases are not produced in a smooth stream; exhaust gases originate in pulses. A 4 cylinder motor will have 4 distinct pulses per complete engine cycle, a 6 cylinder has 6 pules and so on. The more pulses that are produced, the more continuous the exhaust flow. Backpressure can be loosely defined as the resistance to positive flow - in this case, the resistance to positive flow of the exhaust stream.

III. Backpressure and velocity

Some people operate under the misguided notion that wider pipes are more effective at clearing the combustion chamber than narrower pipes. It's not hard to see how this misconception is appealing - wider pipes have the capability to flow more than narrower pipes. So if they have the ability to flow more, why isn't "wider is better" a good rule of thumb for exhaust upgrading? In a word - VELOCITY. I'm sure that all of you have at one time used a garden hose w/o a spray nozzle on it. If you let the water just run unrestricted out of the house it flows at a rather slow rate. However, if you take your finger and cover part of the opening, the water will flow out at a much much faster rate.

The astute exhaust designer knows that you must balance flow capacity with velocity. You want the exhaust gases to exit the chamber and speed along at the highest velocity possible - you want a FAST exhaust stream. If you have two exhaust pulses of equal volume, one in a 2" pipe and one in a 3" pipe, the pulse in the 2" pipe will be traveling considerably FASTER than the pulse in the 3" pipe. While it is true that the narrower the pipe, the higher the velocity of the exiting gases, you want make sure the pipe is wide enough so that there is as little backpressure as possible while maintaining suitable exhaust gas velocity. Backpressure in it's most extreme form can lead to reversion of the exhaust stream - that is to say the exhaust flows backwards, which is not good. The trick is to have a pipe that that is as narrow as possible while having as close to zero backpressure as possible at the RPM range you want your power band to be located at. Exhaust pipe diameters are best suited to a particular RPM range. A smaller pipe diameter will produce higher exhaust velocities at a lower RPM but create unacceptably high amounts of backpressure at high rpm. Thus if your powerband is located 2-3000 RPM you'd want a narrower pipe than if your powerband is located at 8-9000RPM.

Many engineers try to work around the RPM specific nature of pipe diameters by using setups that are capable of creating a similar effect as a change in pipe diameter on the fly. The most advanced is Ferrari's which consists of two exhaust paths after the header - at low RPM only one path is open to maintain exhaust velocity, but as RPM climbs and exhaust volume increases, the second path is opened to curb backpressure - since there is greater exhaust volume there is no loss in flow velocity. BMW and Nissan use a simpler and less effective method - there is a single exhaust path to the muffler; the muffler has two paths; one path is closed at low RPM but both are open at high RPM.

IV. So how did this myth come to be?

I often wonder how the myth "Hondas need backpressure" came to be. Mostly I believe it is a misunderstanding of what is going on with the exhaust stream as pipe diameters change. For instance, someone with a civic decides he's going to uprade his exhaust with a 3" diameter piping. Once it's installed the owner notices that he seems to have lost a good bit of power throughout the powerband. He makes the connections in the following manner: "My wider exhaust eliminated all backpressure but I lost power, therefore the motor must need some backpressure in order to make power." What he did not realize is that he killed off all his flow velocity by using such a ridiculously wide pipe. It would have been possible for him to achieve close to zero backpressure with a much narrower pipe - in that way he would not have lost all his flow velocity.

V. So why is exhaust velocity so important?

The faster an exhaust pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The guiding principles of exhaust pulse scavenging are a bit beyond the scope of this doc but the general idea is a fast moving pulse creates a low pressure area behind it. This low pressure area acts as a vacuum and draws along the air behind it. A similar example would be a vehicle traveling at a high rate of speed on a dusty road. There is a low pressure area immediately behind the moving vehicle - dust particles get sucked into this low pressure area causing it to collect on the back of the vehicle. This effect is most noticeable on vans and hatchbacks which tend to create large trailing low pressure areas - giving rise to the numerous "wash me please" messages written in the thickly collected dust on the rear door(s).

VI. Conclusion.

SO it turns out that Hondas don't need backpressure, they need as high a flow velocity as possible with as little backpressure as possible.

 

That's a honda, and I said torque curve, 'nuff said

 

Very informative post.

 

Narrow pipes with turns and folds create backpressure. Not to mention the mufflers and cats also add to this.
It is this backpressure that is used to tune torque curves.
As I mentioned before about scavenging, this of course works with exhaust velocity and pulses to reduce the effects of backpressure. Like the article mentions.. backpressure is a factor in any exhaust.. its how you size it according to the desired torque curve and rpms. It's all about compromise..

 

ya i think this car is buffed out already. anything else would just be on an aesthetic level.

 

very informative post thanks paul

 

I will also agree that considering the state of tune the engine is in from the factory, and the design of the intakes... there will be little gained from a CAI.

Remember, it's making 306 HP under the new SAE rules... so that's probably about 315-320 under old rules, to compare to the old 298 HP. So they basically squeezed another 20+ HP out of this engine...

Kind of like the M3 engine... 333HP out of 3.2 liters... the aftermarket couldn't really do squat for that engine in terms of bolt-ons.

I'd think about 25HP max from HFCs, an exhaust and maybe ECU tune is possible... that's hitting about 100 HP/liter... pretty good for N/A spec. Seems I remember that the exhaust manifolds are a pretty good design as well, so not much to improve there, either. The only other thing to consider is maybe the intake plenum... see what people come up with for that. Hydrazine posted a dyno of the '07... and it seemed to run out of breathing up top from the looks of things. There may be some improvement there.

 

The upper RPM power drop is likely due to trimming the throttle opening as redline approaches. A conservative measure to minimize the impact of bouncing off the limiter. Nissan did this on the 1st gen tune and probably did so again. Technosquare did away with it after raising the limit from 6600 to 7100. A TS reflash on the '07 will probably do some good once they crack the code.

With a dual intake and split plenum, I doubt VE loss is due to induction design. Its in the tune.