no no no! the experts here (that never dyno'd for comparison) will tell you, HEMI'S NEED BACKPRESSURE!

lol! thanks for posting. maybe the HEMI'S NEED BACKPRESSURE crowd will shut the heck up now (but i doubt it)

It is generally accepted by automotive engineers that for every inch of Hg of backpressure (that's Mercury - inches of Hg is a unit for measuring pressure) approximately 1-2 HP is lost depending on the displacement and efficiency of the engine, the combustion chamber design, etc. Our sources indicated that in the case of the L67 3800SC, 1HP per inch of Hg is reasonable.
1 inch Hg backpressure = 1 HP lost

 

I don't know about the Hemi because I don't own one but I do know with the 3.6 Penta that when I removed the Mufflers and Resonators the car was slow. The car sounded great though with a deep roar that could be heard from the next block over. I had no get up and go 0-60 was around 8.7 Sec. After feeling such lag I wasn't even interested in checking the top end. When I installed the Magnaflow exhaust I regained my low end and my top end was much, much better.

I agree that maybe you don't need backpressure but with these cars you need a computer to adjust mixture settings and other features but currently the 3.6 doesn't have a tuner out in the market.

 

I think this is mostly just a case of inaccurate use of terminology. If you just replace the term "back pressure" with "velocity", then the old adage statement makes perfect sense. It just so happens that both velocity and back pressure are conceptually associated with a smaller exhaust pipe dia, hence the laymen may find them to be interchangeable terms. Velocity (which can be likened to be an aspect of "dynamic" back pressure) is at the very core of choosing the appropriate size exhaust size (not too big and not too small)...static back pressure, not so much as far as being good for the engine.

Ideally, the exhaust pipe size would scale up or down in realtime based on engine rpm (in conjunction with displacement property). Obviously, it can't, so the next best thing is to choose a size that still gives good velocity at lower rpm, while not being too restrictive (from classic back pressure) at higher rpm. The next best thing is an exhaust that could go from a single tube to a double tube configuration at a prescribed rpm. So you get the best of velocity benefits at lower engine speeds, while also getting the best velocity benefits with an exhaust that flows well at higher engine speeds. Some fancy-pants cars do already try to employ this concept on the exhaust side. Obviously, the same idea on the intake side has existed for some time.

The obvious implication here, is that the wider the rpm range that a modern engine has, the less suitable a fixed-size exhaust system can accommodate the engine, w/o sacrifice at some operational extreme.

As far as exhaust modification, probably the most basic guidance is to keep the exhaust pipe size you have to maintain optimal velocity that the engine designers intended, but seek an (oversized?) muffler with the least amount of back pressure to yield max performance. The assumption here is that the flow properties of the exhaust pipe are probably right where it needs to be, and you are essentially shopping a muffler that can meet the same flow properties as the pipe it connects to (because that muffler will be the limiting factor as far as flow and back pressure). The mistake that is most often made is to upsize the exhaust pipe and muffler, which is totally a shot in the foot. The exhaust system flows great, but the engine will never benefit from it unless it is screaming at its last 1500 rpm of powerband (probably just fine on a car that will live on competitive road courses, but not so great for a car meant for transportation on public streets or hwy). The exhaust system is essentially suited for a significantly larger engine, at that point.

Now if you are making significant engine changes (forced induction, boring, stroking, etc), then that changes where the exhaust size needs to be altogether...

 

yep as you said:saythat:

 

Uh oh....backpressure chatter! DUCK! INCOMING!:icon_razz:
Backpressure....almost as dreaded a thread as "what oil do you use?"........:icon_razz:

 

I think the practical butt dyno is explaining things that don't contradict anything else said. Sure, maybe back pressure looses you some horsepower up top... but as demonstrated by people who have eliminated it, removing the backpressure kills down low torque. Torque in the bottom range is exactly wht makes a car feel fast. Loosing it will affect drivability, and fuel economy as you'll have to really get on it to feel like you are getting anywhere.

 

Urban Myth Number 42: "Hemi's need backpressure"


It is easy to see how this misunderstanding arises. Lets’ say that Max puts a 3-inch system on his normally aspirated car. He soon realises that he has lost power right through the power band. The connection is made in his throbbing brain…. put on 3" pipe = loss of backpressure = loss of power. Max erroneously concludes that you need backpressure to retain performance. He has ignored the need for exhaust gas velocity to get that scavenge effect.

If Max had chosen a smaller pipe he would have achieved better performance in the mid- to high-RPM power band. You need the combination of the least positive (close to zero) backpressure possible with the highest gas velocity achievable to create performance. The diameter of the pipe (and smoothness of internal finish and bends) will strongly influence if your exhaust change is going to create performance or lose power.

 

When contemplating a modified exhaust system there are those who want the biggest diameter pipe that can be had. Their idea must be that fatter pipes are more effective at venting than narrower pipes. This sounds reasonable but it is not quite correct. Sure wider pipes have greater volume and higher flow capacity, but that is just half of the story. Capacity is one consideration but gas velocity is the other factor.
An experienced exhaust designer knows that the best exhaust is one that balances flow capacity with velocity. A given volume/time of gasses will travel faster through a 2" pipe than the same volume of gas passing through a 3" pipe. So when taken to its extremes we can see that a too narrow pipe will create backpressure (restrictions to positive flow) problems and a too wide pipe will cause a very slow flow with no backpressure.
The optimum is where the fastest velocity is achieved with the least constriction possible.

This situation will arise when the pipe is wide enough so that there is the least level of positive backpressure possible whilst achieving the highest exhaust gas velocity.
The faster the exhaust gas pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The scavenge effect can be visualised by imagining the high-pressure pulse with a trailing low-pressure area behind. The faster the high-pressure pulse moves the stronger the draw on the low-pressure gasses and the gasses behind that. The scavenge action is like (but not exactly) suction on the gasses behind.

The greater the clearance burned fuel from the combustion chamber the less diluted the incoming air/fuel mix is. Scavenging can also aid intake on overlapping valves (where the exhaust and inlet valves are open at the same time) by drawing in the intake. These are good things to happen.

So instead of going for the widest pipe possible we should be looking for the combination of the narrowest pipe that produces the least backpressure possible. In this scenario we achieve the least restriction on positive flow and the highest gas travel speed.

Exhaust pipe diameters are best suited to a particular RPM range. If we used a constant RPM engine this would be easy to specify. But a variable RPM engine will mean that not one size suits all. It is possible to vary the size of exhaust volumes according to rpm but it is very expensive (Ferrari has done it). The optimum gas flows (volume and speed) are required at the RPM range that you want your power band to be located. For a given engine configuration a small pipe diameter will produce higher exhaust velocities at a low RPM (good) but create unacceptably high amounts (bad) of backpressure at high rpm. If you had a car with a low RPM power band (2,000-3,000 RPM) you would want a narrower pipe than if your power band is located at 5,000-7,000 RPM.

 

Very well said my friend. SO from this i am concluding that having magnaflows is better than flowmasters if we compare the backpressure. Because magnaflows are hollow and straight with perforations that cut noise whereas flowmasters have chambers. So to get a good velocity by keeping minimal backpressure we go for small dia pipes + magnaflows? :armed:

 

Yes, that is essentially the concept! ...or you can use a chambered FM, if you go with an upsized intake/exit size (but keeping your original exhaust pipe size...using adapter couplers, as needed...essentially, you are fitting a muffler designed for a larger exhaust system with higher flow demands). Basically, you want the muffler cfm to at least be close to your exhaust pipe cfm, in order to benefit from the flow and velocity capability of the pipes. The straight-through bullet muffler (i.e., Magnaflow) sort of just takes care of that in its inherent design.

 

Dynomax turbo is a chambered muffler that offers great flow. The original "straight through" glass packs actually had more back-pressure than a well made baffled muffler. I have yet to see a db sound dampening vs back-pressure that favors a 'straight through" design. If it were true: straight through gives the least back pressure and dampens or muffles the sound the best all OEM in the world would go to this design. AFAIK, no OEM has straight through mufflers on any car from ma's to a Z06... don't know about a F50, at best the "staight through" designed are used as cheap inline resonators (another misnomer). A straight through muffler dampens or muffles the sound by causing perimeter turbulence. Visually this will look like the best flowing design, but in practice it will be come VERY restrictive it were to be used to get OEM sound levels. So IMO nothing on the market can beat a Dynomax turbo muffler in drone vs flow. PERIOD.

http://www.dynomax.com/assets/2008catalog_pdf/superturbo.pdf

dual 17767 will support 368 HP at street sound levels for almost zero loss in HP, it does not get better than that.

I'm now running the VT, on my Type R, and let me tell you it is quite, like stock untill I get on it, it does have a little of mid throttle resonance, but nothing like a 70mm fart pipe. About 235 Hp 1.8 liter NA, and the VT is rated at 841 cfm on 2.5 inches, vs 588 for the Magnaflow "straight through".


http://www.dynomax.com/press_releas...ormance-exhaust-systems-now-available-with-drone-free-dynomax®-vt™-muffler.html

 

I've been wondering about the VT muffler. I went with Thrush for my experiment and its not great, but the experiment worked well enough that I will probably just get the Pypes kit that matches my semi-homemade xpipe pipe system but with nicer materials and actual tail pipes.
For anyone wondering I kept all piping at 2.25 and picked up MPG and improved acceleration in the quarter mile