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SilverArrowZ
05-15-2004, 08:07 PM
They say free flow muffler doesn't work well on automatic transmition as they need some back presure to perform optimumly..

Can anyone clear the doubt for me?

Egg Nog
05-15-2004, 08:13 PM
I've never heard of this problem before, but it makes sense.

It would have nothing directly to do with the transmission, but because of the way autos perform, it might be problematic.

Free flowing exhaust systems will tend to give you more pull when you're in the top of the rev range and less elsewhere. Automatics, for efficiency reasons, don't like to stay in high rpms. I can see how this would be a problem with automatics and not really with a manual because you can effectively keep the revs wherever you want with the latter.

This is all just from my own logical analysis... if I'm wrong, someone correct me...

Sweeney921
05-15-2004, 08:37 PM
sounds logical

SilverArrowZ
05-15-2004, 10:23 PM
yeah.. this topic is starting to get people's attention in Malaysia. Local mags are all talking about it and some even try different setup of mufflers on their automatic transmition project car to make a conclution.

For one thing, a friend of mine swap the stock muffler of his AE110 with 4A-FE engine. It sounds cool but somehow feel lacking power than before.

megotmea7
05-16-2004, 01:16 AM
actually "backpressure" doesnt add low end torque as commonly belived(*gasp*). lets think about this logicaly, how can a restriction in your exhaust increase its performance anywhere in the rev range? ppl typicaly take the loss of low end power with the change to a higher diameter exhaust as the work of backpressure(or the lack off). in reality it is the lack of EG velocity and exhaust scavenging that goes with it in a smaller diameter pipe. at lower rpm the smaller volume of EG's exits at a a higher speed in a lower diameter pipe as oposed to a higher diameter pipe where little to no scavenging might occure. at higher rpm's the engine is pumping more air so the smaller diameter pipe starts to become a bottleneck and hurts power, the higher diameter pipe flows the EG's more efficiently and at a higher velocity compared to at lower rpm's and doesnt bottle neck the gasses so more power is produced. if exhaust backpressure is required for low rpm torque then why dont you see variable opening butterfly valves in exhaust systems? starting out slmost closed at lower rpm and opening up for high rpm power?. it simply would create a restriction to flow where flow is the very thing needed. so until variable diameter computer controlled exhaust piping is developed you'll just have to go with the right sized exhaust for you application (i.e. putting a 3 inch plus exhaust on your civic wil make it even more peaky and probly wont produce as much peak power as a nice 2-2.5 inch exhaust that would allow low rpm performance also. and dont forget the no no's of crush bent piping and perferated core mufflers...

p.s. to answer the question you actually asked... a strait thru non perforated core muffler matched to teh diameter of the rest of your system will offer the best performance no matter your aplication, standard or auto. :)

Matra et Alpine
05-16-2004, 02:43 AM
actually "backpressure"
are you meaning something else ?

exhaust scavenging
THis is the key in the exhuast design adn SOME call it back-pressure :0
Effective scavenging is using one exuahst puls to "pull" the next cylinder gasses bye virtue of the lower pressure area created behind it.
Without some 'backpressure' in the exhaust design their is no significant low-pressure area.
If "back presure" is somet other phemonenan can you please explain as they're the same here :)
Road cars design their exhausts gas flow to optimise emisssions and performance at curigin speeds - where mpg is measured. So they're not designed for performance.
The mistake often made is just adding a BIG pipe in the mistaken belief that bigger is better.
WHat needs to be done is decide where the performance improvement is needed most and then design the pipe diameter and flow to maximise the lowest-pressure point to be at the exhuast header as the exhaust valve opens. THis is especially critical if their is a large valve overlap.

flows the EG's more efficiently and at a higher velocity compared to at lower rpm's and doesnt bottle neck the gasses so more power is produced.
only at high revs where there are more exhaust pulses and less 'spread' of the wave and gasses are more energetic.

if exhaust backpressure is required for low rpm torque then why dont you see variable opening butterfly valves in exhaust systems?
You do, just not in cars where it is VERY difficult to tune the exhaust and they don't rev so highly. Large rev range makes exhaust tuning much more complex.
The Yamaha X-UP valve on their performance bikes does EXACTLY what you described.
Motor racing formulas don't allow exhaust valves and at Very high power the materials and reliability are very expensive to achieve.

it simply would create a restriction to flow where flow is the very thing needed.
Not, it creates timed exhuast pulses at the point in the rev range it is most effective.

so until variable diameter computer controlled exhaust piping is developed
The peak of race engine development and engineering - MotoGP - have played with different exhuast flares and chambers for decades and continue to produce different shapes to maxmimise power in different ciucumstances. If anyone decides the extra weight of all the computer-controlled piping gave enough power to balance it then it would happen. ( Quick get th epatent in just-in-case they've missed it :) )

p.s. to answer the question you actually asked... a strait thru non perforated core muffler matched to teh diameter of the rest of your system will offer the best performance no matter your aplication, standard or auto. :)
Not if the auto is changin gears in the 2500-4000 rev range.
Typically in race engines you tune the exhaust to give you the usable power where you need it dependant on corners, gears and straights.
I can see exactly how the lowering of mid-range pwerformance and increase in power in the upper range would be a pointless exercise and a backward step if the auto shifts before the higher power advatange is accessed having just given up the mid-range few bhp.

cp01ae
05-16-2004, 10:59 AM
But if your driving an auto and it shifts before 4500 RPM, you really aren't driving it like you need the performance anyways. If you give it more gas you can get it to shift at higher RPMs where more power is made. I'm not sure if that makes sense to anyone, but I think it sounds about right. What you said Matra seems to make sense too.

KnifeEdge_2K1
05-16-2004, 01:43 PM
i think ive said this before but im still not 100% sure im correct

a little bit of back pressure is actually good since it keeps the exhuast moving at a constant velocity, this way after each stroke there will be a small vacume at the exhaust manafold which helps extract the exhaust gasses from the cylinder

megotmea7
05-16-2004, 09:38 PM
the defenition of backpressure im familiar with and all the ppl i talk to is a restriction in the exhaust such as an exesive amount of bends, crush bent pipes, larger to smaller diameter piping(creating a bottleneck) preforated mufflers. restrictions in the exhaust that would lower velocitys and create turbulance. hope that helped

SilverArrowZ
05-17-2004, 02:52 AM
=.="
I start the tread to ask a little question about muffler choice for auto transmition and now i'm confuse about muffler design instead :confused:

Matra et Alpine
05-17-2004, 04:01 AM
=.="
I start the tread to ask a little question about muffler choice for auto transmition and now i'm confuse about muffler design instead :confused:
:) Welcome to the world of UCP :)

Matra et Alpine
05-17-2004, 04:16 AM
the defenition of backpressure im familiar with and all the ppl i talk to is a restriction in the exhaust such as an exesive amount of bends, crush bent pipes, larger to smaller diameter piping(creating a bottleneck) preforated mufflers. restrictions in the exhaust that would lower velocitys and create turbulance. hope that helped
That's why I wanted to ask.
This definition of backpressure starts with the premise that it is bad.
Backpressure is managed in race exhausts to enhance performance as we'd already covered.
So it's NOT all bad, you need some of it to get the most from a highly tuned engine.