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smaller exaust pipe for mpg
since exaust is a bunch of pulses that line up in a row and flow nicely out the exaust system in an orderly fashon, (in fantasy land) at the perfect diameter pipe and rpm,. and we know the manufaturer has to compromise on the size to allow operation across a broad rpm range, what would be the benifit of installing a smaller pipe size from the header collector to the tailpipe, including finding a smaller size muffler and cat converter (if they exist) would you also have to get/make smaller primary tubes and collector or could you start the smaller exaust at your stock size header collector? i,ve read other data/posts that talk about restricting the exaust or creating backpressure. that is not the same thing as a smaller pipe, putting a singlar restriction in any position would only serve to create a (trubulent mess) im talking smooth sailing for the pulses, targeting around 2000-2500 rpm (where i live) for most of my driving. i dont care if my hp drops from 108 to58 ,if my mpg goes up enough,, i could be barking up the wrong tree but im interested in some of you engine guys opnions as to the possible benifit/problems.
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you want this in your tail pipe. guaranteed to produce results.
http://t2.gstatic.com/images?q=tbn:G...st_whistle.gif |
i might "give" one of those to a fellow coworker. he deserves it!
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LOL
Despite popular belief, everything that kills performance doesn't automatically improves fuel economy. |
yeah... stick it WAY up into the pipe... and add a bumper sicker.
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Never thought about MPG for a smaller pipe but i can tell you this:
On motorcycles the exhaust length and dia. are very critical to how the bike runs. Longer/smaller pipes give better low&mid range torque while short "drag shorties" give a higher and narrower HP curve. While most car exhausts are much longer, and many consider them to be too long to affect things, I'm not convinced they don't have some effect. I just think at the normal length they match s many 2nd and 3rd frequencies, tract tuning doesn't matter as there is still a matching wave length. Others feel at those lengths the waves have died out and turned into nearly steady flow. Now changing the header size & length will have an affect, lowering and flattening the HP curve shouldn't change the FE but the added torque may cause you to lift off the throttle more. :thumbup: |
Stick your hand over the end of the pipe when it's running- that isn't a steady flow.
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Exhaust tuning can achieve specific characteristics. You can choke the engine out a bit at the higher RPMs, and get desirable lower RPM characteristics which will change your VE and BSFC maps, meaning that you'll have to change other things to fully take advantage of it.
It's not "killing performance to get better fuel economy", it's tuning for a specific characteristic. You'll have to do some serious studying to determine the proper exhaust length/diameter configuration for the characteristics you're trying to achieve. |
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One day I was cruising down a highway at about 65. The (then current) muffler had been getting progressively louder, but I didn't worry, because at worst, it typically would rust out and break off right where the pipe entered the muffler. This time however, I heard a loud BANG! from the rear. I looked in the rear view mirror to see pieces of muffler flying off behind me. :eek: Yep, it exploded. Now you know what can happen from having a partially clogged exhaust. Purposely restricting the exhaust? No, thanks... :rolleyes: |
I only have a little expirience and it's certainly not scientific. I went from a 1"7/8 to a 2"1/4 exhaust, cat back, on the ZX2 and didn't really see any noticable change in FE. Then changed my cracked cast iron log manifold to a nice SVT 4-2-1 header and didn't see any big changes in FE. During that time I was not tracking mpg numbers real close, just kinda observing miles and gallons used. However I did put in a catalytic converter where the orginal had been removed and the difference after many months of tracking FE was nil. During this whole time my FE has steadily improved from many other much cheaper mods. Like ChrisT, I believe with enough time and testing you could probably see a measurable benefit from tuning an exhaust to a certain rpm range, but the change would probably be very small unless coupled with a complete tuning of the system ie. intake tract, ports, valves, cams, and the electronic tune.
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Auto Exhaust Science Good read about exhaust.
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Reducing the diameter of your exhaust pipe will do nothing for your torque at low rpm. That's a myth.
The myth got started because back in the carburetor days, adding headers and a performance exhaust to a car leaned it out, causing a loss of torque at low rpm's. Thus the myth that if increasing the size of your exhaust lowers low end torque, then decreasing the size of your exhaust must then increase torque, right? The real solution to the issue of lowered torque with a big exhaust was due to the headers actually doing their job and lowering backpressure, which leaned out the air/fuel mix. Once you re-jetted the carb to account for the headers, you INCREASED torque all the way across the rpm band. With modern fuel-injected cars, the computer will compensate for the reduced backpressure automatically so there will be no loss of torque. About the only thing you could do that would hurt your torque output would be to reduce the diameter of your exhaust system. It will increase your backpressure, causing your engine to work harder to pump the exhaust out the back. |
One thing that has stopped me from pursuing this further is not wanting to have the system "neck down" as IMHO that would be bad. Does that mean the ports in the heads are too big? Exhaust valves too? Probably.
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i think the valves are a completly different animal frank, because at that point the other cylinders dont affect its individual flow as much as when all the pulses mix together at the collector, im sure you are correct that changing all the paramiters would be best ,i just keep thinking that by changing the exaust it could make an improvment, because at low rpm's not that much air would be tring to get past the valves ,, but i have only modified v-8 carburated cars so i will defer to the more modern builders advice,i have seen big changes in v-8 cars depending on header primary diameter/length, and collector diameter/length, some of those cars highly modified, some stock,,but all 302+ cubes :confused:
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Reducing the diameter of the pipe can allow for less turbulent flow through the tube if the volume of flowing gasses isn't enough to "fill" the tube at it's current temperature/density. There's alot more to it than that simple phrase, but basically, exhaust tubing follows the same set of rules as an external flow, except with a much thicker boundary layer attached to the walls of the tube. In other words, you'll get more bottom end if you have a properly sized pipe, as opposed to an obnoxiously large one. Regarding carbs, once you retuned the carb, you were adding more fuel to the new amount of airflow to acheive more power across the board, which still means that you're burning more fuel at a specific RPM than you were. If the amount of fuel you're burning extra is less than the VE increase, you get a net gain in efficiency. If it's not, you're either breaking even or worse for efficiency, even though you could still be making more power than you were before. Our purpose isn't normally to increase low-end torque above what's already there, it's to make the engine more efficient at creating it. That means that anything which requires additional fuel, with few exceptions, is going the wrong way for our intentions. Most vehicles already have plenty of torque to do what they need to do, so adding more isn't really going to help, unless it's a byproduct of increased efficiency. |
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R = QD/vA Q = volumetric flow rate D = diameter of the pipe v = viscosity A = cross-sectional area of the pipe Since the volume of flow and the viscosity don't change, the only variables are the diameter and the area; and for a circular cross section, the area increases as a function of the square of the radius * pi; so the Reynolds number is always going to be lower for the larger diameter pipe. |
IF there isn't enough fluid to fill a vessel, it will not flow in a linear pattern.
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There are also modern cams to consider, as well as egr function. The net benefit of running an efficient exhaust at cruising speeds is to reduce the amount of energy required to push the exhaust out the tailpipe. I think for hypermiling purposes the only thing you can do that would even possibly have a noticeable effect would be an aftermarket muffler, since the stocker is nearly always a POS. You can see in this pic that the exhaust coming into the stock muffler has to make a right angle turn to get to the exit, and this is actually a pretty good stock muffler: http://www.roadraceengineering.com/3...ufflerguts.jpg On the other hand, a good performance muffler has a straight-through design that is a lot less restrictive: http://image.off-roadweb.com/f/94008...er_cutaway.jpg The reason stock mufflers don't look like this is because performance mufflers won't pass the noise requirements the govt. places on the OEM's. |
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For mufflers, PVC is acoustically inert, and with properly placed dimples, could be tuned so that sonic frequencies cancel themselves by reflection without compromising flow. Of course, nobody wants a plastic muffler, cuz "it could melt". |
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In a performance situation, you worry about filling the cylinder as much as you do about emptying it, so you make both valves as big as possible, with the exhaust valve ideally flowing about 75% of the intake flow. In a hypermiling situation, the intake valve is always going to be plenty big. So if I were building an engine from scratch to increase FE, my focus would be on making the exhaust flow as much as possible at low lift numbers. That means as big an exhaust valve as I can fit in the head, with regard to other parameters such as shrouding. |
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Thus it's a perfect example. |
You're trying to tell me that at 2,000 RPM there is no pulses in the exhaust? That's funny, I seem to remember watching smoke puffing from the exhaust at idle. Maybe I was seeing things?
There is no pulse scavenging or harmonic tuning because it's not done from the OE's. That doesn't mean it can't be done. BTW - That calculator suggests that at 2,000 RPM for a 1.0 with a 12* EVO, the ideal pipe diameter is 0.00". I'm pretty sure it wasn't designed for the type of information we need, here. |
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The pipe doesn't have to be that long for harmonics to take effect, and would only need to be that long for ideal pulse scavenging, and then that would also depend on the amount of flow and the speed/density of that flow. |
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Once again, the pipe doesn't need to be 18 feet to benefit from scavenging. That's the "ideal" length for a given engine size at WOT flow, assuming (I believe) 100% VE.
In reality, that wouldn't be happening most of the time, and considerations could be made to take advantage of pulse scavenging, although not in the most ideal sense. Harmonics are a different beast altogether, and can be taken advantage of on increasingly smaller amounts after primary harmonics, but can still be usable by length/cross section tuning. |
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I learned this from a mechanic.
4 cyls have no overlap, exhaust has to be precise for homogeny. That is many times a locale, the common fuel, turbo, no turbo, carbed, injected, low compression, high compression, big volt ignition, low volt, single fire , double fire, and how much air from idle to full throttle? and then you know what I learned..the same exact exhaust can be swapped through all engines, because there is no two cylinders firing at the same time. in the 8 cylinder world, and beyond, there is 90 degree overlaps, then in the 10 cyl it is 72 degrees, the 12 it is 60.. if you are running deep, there is reasons, very rare, to have what look like stove pipes, like a diesel... other than that, it is trial and error. even ignition strength alters what is needed. The length of the exhaust sytem may call for something else..my own is 1781cc, with a 2.25 axle back, on a carbed engine...sounds bizarre, until mentioning 14.6 feet to get to the back..the little pipes it could use, are bad because of it. and that brings how much heat is sustained, are pipes covered? going by a spinning driveshaft drops temps..fwd does not have to worry of this..etc etc etc etc there is a precision after all. |
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The WOT point was made to convey the fact that the calculator was based on assumed figures, including WOT. In reality, at 20% throttle, 1.0L is not even close to 1.0L. The pipe could be much smaller, increasing deltaP, without having much impact on friction, to a given extent. The idea is to get the fastest flow with the least friction for a given RPM. This means that there is always a compromise between diameter and length. At any length, pulsed flow can be used to scavenge flow on another cylinder, since as the flow expands and speeds up, it's relative pressure is reduced, which creates a sort of vacuum at another opening, drawing against the other opening's exhaust valve. Harmonics tuning creates a wave-driven effect in which exhaust gasses can be forced out of the pipe at a higher speed by pressure reflection, which also reduces pressure, increasing deltaP (desirable). |
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I suggest you actually read the posts on that forum.
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This calculator is a little more effective when determining exhaust length/diameter, as well as intake length/diameter for a specific tuning characteristic.
Many more inputs, as well. Pipe Sizing Calculator |
Bernoulli's principle only covers pressure PERPENDICULAR to the direction of flow, thus carbs still work.
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You are trying to argue that forcing a gas through a smaller pipe is somehow easier than forcing it through a large pipe.
If that's the case, then why do turbo cars, whose turbos operate off of the delta P before and after the turbine, have huge exhausts? Even more to the point, turbo diesel trucks, which operate in the rpm range we are discussing, come with 3 and 4 inch exhausts from the factory. |
I am informed by my engineering friends and family that the general rule for exhaust tubes is that they should have the same cross sectional area as the port at max valve lift. As I understand it for most of the exhaust cycle the port area varies with the valve lift, and changes with respect to time. Therefore for most of the exhaust strock the major empedament to flow is the exhaust port. so that at low rpm there is relatively lots of time for the gases to get out, even if the tube is smaller diameter. I agree with stone bracker that a larger tube diamerter is best, however the more important issue is how much differance will it make to a FE engine at low rpm? not much, unless the tubes are cosiderably smaller than the port area at max lift. If you really want to know what is best you will have to do a lot of testing to find out. Most "high performance" stuff is to maximize toque or power at a specified rpm so that the vehical will accelerate faster... what we are looking for is minimal fuel consumption. what we share with the HP folks is a desire to reduce pumping loses and to improve over all engine efficiency, so that we can burn less fuel, fast acceleration uses lots of fuel!
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