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04-27-2009, 01:30 PM | #1 |
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Drives: 2007 Yaris RS Blazed Join Date: Oct 2006
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does BACK-PRESSURE increase MPG vs straight thru...
thinking of straight thru muffler but stock pipes..worried about drop in mpg. anyone???????????
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04-27-2009, 01:42 PM | #2 | |
Drives: 2008 Yaris Liftback (Sprocket) Join Date: Oct 2008
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This was written for Hondas but I think the same principle applies...
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04-27-2009, 01:47 PM | #3 |
toyota
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still confuse does that mean that if i used stock pipes and just change the muffler to a straight thru that i wont lose mpg at all but better on top end cause its less restrictive...
thanks by the way for this input. |
04-27-2009, 02:00 PM | #4 |
Drives: 07 metmet LB Join Date: Oct 2008
Location: VA Beach
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basically if you go with a straight through muffler alone you'll probably lose a little bit down low and gain a bit up higher in the RPM. I cant see a muffler alone affecting your MPG that much. I would say 1 Mpg for 2-3 HP is worth it no?
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04-27-2009, 02:11 PM | #5 |
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1 mpg is ok i read somewhere in here 2 weeks ago that putting his silencer on gave him extra 3 mpg...does that make sense at all??
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04-27-2009, 02:13 PM | #6 |
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[1] Go dyno your car
[2] Do what you want to do [3] Dyno it again [4] Let us know |
04-27-2009, 02:26 PM | #7 |
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Drives: 2007 Yaris RS Blazed Join Date: Oct 2006
Location: toronto, canada
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its not the horsepower that im after..i just like the looks of this muffler and sound but dont want to sacrify mpg..not significantly atleast. 3 mpg drop is pretty huge if that was accurate..
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04-27-2009, 02:27 PM | #8 |
Drives: 07 metmet LB Join Date: Oct 2008
Location: VA Beach
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hmmm i would say 3mpg is a little high...maybe a little change in his driving style along with it? But its not impossible either.
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04-28-2009, 12:11 PM | #9 |
Drives: 07 Yaris sedan Join Date: Jun 2007
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Now that we have computers doing all the management in our engines, much of what used to be Gospel is immaterial now. I have built exhaust systems that added 20% to the performance (of a very choked engine), and ended up with a substantial increase in FE as well. If one has an efficient engine, then it stands to reason that it will use less fuel to do the same work; that is what efficiency is. Whether you use the increased efficiency for better acceleration or better fuel economy is your choice.
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04-28-2009, 12:12 PM | #10 |
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Drives: 2007 Yaris RS Blazed Join Date: Oct 2006
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???
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04-28-2009, 12:21 PM | #11 |
Drives: 07 metmet LB Join Date: Oct 2008
Location: VA Beach
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meaning if your exhaust gives you 5hp but you drive it exactly the same as before, theoretically your mpg should increase because you have more power pulling the same weight. However, most of us like to use the extra power that we get so it will basically cancel out any mpg gain. Think thats what he meant basically?
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04-28-2009, 12:23 PM | #12 |
Drives: 07 metmet LB Join Date: Oct 2008
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BTW i have the megan axle back and header, and fujita CAI, and i drive pretty aggressively and still get 36ish highway so i wouldnt be worried too much!
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04-28-2009, 12:32 PM | #13 | |
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got it thanks..it does make sense...its just there is a theory that back pressure allows the engine to reuse unburn fumes that stays on the enginedue to backpressure.to get a better gas mileage..???
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04-28-2009, 12:49 PM | #14 | |
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Quote:
Reducing back pressure can get you a slight increase in gas mileage, but there is a point of diminishing return. That is what we don't know. For that you would need to consult an engineering expert. |
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04-30-2009, 08:38 PM | #15 |
Drives: 2008 Yaris HB Join Date: Oct 2008
Location: University Place,WA
Posts: 828
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I was the person who put the silaencer in my Greddy and gained about 2.4-2.6mpg,same road,same drive and same speeds each tank for 3 tanks now back and forth,weather about the same temps and humidity each day,no reall change in speeds or traffic.....I am still baffled by it.....going to try a test withthe stock muffler to see if it goes up or down.
Honestly I am super happy to average the 32mpg I was gettign before the silencer,but now I am right at the 34-36 range,with only about 3 miles aday driven at 35mpg in town to the freeway,60-63mph on freeways....... I do not know why it went up,but it did........the silencer on this exhaust is only intering the can about 4 inches too so I am unsure of how it is changing the flow short of it has to push the gasses back to the rear of can to exit and that is causing a pressure point and moving the air faster......I do not know...... On tank 4 now and still gettign the same milage,car is at 14,600 miles now and running on stock rims,no hubcap,air in tires is normal and not over inflated or anything....... Honestly I think our little motor is affected by having a big can on it in a bad way for normal driving,under WOT it must have benifits other than sounding great,but for normal driving it might hinder MPG......like I said I do not know,that is why I asked the original question a few weeks ago...... |
05-01-2009, 04:07 AM | #16 |
Drives: 2007 Toyota Yaris Sedan Join Date: Feb 2009
Location: Orlando, USA
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hmm... i forgot about the excuse of telling "the wife" that the mod is a gas saver...
I might need to add a muffler to make it still look stock for her. LOL |
05-03-2009, 12:50 AM | #17 | ||
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Quote:
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Come along with me, visualizing the exhaust system and motor.... 1. Gas velocity through the exhaust pipe is not continuous. The person who wrote your guide seems to treat exhaust gas as a continuous flow, which is not so. They paid lip service to the idea of pulses but then modeled their explanation in terms of pipe diameter and not geometry of the system, which is a big factor in "tuning" it so that pulses do not interfere with one another or form resonances that MAY cause increases in backpressure. Exhaust gas "pulses". A pulse will contain a mixture of sine waves and thus different frequencies and thus will interfere with each freqs and pulses from other cylinders (and probably from previous pulses from the same cylinder - depending upon the transit time from the exhaust port to tail pipe. The pulses will have SOME inference with one another. They may act like waves going down a canal, either reinforcing or canceling one other to some degree. This is where the frequencies present in the pulses matter, some will interfere, others will not. The shape of the exhaust, both in terms of the length of pathways but also a given diameter of pipe, matter. In a continuous flow bigger is better, but that may not be the case with pulsatile waves. There will be turbulent flow through the interior of the pipe that will effect some viscous drag, as well as boundary layer conditions. This is dependent upon the diameter of the pipe though boundary layer conditions close to the side of the pipe are influenced by gas temperature and heat transfer, how much I'm uncertain. There be transference of heat between the gas pulses themselves. There is also mechanical transfer of vibration from the exhaust pipe to the gas and vice versa. Don't know how much of an effect that this will have on pressures and hence back pressure and flow rates. The rate of gas pulsation is dependent upon the RPM of the motor. My intuition is that there will be an optimal rate of pulsation for a given exhaust system, one that causes the least amount of back pressure. Might even lie in the motor's normal sphere of operation. We would have to model these effects to come up with a exhaust system shape and diameter of exhaust pipe. The plot thickens..... You have a catalytic converter which will alter the chemistry and thus the molality of the exhaust. That will alter back pressure to some degree. To make things even more interesting, you have two oxygen sensors which feedback into the ECU, which will then alter the chemistry of the exhaust, and this matters because fuel efficiency is a measure of average fuel consumption and not any particular singular combustion event. Chemistry is a big deal for fuel efficiency. There may other emission controls like EGR, which will be influenced by the ECU. We have to view the exhaust as a coupled system, starting at the exhaust port, with influence from the other three exhaust ports, the effects through the exhaust manifold, through the catalytic converter, resonators if any and the topology of the exhaust system itself. PLUS the ECU!!! I cannot assign weights to some of the effects, especially the catalytic converter, but I think it's safe to assume the follow effects go in this order of precedence... (however one must gather data rather than guess...) 1. RPM, which will be a factor with along displacement to govern total gas volume. We can view at this first approximation a simple "volume of displacement times the frequency that the piston compresses on the exhaust stroke divided by 2 (four stroke engine, every turn a piston rise and fall). You will probably never get any more gas into the system than the motor can deliver - though that Catalytic converter could create pressure through its chemical action. Changes the enthalpy of the gas either more or less to some unknown degree. The frequency of the gas pulsations and their chemistry will affect resonant frequencies of the system and the interference of the gas pulsations. 2. The ECU will govern the chemistry of the exhaust and its temperature. I think the ECU is where anyone ought to start modifications, if it's legal in your locale. 3. The Yaris's VVT will have some effect upon valve timing, and hence the rate of dwell for the exhaust valve. Since the VVT is under the control of the ECU its effects will be fedback from the exhaust gas itself. Back pressure from the catalytic converter, which will be governed in part by its temperature and the chemistry of the mixture and changes to the mixture. I've just described backpressure, by the way, and not mileage. Mileage is a function of quantity of fuel per mile, which is dependent upon the efficiency of the motor, which will be moderated by the ECU. Where backpressure, inside of the combustion chamber, comes into play is how it influences the combustion process, and indirectly influences the following combustion (since the exhaust gas carries away heat from the combustion chamber and thus influences how the next mixture will behave and influence the ECU through the O2 Sensor). So there is a thermal history effect at play here. Unless you got a lab, lots of time and professionals to set it all up, I'd leave this problem to the Pros. Gene Last edited by GeneW; 05-03-2009 at 01:33 AM. |
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05-03-2009, 01:09 AM | #18 | |
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Gene |
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