Modifying exhaust to increase mileage?

[Matt Timion]

Will modifying the exhaust pipe yield better gas mileage? Some people have said that putting on a larger pipe will in fact help with fuel economy, as the exhaust is less restircted.

There is a Civic EX at my local junkyard with a dual point exhaust. it should bolt right up to my car. Do you think that switching from single point to dual point will help increase my gas mileage at all?

[Flatland2D]

There was a thread about this not too long ago over at honda-acura.net that talks about exhaust diameter and performance (or efficiency in our case). Conceptually it's easy to see why a larger diameter pipe would allow more exhaust flow. However, there is a practical limit in this. The details are in the article I referred to, but the basic idea is that it's about balancing cross sectional area with velocity. An exhaust pulse traveling down a pipe will go faster than an identical pulse going down a larger diameter pipe.

I just searched for the thread because I think it might be a worthwhile read everyone here.

<a href="http://www.honda-acura.net/forums/showthread.php?t=201281">Backpressure: The myth and why it's wrong</a>

[SVOboy]

Will modifying the exhaust pipe yield better gas mileage? Some people have said that putting on a larger pipe will in fact help with fuel economy, as the exhaust is less restircted.

There is a Civic EX at my local junkyard with a dual point exhaust. it should bolt right up to my car. Do you think that switching from single point to dual point will help increase my gas mileage at all?

Lower back pressure is better for mpgs is there theory, but then why did honda make our pipes like 1.25"? I dunno, mehbe cheapness since it's not a preformance car and mehbe something else. Either way, the real question is how it the cross sectional area of the entire system going to flow? If you have a 2" pipe going into a muffler with a 2" tip, then it is the same as a 2" pipe going into a muffler with two 2" tips. The resistivity of a system is governed by the section with the smallest cross sectional area, if you add some big piping on the end but leave smaller stuff somewhere else it'll come that same speed as always out the 2" and then just sit around cuz the flow from the 2" pipe is not enough to use the possibility of the 4" pipe. I hope that makes sense in the case of getting a dual exhaust thingy.

PS: any luck on the knuckles?

[chasgood]

I have always been told that back pressure is good at low RPM's. It seems to help torque.
Nissan has a muffler on the 2.5 L cars with a spring loaded valve. At low RPM the valve is closed. As pressure increases (at higher RPM) the valve opens increasing flow. Best of both worlds.

[JustMe]

Here is a link for a jet engine car exhaust:
http://www.aero-turbine.com/how/

Anybody know how to add an afterburner?

[Compaq888]

I have always been told that back pressure is good at low RPM's. It seems to help torque.
Nissan has a muffler on the 2.5 L cars with a spring loaded valve. At low RPM the valve is closed. As pressure increases (at higher RPM) the valve opens increasing flow. Best of both worlds.

Are you talking about the 2.5 altima? I don't know if it has it. But when you open it up it sounds like a police siren.

[diamondlarry]

I'm not sure how effective it is but I hear that you can weld a sparkplug into the exhaust tip and plumb in some propane with the appropriate controls and flip a switch while tou're gunning the engine to shoot flames out the tailpipe.

[rh77]

Will modifying the exhaust pipe yield better gas mileage? Some people have said that putting on a larger pipe will in fact help with fuel economy, as the exhaust is less restircted.

There is a Civic EX at my local junkyard with a dual point exhaust. it should bolt right up to my car. Do you think that switching from single point to dual point will help increase my gas mileage at all?

I'm still at a loss with trying to find out info about exhaust diameter. I have exhaust leaks and need to replace all but the header and cat as soon as I can make a decision. I'd hate to stay stock and find out that a different muffler or diameter could have yielded better economy, but it's tough searching the Net. For a larger diameter, free-flow muffler, the marketing is "More Power, Better Fuel Economy!" -- but that's a but of a contradiction.

I'm trying to look at the design of fuel-efficient Hondas for direction. It seems like the more restrictive, the better.

Since the EX had a more powerful engine, I'm guessing that it has a larger exhaust system. My gut feeling is that it would yield less torque and a loss in economy.

This is all just thinking out-loud, but if you lose torque, you press on the throttle more to get going from a stop, which means more fuel used. Once out on the highway, free-flow may mean better economy since the RPMs are up and it would take less energy to push the exhaust out -- but for some reason, you need backpressure at lower RPMs.

If I had to decide right now, I'd go with smaller diameter piping and a stock muffler, or one with a valve like the 2.5 Altima or Lancer Evo. A device called the "Turbolator" is on the market that does the same thing (separate thread). I'd really like to understand the science of backpressure better...

RH77

[Matt Timion]

You may want to just visit a local muffler shop and ask them what the correct pipe size is for various cars. I'm certain they have this information on hand.

[kickflipjr]

I think that the stock pipe diameter on most cars is the best. If it's possible you can install maderall bent pipe for a tiny increse in mpg.

The turbolator (sping valve exaust tip) looks interesting. I think it would help at idle.

[MetroMPG]

this is another one of those things that could be easily tested with the scangauge if we had a warm-weather winter tester. do a bi-directional run, record data, unbolt part of the stock exhaust, repeat run...

[SVOboy]

this is another one of those things that could be easily tested with the scangauge if we had a warm-weather winter tester. do a bi-directional run, record data, unbolt part of the stock exhaust, repeat run...

That's true, you could just make a can hole or something to reduce exhaust size.

Anyway, back to rh77's stuff. The way I understand bigger exhausts work for power is that they almost always increase high end horsepower at the cost of low end torque, so I think you might be on to something there.

[rh77]

I found this article -- some valve overlap and scavenging is interesting (it's written for big V-8's, but the science is the same)...

http://superchevy.com/technical/engines_drivetrain/exhaust/0505phr_exh/index.html

RH77

[kickflipjr]

With my old car I acutually put a smaller sized tip (1.5 vs 1.75) on it to try to get more backpressure. It didn't do much of anything but i did notice loss of power over 3k rpm

[JustMe]

Here is a link for a jet engine car exhaust:
http://www.aero-turbine.com/how/

Anybody know how to add an afterburner?

Seriously, check this out. They say their exhaust improves mileage. Here is another one:
http://www.spiralturbobaffles.com/semiinfo.html
.

[kickflipjr]

It seems we agree that backpressure is needed during normal operation.

Do you think there would be benifit for strait pipes / free flowing muffler for extended highway driving (at 2,500-3,000 rpm)?

I would say yes, but let me se what i can find on the net.