high flow exhaust = 3-5% mpg gain?

[mcrews]

jtbo,
You cant dispel a 'myth' with a non-documented article by someone with no last name.

niky.
I am sure there are a lot of 500 hp engines running 2". The dameter size is rcommended of an optimal size given the max hp being run.
just like you can run 2" with a 90hp motor. you just wont be optimal.

The point here is that posters keep providing their 'personal' experiences saying larger is better. It's not. period.
If a poster disagrees with this chart, then find a chart(or study) to disprove it.
It really is that simple.
And all the talk about the exhust manifold is all well and good, except noone is going to go uot and buy a tuned manifold. they are just going to fix the pipe and muffler.

[jtbo]

Originally Posted by mcrews

jtbo,
You cant dispel a 'myth' with a non-documented article by someone with no last name.

niky.
I am sure there are a lot of 500 hp engines running 2". The dameter size is rcommended of an optimal size given the max hp being run.
just like you can run 2" with a 90hp motor. you just wont be optimal.

The point here is that posters keep providing their 'personal' experiences saying larger is better. It's not. period.
If a poster disagrees with this chart, then find a chart(or study) to disprove it.
It really is that simple.
And all the talk about the exhust manifold is all well and good, except noone is going to go uot and buy a tuned manifold. they are just going to fix the pipe and muffler.

Let's do a little search, you can pick any number of sources from this search:
https://duckduckgo.com/?q=backpressure+myth+stroke

Understanding how engine and exhaust system works is crucial when planning exhaust modifications or changes, so much is done because it is always done and because there are beliefs.

Also it is important to understand why backpressure is a myth and why smaller exhaust is good only for noise reduction side, same goes for bigger exhaust pipe, it often is just increasing the noise and as we know, noise = power, at least if you go to any parking lot and listen to teenagers and their cars.

There is difference with carburetor and fuel injection, with carburetor you can go too well flowing exhaust if carb is not tuned right, it is much more sensitive system.

People often can't separate different needs of fuel injection and carbs, there are really lot of such myths living partly because of that, even race car suspension building has lot of myths that are taken as a truth as everyone always has done so and has won.

I have not found anything that says that backpressure would be needed, other than opinions of individuals. When building 2 stroke engine, there backpressure plays important role, but that is about it, even with carb car engines don't really need it, but if those are tuned for restricting exhaust, then it would be needed to adjust carb to match new condition if exhaust is made larger, but often even that is not needed as exhaust often are not very restrictive.

Exhaust manifolds are there around 150 I guess? Custom made bit more?
I would put my money on that, not the rest of exhaust system if rest is still ok condition.

But that backpressure being myth is common knowledge, imo.

[niky]

Backpressure may be a myth, but the need for laminar flow is not. Simply put, too big of a pipe introduces too much flow turbulence, which robs you of power and efficiency at low rpms. Eventually, you get to a point where too much pipe robs you of power at higher rpms, too.

The only time running a 3" system will be the same as dumping to atmo is when that 3" system dumps out right after the exhaust manifold. For low powered engines, keeping flow even through the system is important.

Instead of thinking of it as backpressure, think of it as drinking through a straw. If the straw is too big, you can't generate enough suction to pull the water up into the straw. If it's too small or too bendy, you exert more effort for less fluid intake. Basically the same with exhaust. You want the exhaust to flow out as fast as possible. If the straw is too small, you have too much backpressure, and waste power pushing air out. If the straw is too big (and too long), flow will stagnate, causing power loss.

As for intake tuning... everything matters. You CAN do pulse tuning before the intake manifold... which is part of why CAIs are actually effective (it's not just the cold air). We were playing around with a WAI on the dyno and just a simple tube extension of 8", with the filter in the same place, increased horsepower by 10 hp. A non-inconsequential thing on an engine with just 140-150 whp. Even keeping the tubing the same and fiddling with the shape and position of the airbox itself can net a 3-5 hp gain in power.

The only reason power gains from intake tuning are lower with newer cars is because of increasingly adaptive engine computers... which is why some newer CAIs and intakes now feature MAF signal modulators.

[jtbo]

How much your throttle butterfly is open at cruising, what is angle of throttle butterfly?

I would consider it being kind of wall that stops so much of pulses that it has no effect, also often there is 90 degree direction change with intake runners and plenum.
Problem often comes when intake piping is long and still original diameter, which makes it more restrictive, longer you go, larger diameter it must be to not be more restrictive.
Full power is another thing again, it is hardly ever used on street except with young ones.

I have not seen any negative effects of bigger exhaust at dyno (not any gains with otherwise standard vehicle), well except noise. Surely everything is best made so that there are balance, but of course with everything one must seek balance and optimal solution, as with everything going too far is going too far.

With exhaust more effect than size is what kind of silencer boxes there are, often those are such that gas turns 180 turn twice before exiting, especially with standard family cars, making distance gas travel further and also any turn is restriction even it would be made smooth, which sadly often is not the case.

I got 0.5l/100km gain with one of my cars that had silly back box where gas did enter from same edge than it did exist, then pipe did go around the box something like 270 degree turn after it made 180 degree turn at box and it was quite sharp turns straight pieces between them. I put see trough box there, it was bit loud for sure after that, but there was also some gain. Maybe some of it was because it was easier to judge engine load from exhaust note, maybe it was so loud that I did use lower throttle subcutaneously, who knows, I was not much to hypermiling then, I met rev limiter daily

One could of course make exhaust larger, found this silly pic from the interwebs

[niky]

On the contrary, I've seen many dynos showing a loss in low-end (and sometimes midrange and even top-end) power with a big exhaust. It's all relative. Some engines (particularly Hondas) seem to never have enough exhaust... and you can go pretty big with them with no losses at all.

But others have such restrictive exhaust ports and are so sensitive to exhaust scavenging that going just a little too aggressive can lose you power. Just because some motors can take way more doesn't mean all of them can.

-

I've experimented with so many different iterations of airbox and pipe on my car that I can safely say that it still does matter, even at part throttle cruising. What we finally settled on was a hybrid WAI, with the filter all the way behind the headlight, attached to the MAF with a stock diameter tube to promote laminar flow at the sensor mount, followed by another smooth, stock diameter tube going to the throttle body. This produces the best power at all rpms, colder running on the highway, and no danger of water ingestion as with a full-on CAI.

Likely a stepped diameter tube would do better, but this would require a MAP conversion, which is too involved a project for me to consider on a daily driven car.

[jtbo]

It is interesting that result that you have got with air pickup changes.

Could some of effects have to do with angles of intake piping? I did read some time ago from results done with pipe and different angles in it, there was more loss of velocity, more there was angle, even small angles seemed to cause some effect, so that is something what comes to my mind that can also be part of it.

Problem I have with intake pipe and pulse tuning is that it would be at least XXth resonance so far away (it is Hz of intake valve opening/closing and distance comes from speed of sound, basic idea is that pressure wave goes back and forth in intake runner and one tries to match dimension of runners for Nth pulse so that pulse is at intake valve when intake valve opens), with direction changes, with restriction valve (throttle butterfly) I doubt that there will be anything left from pulse. Without any turns or restrictions there could be more chance for that effect, that is why I'm skeptical and think that there can be some other feature that causes improvement.

But there was ram air topic where I posted link to test and results to pipe angles somewhere in here.

If one has kind of block exhaust manifold, then there can be of course lot of odd effects happening with larger exhaust.

My recipe for economy tuning would be to get exhaust manifold that is tuned to max torque rpm, then swap silencer boxes, keep diameter of pipe standard, if one is really adventurer, new individual runner intake manifold with runner length matched for max torque rpm could be put in, but that is expensive one, however anything japanese and relatively modern, it should already have that (some even have good exhaust manifolds as standard), performance models might have intake tuned to max power however, some might have adjustable intake runner length.

Easiest first, silencer boxes to something more flowing, but still tastefully silent.

Chart posted earlier in my opinion can be perfectly good guide as especially with gasoline burners you are not moving that much of exhaust or air when cruising speeds are used, if you think that on highway car is using 20hp, rpms are relatively low, you don't really need much of diameter, with no restrictions from silencers it would spit out gases really well and more efficiently which leads even less exhaust gases.

There is difference of exhaust manifold and exhaust pipe size, exhaust manifold is critical in length and diameter, but I consider exhaust pipe to be such that exhaust manifold just carries evenly organised pulses out and as long as there is no restriction from rest of pipe thing works well, that is why I would attack first to those silencers.

[mcrews]

jtbo said
Chart posted earlier in my opinion can be perfectly good guide as especially with gasoline burners you are not moving that much of exhaust or air when cruising speeds are used, if you think that on highway car is using 20hp, rpms are relatively low, you don't really need much of diameter, with no restrictions from silencers it would spit out gases really well and more efficiently which leads even less exhaust gases.

that was my point also. I just watched my scangaugeII at 60 mph, 1812rpm and 30hp.
Most of us are using very little of the MAX hp availible on our engines. To use a diameter size based on the max is just silly. You want to be down a size or two.

[Blu3Z3rg]

Ok guys, this is why I suggested using a calculator.
If you put in your engine specs AND RPM it will tell you an optimum. There is also an entry for horsepower.
IF you put in your specs, put it at the rpm you want on the highway, and tell it how much horsepower you are making, it will compensate and tell you the proper diameter. If you only ever plan on making 30 hp at 2k rpm, then fine, install a 1/2" exhaust.

I run a 2.5" exhaust even though it recommended 3" for max power. Why? I switched it around to a 2500 rpm rangine instead of a 5000 rpm since i plan on towing.
Result? Just under 2.5" exhaust. On a civic or something smaller, you don't tow. But your rpms are right around there.

Lets face it, the EASIEST way to SIMPLIFY the exhaust situation and keep it IN COST is to just do a simple catback in the same diameter pipe that factory has, but straight throguh.

Any mufflers = disturbed airflow.
turns = disturbed airflow.

Just take it from the cats and run it out.

[sendler]

Getting back to the original question which was about just changing just the muffler, not the header pipes, My motorcycle would be a good test sled. I haven't changed the muffler yet because the reports from people that have are that it becomes so loud that you will start getting tickets. I also like to keep the cat which is very large and functional in the muffler of the CBR250R. Many dynos have been posted by many different sources and all the different mufflers seem to have the same effect. Big gains in power to the bottom and mid range with less gained on top. If I gain 9% torque at 4000 rpm where I begin my pulses will I gain 9% fuel economy? For the cost of $160 and some extra noise, I might have to find out before my next competition.

[Frank Lee]

Any mufflers = disturbed airflow.
turns = disturbed airflow.

Just take it from the cats and run it out.

No mufflers = disturbed neighborhoods.

Anyone contemplating open pipes should do everyone a favor and run the outlet into the cab or at least, within 6" of the driver's ear.