Exhaust Backpressure: Fact or Fiction

[The Toecutter]

Man i'm just sayin :BLAA: None of these Fast guys seem to understand Back pressure!!! [ame=http://www.youtube.com/watch?v=GpSObv5vhtU]YouTube - Top Fuel Nitro Harley Mashup 2010[/ame]

 

[jtarkany]

Man i'm just sayin :BLAA: None of these Fast guys seem to understand Back pressure!!!

:scared: I think I wet myself a little.

 

[iSteve]

I'm pretty sure none of those fast guys are concerned about low rpm torque smooth throttle response in traffic and 0 to 60 times.

 

[FinalImpact]

Just a thought!,
Does a Top fuel funny car have back pressure, does any real high performance vehicle, such as a motogp bike have back pressure ? seems to me that all these top performers have 1 aim, get the exhaust gases out as soon as possible, that's why the exhausts are so short!

Just a thought

It's cause they only have one Throttle Position for 6.857 seconds => WOT!!!!!

Lots of good info in here; what it comes down to is called trade offs. What do you want?

Stump pullers = small long exhaust, short stroke cranks, low rpm
Midrange = increase the stroke to near square (bore > stroke), exhaust gets bigger and shorter, usable power band is friendly for commuting
High output = short large volume exhaust, stroke increases, cams dump tons of raw gas right into the exhaust at low rpm, engine makes no torque below full throttle, but at full throttle, Hold On!

Yes, a ton of variables left out; the point is what do you want? If you want the usable torque range to be 9000 to R/L open the exhaust up so it ends past your skull.

However, most people don't even know they want and need midrange torque and pull to accelerate a vehicle until you take it away! Hence the reason back pressure and proper cylinder filling become so important to a daily driver . . . a compromise giving lots of torque throughout a wide range of RPM.

My suggestion is to ride something you like that has the power when and where you want it. Now make yours like that one. We don't have the big lab, dyno, test track, and engines to destroy figuring out what doesn't work! Go for the juggler, what does work for you! Copy it as close as you can!

 

[FinalImpact]

I'm pretty sure none of those fast guys are concerned about low rpm torque smooth throttle response in traffic and 0 to 60 times.

Very True! Well Said!

 

[bikerdude]

So is everyone saying it is best to just leave the stock exhaust alone? I was just thinking of drilling my 07' muffler out with a 3/4" bit just for a little more sound. Will it burn the valves or heat up the engine or have any ill effect on the engine? I have read alot on this subject and some say it will hurt and some say it won't. Please help

 

[iSteve]

Aftermarket exhaust are tuned to a engine. Simply drilling holes may or may not make a difference in performance. And without testing you won't know if it's for the the better or worst. I'm pretty sure it won't hurt your engine and it probably won't be that much louder. Probably best to save some money and buy a exhaust if you want loud.

 

[Marthy]

Exhaust tuning is all about primary tube diameter and length. Having a good scavenging effect is what you need, but it work on a very small range. Usually in the max torque range. Imagine a "ball of fire" coming out when the exhaust valve open... by the time that ball of fire reach the collector, the next exhaust vale open... and so on. Pass the collector it all depend how much you want to kill the sound. Tri-Y exhaust are a bit better in the mid range to maximize scavenging effect. Any engine can be tune to an all open exhaust, the most important thing again is the primary pipe all the way to the collector.

So on your cycle, unless you swap cams, intake... primaries should stay about the same.

 

[abraxas]

I've wondered the same for years, will doing anything have an advantage?

The answer i keep coming up with is NO. That being said i'm very happy with the sound, i like quiet.

You could drill out the baffles, but i've heard one and it sounds really tinny, the solution is to drill more, but it's not something you can undo.

From reading here and elsewhere, theres also no performance advantage to drilling out the cat.

Yamaha made the fazer what it is, and all round i haven't found any reason to change it.

 

[Nelly]

Any one who ever rode or modded their 50cc moped knows the importance of the all sought after "Micron Expansion chamber" over stock.
After all this was the basis for all small bore two stroke tuning.
Hand polished inlet and out let ports. Racing reeds.
Those were the days when working on an engine was simple.
For single cylinder engines back pressure was pretty important.
Nelly:thumbup:

 

[Phil_RC_1]

My biggest problem with the original post is that only mom-and-pop shops were contacted to support the "Fiction camp" and A professional, "knows-what-they're-doing" shop was contacted to support the "Fact camp". There are plenty of professional, "know what they're doing" shops the would support the fiction side of this pole.

I think one of the main issues is a misunderstanding of the term "Back Pressure". As an example: A smaller diameter straight pipe in the system might be considered adding back pressure, which really isn't. It's increasing exhaust gas velocity. Also, an obstruction, like a vertical disc or valve (similar to a throttle plate in a carb or throttle body) might, and it is. For low end torque, the small diameter straight-through pipe is a good back pressure, the disc,,,, not so much.

There's a ton of info out there on the net. Just use some common sense and don't take average Joe's Home page as science fact over an established manufacturers white paper, or a university web sight.

 

[jtarkany]

My biggest problem with the original post is that only mom-and-pop shops were contacted to support the "Fiction camp" and A professional, "knows-what-they're-doing" shop was contacted to support the "Fact camp". There are plenty of professional, "know what they're doing" shops the would support the fiction side of this pole.

I failed to make it clear in the original post that I did not approach Mom & Pop or professional shops with a predetermined agenda. There is only one professional shop near me and several Mom & Pop shops.

I approached them all with the same question, "Do you think backpressure is necessary for my motorcycle?" At which point I would turn and point at my bike, then describe to them what I was thinking about doing.

The end result was the Mom & Pops felt it was fiction and the professional felt it was fact.

 

[mave2911]

My understanding is that the headers, or exhaust manifold, generate the necessary back pressure - so opening the exhaust after that has little effect.

So if my understanding of your proposal is accurate, then it should work.

Cheers,
Rick

 

[jtarkany]

My understanding is that the headers, or exhaust manifold, generate the necessary back pressure - so opening the exhaust after that has little effect.

So if my understanding of your proposal is accurate, then it should work.

Cheers,
Rick

Well, I did mount it and have had no adverse effects :thumbup:

 

[lrojasma]

I dont understand why you got such results.

Some days ago my exhaust got damaged form a fall and i run without any mofler, any cat, nothing from above, for about two weeks, and those were my fastest weeks yet, i felt a lot of guts on the upper range of RPM, but it sounded like the devil is coming to get you.

Could be that different bikes has differents results?

Weird.

 

[Gelvatron]

It's very much a fact if you have not studied this topic please don't comment I think you are confusing the people who are asking, each cylinder has an out put once the exhaust valve opens and exhaust is forced out from the intake air on VVT or interference engine the exhaust leaving through the headers ect allows a draft to form behind it like NASCAR the exhaust from the next cylinder that was fired now drafts of that backpressure is now created based on the resistance that this exhaust traveling experiences and this "fictional" (being a smart a$$) back pressure now creates a vacuum for the space in between the exhaust from each cylinder actually helps pull the exhaust out of the next cylinder that has its exhaust valve open so yea it's necessary the amount I don't know that's per application every engine is designed with this in mind and is meant to run around it I'm all for modding and slip ons will probably not really. Effect this so I wouldn't denounce getting some, where exactly this pressure is made is guess but don't know for sure the headers and prior to the cats I'd say don't mess with that stuff unless you don't care, ever see a muscle car with 4 inch exhaust and as the tool slams the accelerator his car backfires like a mofo I rest my case

 

[Marthy]

The most important part of a headers is from the header flange (cylinder head) to the collector. The diameter of the primary pipe need to flow enough air out at peak cfm (rev limiter) and keep the exhaust air speed under around 70% of the speed of sound not to get the air turbulent.

Imagine that every time the ex. valve open, it throw out a "ball of fire". At peak HP you ultimately want that "ball of fire" pass the collector at the same time the next exhaust valve open for maximum scavenge effect. That "pull" the next "ball of fire" out and help bringing A/F mixture in during the overlap period.

So basically once pass the collector... it doesn't matter. Since every time you ear a pop from the exhaust. You ear the shock wave from the exhaust hot air hitting the ambient air. The purpose of a exhaust pipe is to cool down that air to reduce the pop and kill the sound.

As you open up the exhaust and improve the exhaust scavenging... you need to add some fuel, remap. In simple words this is how it works. (Or the way I understand it... LOL)

Cheers!

 

[Motogiro]

Also these concepts in the physics of hot air flow change with frequency and volume so there is no one formula for flow and wavelength that will fit the motor's output and different volumes and varying frequency of pulse in the collectors.

Same thing on the intake. That's one of the reasons you'll see the new R1's changing velocity stack length at different volumes of intake. Same with the ex-up valves on exhausts.
If you're going to stick a bike on a track and run it near maximum output where the volume of a collector is tuned for that output then low back flow is probably closer to the efficacy of motor. On a street bike the same exhaust will not be near optimum. You can even put a bike on a dyno and show how the loss off back pressure with give more ponies at a high RPM but it may be more inefficient at most all of the useable rpm ranges that the bike lives in.

I really can't conceive how you can chop off an exhaust system in your driveway, run around a hiway and neighborhood at 10-14k rpm and say my bike will bike run better doing this. I mean, yeah, I understand the emotion in it but not the science in it.
I changed my exhaust to some slip-ons made by a company that does dyno and I pretty much trust their numbers. When I took the bike out it did feel like a big difference everywhere in the powerband. It's a different motor than an inline 4 and so the results are going to be different. I don't have any numbers to prove it but I do love the sound.

 

[SweaterDude]

I know backpressure is a bigger deal on 2-Strokes than 4-Strokes and I understand the whole path of least resistance spiel that would indicate the need for a 4-2-1 on an I4 head. but as stated i think that after the 4 goes to 2 you should be able to use a dual set-up without any major side-effects as you have. its not gonna be any race engineered set-up but i doubt 99.9% of us on the forum have the time, let alone the resources to accomplish that feat.

MV Agusta has a department that only researches valve angles and the result on power output. they spend months playing with valve angles trying to get that extra .25HP out of a machine. sometimes the change of .02° can ad a whole 1HP imagine what the race teams go through when designing a full system for their 240HP GP bikes trying to: save weight, increase flow, keep backpressure, not temper with aerodynamic flow around the bike, etc.

its madness.

 

[FinalImpact]

It's very much a fact if you have not studied this topic please don't comment I think you are confusing the people who are asking, each cylinder has an out put once the exhaust valve opens and exhaust is forced out from the intake air on VVT or interference ((INTAKE DOES NOT FORCE OUT EXHAUST & adding Variable Valve Timing which our bike does not have is not helping to clear this matter)) engine the exhaust leaving through the headers ect allows a draft to form behind it like NASCAR the exhaust from the next cylinder that was fired now drafts of that backpressure is now created based on the resistance that this exhaust traveling experiences and this "fictional" (being a smart a$$) back pressure now creates a vacuum for the space in between the exhaust from each cylinder actually helps pull the exhaust out of the next cylinder that has its exhaust valve open so yea it's necessary the amount I don't know that's per application every engine is designed with this in mind and is meant to run around it I'm all for modding and slip ons will probably not really. Effect this so I wouldn't denounce getting some, where exactly this pressure is made is guess but don't know for sure the headers and prior to the cats I'd say don't mess with that stuff unless you don't care, ever see a muscle car with 4 inch exhaust and as the tool slams the accelerator his car backfires like a mofo I rest my case. ((that's about "tune" more often than not as the Primary and collector pipes are effective at higher output. Again - the picture depicted here is it doesn't work! But it wasn't meant too as its effective RANGE IS NOT from idle to 3000 RPM. Its from 4500 RPM up.))

yes, i have heartburn with this, but please don't take it personal.


FWIW: As the intake opens, the piston is going down, this promotes cylinder filling by drawing in the charge on N/A engine. Its what makes the vacuum in the intake manifold. On the way up, it compresses the AF mix (valves are closed), ignition occurs and the piston travels downward on its power stroke. On the way back up, the exhaust opens and begins its path out of the engine. The piston begins its descent and the intake opens to bring in a new charge.

As for the actual topic; I reiterate that its all about trade offs. No single design works for everyones needs. Increased effectiveness in any given area will likely compromise another area. Torque at the bottom vs Torque at the top or mid range. Our 600 lacks torque on the bottom and anything changed to "help it" exhaust wise is likely going to hurt it on the top end.

Please for Christmas give me a cross-plain crank, new cams and an ECM which can manage ignition and fuel for my new crank! :rockon: :rockon: