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Stop saying exhausts need back pressure.

6K views 35 replies 16 participants last post by  Harleymarlboroman 
#1 ·
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#4 ·
Cam overlap comes into play as well. If you have high overlap cam combined with less than ideal header lengths/diameter, scavenging at the wrong moment will pull fresh mix out of the cylinder and you lose power. Low overlap cams are by nature not sensitive to a high scavenging exhaust. Anyway, pretty complicated stuff that only experts can model and design to make good use, Throw in some variable cam timing and you are at another level of complexity. Add a turbo and all previously established rule are irrelevant.


Our big K has a big, dumb and lazy engine. lets keep it that way
 
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#5 ·
This is just one of the many reasons why the penchant for bolt on power is often counter productive. I've been to a few dyno day type events where my bone stock bike made more power than bikes on which had been spend many thousands of dollars. Those guys hate it when I bring a stock bike to a dyno day.
 
#6 ·
My first road bike was a basket case (mostly the engine) '72 Norton 750 Commando Combat that I built back to riding condition. Our first road trip was from Iowa to Colorado in '76. We made it into Colorado and while riding, my bike lost almost all power. It ran but I couldn't accelerate and the bike just kept going slower and slower as I tried to work the throttle. After pulling over one of my buddies asked what the problem was and when I told him I had no power his first comment is he thought I had lost back pressure. I checked and found the collars that hold the headers to the cylinders had come loose. After tightening them back up the bike came back to life and I had power again. We evidently didn't know all the proper terms but that back pressure comment made sense, at the time and it stuck with me. It may be semantics but the exhaust needed some kind of passage way that the pipes created compared to having the exhaust leaking a bunch right out of the cylinder heads. I don't like to use the term scavenging because my bike was no buzzard, it was a Snortin' Norton! :)
 
#7 ·
I've had two buddies buy Commandos in the last 6 months. One's a '73 750 ... right hand shift, left side brake, no magic button. Might even be a four speed (haven't ridden it ... yet)


The other's a '75 850 ... standard controls and has the estart. Five speed I believe


Beautiful bikes


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#8 ·
It is not about "back pressure". Every exhaust system, not matter what the design(muffler or not) has a reversionary wave. This is created at the point the exhaust wave travelling away from the exhaust valve meets an interruption, usually the end of a tube or some other significant object. This length is how an exhaust system is "tuned". The reversionary wave, on it's return to the exhaust port would dis-allow the escape of un-burned air-fuel mixture from the combustion chamber at the point of valve overlap. This has been addressed in many ways by the Asian manufacturer. Usually with some form of exhaust system modifier. Now, with variable valve timing, this reversionary wave, and its impact can be addressed in other ways. As time has progressed you may have experienced diminishing returns from adding aftermarket exhaust systems to you brand new bike. This is one of the major contributing factors. Just sayin'
Rick
 
#12 ·
This subject comes up sometimes and well....... here ya go.

https://youtu.be/jjPeP_Nn2B4

Yeah that guy really likes to hear himself talk. Let's talk about a real time installation and conclusion. Obviously BMW did their homework when they designed the exhaust for the B and GA because the net gain from installing 2K worth of Remus slip-ons is 1 HP and 1/2 lb/ft of torque. Hardly worth the expense you might say, that depends on what the actual net gain was. The looks and sound was well worth the expense. And the look on the HD guys face when you effortlessly pass his ass is priceless.
 
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#13 ·
My understanding is thus; (just my understanding, not my engineering half talking although it does pass the sniff test).

We call it backpressure, we really mean back propagation of a wave, the direction of gas is always away from the cylinder (4-stroke), 'back "pressure"' would require the direction of forces to change mid-cycle. That is not what is happening from the perspective of the cylinder. Its more useful to think of the exhaust gasses as always moving forward, then you tune the expansion chamber (referencing the above 2-stroke pipe) to carefully time the back propagating positive wave. The canceling effect created by the reflection of positive waves as seen in the 2-stroke pipe RL Lemke shows is required in 2-strokes in order to limit scavenging. Too much it won't make power, too little it won't run. It isn't 'backpressure' so much as a cancelation of the negative pressure region (low pressure part) of the exhaust wave, stalling the pressure differential. TANSTAAFL so you end up with a very limited RPM range of power when you do this as anything over or under that RPM range (fc) and the timing of the back propagating wave is off and therefore less effective. At least that is how I understand it.

My understanding of performance engineered four strokes is that the limited overlap of the intake and exhaust valves being a good thing to help manage velocity/time in the intake, the focus is on the header design and tuning. (and valve timing when possible) You tune headers, you engineer resonators for the most pleasing effect. Its hard to improve on what the factory has already done because the factory wants to get the most performance while still meeting regulations with a margin of error. The factory also has cost of manufacturing considerations to deal with. Overly complex header or resonator designs just aren't going to get past the penny counters. The challenging part is the difference in volume over time when you move up/down the RPM range. When you add an aftermarket exhaust you are always creeping towards more volume (louder) or for just a different sound which is purely subjective. I don't know of too many (any?) aftermarket kits which improve performance and make the engine note quieter. You might improve performance but no free meals here, it always cost something.
 
#14 ·
There's too many variables to say for sure, back pressure and scavenging have to work together. Not all engines or systems are the same. I do think for the most part the OEM's do a pretty good job now days. Sure, at some point the bean counters come in and have an affect.

My buddies 65 Mustang bracket racer runs quicker and faster times through mufflers then without. Wednesday nights are test and tune nights but mufflers are required. Screws things up for a guy just wanting consistency.

I love all the new Chargers, Challengers with loud exhaust. Most get on it and you look around to see which car it is cause ain't nobody really going anywhere fast. Had a new Super Bee get on it in front of my Jag on the highway. I drop 2 gears climb on his bumper while he's giving it all it's got and I absolutely could not get this guy to go again. That supercharged 5 liter Coyote engine is no slouch!
 
#17 ·
Everything beyond the header is a resonant chamber, just not a very good one since it will only be really successful at its critical frequency (a function of diameter and length). When your buddy added the muffler, he wasn't adding backpressure, he was changing the tune of the primary resonant chamber. By adding volume (the muffler) he was changing how the back propagating wave effected the tune of the exhaust system. He could have achieved similar results by changing the diameter of the primary exhaust or its length, he just happened on a tune that was made more efficient when he added the muffler. Its not rocket science to get to 90% right but that last 10% is a b*tch.

The coyotes are overachievers. Those engines are amazing performers and I just love what Jag has done with exhaust tuning (sound) on them. Our CFO drives a Maserati, the COO drives a jag. They park opposite each other in an underground garage, when they start their cars together it sounds like a GT paddock. awesome sound.
 
#16 ·
Airplane 4-stroke engines have LOUD unrestricted exhaust pipes. I have to wonder if a tuned exhaust would have saved the lives of B-29 airmen lives when so many of those radial engines caught fire?


BTW: The B-29 development program cost American taxpayers more money than the Manhattan Project.
 
#19 ·
Thank you, we have a winner!!!

When it appeared that Darryl's thread was about to disappear from the active topic list with no responses, I decided to watch the video and bump the thread for him (yer welcome HMM) :)

Anyway, my post was pure gibberish. Can't believe it took this long to get trashed ;)




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#27 ·
Thank you, we have a winner!!!

When it appeared that Darryl's thread was about to disappear from the active topic list with no responses, I decided to watch the video and bump the thread for him (yer welcome HMM) :)

Anyway, my post was pure gibberish. Can't believe it took this long to get trashed ;)

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Thanks, Dan. My threads never take off. They die much like Jack Riepe's love for his ex-girlfriends. Glad I can count on your gift of gab to bring this one back to life. Your post was funny. Can't believe some took it seriously.
Best.
 
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#20 ·
The 2200 -2500 rpm comment was added as a very obscure reference in response to the (at the time) handwringing on another thread about our bikes surging/bucking/thrashing about ... at revs that most people never visit on this bike. Or shouldn't :mad:


Lol




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#21 ·
I like your sense of humor:smile:
 
#35 ·
George Carlin is mine. Rip George
 
#36 ·
Not a great video, but interesting nonetheless. Old times are now new.



 
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