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#239678 - 02/24/09 9:14 pm Exhaust backpressure, what is the truth???  
Joined: Dec 2007
Posts: 468
Ducknaldo Offline
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Ducknaldo  Offline
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Posts: 468
Denmark
Several times Ive been advised never to run these old brits with a straight through pipe.
I have taken these warnings seriously and thought about how to prevent valve burning and preserving power and for the last season Ive been riding with peashooters.
I, like many others have found that the Triumph (650) engine runs great with those mufflers, even though Id prefer a little bit more sound smile
I think I can actually feel a difference in power output at lower revs(from straight through), but it might be in my head.
I just stumbled upon a very interesting article describing myth and truth about exhaust backpressure and would like to share it with you guys and hear some opinions, seen from a brit-riders point of view.

Ill copy it in full length here since it has no copyright information anywhere, with source info ofcourse.

Here goes.:

Destroying a myth.



Some say that "an engine needs backpressure to work correctly." Is this true?

No. It would be more correct to say, "a perfectly stock engine that cannot adjust its fuel delivery needs backpressure to work correctly." This idea is a myth. As with all myths, however, there is a hint of fact with this one. Particularly, some people equate backpressure with torque, and others fear that too little backpressure will lead to valve burning.

The first reason why people say "backpressure is good" is because they believe that increased backpressure by itself will increase torque, particularly with a stock exhaust manifold. Granted, some stock manifolds act somewhat like performance headers at low RPM, but these manifolds will exhibit poor performance at higher RPM. This, however does not automatically lead to the conclusion that backpressure produces more torque. The increase in torque is not due to backpressure, but to the effects of changes in fuel/air mixture, which will be described in more detail below.

The other reason why people say "backpressure is good" is because they hear that cars (or motorcycles) that have had performance exhaust work done to them would then go on to burn exhaust valves. Now, it is true that such valve burning has occurred as a result of the exhaust mods, but it isn't due merely to a lack of backpressure.

The internal combustion engine is a complex, dynamic collection of different systems working together to convert the stored power in gasoline into mechanical energy to push a car down the road. Anytime one of these systems are modified, that mod will also indirectly affect the other systems, as well.

Now, valve burning occurs as a result of a very lean-burning engine. In order to achieve a theoretical optimal combustion, an engine needs 14.7 parts of oxygen by mass to 1 part of gasoline (again, by mass). This is referred to as a stochiometric (chemically correct) mixture, and is commonly referred to as a 14.7:1 mix. If an engine burns with less oxygen present (13:1, 12:1, etc...), it is said to run rich. Conversely, if the engine runs with more oxygen present (16:1, 17:1, etc...), it is said to run lean. Today's engines are designed to run at 14.7:1 for normally cruising, with rich mixtures on acceleration or warm-up, and lean mixtures while decelerating.

Getting back to the discussion, the reason that exhaust valves burn is because the engine is burning lean. Normal engines will tolerate lean burning for a little bit, but not for sustained periods of time. The reason why the engine is burning lean to begin with is that the reduction in backpressure is causing more air to be drawn into the combustion chamber than before. Earlier cars (and motorcycles) with carburetion often could not adjust because of the way that backpressure caused air to flow backwards through the carburetor after the air already got loaded down with fuel, and caused the air to receive a second load of fuel. While a bad design, it was nonetheless used in a lot of vehicles. Once these vehicles received performance mods that reduced backpressure, they no longer had that double-loading effect, and then tended to burn valves because of the resulting over-lean condition. This, incidentally, also provides a basis for the "torque increase" seen if backpressure is maintained. As the fuel/air mixture becomes leaner, the resultant combustion will produce progressively less and less of the force needed to produce torque.

Modern BMWs don't have to worry about the effects described above, because the DME (car's computer) that controls the engine will detect that the engine is burning leaner than before, and will adjust fuel injection to compensate. So, in effect, reducing backpressure really does two good things: The engine can use work otherwise spent pushing exhaust gas out the tailpipe to propel the car forward, and the engine breathes better. Of course, the DME's ability to adjust fuel injection is limited by the physical parameters of the injection system (such as injector maximum flow rate and fuel system pressure), but with exhaust backpressure reduction, these limits won't be reached.

- Adapted from Thomas V.

http://www.uucmotorwerks.com/html_product/sue462/backpressuretorquemyth.htm


*******************
Run `em...
*******************
1969 T120
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#239708 - 02/24/09 11:03 pm Re: Exhaust backpressure, what is the truth??? [Re: Ducknaldo]  
Joined: Dec 2007
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douglas Offline
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australia
Nothin' like a Melin with straight out pipes!!

#239710 - 02/24/09 11:09 pm Re: Exhaust backpressure, what is the truth??? [Re: douglas]  
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douglas Offline
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australia
I tried to edit it to put the"r" in but it wouldn't let me!!

#239739 - 02/25/09 1:30 am Re: Exhaust backpressure, what is the truth??? [Re: Ducknaldo]  
Joined: Aug 2005
Posts: 1,211
Steve in Tulsa Offline
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Steve in Tulsa  Offline
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Tulsa, Oklahoma, USA
It's not back "pressure". It is reverse wave. An open pipe has no reverse wave until the end of the pipe. A tapered exhaust creates a reverse wave at every change in diameter. A baffled exhaust, straight thru the middle still creates the reverse wave by virtue of the baffle. This wave helps purge the combustion chamber of spent fuel and gases while the exhaust valve is open.
Some of the Harley guys have learned a completely open 2" exhaust is NOT what you want which is why there is a 3/8 diameter rod available for the end of the open exhaust. It's called a powerplug and sits vertically in the middle of the exhaust ( it screws in top to bottom and it works).
It gives the jerks the noise they want and the back wave they need.
The cross over exhaust on the triumph, while splitting the exhaust noise on two mufflers instead of one, does the same thing on short timing to the exhaust valve improving bottom end.
HTH


Steve in Tulsa
#239763 - 02/25/09 4:47 am Re: Exhaust backpressure, what is the truth??? [Re: Steve in Tulsa]  
Joined: May 2006
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IAMKEITH Offline
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IAMKEITH  Offline
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Cincinnati, OH
I'm running baffled mufflers and velocity stacks on my 650, no problems in 40 years, the sound is unbeatable!


1967 Triumph Bonneville T120R

2012 Harley-Davidson Iron 883
#239783 - 02/25/09 10:25 am Re: Exhaust backpressure, what is the truth??? [Re: IAMKEITH]  
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dave jones Offline
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dave jones  Offline
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Emsworth, sunny south of Engla...
The normal US spec exhaust system of the 60s is practically straight through anyway. They seem to run just the same with the UK resonator silencers. I have the original handbook for my 67 650 and it gives the horsepower figure with "straight through exhaust system".

dave

#239785 - 02/25/09 10:35 am Re: Exhaust backpressure, what is the truth??? [Re: IAMKEITH]  
Joined: Dec 2007
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Ducknaldo Offline
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Ducknaldo  Offline
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Denmark
It seems there are still different opinions on this matter.
I dont know much about what really happens in the engine when gases leave.
It sounds weird to me that a back wave of burned fuel, should be helping getting burned fuel out of the cylinder.
Now I dont claim to know what really happens, thats actually why I opened this thread.
The question is not whether some people like loud bikes or not.
The question is, is it really dangerous for the valves, WHEN the carbs are adjusted for it?
Does it really give you a little extra torque to have a little resistance in the exhaust?
It sounds more logical to me that resistance consumes power from the engine.
I dont think factory mufflers is the optimal resistance for the engine(if needed).
Most race bikes have very much open exhausts and its kinda unthinkable that this should be merely a show for the crowd.


*******************
Run `em...
*******************
1969 T120
#239881 - 02/25/09 9:57 pm Re: Exhaust backpressure, what is the truth??? [Re: Ducknaldo]  
Joined: Aug 2001
Posts: 1,852
dave - NV Offline
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dave - NV  Offline
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Elko, Nevada USA
As the gent has mentioned here, "The need for back pressure is a myth". It's the 'reversion pulse' with open pipes or a mega that messes up the low/mid range running of an engine.

I Did Not 'discover' any of this! I'll merely parrot what you can read in any number of books and notice what many tuners have done to overcome the problems.

Do Not confuse the relativly slow moving reversion pulse back up the head pipe, with the sonic reflected pulse. The reflected pulse timing is used in 'tuned' engines with long cam overlap to help clear the combustion chamber of burned gases at the end of the exhaust cycle. A whole nuther deal.

There are many methods used to minimize the effects of reversion pulse and this problem has been dealt with for many years by engine builders and mfgs. The reversion pulse flow back up the headpipe is generated when the hot burst of exhaust gas arrives at the end OF All exhaust systems.
When the hot gases flow through the system a vacum is created in it's wake. When the hot gas pulse reaches the open air, cold air flows back up the pipe carrying some residule exhaust gas with it. Without any restiction this flow will go into the combustion chamber through the still open exhaust diluting the incoming fresh fuel/air mix. It will also continue to flow through the just opening inlet valve and through the carb on engines with fairly radical cams with some overlap.

The reason engines with a long exhaust system and/or a complex muffler don't have reversion problems is because the reversion flow can't reach the engine before the next exhaust pulse is initiated. The reversion flow is also blocked in the muffler. This isn't too much of a problem with the modern 4 valve engines with their relatively mild cams.

Some of us have seen the 'ball' of fuel rich /air mix at the mouth of a racing engine's carb at low RPM. As the reversion flow backwards through the carb it again picks up fuel on each transistion, causeing the 'blubber' of 'megaphoneitus'. This richness caused by double/triple fuel loading at lower RPM and smaller throttle openings is the reason a 'lean needle' was used in many racers.

The fix? As this reversion flow for some strange reason seems to follow the 'tight curve' surface of the exhaust headpipe, a simple ledge at the bottom of the exhaust port will stop it. BTW it's common with racing auto engines to have a header miss match which does this for them.

I got the idea for my anti reversion fix on the Goldies from a Triumph tuner many years ago. It really peed him off that I had discovered his secret! I'll share with you guys ..
My proven modification takes care of two issues.
It's a well known fact that the exhaust ports and head pipes in our BI engines are too large to maintain gas speed to more fully evacuate the combustion chamber at the end of the exhaust cycle.

I "Stub" my exhaust with a piece of tubing ~ 3" long who's OD is smaller than the ID of the head pipe. The stub spigots into the exhaust port fitting intoa shallow counter bore ~ 3/4" beyond the end of the head pipe. It angles upwards toward the top of the head pipe curve. The stub is welded in place in the headpipe. The mismatch at the end of the stub in the pipe creates a 'pocket' to stop the reversion pulse from flowing back into the port.

I've seen the washer affairs for sale to stick into the ports in the after market catalogs for the HD guys to use with open exhausts.

Another interesting way to deal with AR was told me by a gent here on bb.com some time ago. He had used auto exhaust pipe reducing adapters to fab up a bulge in the head pipe up near the port. He had cut slots in the head pipe inside the bulge to divert the AR pulse into the bulge area. BTW, have you ever noticed the "Power Bomb", bulge in some of the after market racing pipes? heh.

Try it, You'll like it!

BTW, a couple of years ago one of my much admired tuner/writers had a great article on "Stubbing Exhaust", in John Healey's 'Vintage Bike' newsletter. I'm sure I'm repeating many/most of what he described.
All of you should be subscribed to John's newsletter.

whew ...

Last edited by dave - NV; 02/25/09 10:02 pm.

dave - NV
#239904 - 02/26/09 12:27 am Re: Exhaust backpressure, what is the truth??? [Re: dave - NV]  
Joined: Jun 2007
Posts: 5,461
HawaiianTiger Online content
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HawaiianTiger  Online Content

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Maui Hawaii
I'm not inclined to read so much science on my hobby bikes but I am definitely interested in any empirical data gathered by the those who post here. I have some practical knowledge based soley on seat-of-pants and many examples of modified exhausts over the years on my bikes and others'. For the most part, the stock exhaust found on Triumphs gives nice usable power throughout the rev range without endangering your engine with flat spots or lean spots that might cause damage such as seizure. If you try something else you'd better pay attention to your engines' needs in order to avoid costly repairs. A gentleman I know bought a hot rod '52 Thunderbird and showed it to me. It had open megaphones on it and he claimed the bike was very fast for what it was. However, you could barely tolerate standing beside the thing even at idle. I advised him to put the stock exhaust back on it but he ignored my advice. A few weeks later I was doing a top-end job on it when he holed a piston. 'Nuff said.
Bill


Bikes
1974 Commando
1985 Honda Nighthawk 650
1957 Thunderbird/T110 "Flying Tiger"
Antique Fans: Loads of Emersons (Two six wingers) plus gyros and orbiters.
#239916 - 02/26/09 1:52 am Re: Exhaust backpressure, what is the truth??? [Re: HawaiianTiger]  
Joined: Oct 2007
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Kenneth Million Offline
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Kenneth Million  Offline
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Jonesborough, TN
Triumph put straight exhaust on their TT Specials when they built them. I know that the carbs were jetted to run them, but why the straight pipes if there was something better? Just wondering.


Kenneth Million
1977 T140v
1971 TR6R
1971 T350 Suzuki
1968 CA160 Honda
AHRMA # 653
#239926 - 02/26/09 2:47 am Re: Exhaust backpressure, what is the truth??? [Re: Kenneth Million]  
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MarksterTT Offline
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Triumph twins run the best with straight open pipes for all around performance increase (all around key words). Was surprised in the late 1960's when my rt side muffler fell off of my '66 Bonnie, felt like someone had pushed me from behind. So, I stopped and took the other one off, wow! instant hop up and lighter too. Ran the next couple of years around Las Vegas with straight 1 3/4" TT pipes, 750 Routts kit, PM 1060 cams. Also tried straight TR6 1 3/8" dia. side pipes but lost power most everywhere, esp. top end. The TT's should have been 1 1/2" or 1 5/8" dia. for better low to mid or possibly a stubb like Dave recommended in the 1 3/4" pipes. Can't believe I made that much noise, stupid kid.

#239954 - 02/26/09 8:47 am Re: Exhaust backpressure, what is the truth??? [Re: MarksterTT]  
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Ducknaldo Offline
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Ducknaldo  Offline
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Denmark
Now its actually starting to make sense to me whats really happening, thank you for the parroting.

But if this is really whats causing burned valves or holes in pistons, then the solution for curing the lean mix shouldnt be jetting the carbs for it, but raising the needle.
Jetting would also affect the higher rpm ranges, in which this problem shouldnt be occuring due to higher gas speed in the exhaust.

Of course, minimizing the reversion pulse with straight through pipes, would then eliminate the need for both solutions regarding carb settings, or?
This would for sure contradict general belief.

Dave.:
Thank you for the explanation, it sure put the grey matter at work here.
Do you have pictures of your pipe conversions?
Now Ive also been thinking about those old brits having too big exhaust ports and pipes.
What would actually be the correct size, for, lets say a 650 Bonnie, to maintain good gas speed in the pipes?

Hawaiian Tiger.:
I dont read science to create a race bike out of my hobby bike, but I sure would like to build an extra engine for the Bonnie some day with (almost)max possible power for this engine type. just for fun and hobby.
Now its not that I do not respect experience, but in the example you mention there could be lots of other reasons for these pistons getting holes in `em.
Eventhough, Im quite a young lad compared to many in here, Ive also seen a lot of Triumphs with holes in pistons, burned valves, seizure and other heat related problems.
In every one of those engine crashes though, jetting, air leaks or bad driving seems to be the cause, more than an open exhaust.

Kenneth Million.:
The factory putting TT pipes on their racing bike also tells me something, at least about performance.
Of course, these engines were probably not expected to have a long life, before next overhaul, so it doesnt really count, cause on a street bike, we usually like it to be somewhat stable over longer periods of time.
Id say one of the reasons for using TT pipes would be weight saving also, but that would be a guess.


*******************
Run `em...
*******************
1969 T120
#240005 - 02/26/09 5:47 pm Re: Exhaust backpressure, what is the truth??? [Re: Ducknaldo]  
Joined: Jun 2007
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HawaiianTiger Online content
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HawaiianTiger  Online Content

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Maui Hawaii
I think I should have finished telling the story I started....When I rebuilt this man's top end I finally convinced him to install the stock exhaust system and he was pleased beyond description. His bike was transformed into a bike that made more power everywhere except at the top end and that translates into a bike that in everyday use is just quicker and easier to ride. An old tuner I once knew gave me the pricise measurements of the most effiecient exhaust system for half mile and mile dirt track bikes. It was exactly so many inches header pipe and length of reverse cone megaphone and also the distance from the jet tube to the inlet valve. I have summarily ignored this data in favor of tuning my bikes for maximunm usable power throughout the rev range and despite what other have said here jetting and open pipes will only net you an increase in higher rpm power with an engine in otherwise stock condition.
And by the way, parroting is certainly acceptable if the original bird does not chime in.
Bill


Bikes
1974 Commando
1985 Honda Nighthawk 650
1957 Thunderbird/T110 "Flying Tiger"
Antique Fans: Loads of Emersons (Two six wingers) plus gyros and orbiters.
#240014 - 02/26/09 6:21 pm Re: Exhaust backpressure, what is the truth??? [Re: HawaiianTiger]  
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JBMorris Offline
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JBMorris  Offline
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Farmington,Connecticut,USA
Back in the day i decided to fit a pair of Dunstall Decibels just because i liked the looks over stock.
On the first ride it became readily apparent that something was wrong.
Take a look at the photo and notice the dark brown exhaust on the drive side.
The pipes were shipped with a plastic wrap around the baffles and someone forgot to remove it from one of the two.


1978 Bonneville T140V PX
#240170 - 02/27/09 2:51 pm Re: Exhaust backpressure, what is the truth??? [Re: Ducknaldo]  
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Phatt Bob Offline
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Cheshire UK
I think I saw an article once where a simple saw cut was made in the inside curve of the header pipe, and the pipe knocked in (head side) fillet welded into the resulting gap flat face towards the silencer.

Same principle?

Cheers

Bob


Phatt Bob
'95 Daytona 1200
'98 Daytona 1200 dragbike
ex-850 T140 Caff Racer, 850 Triton, Morgo T120, Starfire and Pretend Daytona 500 owner
#240376 - 02/28/09 1:42 pm Re: Exhaust backpressure, what is the truth??? [Re: Phatt Bob]  
Joined: Dec 2007
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Ducknaldo Offline
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Ducknaldo  Offline
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Denmark
Brimor, Im not really sure what you are saying, but nice bike you got there.
Bob, To me it seems like the same principle.

Im actually still not sure whether its bad for cams, pistons, etc. to have an open exhaust, IF you jet it and conquer the reversion pulse, in any mentioned way.
To me is seems a bit religion-like.
Some people like it and some people dont, but i can assure you guys, that my intention is only to shed some light over the problems connected to it and nothing more.
Would it be fair to say that all the talk about back pressure and reversion pulse really boils down to. "Restricting airflow backwards in the pipe, while maintaining fast gas flow the right way?
If that is the case, why not put some sort of valve in the exhaust pipe?(theoretically, Im not planning on doing it)
What about Supertrapps, are they only limiting noise or actually doing something for the torque also?


*******************
Run `em...
*******************
1969 T120

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