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#25892 06/10/07 1:02 pm
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Lannis Offline OP
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Rolled the Firebird out of the shed for a 300-mile bimble yesterday to see a fellow Britbiker (he's 150 miles away, we're certainly thin on the ground here).

Running fantastic, but about 15 miles down the road it suddenly got loud and power dropped off by about half, literally half.

There's a bracket on the Firebird Scrambler that holds the high-mounted mufflers on, it bolts to the seat rail and down to the passenger footpeg mount, with a plate in the middle to which to bolt the pipes. This bracket had fatigued and fractured just below the muffler mount, which meant that the mufflers were "hanging" from the top mount, and they came unclamped from the header pipes and swung back, the pipe-to-muffler clamps alone will not hold the mufflers on.

So I figured, well, I'll just ride with open pipes, strap the loose mufflers onto my son's bike who was riding with me.

But even though the Firebird mufflers are "straight through", with a 1-1/2 hole right through the middle and baffling on the outside, the bike WOULD NOT RUN without those mufflers. I went about 3 miles and couldn't pull any hills in top gear, it would slow down to 35 miles an hour, and I knew I couldn't ride the whole day like that.

It went from 40 horsepower to about 13 horsepower just going from mufflers to "open pipes" without changing the jets. I need to learn more about the mechanism that makes that happen. I know about "reflected exhaust waves" helping extract burned mixture from exhaust, but surely that can't explain all this difference?

At any rate, I nursed it home, got out the old Suzuki instead, and still had a nice day out, seeing old Clymer Indian-Velocettes, Broughs, and Vincents in a basement shop you'd never think you'd find them in and having a lovely cookout of hamburgers, sausages, beans, salad, pickles, and brownies .... ! Blazing hot day but got caught in a lovely 5-minute downpour on the way home that cooled it down nicely.

It really made me wonder how many people are riding around on Britbikes that have no power at all and run like crap, but think that they are the horsepower kings because their bike is making a huge racket and deafening everyone ....

Lannis


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In Remembrance
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OK , I'll take the bait .....
First off , you know darn well you cant just drop in with a story about a secret subterranean stash of serious classics like that and not elaborate further .....

Next , yu dont mention the outcome of the FB?? Is it repaired and running as it should be ? I am seriously surprised by this report , I have not seen an A-65 fitted with Concentrics make such a drastic change in power due to the exhaust becoming less restricted . I cant trust my memory as well as I would like to , but I seem to recall switching from 1 3/4" "TT" pipes over to a set of stock replacement Togas with no , or maybe only slight , jetting change, maybe needle setting or slide cutaway was changed , but I seem to remember being amazed that the same basic jetting was so versatile .


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I don`t know why it happens but here`s my experience:

Had problems getting my 1971 Lightning to perform right. Tried different main jets, adjusted and rechecked timing with a stroboscop - still had low power, compression was good by the way. Was told that it could suffer from to restricted exhaust, original mufflers, even though it was quite loud too me.

Took the mufflers off, result; very bad low and middle range response but outstanding high rpm response.

Did you try to give it a lot of rpm ?

I believe that the general rule is that you have to enrichen the mixture when your pipes are more open, since the open exhaust know is letting a lot more air both in and out of the engine. Maybe enrichening would have helped you?


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Longer pipes, even when they're wide open, will offer more "back pressure" (which is probably a crude way of describing the length of the waves set up by the exhaust system) than shorter pipes.

Case in point, FBs with straight through (but longish) pipes specify the same jetting as Lightnings with silencers.


Mark Z

'65(lower)/'66(upper, wheels, front end, controls)/'67(seat, exhaust, fuel tank, headlamp)/'70(frame) A65 Bitsa.
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Lannis Offline OP
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Bonzo, Rocker, MarkZ -

Sorry about the delayed response, this new job's got me on the run (I'm a genuine laptop-carrying, cell-phone-toting, carry-on-bag humping, catch-the-next flight Road Wh*reior now!! Enjoying it though so far, it's different!)

Haven't had a chance to even take the broken part off yet, I've got to get to bed, have to get up at 0300 tomorrow to drive 4 hours North to Fredrick Maryland for a meeting, then drive 3 more hours North to see a man about a Moto Guzzi SP-III (Fay is coming with me to try out the co-pilot's seat and pass judgement on its suitability for long two-up rides). Then back home about 6 hours Friday morning, half-a-day at work, then to a retirement party for a friend, THEN pack Friday night to take off on a 300-mile round-trip camping outing on the M21 (the Monterey VA "Bluegrass Camp") for some pickin' and grinnin' and kickstartin' old bikes on a dewy morning, THEN the Virginia Guzzi Rally is next weekend so I don't know when I'll be able to braze that bit back up.

As far as the bike stash goes, names have been withheld to protect the innocent, you might know him anyhow, I'll get with you next time I see you and spill the beans .... !

It'll be interesting to get the pipes back on and see if that power comes back. Revving it did no good at all, it wouldn't pull the skin off a rice pudding except just coming off idle, it would jump good but then just give up and die ....

Lannis


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Let's not be confused ...
The "sonic negative reflected wave" back up the head pipe toward the exhaust valve is generated when ever the exhaust pulse contacts a larger or a smaller change in the pipe volume. This wave is what's used to 'tune' the exhaust for best extraction and efficiency. If this reflected negative wave traveling at ~ the speed of sound is timed by the shape and length of the exhaust to arrive at the exhaust valve just before it closes it will help clear the combustion chamber of burned gases.
A megaphone with it's constantly changing diameter will generate a series of negative reflected waves that will 'tune' the exhaust over a fairly large range of upper midrange RPM if done right.

But ... Exhaust Reversion, aarrgh.
There's another exhaust characteristic that causes PITA problems with All engines. It's called Exhaust Reversion, aka ER.
When the high temp and high pressure exhaust charge reaches the open end of the exhaust system it creates a slight vacum in the pipe.
Atmospheric pressure then causes air to flow Back Up the pipe, carrying with it some residual burned exhaust gas.
Note:
This pulse Does Not travel at the speed of sound but much slower and is Not a function of a reflected sound wave.

If the exhaust head pipe is short enough and the exhaust valve duration is long enough, this pulse will flow back up the exhaust pipe, through the open exhaust valve into the combustion chamber and during valve 'overlap', out through the open intake valve and through the carb.
This is why you sometimes will see a 'ball' of fuel fumes at the carb mouth of a race engine at low revs. It's also why you need to lean the needle and idle circuits on a racing engine with a short tuned exhaust as at low RPM the intake charge is being double and even triple charged with fuel, being pulsed back/forth through the carb, picking up fuel each time.

BTW, this phenom was first noted about 1896 by gent experimenting with internal combustion engines! He was quite happy curing the problem with a 28 foot long exhaust pipe!

This problem with the ER screwing up the low rpm operation of a racing engine is commonly called "Megaphonitus".
If you have a longer untuned exhaust system as many/most road bikes have, it's not an issue. The ER pulse won't arrive back at the exhaust port in time while the valve is open. This is why when you radically shorten your exhaust, the bike/car runs like crap.
And as you can readily see, engines with shorter exhaust duration with milder cams, will also have a less problems with ER. This is also why the 'modern' 4 valve engines don't have much of a problem.

What to do? ...
For some strange reason the ER pulse seems to flow along on the inner surfaces of the exhaust header and seems to be most prevalent flowing along the 'short side' of the header pipe curve.
This is why it's a common 'tunner trick' to have a missmatch ledge/edge on the bottom of the joint at the pipe to head junction.
Some engine cylinder heads have had a step cast into the bottom of the exhaust port. When the ER pulse slides up the pipe the small ledge will stop it from reaching the valve.

Now 'The Fix'.
The first I heard about this issue it was 'deep, dark secret' held near and dear by certain Triumph race engine tuners in The Era.
A "If I tell ya, I'll have to kill ya", sort of thing. Those early Triumphs had a problem with over sized exhaust ports and pipes as did many engines of that time.
"Bigger ain't always better"!
So ... these guys were welding a smaller diameter piece of tubing into the exhaust port, to. This decreased the port volume which increased flow velocity and improved chamber scavanging.

This piece of tube in the port is commonly called "stubbing", and ended up doing two things.
The power went up due to improved scavenging and the end of the 'stub' protruding into the header pipe acted as an effective block to that nasty Exhaust Reversion pulse.

Gee I wonder:
Why do some of the Goldies running around have such great midrange power, 'pulling like a train' without a hint of megaphonitus? hmmm.


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Phew Dave,
Could you go over that one more time. Seriously I have saved an article which was about 10 pages long on the same subject. When I got thru it I had no real idea what to do on my B50 exhaust so I left it up to Ed V. I believe my b50 will outrun my A65 to about 60 mph but after that the A65 probably has the edge.
Lannis, sounds to me like you need to put those pipes back on first chance you get.
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Lannis; ever noticed that banjo players tend to love ol' brit bikes? Must be all the tinkering involved.

Eric Weissberg (of Duelling Banjo fame) loves them (ask him about Isle of Man). He might be in VA this weekend; who knows. Hell of a nice guy. Musical genius, too.

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Thank`s Dave, well written !


1940 M21
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From Dave. Quote

"But ... Exhaust Reversion, aarrgh.
There's another exhaust characteristic that causes PITA problems with All engines. It's called Exhaust Reversion, aka ER.
When the high temp and high pressure exhaust charge reaches the open end of the exhaust system it creates a slight vacum in the pipe.
Atmospheric pressure then causes air to flow Back Up the pipe, carrying with it some residual burned exhaust gas.
Note:
This pulse Does Not travel at the speed of sound but much slower and is Not a function of a reflected sound wave."

Sorry I dont follow this and would love to have someone explain to me what Im getting wrong.(Follow may be the wrong word - actually I dont believe this but am very interested in being re educated)

About a year I asked about ER or AR (anti reversion devices) and got an abrupt comment from a contributor to the effect I was a simpleton and shouldnt ask silly questions. Maybe I am but Im also thick skinned enough to ask again.

So lets go. - Im totally on board with the "sonic velocity" wave behaviour and have read the SAE papers and Blair and inspected his dynoscope plots etc etc. I have also read the Fueling patent papers.( His patented AR device is another version of your "stubbing" I think). I have also inspected the NZ modified "D'd" exhausts in my Dommie motor and even the "D'd" exhausts in an early version of the G50 motor which has won the Australian 500 Classic title for the last few years.

So I know AR devices are used in successful bikes.

But if you read Blair you will not find a word about them.

I cannot see that this is a phenomona caused by " Atmospheric pressure then causes air to flow Back Up the pipe, carrying with it some residual burned exhaust gas...much slower and is Not a function of a reflected sound wave."

For me revision is just the flip sign of the same coin that gives you the ability to " 'tune' the exhaust for best extraction and efficiency to ... help clear the combustion chamber of burned gases. ie it is caused by "sonic speed"

I can only imagine that reversion devices dampen reverse or other wise ameliorate this higher pressure pulse which causes reversion during overlap. But then surely they would do the same for the lower pressure pulse which helps pull the full air mixture into the motor giving the greater than 100 % charge efficency from pumping alone. ie why doent it kill the good with the bad?

Enough of that. To go back to Lannis's first posting. Like Rocker I would have expected the problem to go away with high rpm. On a short pipe or megaphone equiped bike you get the poor power low down then the popping and spluttering as you go through reversion, then if you can nurse it through you will eventually hit power and off it goes. But you may have to slip the clutch a lot to pull through into the power band.

Does this bike have a cam with a lot of overlap? Large diameter pipes will not help either.

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johnm from NZ .. I would enjoy discussing exhaust systems with you, but first I need to know 'what level you are coming from'. My little write up is merely parroting commonly known and understood facts. Not my opinions or inventions.

Are you a successfull 'tuner'?

What engines are you familiar with?
Modern or Vintage? Euro, US or Brit?
Are they fast?

Why do you believe it's normal to have "poor power low down", with a tuned exhaust system.?
Gee, didn't I explain how to eliminate that issue?

You remark someone of consequence doesn't believe there's such an issue as "Exhaust Reversion". That's also your feeling?

I'd bet I could look around and find someone who doesn't believe in brushing his teeth or another person who showers once a month.
The world is full of the 'unwashed and uninformed' you know. Doncha?


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Hi Dave,

I don't think I am a very good tuner unfortunately. I am most familiar with 1950s English twins and singles. This is a dyno plot of my before and after engine development on a 500 Norton twin. It stops in Oct 2003 when I had a bit of a crash.

[Linked Image]

This Dommie is reasonably quick with rear wheel 47 bhp at 6000 rpm. The lower run had a standard street exhaust. The second run actually has a milder cam with less overlap - a PW3 versus 4S in the first run. You can see the characteristic reversion peak as it comes out of megaphonitis at 4400 rpm on the higher run. The engine was massively rich at this point from reversion through the carb.

Sorry I didnt mean someone of consequence (Blair) didn't believe in reversion (the opposite in fact - he spent much of his career modeling such phenomena). What he does not mention at all is AR devices, "stubbing' etc.

To answer your question of where I am coming from. I guess I'm happy to do the reading and the testing, but only within the context of a "normal career, family, other things to do" time allocation ie a few hours a week. I have a basic understanding of first year level university physics, I have played on bikes for 35 years but am not a professional, I have raced in NZ Clubmans classic racing for 6 years. In the hands of a good rider (not me) my bike has finished in the top three in NZ. I have read several of the standard textbooks and have used Blair and Lotus engine modeling software to design my exhaust system followed by a dyno based development programme.

To be absolutly clear with my question. I understand (mostly) and agree with exhaust modeling pressure behaviour described by for example Blair. This sort of thing from his book

[Linked Image]

What I cannot understand is how to reconcile the "fix ' of an AR device or pipe stubbing with this theory. It seems to me that they are a mechanical fix to a problem at one rpm range which inevitably must lead to a loss at another rpm range.

Modern engines have ways to fix such problems but they usually involve real time computer monitoring to adjust ignition, carbs, valve timing, inlet and exhaust manifold switching to match the rpm, throttle engine load etc - all of which is illegal in my class of racing anyway.

As to the idea that there is a physical movement flow back up the pipe. I cannot see that either. Do the maths. For an average pipe lenghts at 3000 to say 5000 rpm you are talking of speeds of 500 to 1000 mph to get to the end and back in one engine cycle. Thats close to sonic speed. For example Jennings suggests around 1700 ft/sec or 1160 mph for the speed of sound in an exhaust system.

Im not trying to be difficult. It just that I cannot make the sums work.

I guess I could also add one more thing. After I did the dyno runs in 2003 I had the opportunity to talk for a couple of hours with all my dyno run graphs with Peter Williams and Norman White and asked them about the reversion peak on the dyno. They were pretty interested because in the 1970s they didn't have good dyno access and hadnt seen a complete set of runs on a 500 twin. Peter was a bit surprised with the reversion because he designed the PW 3 to work at lower rpm but that was for a Commando which has a lower rod/strke ratio. He suggested I advance the cam a bit to try and get rid of it. But he didn't see anything wrong with the exhaust design.

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Howdy johnm...
There are many articles etc. around on Exhaust Reversion and the different methods to cure the problem.
Doing a Google search I came up with this interesting patent for a AR exhaust system.

here's a partial quote:
"Internal combustion engines generate exhaust gases in a series of positive pressure pulses. Between each positive pressure pulse, negative pressure conditions are created which result in exhaust gas reversal through the exhaust system. If the back flow reaches the internal combustion engine, the performance thereof will be adversely affected. In particular, reversionary gases can enter the cylinders of the engine upon opening of the intake valves. This will result in a lowering of torque and peak horse power as well as in a fall off of the fuel economy of the engine".

Anti Reversion patent descrtiption


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Commerically available "exhaust stubs", listed. I don't agree with this design, but it's a AR quick fix for riding your HD with open exhaust.
"Exhaust venturis".


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Here's an interesting article on engine tuning. See the 'Exhaust system' section for this gents description of the Exhaust Reversion issue.

Intake and Exhaust system Tuning


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A classic read on engine tuning is "Scientific Design of Exhaust & Intake Systems", by Smith and Morrison. Here's a short quote found on pg 27.
"Reverse flow:
To indicate the magnitude of this interference, it should be mentioned that in really bad cases exhaust gas can pass right through to the induction system and become mixed with the fresh charge which is fed to all cyclinders".

Many of us have found this book interesting. It gives good descriptions of how an engine works. It's a bit old but still available I believe. It's last reprint was in 2000.

Another 'classic' read on engine tuning is "Four Stroke Performance Tuning", by A. Graham Bell


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Hi Dave,

Thanks for the references. I will look at the links. I own both Smith and Morrison and Bell.

Have you ever read any of G Blairs work? He has done a lot of research on gas flow through four stroke engines, much of it on our sorts of engines. G50 Matchless motors etc.

This is reference to his book which has a long chapter on gas flow through four stroke motors with lots of engine test data matched to computer modeling and race track results. He takes Smith and Morrisons work forward about 30 years.

http://www.amazon.com/Design-Simulation-Stroke-Engines-R-186/dp/0768004403

There is also an interesting Society of Automobile Engineers paper # 2001-01-17974218 Exhaust Tuning on four stroke engines, Experimentation and Simulation, Blair, Mackey, Ashe and Chatfield. They do engine design work for F1 and MotoGP Teams. (Blair and Associates and Optimum Technology.

http://www.optimum-power.com/

http://www.profblairandassociates.com/

My issue is that I cannot match this work to the claims made for AR devices. The people who promote AR devices to do not provide dyno results or engineering and scientific back up for their claims. They assert things but provide no proof, either in mathematics or on the dyno or track. (or none I have been able to find) - For example the references you gave are long on promises but short on facts. On the other hand Blair and others provide real hard test and track data backed by peer reviewed engineering and mathematics.

However a friend of mine has just pointed me to a paper on dyno testing of ARs by Elliot, Vizard and Johnson. This will be very interesting I think when I can see it because these guys have excellent reputations and real credibility.

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aahh johnn ... Obviously you have gone to some serious efforts 'tuning'. I like and admire your researching and studying the information given by the experts on the subject. There are many hundreds of 'tuners' out and about that are so secretive.
Many of us are working merely 'on the edge' of efforts to build up a racing engine and are dealing with a limited and marginal level of knowledge that I find often times a bit frustrating.
It's so easy to 'talk fast'! We (me!) are often lulled into thinking we have 'the answers' just because our bikes are fast and win.
Enough power is never enough eh?

I must admit to not having read G Blair's books and I would be very interested in a source for his writings. I'll be looking at our book suppliers.
I appreciate your web references and I'll be checking them out. Thank you so much for your interest and sharing your knowledge and references. I like that.

I'm curious .. did your Norton make 47hp on gas or alky as many of you 'down under' racers use?

And speaking of running on alky ... the dirt oval 'short tracker' I'm building up for racing in the SoCalifornia 'ain't no rules' dirt tracking has a GM speedway engine that's advertised as having 65hp. Hopefully with a swing arm frame and ~ #200 bike weight with traction the rider will be able to keep it off the wall. We'll see ...
I'm trying my best designing the exhaust system with this engine. OEM the 4 valve GM engine had a short alu collector type exhaust manifold feeding into the 1 7'8" head pipe. To fit things into the twin down tube frame we're using I must change things a bit. I'm making a short paired tube 'manifold' set up feeding into a single headpipe and the junction lengths are still up in the air. aarrgh. All I have to go on is what's been done with other engines.
'Always sumpin Rosana Rosana Danna'.


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Hi Dave,

My bike was on pump gas 98 octane. It is run in our clubmans class which means the most we can use is Race Fuel which is leaded 100 octane I believe. The octane numbers may not be tranferable to the States. I have read about different rating methods but have never sat down to work it all through. The bike only has about 10.25:1 compression so I don't think it needs the race gas.

I beleive the hp reading was legitimate. It measured the same peak HP as a good street MK111 850 Commando on the same modern Dynojet inertia dyno. The Commando had a much better power band of course. Most real life street Commandos measure in the high 40s rear wheel bhp on the dyno. The Commando bhp numbers you read about in the 60s are for very well prepared tuned bikes I think.

Blair's book is also available at the Society of Automotive Engineers. It is about 800 pages long and has a lot of maths but also a lot of real data and a chapter on empirical advice for tuning including inlet and exhaust. This chapter is very very good for an non engineer. It is expensive about $90. He was a Doctor and Professor of Engineering but also a bike nut and worked around the British race scene for many years. Some of the book is heavy going and parts of it are engineering degree level maths but if I read it a few times I got lots of good ideas.

I havnt seen the Vizard AR paper yet but will let you know how I get on.

cheers

John


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