building an exhaust.. any guru advice or tips?

[monsta]

yes, I think I get what your saying. And I think your totally wrong... no offence...

"if those two pipes did somehow have a scavenging effect on each other it would be to no purpose as it would effectively be scavenging from itself... this is because a scavenging effect (or negative pressure) at that point would have its effects on the whole system and not just the opposite cylinder because the two cylinders pressures have already met and are both already at play"

I think, without a doubt there is some scavenging.. whether its miss-timed because of lenght prior to the cross-over is another thing.. what you say would be true if both cylinders fired at the same time...

"as for the theory on scavenging, my understanding is that it wont make any difference changing that end part of pipe as any real scavenging effect will take place from where the two cylinders headers first meet. this will still be at the same point as oem for you as you have the original headers and joiner in your design. so with those pipes your effectively dealing with the length of tailpipe on a car when the scavenging is happening at the headers."

'my theory' is that scavenging would need some follow-on from the cross over to provide a negative pressure at the opposed cylinder??  thats why I welded the radius bend from the cross-over. Otherwise I would have just let it exit straight into the box.

[MonsterHPD]

yes, I think I get what your saying. And I think your totally wrong... no offence...

'my theory' is that scavenging would need some follow-on from the cross over to provide a negative pressure at the opposed cylinder??  thats why I welded the radius bend from the cross-over. Otherwise I would have just let it exit straight into the box.

Are you going to test this on the dyno when it´s finished? I´ve never really understood the reason for the cross-over design, so your experiment is very interesting. Especially since it is somewhat opposed to my own thinkings on the subject    (also no offense).

I noticed on my M900 when I still had the stock headers on that the right-hand silencer was louder and getting warmer than the lefthand side, indicating more waste products ended up through that exit. Looking at the header entries, I did not find that surprising.

I would tend towards very little scavenging effect through the cross-over design, but I have no hard evidence one way or the other ... so please keep us posted. Exhaust systems are always good for a surprise or two .   
   

[monsta]

Interesting that there is now 2 people that dont think there is scavenging...  I may have to eat my words! 

But, I wonder why Ducati would use a cross-over if it wasn't for a purpose?

Anyhow, yes I will be dynoing and yes I will keep you posted...   

[koko64]

 
I like it when people design and test stuff. You never know if you dont have a go. Goodonya Monsta.

There are tuners that have shown that the stock headers work surprisingly well. The stock crossover and twin exhaust muffler set up provides ample volume for big bore twins. Exhaust scavenging is another issue again related to engine configuration.
 
I have been surprised by dyno tests (in relation to airbox mods),  when one principle you think will influence an outcome is overshadowed by another principle you may not have considered.

Those into this thread should check out Doug Lofgrens MPS site. Brad Blacks bikeboy site also has a saga or two.

Looking forward to the testing results.

[MonsterHPD]

Interesting that there is now 2 people that dont think there is scavenging...  I may have to eat my words! 

But, I wonder why Ducati would use a cross-over if it wasn't for a purpose?

Anyhow, yes I will be dynoing and yes I will keep you posted...   

We might have different theories, but we have the urge to build and test in common 

Looking forward to your dyno runs, as well as those from Roaduser 

[Roaduser]

I'm not saying there is no scavenging effect at play in this setup, I'm saying the scavenging effect is not at play in the area you are playing with. this may simply turn out to be more of a terminology difference between us, but after reading what your doing and your explanation i still disagree that those pipes after the merge are having any effect on scavenging.

when one header has a pressure force that passes the merge it creates a negative pressure in the non firing header and helps remove waste and, depending on cam profiles, possibly suck more inlet gases into the non firing cylinder. that theory prevailing, your scavenging is happening at the merge and dependant on the header pipes before the merge and the characteristics of the merge itself.

another theory is that where the pipe area increases dramatically (commonly the merge area, or in zoomies/split pipes the end of the pipe) there is a wave that returns back up the firing cylinders header till it hits the valve or cylinder, then returns again out the header and that can aid in scavenging of a firing cylinders own wastes/aid inlet gases. this theory is the tuning of a header pipe and thus the timing of the return wave is effected by the length and volume of the header pipe before the merge/termination of the pipe. this is thought to then only have a positive effect at one point in the rev-range and thus a different length of header can increase performance in a different area of the rev-range and have a negligible or negative effect at others. as the wave will be at a constant speed and timing from when the valve opens, it depends on the engine speed/valve timing as to when and wether the wave is synchronised and having a positive effect on the cylinder.

my thought on your design is that since both of your outlet pipes are after the merge and the same length/volume, they will theoretically both have the same pressures and waves that are split between them from both of the header pipes and will then effectively act like two simultaneously firing cylinders having no positive/scavenging effect on each other. its a difference in pressures and waves that causes the scavenging effect.

whats your theory on how those pipes will effect your exhaust? as others, I'm intersted in other peoples ideas and theories as much as anyone

[monsta]

ok, its very difficult to explain my thought's without drawing a diagram.
but... as the gas from one cylinder is split at the merge point (crossover) into 2 streams it interacts with the other cylinders also split stream (at the merge) to creats a venturi effect.
this will create a negative pressure, which will draw gases through the now open valves (valve over-lap) which helps remove spent fuel and bring in a fresh charge of fuel/air to the cylinder.

Without the extra length from the exit of the crossover, 'my thoughts' are there will a chance that the venturi wont be a efficient...

Anyhow, as I've stated previous, I know that the standard exhaust works better than the quat d.
What I'd like to achieve is similar performance as standard, with the under engine exhaust look...

Time will tell!      

[MonsterHPD]

.........
but... as the gas from one cylinder is split at the merge point (crossover) into 2 streams it interacts with the other cylinders also split stream (at the merge) to creats a venturi effect.
this will create a negative pressure, which will draw gases through the now open valves (valve over-lap) which helps remove spent fuel and bring in a fresh charge of fuel/air to the cylinder.
........

Just speculating on the scavenging theme .... I remember I once read an interview or something with Rob Muzzy (I think it was a Kevin Cameron story in an old Cycle Magazine) that the angle between the four headers as they joined the collector had a definite effect on torque / power. One theory as good as any other since I don´t know could well be a venturi effect beteen the active and passive pipes.

Just speculating again ...Should the cross-over design not work satisfactorily, one possible alternative design could be to join the 2 headers in a "regular" Y-piece inside the exhaust box. This would potentially also create a venturi effect. On the other hand, that would just be a normal 2-1 with a slightly different collector / silencer.

I don´t know what silencing you´re planning, but maybe some perforated inner skin with damping material between the walls would help dampen out "drum-skin" resonance of the flat-ish box walls?         

[monsta]

I don´t know what silencing you´re planning, but maybe some perforated inner skin with damping material between the walls would help dampen out "drum-skin" resonance of the flat-ish box walls?         

that's another thing I have to work out.
At this stage I'm thinking of just, in... change of direction (twice) and out. Thats how the quat D is... I put a bore scope (camera) inside when it was new to have a looky...

I'm planning on have an inspection cover screwed onto the top side so that if I need to I can pack it with some stainless swarf.. or something.. to quieten it if needed.  That may well be my downfall as well...

Your idea of an inner skin is a good one, I'll have to have a think about that, see what I can come up with.   

[Raux]

Don't make the mistake danmoto is making using loose packing with too large holes on the perforated tubing.

[monsta]

so... this is where I'm heading.



At this stage I only have one 90 deg elbow (the 2 offcuts lying in the center are just supporting the elbow sticking through the hole), I'll see if I can get something tomorrow. As you can see the gas has to change direction twice.

[MonsterHPD]

so... this is where I'm heading.

This is really nice 
Really shows how much effort has to go into any idea you might want to try out. 

It´ll be a while, but I´ll add to this story when I get around to making up my 2-2 megaphone exhaust for my track St2 bike.

   

[monsta]

looks like I'm doomed to failure! 

I wish I had thought of this before I started...

I've blown air down the exhaust to see what sort of vacume there is on the other exhaust inlet.
I used a little bit of paper and it sucked straight in then blew straight out the the outlet.
proves there is a vacume.

Problem is, when I do it to the exhaust that I've fabricated there is no vacume, it seems very neutral.
And when I put an extension on the bends that I welded onto the crossover it actually starts to blow out the other inlet!  :'(

So, I guess the bends that welded on are too tight and causing a bit of restriction?
I might have to just cut them off bit by bit to try and get them to mimic the longer radius bend of the standard exhaust?
If that dosn't work... throw it in the bin and put it down to experience! 


 

[monsta]

ok, so all's not lost...
I was playing around today trying to suss a solution and blew air down the quat d, same as I did to the standard and my fab'd one...
Its actually blowing air back up the other exhaust inlet at about the same thats coming out the outlets...
that cant be a good thing??

so.. if mine's is neutral, IE; is not blowing out the other inlet, it's gotta be better than the quat d??  anyone?   

[BastrdHK]

Great stuff guys!  I have always wanted to do something similar, but did not think that header lengths, radii and crossovers were that important.  Guess I should've been clued in by the huge exhaust aftermarket!

So Monsta,  why make your exhaust gasses bend again in the outlet?  Why not just have them cross again with perforations or whatever, so that the right side of the original crossover outlet exits out of the left side of your fabbed box and vice versa.  That would seem like an easier to build/less restrictive flow to me....thoughts?