Exhaust back-pressure questions

Hi,
I've been trying to come to grips with exhaust back-pressure and so far haven't been able to find any straight forward information, just quite a lot of "it might affect the back-pressure" type of posts.
I'm pretty sure I don't have the right tools to measure or even apply a measured change to the exhaust pressure but I can alter the exhaust at a physical level so in saying that my question is...

If you look at a stock exhaust system 4 into 2 with the 2 stock mufflers would I be close in assuming that the greatest restriction to the exhaust flow is the actual diameter of the hole where the exhaust is released into the atmosphere from the muffler? or does the internal baffle system of the collector and mufflers play a large part in the exhaust back-pressure also?

 

If you change either of the things you mentioned, it would alter the back-pressure. If you're wanting to know which would alter it the most or to what degree, I don't go in water that deep...

skillet

 

I'll re-phrase.
What is the ID of the stock muffler pipe at its smallest point?
I'm pretty sure my pipes(which are non-standard and look like hot-dogs) are causing hesitation and running hot/lean which is also causing hard starting when hot (fuel evaporation in the bowls). Either way I have nothing to lose creating a restrictor for each pipe to test the theory but I was hoping someone had some hard data that I could make some sense of before launching into the experiment.

 

It's complicated, but you have to think of exhaust as two completely different things, a high energy pulse wave that travels at up to twice the speed of sound, and a mass-flow of the bulk of the gasses.

You also have to consider how the cam overlap and header length interact at certain RPM's to create the scavenging effect.

Too little back pressure, and you ruin your MPG and even hurt mid-range performance. You even need to jet rich just to make it run right.

Restrict your "hot dogs" to about 16 MM holes (2 of them) and see what happens.

 

I'll give it a whirl!
I just measured the current ID and it is 38mm on each so reducing down to 16mm should create an interesting difference.
Thanks!

 

This is kind of funny...

After much deliberation I decided to make a cone shaped 38mm->16mm restrictor out of some stainless steel sheeting so... geometry.

I went to this website
http://mathforum.org/dr.math/faq/formulas/BuildFrustum.html
and felt my head was about to explode so looked at it in a real world fashion and came up with this.

The pattern for a cone is basically a quarter of a circle, like a 25% of a pie-chart, so... obviously the large arc length should equal the circumference of the large hole and logically the small arc length the circumference of the small hole.. in this case the restriction from a 38mm diameter hole to a 16mm diameter hole.
Now this is close to the funny part... i swear (and if you've gotten this far it might not actually be that funny but, onward)

The math.
38mm(large diameter) x Pi = circumference of the hole (ie. 119.38mm) cool!
119.38 x 4 = circumference of the full circle or pie so to speak (ie. 477.52mm) excellent!
477.52 / Pi = diameter of the full circle or pie (ie. 152mm)
do the same for the small diameter ... you end up with 32mm.

Now at this point I'm thinking, "man this is cool and who needs geometry" and so now I realise the diameter needs to be halved to become the radius which is the length of the right angles of the 25% of the pie.
Rockin!

What the! That equals 76mm!

Now I'm sure that is probably common knowledge to someone out there but it was sure long winded getting to that solution... so when i looked at the equation in algebraic terms it was,
Diameter x Pi x 4 / Pi / 2 = Radius so...
Diameter x 2 = Radius is the same thing since everything cancels itself out...

and now that I've typed out this convoluted post I'm wondering if anyone will actually find it funny or useful... but to make a cone of a certain diameter just double the diameter and use it as the right angles of the pie-piece and away you go.

 

very cool lol, so let me repeat that, to make a cone half the shape of the diameter of a hole, get a piece of whatever twice the length and then start shaping it, correct? Well I guess you would still need to use the first equation with pie (diameter x pie x 4) to get the circumference and then you measure your crafted piece, right again? lol

the inside of my stock silencers have a large plate welded inside restricting about 65% of the flow, they are megafone shaped and the restrictor plate is somewhere towards the front. Let me know if you want the diameter where the plate is

 

Yeah, the diameter of the smallest pipe + plus the friction and turbulence on the walls of the pipe will affect the mass flow. But if you increase the area of a pipe and allow the gases to slow down, then narrow down the pipe and speed the gases back up...the velocity and mass flow out the back end will be the same (of course mass flow will be the same) as through the header pipe. However, the shockwave from the pulse can "bounce" off this slower moving lower density, lower pressure air. It's kind of like the index of refraction through a translucent medium. However, a gas cannot move faster than Mach 1 (the speed of sound). That number is not fixed however, and is dependent on the temperature, molar mass, and the specific heat ratio

 

This thread reminded me of a link I came across a while back, might have even been from this forum:

Solving the big problem with drag pipes

I never tried it myself but was tempted!

Another link with dyno testing and documented torque/horsepower curves:

Making Drag Pipes Work

 

Yes, I meant the pulse travels faster than the "regular" speed of sound that is 1,110 feet per second.

My previous Seca 900 had a 4 into 1 Kerker that had an internal restrictor that looked like an hour-glass. 2 joined funnels, with a minor diameter around the size of my thumb. All of the exhaust had to go thru this neck.

 

before you pull you hair out with all that fancy arithmetic, just make one out of cardboard and then copy to metal
i just changed the rings on my SuperTrapp muffler and went from 9 rings to 8 and can tell it runs different so math that out and see what kind of adjustments your going to have to make

 

The tunable baffle is a very cool idea from tibor's links... like the exup system in principal but manual... very cool indeed!

I made the cones, welded them up with the oxy and poked them in to the hot-dogs and welded them in large side facing out at the world.
Hell of a difference in volume but I will take it for a ride to do a quick check to see if the bike can hack it and then colourtune if it looks all okay!

The hour-glass sounds a lot more aero-dynamic, if it all works out I think I'll grind the cones out and redo like that and I oughta be a cone making expert by that time.

I'll let you know if it corrects the flat spot hesitation and the overheating(hard to start when hot) problem at the same time also.
Thanks all for the input!

 

Hehe that's the thing if you forget about Pi and just get the diameter of the hole and double that figure... that's the radius(or length of the right angles).

Then create a right-angle and use a compass your away!

So hole diameter is 38mm
the right-angle lengths of the cone pattern become 76mm and draw the arc with a compass between! a cone/traffic cone!


repeat the same process with the small hole using the original corner of the right-angle and tada! a funnel/restrictor/reducer.

Handy to know if your in the business of making trendy hat-ware for pupils of hogwarts... im sure.

 

I think there are alot of different ways to slow down exhaust gases and break up the pulses to muffle the sound. I won't profess to know all of the theory behind exhaust tuning (tho I'm thinking it would be worth while to pick it up), but I think this thread touched on many of them.

The point of exhaust systems is to remove exhaust efficiently during the exhaust stroke...and as some people haven't figured out yet, reduce the noise level :roll: . There are many ways of doing it.

You can create restrictions, which prevent the gas from flowing. This works because the velocity of a fluid will increase if the area of the pipe it passes through decreases - in order to preserve conservation of momentum and; therefore, mass flow. However, there are boundary layer effects, and the gasses cannot exceed Mach 1. If you sufficiently narrow the opening such that the gases will have to travel close to or beyond Mach 1 to preserve the mass flow, the mass flow will have to decrease. That is how your cone baffle works.

You can also change the density of the fluid. Changing the density changes the Mach number (if the expansion is assumed adiabatic, meaning no heat transfer, then the temperature decreases). This causes the shockwaves to bounce of this "wall" of less dense fluid, and the pulses can be timed for scavenging, and for destructive interference of the pulses (muffling). The pulses can also be absorbed by packing a baffle.

All of these principles are used (with quite a bit of math and modeling) when designing an exhaust system. And then some PO rips the baffles out to make it all louder :evil: !!!! Oh well, I think I'm getting close to tuning it

 

Here's some boring stuff about mufflers- -

http://naca.central.cranfield.ac.uk/rep ... t-1192.pdf

 

That's some great info, I can see a muffler creation workshop coming up.
If I had more material I would probably build the whole muffler from scratch after just leafing through the PDF.
hmm hmm...
Must not get ahead of the original reason for fiddling with the back-pressure though at this point.

 

Progress:
RTV'd the inlet manifolds, redid the inlet manifold gaskets, made a YICS tool that seemed to work and colourtuned again.
2 of the carbs were slightly rich but all are now looking good... 5 and a smidge turns out on average!

According to the XJCD the standard jets are 110 and 40 but mine are 118 and 43 so I'm thinking the PO(or one of the POs) rejetted for the hot-dogs.

Took it for a spin and it was almost perfect above 3K and with linear acceleration worked great but with hard throttle there is bunny hopping almost(when under 3K that is).

I'm pretty happy at this point with the suppressed exhaust noise now too but I'm thinking the jets may need to be re-stockified if I'm going to continue on this path... Sound like the right way to go?

 

My carbs have 118 and 43, thats what came stock on it. Th whole bike was stock when I got it, th stock pipes rusted out real quick...now..stock headers into basically stock collector into one Supertrapp megaphone type muffler. Runs well, good plug colour,good sound not too loud.
But from my info gathering about them they are different to what th US got, I guess those jets are what we got in Oz

 

Aha! Thanks bushy.

 

Hi All,
An update question!

With the current back-pressure and colour-tuning the bike runs with heaps of power and responsiveness when hot but...
when it's cold and I use the choke(enrichment circuit) to start, which it does on first hit, if I leave the choke on long enough to get some heat into the engine so it idles properly I end up with black fouled plugs within a couple of minutes.

Going for a ride and pushing the rpms burns it off marginally but by the next cold start I have to really pull the plugs and physically clean them up or the bike misses at low rpms.

It seems to be an across the board issue as all 4 cylinders have fouled plugs after using the choke. The fumes are so thick with the choke on, even in the open environment I start it in, that I think it is running richer than just rich from the enrichment circuit being enabled.

Before the back-pressure experiment, subsequent re-colour-tuning(blue when hot at idle) and the carb boot sealing I didn't have to use the choke to start... but now it will start only if i crank over and over without choke or straight away with the choke.

Except for the fouled plugs it seems like a better arrangement than the original before the exhaust back-pressure test.

Do you think the back-pressure experiment could be causing the super-rich choke situation?

Thanks for any input!

 

Have you allowed for a little bit of slack in th 'choke' cable adjustment at th carb end when in th Off position so it is OFF...maybe you use too much 'choke'..float levels??...have you done th whole valve shim/ synch etc' if thats not all ok it may be why you need to use th 'choke' so much...points to ponder

 

Choke cable is loose when choke is off.
I set the float levels manually when I went through the carbies, but they were pretty much dead on already according to the measuring dry way.
I re-shimmed after I did the head gasket, cam chain and lapped the valves, 2 were out but I already did a re-shim before the head gasket, etc (i'm getting good at using the shim tool from chacal)...

 

From all th talk on here about carbs, th dry float level is for starters but needs to be done wet for a proper level .. like a bench synch needs to be followed by a running sych.. I'll put my bet on thats th problem.

 

Aha! Wet test is next! Thanks

 

Well..once you've got th floats right you'll have to do th pilot adjust and running synch again, then it should be purring for you whenever you choose..good luck..