Exhaust back pressure regulator

G

gt alex

Been Around the Block
Quite round town not so quite wot
Still playing with my muffler. with 3 discs noise was about 92 dba at 4500rpm (94 being legal limit) but after a day of riding the noise dropped more it lost power and the front pipe when blue. so i tried 5 discs again (too loud) so not wanting a repeat problem I have gone to 4 discs (94dba) and made a back press regulator (pop off valve) which starts to open 6000rpm with no load. the trumpet does nothing its a shoot measure (i got it from a opp shop today I was looking for something stainless to use) and the base is the valve. I have cut the trumpet off, maybe it will become part of a ram tube.
 

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do you have glass matt on the internal core? or no internal baffle and core?
 
Hi the spring was the from a old fridge motor cradle, its was the only spring I had with what i felt was the correct tensiones.(long term???)
The muffler is a glass packed with 2" core(way loud), not wanting to stifle the motor (sv650) I made a insert camber with 2 x 1" tubes baffle each end 5" long with stainless wool packing (107dba sounds like a v8) then addind the tunable disc got it legal (too quite and restrictive for my taste).
Noise is measured 1/2 rpm max power so with this 2/3 rpm the back presssure is prevented from building up any more. and the v8 trys to get out but it can't (unfortunatly)
 

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According to David Vizard, more than 2psi back pressure and your losing power. I would look at 'interference baffles to keep noise inside silencer. There is more information online now about exhaust design so it saves you finding the $25.00 book(and reading through it multiple times ;D )
 
crazypj said:
According to David Vizard, more than 2psi back pressure and your losing power. I would look at 'interference baffles to keep noise inside silencer. There is more information online now about exhaust design so it saves you finding the $25.00 book(and reading through it multiple times ;D )
Click to expand...

Yup. Pressure in the exhaust system is not a good thing.
 
I think it's from a misunderstanding from automotive world plus various TV 'personalities' building custom bikes using 'buzzwords' without a clue what they mean. Some pressure in exhaust can prevent fresh mixture moving out of cylinder when cam timing is either extreme or just wrong. Anyone building a 'race' motor should know these things, maybe 'Smokey Yunik' started it with his NASCAR motors? (he was know for providing false information for general consumption)
 
any "back pressure" will cause reversion at some point or points in the rpms

if its not evacuating its not getting fresh fuel

if its bad enough it will spit back when the valve overlap occurs

i have some wonderful highspeed photos and clips of flames coming out carbs and getting sucked back in

on the next frame
 
If your exhaust internals are essentially pass-through, then no amount of packing is going to do much for the dB's.
 
We forget that there is always pressure in an exhaust, but it sure isn't constant. As the valve opens there's a slug of high pressure gas moving down the pipe and there's a pressure wave within that gas but moving much faster.

When it gets to the other end it is reflected. If it hits a hard surface, it is reflected back as a positive wave. If it reaches an open end, it reverses and comes back up as a negative pressure wave. And that wave reaches the vale and gets reflected again and so it goes of for a number of cycles. Overall the exhaust will be slightly above atmospheric but as that wave is reflected it can suck a rag back into the pipe.
 
I agree back pressure is not the goal, but having tested many internal baffle designs (with this 2" straight though shorty) most had similar effect on noise e.g. about 107 dba at 4500rpm which is the rpm used on my bike for legallity. so a disc type muffler as a tail peace alowed me to tune the sound but to get down to 94dba.(hopefully less back pressure than the 20mm DB killer supplied the epa yosi I have which still about 105 dba)
Using a small muffler required more restriction than I wanted so the regulator stops the buildup of back pressure at higher rpm.
It is still turnable for track by adding discs.
Maybe in the future I coud test a 1 5/8 straight through muffler no internal baffle or dbkiller with the disc pack tail piece.
 
Go with a 1.25" core and fiberglass packing. Stainless steel wool is horrible for attenuation, fiberglass is worlds better and basalt is even better than that.

The restriction you're worried about makes very little difference to anything. Back in the day I measured the restriction of a CBR900 stock muffler and TBR aftermarket can. The stock muffler had ten times the restriction of the TBR part. With a jet kit, the TBR part gave something like 3% more horsepower. Maybe it was 5%, whatever. Point being, the difference in restriction between a 1 5/8" tube and a 1.25" tube on an SV650 is minimal, so the powerloss will be minimal too. On the other hand, for attenuation you want a thick layer of packing, the more the better. I'd shoot for at least .75" thick (1.5" larger OD than core) but no one does that because it doesn't look cool.

Anywhos, worrying about restriction from the core and then adding disks to restrict it seems a little convoluted to me. You're losing any advantage of a thick packing layer and still adding restriction in the end.
 
Interesting about CBR900 exhaust, I guess you put it on a flow bench? While an inline 4 pumps a lot more air overall, Honda spent a ton of money optimising system. You did measure the weight difference? (probably substantial?) Personally, I think a different diffuser piece with 1.25 inlet side but keep the design as it looks 'cool'. Wadding on a mesh will absorb higher frequencies (the most annoying ones), add a few more discs and a softer spring so you can be 'dead quiet' around town at night (most 'dangerous' time for getting tickets in my experience ;) )
 
crazypj said:
Interesting about CBR900 exhaust, I guess you put it on a flow bench? While an inline 4 pumps a lot more air overall, Honda spent a ton of money optimising system. You did measure the weight difference? (probably substantial?)
Click to expand...

I was doing exhaust development for OEM automotive stuff at the time and the bench was specifially built to measure restriction in an exhaust. I've probably got the numbers around here somewhere but who knows where.

Yes, fiberglass packing attenuates the annoying higher frequencies. Lower frequencies are unaffected by the packing, they act as if the muffler is empty. Steel wool (stainless or otherwise) doesn't attenuate much of anything. Sure it never blows out but then it doesn't do much of anything while it's in there so what's the point? I have seen a thin layer of stainless wool/mesh put over the perfs and then the rest filled with 'glass in an attempt to gain some longevity but the steel still degrades the acoustic performance of the 'glass. OEMs use long strands of fiberglass that has been fluffed up, not the chopped up crap the aftermarket sells, so the orginal material lasts longer then replacement stuff.

As long as we're talking muffler designs - there is no need to use louvers vs perfs (holes) if you are going to pack the muffler. Using perf tubes with nothing packed around them can lead to whistling. If you do use louvers, they don't have to stick down into the flow. That helps with fabrication but not necessarily acoustics. Most OEM louvers are formed outwards, not inwards, and they run around the tube, not down it. Sound doesn't need to be scooped up, it travels in all directions regardless of what the flow is doing. So don't bother forming a bunch of louvers into a pipe for the core, just buy some perfed metal and roll a tube, or buy a perfed tube. Where? No fooking idea. I've always had access to a prototype shop that builds whatever I ask for. ;-) Actually Cone Engineering sells muffler components.

I didn't measure the weight difference but it was significant.
 
DesmoDog said:
I was doing exhaust development for OEM automotive stuff at the time and the bench was specifially built to measure restriction in an exhaust. I've probably got the numbers around here somewhere but who knows where.

Yes, fiberglass packing attenuates the annoying higher frequencies. Lower frequencies are unaffected by the packing, they act as if the muffler is empty. Steel wool (stainless or otherwise) doesn't attenuate much of anything. Sure it never blows out but then it doesn't do much of anything while it's in there so what's the point? I have seen a thin layer of stainless wool/mesh put over the perfs and then the rest filled with 'glass in an attempt to gain some longevity but the steel still degrades the acoustic performance of the 'glass. OEMs use long strands of fiberglass that has been fluffed up, not the chopped up crap the aftermarket sells, so the orginal material lasts longer then replacement stuff.

As long as we're talking muffler designs - there is no need to use louvers vs perfs (holes) if you are going to pack the muffler. Using perf tubes with nothing packed around them can lead to whistling. If you do use louvers, they don't have to stick down into the flow. That helps with fabrication but not necessarily acoustics. Most OEM louvers are formed outwards, not inwards, and they run around the tube, not down it. Sound doesn't need to be scooped up, it travels in all directions regardless of what the flow is doing. So don't bother forming a bunch of louvers into a pipe for the core, just buy some perfed metal and roll a tube, or buy a perfed tube. Where? No fooking idea. I've always had access to a prototype shop that builds whatever I ask for. ;-) Actually Cone Engineering sells muffler components.

I didn't measure the weight difference but it was significant.
Click to expand...
So, what about wrapping with fibergass cloth instead of the batting? Has long strands for longevity.
 
pidjones said:
So, what about wrapping with fibergass cloth instead of the batting? Has long strands for longevity.
Click to expand...

It has to have some "loft" to it so it's more like dense cotton candy or a soft felt than a mat, but yeah, long strands are better than short.
 
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