Topic: "homemade" exhaust

[smccloud]

Ok,
Since I don't want to spend the money to order a Mac 4 into 1 system from Dennis Kirk (even though they are around a 60 mile drive away for me), I am planning on modifying my factory pipes and making a 4 into 2 system (the mufflers are shot).  My question is, what size glasspacks are the "best" choice?  I'm guessing 1 1/2" would be around the best, but that is just a guess.  I'm also guessing I will have to weld a washer of some sort over the end to reduce the output size a little more.  Anyone do something like this already?  Also, I will be adding a "H" pipe between the two sides if it matters (probably not needed, but I want to).

[goon 1492]

I was gonna go this route but with a 4into1 header, many have done this with 4into 1 but not like what you want. I have to say though I finally got my catalogue last weekend from Dennis Kirk(good reading material ) and I noticed they have 2into one slip ons from Mac and they look like they would slip on easily after cutting off the old muffler.
I know that's not what you wanted to hear.....
I tried to get an answer for the cfm rate of one 341 pipe and I was gonna guestimate from there for building and figuring, but never got an answer. I have heard from others that if you do go with glass pack make sure the louvers are facing the the outer side of the pipe so pretty much put the pipe on backwards. (that didn't come out quite like I wanted)

[goon 1492]

Oh yeah go for a "cherry bomb" glass pack and not one like "Thrush" makes, I first bought the thrush one and it has a 5in dia. body on it and the cherry bomb' is like 3-1/2" diameter body.
Here is a pic with the thrush one.

[Grumpol]

I was thinking of doing the same sort of thing a while ago and with a little searching i found this burried in the bowels of the forum, cant remember who posted it so sorry i cant give credit......

here's my learned summary of header design theory for the 4-stroke 4 cylinder CB750:

1) Length and width of header, collector, and megaphone sections
From experience, optimal pipe width has been shown to be around 75% - 95% of exhaust port diameter.  At this ratio forces gases to escape with added speed and allows the sound pressure waves to have some influence as well.  Wider than this and exhaust gas speed and hence heat, is a problem.  Narrower than this and back pressure and loss of power is a problem.

Collector pipe volume = sum of volumes of inlet pipes.  Hence, with 4 pipes, Collector diameter = 2 x header pipe diameter.

Megaphone section length adds to overall resonant length, and when combined with header section length, will determine the system's resonance characteristics.

2) Temperature
Break mean exhaust pressure (BMEP) is a good tool to calculate average exhaust temperature (EGTav).  BMEP = K x power (Hp) / displacement (cc) x rpm, k= 447420.  BMEP x 60 = approx EGTav.   At 6000 rpm, the CB750 may have a BMEP of 6.8, which means an EGTav of 410oC.  At 410oC sound travels at about 58,000 cm/sec.  (Pipes will blue at about 600oC)

3) RPM
Pipes will resonate at precise sound frequencies.  RPM determines the sound frequency at which each cylinder is running.  Resonance is key to pipe design, and that means we must establish at what frequency a motor operates, or more precisely at what frequency we want our pipes to resonate.  I’ve chosen 6000 rpm as the target design point.

Since a burst of sound comes every 4 strokes or 2 revolutions, each cylinder's sound frequency at 6000 RPM is therefore 50 Hz. Because there are 4 cylinders, operating at 2 opposing strokes, the combined sound frequency of the exhaust system at 6000 rpm is 50 x 2 = 100 Hz.

Also note that 6000 rpm = 0.01 seconds per revolution (inverse of rpm) or about 0.005 seconds/stroke.

Putting this together, you get; that in the interval between the power stroke and the beginning of the exhaust stroke, at 6000 rpm, sound would have traveled down an exhaust pipe, a distance of about 72.5 cm (28.5") Call this the Header Tuned Length (TL).  Actual exhaust port timing is more precise than merely assuming ½ stroke (90o), but for this illustrative example its close enough.

Pipes will resonate (that sweet sound) if their length is a multiple of their resonant frequency.  From above, TL= 72.5cm and that means 2/1 or 1/1 or 1/2 or 1/3 or 1/4 etc of 72.5cm (1/1 = 28.5”, ½ =14.2”) are harmonic.  All other lengths will dampen the sound.

4) Sound pressure and pipe length
  Sound waves have pressure - they can move the air (exhaust gas in our case).  Also, sound waves can interfere with each other and can be inverted by bouncing them off of objects.   This can be accomplished by using bends, divergent cones (megaphones), pipe length and in the case of multiple pipe exhaust systems, by timing the arrival of sound from one cylinder with the port timing of another cylinder.  At 6000 rpm, a pipe of TL/2  cm would present a wave front to the opposing cylinder at precisely this moment.  Hence placing the first bend at 36cm in a header pipe length of 72 cm would achieve maximum extraction at 6000 rpm.  Thanks to harmonics, any multiple of TL will also work, just to a lesser extent. Conversely a pipe length that is not a multiple of this length, will have adverse effects.  If actual exhaust port timing were to be used, then a slight adjustment to this length would be necessary.

5) Speed of exhaust gases and pipe length
  Gases will speed up through a smaller diameter pipe and slow down in a larger diameter pipe.  Correspondingly, pressure will rise in a smaller diameter pipe and drop in a larger pipe.  Ideally, as the sound front hits the gases it speeds them up precisely when needed, and a reverse sound front slows them down precisely when needed.

The speed of sound is generally much faster than the speed of escaping exhaust gases.  Stroke of a CB750 is 63mm.  Since all the gases must escape within the span of time the piston moves this distance, we can determine the speed of the escaping gases.  The volume of gases = 63 x 61 stroke x bore = 184.1cc (under pressure).  Under the same pressure, that volume would take up 23 ¼ cm of a 1 ¼” I.D. exhaust pipe.  At 6000 rpm, this volume of gases have about 0.005 seconds per stroke to escape, hence the speed is 23 ¼ / .005 cm/sec = 4,650 cm/sec.  This would only be true if there was no pressure differential in the exhaust pipe.  However, since gases are highly compressible, the actual speed is a fraction of this rate calculated as a ratio of pressure (about 50% actually), in any event, a far cry from the 58,000 cm/sec that sound travels at.  Seen another way, the exhaust gases could have travelled an average of about 5”- 9" down the header pipe when the sound from the next cylinder hits them and forces them to pause.

Summary
CB750 pipes tuned to 6000 rpm
Single pipe dimensions 1 3/8" O.D. - 1 1/4" I.D up to 1 ½” O.D. – 1 3/8” I.D.
Actual Header Length = resonance frequency multiple of TL = 36 cm in our example
Collector diameter = 2 1/2” to 2 3/4”
Megaphone length = resonance frequency multiple of TL = 72 cm (for example)
Total exhaust system length = 108 cm (43”)

So there you have it, 4-stroke exhaust pipe design in a nutshell. Grin

[KingCustomCycles.com]

A picture is worth a thousand words. see post sbove 

[Grumpol]

but the thousand words telsl you what to do, the picture tells you very little

[goon 1492]

The pic does tell me Kewl, and WOW where did you find that info, sweet find!

[KingCustomCycles.com]

OK, so why do all high performance engines use little or no exhaust system at all ?  Everything beyond a 6" straight stack is a compromise for looks, EPA, safety, or some other issue.  2 stroke "tuned" exhaust are well documented.  Four stroke is all about no back pressure. Let the fur fly!

[goon 1492]

Actually I read that anything less than 12 inches will help cause back fire sucking back in oxygen, and learning on old leaded gas heads too that you need that some of that back pressure to help stop valve slap/ping, that was mainly needed because of taking the cush out when we went to unleaded gas. Some engines are designed to have some back pressure to help with the valves and also derive some torque from it helping push the other pistons that will be on the power stroke of their cycle. Did that come out right?
I do like to let the fur fly though! Drive it like you stole it!

[cafeconleche]

OK, so why do all high performance engines use little or no exhaust system at all ?  Everything beyond a 6" straight stack is a compromise for looks, EPA, safety, or some other issue.  2 stroke "tuned" exhaust are well documented.  Four stroke is all about no back pressure. Let the fur fly!

Does this mean the engine will run better if run with open pipes?

[scartail]

Does this mean the engine will run better if run with open pipes?

My understanding is that you want the exhaust to be sized (diameter) to the pulse size. If the pipe diameter is too big, then the pulses would richocet, and not move out the exhaust smoothly. If the pipe is too small, then it will choke flow. The pulse size targeted, is based on where you want the engine to be most efficient.

I know there is alot more to the science of exhaust tuning.

BTW: I'm scheming of making a 4-to-1 system for my 550K. I got an exhaust can for a R6 from CL for free. :-D So, to the OP... Keep an eye on CL, some people just want to get rid of their stock parts. My plan is to take the stock pipe and weld them into a mergecollector, then mod the R6 can to fit. Hopefully it works out.

[Porscheguy912]

Here is mine.
4-1 Its a thrush 2 1/2" ID X 23" long.
It sounds great. A louder than stock while riding but purrs like a kitten at idle.
Has a nice low howl.
Its actually not loud enough for me. I am thinking of making another one but shorter.
I was thinking half as short or maybe a larger diameter.

1 1/2" i would guess should be fine but it depends on how you want them to sound.
Also An H-pipe is not necessary if you route the correct cylinders together.
 

[goon 1492]

That looks pretty good porshguy912

[smccloud]

Here is mine.
4-1 Its a thrush 2 1/2" ID X 23" long.
It sounds great. A louder than stock while riding but purrs like a kitten at idle.
Has a nice low howl.
Its actually not loud enough for me. I am thinking of making another one but shorter.
I was thinking half as short or maybe a larger diameter.

1 1/2" i would guess should be fine but it depends on how you want them to sound.
Also An H-pipe is not necessary if you route the correct cylinders together.
 

i'll be doing 1-2 and 3-4, but i want the h pipe just because i can
maybe i'll get a couple cherry bombs that are 2" since thats the smallest they make.

[Porscheguy912]

That looks pretty good porshguy912

Thanks!
It looks like yours but with a aftermarket header.
Be careful with yours though, I think that the angle of exit of the stock header will put your cherry bomb interfering with things or else sticking really far out the side. Just a guess though> its been a while since i got rid of my stock header.

[goon 1492]

Yeah I took that pipe back it was a trush name brand and twice the diameter of the cherrybomb, I am putting on the stock 4 to 4 that it came with and taking a stab at the cherrybomb next fall...
mcloud look up cherrybomb website they have all kinds of sizes and legnths to choose from, I started a cherry bomb build thread but it went into hibernation until I get more ducks in a row..
http://www.cherrybomb.com/products/glasspack


Chrome tips for the glass packs.
http://www.cherrybomb.com/news/27