that DRONE is the result of the engine harmonics and the exhaust length having a similar frequency at that rpm, changing the frequency will change where in the rpm range the drone potentially occures
adding a few inches or subtracting a few inches can make a noticable differance, something as simply as adding a 6" chrome exhaust pipe tip to the existing tail pipe can make a differance, but to maximize the results youll need to design the system harmonics up into an rpm range above or below the average opperating rpm range
in many cases the (DRONE) is a SYMPATHETIC HARMONIC RESONANCE FREQUENCY that feeds from the engine exhaust pulses at a specific rpm frequency,and is amplified by the muffler design. on most corvettes it falls in the 1600rpm-2200rpm band, above and below its noticably lower,as the exhaust won,t vibrate at a matching level, if you modify the pulse frequency by adding an (X) as close to the headers as it can fit and an (H)pipe, just before the muffler enterance with the tail pipe extending past the rear bumper theres a noticable reduction in that (DRONE) WHAT ID DO FIRST AS IT HELPS CHANGE THE FLOW AND REDUCE RESTRICTION because youve altered the frequency reaching the muffler, high enough that its not entered untill much higher rpms are reached than normal street driving uses. you can also break up the pulse strength by adding these inside the collectors but INSTALLING the (X) which you have alreadyand (H) pipe must still be used for the full effect which you don,t appear to have
now keep in mind minor changes in exhaust length or cross flow or breaking the pulse strength, can make noticable improvements, and about any changes move the freq level in relation to the RPM adding 2" to the exhaust tip can even effect the harmonics, but realize that the effective length is from the back of the exhaust valves to the center of the (X) not all the way to the tail pipe. 1800rpms equates to an exhaust freq wave length of about 4.7" so a 2" change in effective length should be noticable,example placing the (H) a mulitple of 4.7" would have little effect, placing it at a mulitple of 4.7" plus 2" would have a noticable effect
http://www.spiralturbobaffles.com/
http://www.secureperformanceorder.c...tore/vortex.cfm
Exhaust Drone
Submitted by: Richard Kwarciany
Drone in the exhaust is due to the engine driving the exhaust system at its resonant frequency. You can't get rid of it, any length of pipe has a natural frequency, but you can change the frequency (RPM) it happens at. Stock exhaust systems are tuned to put the resonant frequencies outside the normal RPM range the engine is run in. Sometimes they add resonators for this purpose. To raise the resonant frequency of a system, shorten its length. To do this, you can try tail pipes that exit by the wheel (like GN's have), or you can try a muffler with a shorter internal flow path. If you have a true dual system, you can change the natural frequency of the system dramatically by adding a balance tube. If you can't shorten the system, then you can add slightly less than one wavelength of pipe. This will also raise the resonant frequency of the system. The wavelength in a single converter system (not true dual) is a little less than three feet at 3000 RPM. Shortening the system by a foot or so will move a resonance at 2000 RPM to about 3000 RPM.
To move a resonance, calculate the wavelength at the RPM that gives the resonance you want to move. Then calculate the wavelength at the RPM you would rather it be at. The difference is the length of pipe you need to add or delete. Add to lower the RPM, delete to raise it. If you want to raise it but you can't cut any pipe out, then add one wavelength of pipe minus the amount you calculated. This will have the same affect. If you add or subtract a multiple of a wavelength exactly, you will not change the resonant RPM.
Wavelength = 1100 X 60 X 1/RPM X 1/4 X 1/2
or
Wavelength = 8250 / RPM
Wavelength = standing wavelength
1100 = speed of sound in air in feet per second
60 = convert RPM to Revs per second
RPM = RPM
1/4 = four cylender firings per revolution (make this 1/2 for "true dual")
1/2 = standing wavelength is half the wavelength of a "normal" wave
NO ITS NOT A CURE BUT IT HELPS MELLOW the TONE SLIGHTLY..........you may also want to consider a layer of sound/heat reflecting material
heres an old post that may help, YES you NEED TO READ ALL THE SUB LINKS and USE THE CALCULATORS...and the collector size predicted is always too large in dia. try for a collector size of about twice to two and one half the sq inch area of the primairy,tubes ...if the primairys are 1.65" with a 2.14 sq inch cross sectional area, thats NOT a 3" collector with its 7 sq inch area its a 2.25" to 2.5" collector a 2.5" collector with its 4"-5" sq inch area
http://victorylibrary.com/mopar/header-tech-c.htm
http://www.majer-tech.com.pl/Exaust_Theory.htm
http://www.btinternet.com/~mezporting/exhaust_length.html
http://www.racerpartswholesale.com/flomstr1e.htm
http://store.summitracing.com/product.asp?d=8&s=127&p=608&searchtype=ecat&l=FLO
http://www.headersbyed.com/chevsbv8.htm
http://www.stainlessworks.net/Header%20Flanges.htm
http://www.stahlheaders.com/Frame%20Flanges.htm
http://www.headersbyed.com/hdrkits.htm
http://www.cachassisworks.com/header.htm
http://www.mazdatrix.com/fabricators.htm
Ive tried to stay out of this but I might as well jump in and get involvedIve done this many times its not that hard. heres what I do, get some of that plastic smurf tubeing in the 2" dia size and some ceiling hanger wire and some of that hard set constuction insulation foam. now weld 1" stubs of the exhaust tube to the header flange exhaust ports and bolt it to the cylinder heads. clamp the collectors to a 6 foot section of 2x6" wood and jack it up solid under the car (collectors not touching the car anywhere)to position them where you want them under the car, now cut (8) sections of smurf tubeing too about 36"-42" long(theres a formula to figure the exact length)slide 4-6 pieces of ceiling suspension wire in each tube with the ends looped over inside the tube. now startin with the upper inside collector position and the rear exhaust port, bend and fit the smurf tubeing to fit, the next forward exhaust port goes to the lower inside collector port the next exhaust to the upper outside collector port and the furthest forward to the lower outside collector port , once they are all bent to fit shoot the tubes full of hardening construction foam, let it harden and then pull each individual tube off one at a time and duplicate it in steel tubeing,(or have your local muffler shop duplicate it) this method REALLY makes the fit and try time minimal. and assures equal length tube headers.SMURF TUBE is flexable plastic electrical conduit thats normally BLUE or ORANGE and COMMONLY called smurf tubeing by contractors its a plastic version of that metal GREENFIELD tubeing that electrical contractors use but its cheaper and easier to work with, it resembles a canister type vacuum cleaner pickup hose but stiffer, its available at big hardware stores,and electrical supply houses dirt cheap in 10' lengths about $6 each or les you will need (3)BTW the 4-6 wires act like rebar in concrete, the loops keep the wires from moveing in the foam while their incased in the construction foam, the hard plastic foam is what keeps it stiff and no it will not be exact you will still need to tweak it to get it to fit but it will speed up the process of making the tube pattern shapes.
just keep in mind that you can buy headers fairly cheaply (under $300 in many cases) for most cars its when you go and get an odd ball combo this comes in handy, like putting a 502bbc in a 57 vette or a 392 hemi in a 63 falcon, or you need headers for a LS1 in you 67 camaroHERES INFO YOU CAN USE
http://www.headersbyed.com/hdrkits.htm<P><A http://www.ssheaders.com/header.htm" http://www.ssheaders.com/header.htm
heres a quick way to figure your true primairy tube length (use 1250f degs(950k) and 5000rpm for race cars, 1200 degs(922K) and 3000rpm for street cars ) if you want other temps look here
http://www.convertalot.com/downloads.html
heres a quick cross check
http://www.btinternet.com/~mezporting/exhaust_length.html
http://www.slowgt.com/Calc2.htm#Header
http://www.prestage.com/carmath/calc_headerlength.asp
or if your really lazy
http://victorylibrary.com/mopar/header-tech-c.htm
and the answers in meters (about39"=1 meter)
but if you just use 36"-40" it will just move your tuned rpm level up or down slightly, not enough to make much differance except on a very peaky engine combo in a very light car.and this assumes your tube dia. is the same size as your exhaust port size.(never smaller, slightly larger dia. is ok)
btw mild steel is much easier to work with than stainless
http://www.racerpartswholesale.com/flomstr1e.htm
http://www.holley.com/HiOctn/ProdLine/Products/ES/ESH/WeldUp.html
the pipes should be slightly oversized rather than so small that they partially block the exhaust port.
a pipe dia. that is small enought to partly block the exhaust port can hurt the cylinder scavageing by causeing turbulance and restrictions at the port exit, while a primary tube size slightly larger than the port leaves a ledge that partly retards returning pressure waves from reduceing the scavageing effect
,...USE THE CALCULATOR PROGRAMS THATS WHAT THEY ARE THERE FORand keep in mind the displacement and rpm levels effect the dia. and length of both the primaries and collectors, these MUST VARY IN DIA. and LENGTH TO MATCH THE ENGINES EXHAUST PULSE TIMEING so that a NEGATIVE PRESSURE WAVE WILL BE CORRECTLY TIMED to SCAVAGE THE CYLINDERS EFFICIENTLY at the desired rpm range
if you want other temps look here http://www.convertalot.com/downloads.html heres a quick cross check http://www.btinternet.com/~mezporting/exhaust_length.html http://www.slowgt.com/Calc2.htm#Header (
(btw its not((Degrees ATDC)they mean subtract the BBDC from 180 degs. then use the answer and the rpm level should be picked for the expected torque peak http://www.prestage.com/carmath/calc_headerlength.asp [
or if your really lazy http://victorylibrary.com/mopar/header-tech-c.htm
and the answers in meters (about 39"=1 meter
example , my 383 vette has a cam with exhaust cam timing that opens at 83degs bbdc, thats 97 degs atdc, http://dab7.cranecams.com/SpecCard/DisplayCatalogCard.asp?PN=119661&B1=Display+Card
Bore: (Inches) 4.03"<BR>Exhaust Valve Opening Point: (Degrees ATDC) 97 degs
Peak Power RPM: 5500rpm Calculated information appears below
Header Pipe Diameter: (Inches) 1.84"<
Header Pipe Length: (Inches) 37.65
Collector Diameter: (Inches) 3.5 (according to the calcs but use a 2.5" dumping into a 3" exhaust,where the collector extends a few inches inside the 3" exhaust, the step helps block reversion pulses and help scavaging
Collector Length: (Inches) 18.82
