If I can find a way to channel the fumes off to the side with shorter pipes, is there any negative effects to this? If resonators are the answer I am concerned it would quite the car down to much. Also........ Now that I am thinking about it, right after I had my engine R&Red about 10 years ago my exhaust system was Shorty Headers, and 2.25 all the way back with a set of super turbos. No Drone! To answer a few of the questions. I'll try and peak my head under the car in the next few days to take a real good look at the exhaust mounts to see if they are in the factory spots and confirm that everything has some rubber between it. It is a full factory interior with extra sound deadening material in the trunk, in the cabin under the carpet and seats.
Put two turbos on it and it will sound like an electric car.. Sorry.. I guess that was no real help.. I think a real factory style muffler might work well like one of the Walkers that have been recommended on this board for years.. 2HP is really a small price..
Ok windows up... no fumes Windows down fumes are your friend at idle. It would look cool though The resos are your best bet with flowmasters and tail pipes.:beer
Davy, Just stumbled across this thread. Haven't been on the board for awhile. This isn't a new problem. The 2k drone has been a headache for years. Cars and trucks of every make and model. Gas and diesel. Doesn't matter. It's not the pipes or the mufflers. It's not the size of the exhaust or the way it is routed. It is caused when the engine is firing at 137 Hz (2000 rpm) which results in a impulse resonance through the exhaust system. The only way to eliminate it would be to install resonance chambers in the tail pipes. Try Googling 2000 rpm exhaust drone and you can read for the next week about this problem.
I don't think resonators would quiet your car too much. When I put them on my 3" exhaust they helped but it wasn't a big difference. Bob H.
WarHawg75 nailed it. I've had six different exhaust combinations on my boattail and all had this exact same problem at the exact same speed and RPM. The drone was so intense at 45 MPH that it would give me an instant headache, yet it was completely gone by say 52 MPH, or 37 MPH. And these were not necessarily loud systems, outside of the car. Among the decent sounding mufflers that I tested, the Edelbrock SDT suv muffler was the best because it is designed specifically to fight this very issue. But it was still too much for me. And a crossover between the two sides didn't help at all, H or X. The X actually made the problem slightly worse. I first thought it was a resonance in the exhaust system itself and I did battle with the issue on this basis for awhile. But I now think it is almost entirely caused by the shape of the car. The boattail portion couples the trunk quite well to the front of the passenger compartment, but only at a specific frequency. I put a speaker in the closed trunk connected to an audio frequency generator to prove this. This extremely resonant coupling has a very high quality factor, meaning that it occurs over a very narrow frequency range, but with a very high peak(at about 130Hz in this case). All cars have this to some extent, but nothing like a boattail because they don't have a boat shaped tail connecting the trunk to the passenger compartment. Removing the tailpipes prevents the sound from being introduced at the rear edge of the trunk, so nothing is coupled to the passenger compartment through the boat shaped tail. I also proved at night when no one would see me that you can run the pipes about two feet beyond the bumper to get rid of the problem. I took the extra pipes back off and blocked the trunk lid open some and drove down the road, and the resonance was much less peaked and more spread out to the point that it was acceptable. I spent a whole summer on this, not constantly but off and on. Long tube headers reduce the phenomenon quite a bit, but you will need to switch to a tubular transmission crossmember to run them. Shortys aren't any better than manifolds in this regard. The pipe diameter of the system, especially the tailpipes is crucial also. The smaller pipe doesn't carry the low tone as well. I didn't really want long tube headers on this car at the time, so my final solution was a 2.5" system with H pipe into Walker super turbos, then 2.25" tailpipes with glasspacks in the straight section behind the tires with turndowns right before the edge of the bumper. It's too quiet outside of the car though, but you have to really listen to hear the drone at 45 MPH. But as WarHawg said, your only choice is to run a system that better suppresses the problem frequency in the first place. The crossover makes it worse because it evens out the tone from side to side. Each side of a V8 does not produce a pure tone. At 2000 RPM you would have some energy at 130Hz, 65Hz and 43Hz. But when they are connected, especially with an X or through a Y pipe, it's all at 130Hz. You don't want that in this case because leaving the sides isolated reduces the 130Hz component significantly from each pipe. They'll still combine depending on how close they exit to each other, but not as much. Long tube headers also disturb the resonance in the exhaust system itself, so they help measureably. If I did it again, I would dump the crossover, go with long tube headers, then 2.5" to some Edelbrock SDT suv mufflers, so that hopefully I could get away with 2.5" tailpipes without resonators. You could also run the tailpipes out under the quarter panels behind the tires, like on a turbo Regal or a 70s Chevy truck. But I don't think that look belongs on a boattail so I never tried it.
David and Ryall are both on the right track. The drone sound is the result of the the exhaust sound wave frequency matching the vibration frequency of the vehicle. In the cycle of the engine there is a split second when the intake and exhaust valves are both opened slightly. This is when the exhaust pulse is momenterilly reversed causing the sound wave to bounce back towards the engine. The exiting sound waves colliding with the returning waves cause the resonance that drives people looney. Shortening the tail pipe and exiting before the rear wheels will not solve the problem. Removing the tail pipes will not solve the problem. What this does is changes the frequency of the resonance. The drone will still be there but at a different rpm. The only way to truly eliminate the drone is to interupt the sound wave collision in the exhaust system. Hence the resonance chamber.(not resonator) A resonance chamber on a exhaust system would be a length of tube or a small canister teed off the tail pipe and capped. This space would allow the sound wave an area to reflect back into the main pulse stream and interupt the harmonics that cause the drone.
After reading through this thread, I recall the Flowmaster 2.5" mandrel bent exhaust system (at least when I saw it years ago) placed the "H" pipe very close to where the mufflers attached, I had wondered if the extra volume downstream would help reduce the resonance. If changing the mass of the system (w tailpipes, w/o) moves the excitation point, perhaps changing the mass (natural frequency) of the system will help. i.e.: at a hanger location (over restrained, not enough etc...).
Excellent input, Bill. Last night I read about some DIY Helmholtz resonator jobs some people have done to solve the problem. Here are some that I thought were worth noting: http://www.moderncamaro.com/forum/v8-engine-discussion/29848-helmholtz-drone-solution.html http://forums.pelicanparts.com/pors.../432055-idea-rid-exhaust-drone-resonance.html http://www.planet-9.com/cayman-boxs...-drone-resonator-fix-design-construction.html Again, great tip and worth studying. I didn't realize the problem was so widespread and fundamental regardless of year/make/model. Devon
Good info! For those wanting to calculate engine firing frequency to compliment Devon's links, the formula is... engine rpm(x the number of cylinders firing in 1 revolution, 4 in this thread)/60. or (2000*4)/60 = 133.3 Great thread!
The thing I found with the reso teed into the tail pipe is folks that have done it say it made the exhaust too quiet
[QUOTE Again, great tip and worth studying. I didn't realize the problem was so widespread and fundamental regardless of year/make/model. [/QUOTE] Wow Devon. Being in the field, I am shocked that this is a new topic for you to tackle. This has been a motor vehicle problem for decades.:laugh: I believe it was a trucker complaint long before it surfaced in the hot rod world.
I guess I've been lucky to avoid it! On all the stuff I've worked on, the only time I had the issue was I had 2.5" duals w/two chamber flowmasters on my stock 430, no balance tube. When the system was upgraded the issue disappeared, fortunately. Devon
Not running tailpipes completely solved the problem in my case. I had dramatically changed the resonant frequency of the exhaust system itself so that it was at least not at the same frequency of the body resonance, and I was no longer energizing the resonance in the body at the right location either. There simply was no drone. I just didn't like the near asphyxiation that it caused with my Riviera being all rusted out. Alot of cars that aren't boattails have more drone without tailpipes though. If you run tailpipes, they just can't be loud at 130Hz. So you would have to at least eliminate the frequency from the exhaust at which the boattail passenger compartment resonates using the capped 1/4 wave stubs, or chambers that intersect in the tailpipes. It takes alot of trial and error that I didn't have time for, so I just made mine quiet enough and pretty much got rid of the drone that way.
This thread is awesome, lot's of good brain storming! Just to pile on, and back up what is being discussed, I was running open headers for a short time until I got my new exhaust on, and dang you could feel the drone in your chest. BUT, it was not a 2000 rpms, it was more like 1000-1500. With my x-pipe, 3" full exhaust, and SpinTech 6000's, it drones at idle. Go figure!
