I went for a drive during my lunch break. The Good: there is no drone while cruising at 45 mph and up. There is no drone when accelerating at 45 mph and above, or 1950+ RPM. Cruising at 50 and up sounds like the car isn’t even running. At least I can’t hear it over the wind and tire noise. The top is up and the gaskets leak so that is kinda noisy, but the exhaust is eerily silent, stealth mode! There is no 119hz drone which is the frequency that the 29.5” j-pipe is tuned to cancel. The Bad: there is drone at 108hz to 112hz (1400-1650 rpm approximately)if accelerating from 20 to 30mph, back off and cruise at 30-35 and it’s pretty quiet. While there is drone, it’s not so loud that it makes the whole car (and my brain) resonate. If I feather the pedal (think “fuel conservation”) I get minimal drone. If I punch it (think “I’m faster than you are”) I’m hearing exhaust growl and not in that drone zone at all and already breaking the speed limit or stopping for the next light or traffic. The Ugly: the current install leaves a few things to be desired. Some alignment, some better hangers, more isolation, a solution to keep the j-pipes supported, and making sure I don’t have any leaks. The Verdict: I’ll tidy things up and if anything tinker with the j-pipe length to see if I can lower the drone that’s still there around town, but... I do not want to do that and find drone happens again above 45mph because then road trips would be horrible. I think for the moment it’s as good as it gets and I’ll see if I can live with the occasional around town drone because it’s silent on the highway. I’m hopeful that some better isolation and mounts will reduce the around-town drone a little more because I think there’s body and fuel tank resonance happening.
She got a bath. There are many cosmetic and mechanical things to deal with but she looks good at 20 paces. The tailpipe extensions will be shortened a bit.
The fact that there is no upper limit to your quiet band tells me that you could tune it a little lower, meaning add length to your tube. That should be a very easy test compared to what you've gone through so far. Jim
Thanks Jim. I was thinking about adding length and seeing if I can reduce the around town drone. Especially after the work I did today. Last year the power top switch had burnt up in the “up” position in turn cooking the pump. New switch went in long ago, so today it hit 60° so I took a break from exhaust work to change the pump. Then I took a top down drive. The exhaust sound is a little loud but not intolerable. The drone is there and can be heard loud and clear. I’m close to having it where I want it so I’m not gonna sweat it too much. It’ll get there. My top works now! That means a lot to me and makes me happy.
The root of the whole drone problem is due to the fact that our V8s are actually two V4s in the same block. Exh pulses are then spaced at 90,180, 270,180 and 90 degrees. Two cylinders discharging at 90 degrees apart are heard as one large cylinder discharging. If where taking about headers the primary pipes then act as you would expect from a in line 4 cylinder motor, but the collector acts like it’s on a 3 cylinder motor, but with different size cylinders. If your hearing is sensitive in the rate/ frequency range the motor rpm spends the most time in and your running headers then a Try Y type of header can be made to phase cancel a good amount of the big exh pulse rumble. I am done with commenting on this post!
Thanks Steve. My 350 has stock manifolds. But thanks for the info on our firing order/pulses. there are some quarter-wave resonator calculators that use a pulse interval in the formula. I’ll see if I can use what you’ve shared to find out if there’s an even better jpipe length.
Doesn't sound like you've tried the 15" length and I don't know why it would work any better, just that the other guys I know who tried it had success with it. But they were running headers so that might make a difference too. Basically it's mostly trial and error to find what works at this point. The GM engineers had it sorted out though, the OEM rides back in the day didn't have the drone problem. Average Joe Dad would have never stood for it. It was mostly the change in mufflers that created the problem, and the stock muffs were pretty restrictive. They did have some complex baffling internally though and were quiet. Jim
I did some new math based on the last drone frequencies I was hearing. I put together a 32” pipe for each side, went for a drive, and ding ding we have a winner! 32” quarter-wave resonators have eliminated the resonant drone and leaves behind the exhaust sound you’d expect to hear across varying RPM and speeds. I didn’t get a chance to get on the highway, but I did get it warmed up, drove with the top down, and at 50mph all I heard was an exhaust rumble. There are exhaust notes being played but it isn’t droning as far as I can tell. Not at part throttle around town, accelerating, nowhere. Much more pleasant. The pipe length I had before left the around town part throttle drone and made the exhaust silent on the highway. I need to do some more driving but I do believe I could live with it this way.
I'm a firm believer in moderation in all things (including moderation) but I never understood how you could be middle of the road until you hit the limits. Consequently I tend to push the limits both ways until I know where they are before settling for something in the middle. So for me the logical next step would be to see how long the tube has to be before causing more loudness. I'm not suggesting you do this of course, it's just what I would probably do in your situation. My curiosity would insist. Jim
I feel ya Jim. I’m the same way. Sometimes I try to eke out just a little more because maybe it’ll be better, and then I ruin it. Lol. The good news is 32” works great. My wife and I drove all over today, local roads, highways, no drone. It’ll keep me satisfied for a while. I bought some metal I’m gonna use to make some brackets for supporting the j-pipes.
Good to hear! lol Is there any reason for the "J" or is it more to do with fitting them in (packaging)?
“J-pipe” is a reference to the shape most of them end up being because of packaging. The attachment to the exhaust pipe needs to be at 90° for it to work effectively, but after that many just have the j-pipe run parallel to the exhaust, but it can turn in whatever direction(s) it needs to to fit the right length for the drone frequency being cancelled. Just no sharp angles or the sound will bounce back out prematurely. Mandrel-bent smooth curves work. Some have gotten pretty creative with the shapes. If you do a Google search for images of j-pipes you’ll see a variety of solutions. All I did was tee off the main with a big 90° elbow and run the longer end along the frame, so mine is more like an L pipe.
