Exhaust/sound engineer types - resonance question

So I added a cheapo glass pack from Summit racing to the NB (prior to, it had a only exhaust tubing from the stock resonator back), and it's got a fair amount of resonance at 3500 to about 5000 at part throttle. But, mellow at light throttle and an angry bumble bee at WOT (I love it).

I've put several glass packs on various cars and never had a resonance issue at all. Is it possible that part of my issue is related to some differing exhaust tube diameters (as opposed soley to the cheap glass pack)?

Disclaimer: The description below is the result of poor planning and/or surprise.

From stock resonator (which is behind the cat), 2.125 OD mandrel bends to just behind the axle, flanged to a 2.25" OD mandrel bend in to 2.25" ID glass pack, out to 2.25 OD mandrel bend to 2.5" OD tail pipe.

Would it help at all to just run 2.5 OD out from the glass pack? Or makes no difference, scratch the whole thing and buy an FM exhaust (which, I'm cheap, so I don't like the idea per se - besides, there's other FM stuff I intend to grab).

The original intent was run 2.125 OD from the stock resonator back through a muffler with a 2.5 OD tail pipe (all stuff I had lying around). That was too darned quiet, so I went to the local hot rod shop for tubing. Only 2.25 OD or 2.5 OD available. So I chose 2.25, got a mandrel bend and tubing and figured I'd run that out to my 2.5 tip. Ordered the 2.25 glass pack from Summit, which, as it turns out, was 2.25 ID. Ooops.

Yes, I am a member of the Keystone law enforcement community.

You need to build one of these, somebody on here will hopefully chime in on how to do the math for best performance, as I can't find it at the moment.

https://www.google.com/search?q=exhaust+drone+fix&rlz=1CAACAC_enUS629US631&espv=2&biw=1366&bih=701&tbm=isch&tbo=u&source=univ&sa=X&ei=k0L-VLKVOYSSyASsoYGQBQ&sqi=2&ved=0CB0QsAQ

How far away from the engine is it? The further from the engine, the more you will get a resonance.

A good resonance killer is a restrictor orifice in the end of the tailpipe. At the end, the exhaust gases are relatively cool and dense so you really don't need 2.5" or whatever. I had a 1 7/8" restrictor at the end of a 2.5" exhaust on a ~215hp nonturbo rotary and could not measure a difference in power. Very large difference in noise.

In reply to Kenny_McCormic - I have too much fun building an exhaust on the ground, and that contraption, no matter what power it would produce, would cause me to jump off a cliff. Great stuff, but not for me.

In reply to Knurled - it's in the "stock" muffler location, or pretty close; there's about 16 or so inches from the end of the tail pipe back to the glass pack.

That contraption isn't for making power, it's a resonance killer.

I saw someone do it for a 928 on corner-carvers. They had an app for their smartphone that charted frequencies, so they drove it at the engine speeds where the drone was worst, used the app to find the sound frequency of the drone (would be easy to see, big spike on the frequency graph), and some math happened to find the appropriate length of pipe to make a kind of Helmholz resonator in that exact frequency. Or maybe the exact anti-frequency. At any rate, they calculated the length of pipe, stuck a closed ended pipe onto the exhaust system, and reported heaps of success. Raging success. Drone eliminated.

Knurled wrote: That contraption isn't for making power, it's a resonance killer. I saw someone do it for a 928 on corner-carvers. They had an app for their smartphone that charted frequencies, so they drove it at the engine speeds where the drone was worst, used the app to find the sound frequency of the drone (would be easy to see, big spike on the frequency graph), and some math happened to find the appropriate length of pipe to make a kind of Helmholz resonator in that exact frequency. Or maybe the exact anti-frequency. At any rate, they calculated the length of pipe, stuck a closed ended pipe onto the exhaust system, and reported heaps of success. Raging success. Drone eliminated.

I did that with my SE-R. Results were mixed at best, but it was occasionally quieter.

Why not try a smaller tail pipe and see what it does? It's easy to do and you can always go back to the current pipe. Not that it help on a single exhaust system but I know that Ford used different size mufflers on most Fox Mustang V8's to reduce the drone level. It was there but each muffler had it at a different rpm so they didn't work together to increase the sound level. I found all this out after I put an aftermarket catback exhaust system on my '92. This system used the same size mufflers and the drone was worst at 50 mph. I was lucky since I had a non-stock R&P (3.55) so the critical speed was lower then if I had OEM ratios. Who drives 50 MPH? Your either higher or lower.

Exactly. 2.5" tip means probably 2.5" ID, yes? So get the smallest reducer that has a 2.5" OD that you can find, squish it a LITTLE so that it's a tap fit into the pipe, then hammer it into the pipe so the reducer part is pointing forward and go for a drive.

Worst thing that happens is you have to remove it again.

What ends up happening is that the tail pipe ends up becoming a muffler in and of itself, and a bunch of the resonance gets lost/confused by the reflections coming from the restrictor endcap.

I think. This is theorizing based off of empirical data.

Ah, resonance killer, got it. Didn't put the two posts together in the head, and notice that those pipes were capped. But I still don't want to weld up a contraption like that.

Knurled, tip is OD, but I will give the reducer a shot, see what it does. The local O'Reilly has a whole bunch of them, all kinds of sizes. If the resonance was reduced a little bit (maybe 25%), it would be tolerable.

Years ago I used a cheap muffler on one of my Z-cars, and the resonance about as loud as the NB here, but was only there at about 2500rpm, and it was easy to adjust my speed and gear selection to avoid it.

I can do that some with the NB, but that resonance range is fairly wide and unless I really go easy on the gas pedal, I'm in that range often.

Could you replace the glasspack with a cheap "turbo" muffler? That would get rid of the resonance as you'd have an absorptive stock resonator and a reactive muffler:

Cheap turbo muffler also works, they flow better than most OE units, basically being an oe type muffler with no packing, and sound nice if you like glasspacks.

The drone frequency range is typically 70Hz to 180Hz. Glass packs due to the way they are built do not really attenuate that range, unless it is really long. Baffle type mufflers work much better at attenuating low frequencies.

The biggest problem I see with your exhaust system is that there are step-ups in diameter. This is bad for performance as the exhaust velocity goes down with every step-up. Many OEM exhausts actually have step-downs in diameter to boost the exhaust velocity.

An actual turbo makes a good muffler

I went cheap as I wanted to know what it would sound like; turns out I really like the way it sounds.

If the "turbo" style muffler will give the same kind of sound at WOT I'll throw one on there at some point. I'm not opposed to a quality muffler, just didn't want to spend $80 on one and find out I hated how it sounded.

Since I hate trying to mock up the exhaust on the ground, I'm going to take a break for two or three weeks. I'll report back with results.

Add in another GlassPack in-line with the othe one?

ebonyandivory wrote: Add in another GlassPack in-line with the othe one?

That will mostly just attenuate the sound further without doing much of anything about the drone.

In reply to MCarp22:

I that true if one is long and one is short? Maybe it's just diameter that makes the difference?

ebonyandivory wrote: In reply to MCarp22: I that true if one is long and one is short? Maybe it's just diameter that makes the difference?

I'm sure the properties of an absorptive muffler change slightly with length / diameter in ways that I haven't researched. That being said, even 3-4 of them together are still going to be bad at attenuating the peak in the 50-200hz range that we percieve as drone on a 4cyl.

no drone problem here … just too many dB's

'91 CRX, CAI, DC Sport header … formally it had a cat and a resonator and then a 2" straight pipe … always passed sound anywhere …

finally the in pipe to the resonator and the resonator itself rusted out … replaced with a Cherry Bomb and a 2 1/2" straight pipe (cat still there) it's now WAY louder than before .. to the point where I'm border line to failing sound …

as much as I disliked the sound of the turbine muffler I put on my Integra, I'm thinking that I might have to replace the Cherry Bomb (with something)

Minor update - on a whim, I stopped by the local auto parts store and picked up a "cheap" resonator tip ($25! Stainless steel has something to do with that....) and put it on (bolt on type).

The tip did reduce resonance drone (eminence front? It's a put on...), I'd say by about 85-90%.

Not sure why or how, just being at the very end of the system, the smart ones can tell us, but it did make a purposeful difference.

The one I got was ugly, so I took it off again, but I now know the path forward.

Curious, what does this resonator tip look like?

Generally making sure the exhaust exits past the back of the car helps quite a bit with resonance and noise.

Another trick is to aim the exhaust to the ground to help reflect and disperse the noise.

This is a dual tip, but the single tip I picked up had the same baffling/mesh inside of it - I think this particular one (Hedman?) is also glass packed.

The difference/sound abatement was fairly dramatic with it on there.

Thanks!