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302GT
Larry Shimp

(171 posts)

Registered:
11/17/2007 01:13PM

Main British Car:
1968 MGB GT Ford 302 crate engine

exhust modifications; resonator tube
Posted by: 302GT
Date: March 30, 2012 09:15AM

I wanted to get rid of drone while cruising. My engine turns about 2000 RPM at 70 MPH, which works out to about 130 Hz: (2000 RPM x 4 firing cylinders/revolution)/60 seconds/minute. The approximate wavelength of this sound at 70 mph is 96 inches. I applied this to a discussion I found on a Mustang web site about making a quarter wave closed side tube to set up standing oscillations to cancel out exhaust sound waves. The principle is that a closed-end side tube can be attached to the exhaust (it opens into the exhaust pipe) to provide standing waves that are 180 degrees out of phase with the exhaust waves. Being 180 degrees out of phase, the troughs in the standing wave will correspond to the peaks in the exhaust waves (and vice versa) which tends to cancel out the sound with no increase in back pressure. The side tube needs to be ¼ of the exhaust wave length because sound waves entering the tube will reflect off the closed end to give an effective path length of 1/2 the wave length, which is 180 degrees out of phase. So, for the 2000 RPM situation, the tube needs to be about 24 inches long. For maximum sound control, the closed tubes need to be the same diameter as the exhaust pipe. I built two rear exhaust pipes with side tubes to replace my resonators. My side tubes are only 22 inches long because I built the assemblies in NJ (where my tools and welder are) but my car is in WI (where I am living now) and I had to estimate what would fit. It turns out there is plenty of room to make longer tubes… Also, I used 2 inch tubing for the side tubes versus 2 1/4 inch tubing for the exhaust because of the tighter bends available in 2 inch tubing. Still, the tubes work remarkably well. There is some minor drone at 60 mph, but it is gone at 70 to 85 mph (and probably higher, but I have not driven that fast yet). Using fourth gear at 70 mph (3000 RPM) there is some slight noise, but nothing like it would be without the modifications. Noise under hard acceleration is also quieter over the whole range and seems somehow smoother. This is to be expected. While the tubes will give the most noise cancellation at a specific frequency, they will still provide some attenuation over a much wider range until the standing wave reaches the same frequency as the main exhaust frequency. So, to summarize, I removed my resonator glass packs and replaced them with open pipes, yet the sound is significantly decreased. Picture is attached.
exhaust picture.jpg


DC Townsend
David Townsend
Vermont
(406 posts)

Registered:
11/21/2007 12:22PM

Main British Car:
'78 B (almost done) 30-over SBF, dry sump

authors avatar
Re: exhust modifications; resonator tube
Posted by: DC Townsend
Date: March 30, 2012 09:56AM

Would be great to see some additional photos of the fabrication. I'd be interested in trying somethingsimilar on my BV8.


BlownMGB-V8
Jim Blackwood
9406 Gunpowder Rd., Florence, KY 41042
(5298 posts)

Registered:
10/23/2007 12:59PM

Main British Car:
1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS

authors avatar
Re: exhust modifications; resonator tube
Posted by: BlownMGB-V8
Date: March 30, 2012 10:01AM

Nice trick Larry. Wouldn't they be even more effective farther up the exhaust system?

I wonder about the frequency where they become a half wave trap though. Sounds like there is plenty of room for experimentation.

Jim


302GT
Larry Shimp

(171 posts)

Registered:
11/17/2007 01:13PM

Main British Car:
1968 MGB GT Ford 302 crate engine

Re: exhust modifications; resonator tube
Posted by: 302GT
Date: March 31, 2012 07:40AM

There some additional considerations to bring up.

My exhaust system has a crossover pipe; without it, each exhaust will see only 2 firing pulses per engine revolution, which would make the frequency too low (at 2000 rpm) for a practical resonator pipe length.

I put my resonator pipes at the end of the exhaust system because that was the only place where there was room. But it works at just about any point on the system, and it would be interesting to see what would happen if the resonator pipes were placed before the main mufflers.

There is some speculation that a chambered muffler (not a glass pack) can act as a sound reflector so that the pipe from the muffler to the end acts as an organ pipe with a tuned frequency appropriate for the pipe length. In small sports cars, this distance tends to be rather short (in my case 60 inches) so the resonate frequency would come in at a point well above cruise RPM. But if it is a problem, the resonator pipe can be made ¼ the length of this pipe section to kill any tendency for amplification at the natural pipe frequency.

I saw where one person incorporated small glass pack mufflers into the resonator pipes with the intention of making the resonators cover a wider range of frequencies. I am not sure if this will work; the muffler may just act to kill or weaken the standing wave.

The pipes are relatively easy to build and experimentation is always encouraged. I have included a picture of a pipe as removed from the car to give a better indication of the construction. Note that the upper (short pipe) has a closed end; this may not be clear in the photo.
exhaust picture 2.jpg


roverman
Art Gertz
Winchester, CA.
(2860 posts)

Registered:
04/24/2009 11:02AM

Main British Car:
74' Jensen Healy, 79 Huff. GT 1, 74 MGB Lotus 907,2L

Re: exhust modifications; resonator tube, where ?
Posted by: roverman
Date: March 31, 2012 02:39PM

If you dare, when elswhere is no room, perhaps roll cage or unit body tube structure ? In other words, a cap plate could be welded in, to tune the length ? A flex or swivel joint and heat insulation might be needed. Onward, roverman.


BlownMGB-V8
Jim Blackwood
9406 Gunpowder Rd., Florence, KY 41042
(5298 posts)

Registered:
10/23/2007 12:59PM

Main British Car:
1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS

authors avatar
Re: exhust modifications; resonator tube
Posted by: BlownMGB-V8
Date: April 07, 2012 09:16PM

What about a crossover that has a tuned stub somewhere close to the middle? Also, is the resonance a function of the distance traveled by the standing wave, or a function of the volume? If the latter, wouldn't a diameter increase effectively shorten the tube?

Jim


302GT
Larry Shimp

(171 posts)

Registered:
11/17/2007 01:13PM

Main British Car:
1968 MGB GT Ford 302 crate engine

Re: exhust modifications; resonator tube
Posted by: 302GT
Date: April 12, 2012 04:12PM

The resonance is a function of the length; while the intensity is a function of volume (diameter).Think of how a pipe organ works...

I received a private post from a Mustang owner who says he eliminated drone by installing different sized mufflers on each side of the dual exhaust system (note that all Mustangs are factory equipped with cross-over pipes)..



MGB-FV8
Jacques Mathieu
Alexandria, VA
(299 posts)

Registered:
09/11/2009 08:55PM

Main British Car:
1977 MGB Small Block Ford, 331 Stroker

Re: exhust modifications; resonator tube
Posted by: MGB-FV8
Date: January 03, 2013 09:55PM

This post has had my attention since Larry posted it. About two months ago, I had my neighbor's 2004 Honda, S2000 on my lift to help him change the oil; while the oil was draining, we've looked over the undercarriage. I was amazed at the high tech engineering of the car and while looking at the exhaust I observed two short pipes sticking out the side of each pipe running to the back (4 banger with dual exhaust). The pipes were about 6 inches in length and were mounted at about the center of the car.

If the pipes were connected together, you would think that it was an "H" pipe, however, the pipes stood well apart by themselves on each run and were capped. I concluded that they were exhaust tone tuners, not quite sure.

On my MGB built with a SBF 302 stroker (331), I plan in using two back mounted stainless mufflers with spiral technology (almost new); there's a weight saving and exhaust flow increase factor. Since I also plan in using two high-flow catalytic converters, I assume that it would tone the loudness down a bit.

I've purchased the pair used from a 2010 Mustang with a 5.0 Liter "Boss" modular engine. The Mustang owner stated that he wanted to sell them because they were too loud for his taste; based on his statement my question is; would a pipe (resonator tube) such as in Larry's posting, help in designing a desirable sound and loudness? your input would be appreciated.


MGB-FV8
Jacques Mathieu
Alexandria, VA
(299 posts)

Registered:
09/11/2009 08:55PM

Main British Car:
1977 MGB Small Block Ford, 331 Stroker

Re: exhust modifications; resonator tube
Posted by: MGB-FV8
Date: January 05, 2013 11:51AM

What, no input/comments? Larry, have you done more research on the subject? I would never have revisited the idea if I didn't see the Honda S2000 set up :)


BlownMGB-V8
Jim Blackwood
9406 Gunpowder Rd., Florence, KY 41042
(5298 posts)

Registered:
10/23/2007 12:59PM

Main British Car:
1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS

authors avatar
Re: exhust modifications; resonator tube
Posted by: BlownMGB-V8
Date: January 05, 2013 12:49PM

You might have to wait a few days for Larry to see it. Sounds like something worth experimenting with though.

Jim


MGB-FV8
Jacques Mathieu
Alexandria, VA
(299 posts)

Registered:
09/11/2009 08:55PM

Main British Car:
1977 MGB Small Block Ford, 331 Stroker

Re: exhust modifications; resonator tube
Posted by: MGB-FV8
Date: January 05, 2013 05:33PM

Thanks Jim; I hope that all is well with Larry.


IaTR6
Dennis Costello
Central Iowa
(104 posts)

Registered:
12/29/2007 02:53PM

Main British Car:
'73 TR 6

Re: exhust modifications; resonator tube
Posted by: IaTR6
Date: April 12, 2017 12:48PM

Just did a search, and found this discussion. I thought I remembered Larry Shimp doing something to his exhaust, and this is the article.
My 302 in my TR6 has separate "resonator" mufflers (I expect just a straight glasspak) just off the headers, with no H pipe. At the rear, the
two 2 1/2" exhaust pipes go into a two in, two out Magnaflow. The drone at 2200 is unpleasant, and would be easy to work around if it was only
a cruise problem, but it occurs in every gear as I work up to speed. I will discuss this idea with the muffler shop, and see what happens.
I also need to look into a chambered muffler.
On another note, I ran a data log on my Megasquirt, and running up in 2nd and 3rd, I see 19.5ms injector PW. Am I in need of bigger injectors?
Have not done the dyno tune yet, I wanted to have the exhaust worked out first.
Dennis


BlownMGB-V8
Jim Blackwood
9406 Gunpowder Rd., Florence, KY 41042
(5298 posts)

Registered:
10/23/2007 12:59PM

Main British Car:
1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS

authors avatar
Re: exhust modifications; resonator tube
Posted by: BlownMGB-V8
Date: April 12, 2017 03:19PM

Dennis, I believe you can go up to 25.4 ms on the injector pulse so at 19 you should be fine.

Jim


IaTR6
Dennis Costello
Central Iowa
(104 posts)

Registered:
12/29/2007 02:53PM

Main British Car:
'73 TR 6

Re: exhust modifications; resonator tube
Posted by: IaTR6
Date: August 13, 2017 08:14PM

Some of this conversion business takes a lot of time! I'm still displeased with the drone in my exhaust, but have space issues.
Jim mentioned resonator tube perpendicular to an H pipe. I'm thinking that having an H tube just at the back of the transmission, where the tubes are closest, with a resonator connected to both exhaust pipes might be a solution. Do you think the cavity resonance formula would result in a tube that would damp the resonant frequency?
Dennis


BlownMGB-V8
Jim Blackwood
9406 Gunpowder Rd., Florence, KY 41042
(5298 posts)

Registered:
10/23/2007 12:59PM

Main British Car:
1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS

authors avatar
Re: exhust modifications; resonator tube
Posted by: BlownMGB-V8
Date: August 14, 2017 11:26AM

Might be worth trying, but of course the pulses from each side do not coincide so a crossover probably would not have the same effect as a tuned cavity where the pulse reflects off the end wall.

On your crossover, it's generally considered a good idea to identify the hot spots in each tube and connect them. They may move depending on rpm.

Jim



88v8
Ivor Duarte
Gloucestershire UK
(498 posts)

Registered:
02/11/2010 04:29AM

Main British Car:
TR6 74 2.5 injection six

Re: exhust modifications; resonator tube
Posted by: 88v8
Date: August 14, 2017 02:07PM

On my 6-pot TR6, I had problems with drone/resonance. Ended up swapping out a twin system for a single system.

On my Land Rover Lightweight (215 V8) there was a good home-brew mild steel exhaust, loud but rusted until it fell off, then I had a custom stainless system that had horrible boom and became very loud when the glass blew out, then I changed to a glasspack Flowmaster that has no drone but sounds like a Gatling gun, now I'm going to a Hooker Aerochamber that I hope will give the boom without the bark. Single box, 14" long.
[www.jegs.com]

It can be a long journey, finding the ideal exhaust.

Ivor


IaTR6
Dennis Costello
Central Iowa
(104 posts)

Registered:
12/29/2007 02:53PM

Main British Car:
'73 TR 6

Re: exhust modifications; resonator tube
Posted by: IaTR6
Date: August 14, 2017 04:15PM

Regarding the crossover, what I'm thinking, is adding one and putting the resonance chamber "T'd" in the center of the crossover.
The resonator should still see 4 pulse per cycle, keeping the 1/4 wave short enough. My current install is separate pipes all
the way back to the Magnaflow "X" muffler in the rear. I was talked into this, and I'm not convinced it was ideal. I'd replace the whole thing, but it was pricey.
Dennis


BlownMGB-V8
Jim Blackwood
9406 Gunpowder Rd., Florence, KY 41042
(5298 posts)

Registered:
10/23/2007 12:59PM

Main British Car:
1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS

authors avatar
Re: exhust modifications; resonator tube
Posted by: BlownMGB-V8
Date: August 14, 2017 06:54PM

I think that might work.


302GT
Larry Shimp

(171 posts)

Registered:
11/17/2007 01:13PM

Main British Car:
1968 MGB GT Ford 302 crate engine

Re: exhust modifications; resonator tube
Posted by: 302GT
Date: September 22, 2017 08:47AM

I am still satisfied with my resonator tubes, but I am interested in how the crossover resonance tube will work.

By the way, I repacked my Magniflow mufflers several years ago. All of the original packing had blown out. I cut a round hole in one end with a hole saw and put in packing made for dirt bike mufflers; it had much longer strands than the original and so should last longer. I closed the hole with an electrical junction box blank off that I bought from McMaster Carr. This will make it easy to replace the packing again if needed.


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