Hi guys,

After discussing mufflers and exhausts with others on the forum. SR20Datsun and I would like to see who else can contribute to an online DIY muffler building and technical thread. I will put up some pics later to make it more interesting.

Basically, I'm on a quest to create the quietest 3'' straight through muffler for a (in my case a 1600) datsun. This isn't about ''putting up with the noise'' because i already do that, this is about ideas and development! Hopefully we can come up with something that the main manufactures don't offer.

To get things going, I have a S13 SR20DET motor. Initially I ran a 3.5'' dump pipe to a 3'' straight through exhaust with a high flow catalytic converter and a 3'' supercat muffler. All stainless.

I want to limit the annoying low frequency waves created as they are the ones that are most frustrating to me. I think they are below 200hz?

Over the years, I've necked the pipe size down with mild steel to 2.5'' from the gearbox crossmember to the rear to reduce noise. However, I've discovered that this does not eliminate the very low frequency waves that are present that a typical OEM chambered muffler reduces through reflective sound cancellation. Which is restrictive.

I've experimented with two different sized hotdogs/resonators. Both 4'' diameter and 3'' exhaust size. One 350mm, the other 600mm long (very tight fit).

I'm now using a Hi-tech muffler (packed with 2kg of long stand fibreglass). I then cut the guts out of it and made the perforated tube into a pronounced 's' shape to slow the gases down and try and absorb some more noise.
I also changed from a 3'' to 2.5'' tip.

My findings:
The 2.5'' center pipe lowered the overall volume but the low frequencies still remain and dominate the interior volume. Cruising seems ok.
Both the hotdogs removed more high frequency waves and gave the exhaust note a 'deeper sound'. Could be just my hearing but this appeared to amplify the lower frequencies.
Changing to a 2.5'' tip reduced the lower frequencies a bit and made the exhaust sound more 'throaty' (which was done at the same time as adding the 's' into the muffler).

I've heard that rockwool packing is better at absorbing sound because it is denser, but also has a 'break in period' because it also takes on water and loses its effectiveness.

A 1/4 wavelength resonator will cancel out a particular frequency and a bit either side, unfortunately the length is too long to place it on the car for the given frequency.

I've also looked at the HKS Super Turbo's slotted tail design (and i really line the sound of this muffler anyway but it's too big for the space available) which they claim eliminates lower frequencies though equalizing sound pressure though the slotted tip. However their tips on this muffler are 5'' in diameter.

Twin tip mufflers seem to be quieter also.

I am considering making the following:
Make a muffler like a twin tip, that divorces one main pipe into two 2'' pipes within the muffler case then join them back to a single outlet. This will equal an area slightly smaller than a 3'' for flow, but will hopefully double the sound absorptive abilities and having a smaller pipe size will also lower the lower frequencies. Then wrap with ss wool and pack it with rockwool insulation. Thoughts?

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Ongoing Project: 1972 Datsun 510 S13 SR20Det. http://ozdat.com/forum/viewtopic.php?f=33&t=17898
New Daily: D22 Navara (The new workhorse)
Retired 12/2016: MY98 Subaru Impreza RX
Previous Car: Restored Green 1972 Datsun 510, Hot L18

supertrapp using reverse cone inserts rather than discs much like the tip of a bike muffler cone shape
but many stacked inside the opposite direction for a loud speaker trumpet etc.

Have you got sound proofing on your fuel tank? As this can push a lot of noise through the car

Not on the fuel tank. But underneath it and along the boot and all over the floor of the car.

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Ongoing Project: 1972 Datsun 510 S13 SR20Det. http://ozdat.com/forum/viewtopic.php?f=33&t=17898
New Daily: D22 Navara (The new workhorse)
Retired 12/2016: MY98 Subaru Impreza RX
Previous Car: Restored Green 1972 Datsun 510, Hot L18

Interesting! I wonder if there is one i could see the inside of?

_________________
Ongoing Project: 1972 Datsun 510 S13 SR20Det. http://ozdat.com/forum/viewtopic.php?f=33&t=17898
New Daily: D22 Navara (The new workhorse)
Retired 12/2016: MY98 Subaru Impreza RX
Previous Car: Restored Green 1972 Datsun 510, Hot L18

Tried to make something similar to the HKS slotted tail design using 3.5" pipe and slots in the tail pipe at about 3mm diameter, flared to match the 3.5" pipe. Made no difference. I'll cross that idea off the list!

_________________
Ongoing Project: 1972 Datsun 510 S13 SR20Det. http://ozdat.com/forum/viewtopic.php?f=33&t=17898
New Daily: D22 Navara (The new workhorse)
Retired 12/2016: MY98 Subaru Impreza RX
Previous Car: Restored Green 1972 Datsun 510, Hot L18

I'm running a 3" dump (bell mouth off the back of the turbo) in stainless steel down to a high flow catalytic converter.
From the cat it then passes to a large 4" OD, 3" ID resonator/hot dog and then through the cross member into a 3" magnaflow out the end.
I've tried numerous bits and pieces including necking the internals down to 2.5" and packing sections with steel wool.

One of the things that I was going to try was to make an interference style system where the resonator goes, as that is the only area where there is enough room! It would be better if it was at the back of the car.

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1972 Datsun 1600, S14 SR20DET Engineered (204rwkW @ 17psi.)
viewtopic.php?t=6579
#SR20Datsun @SR20Datsun

I've got 3inch to the rear crossmember (and a old hot dog which is probably doing nothing) and then 2inch from the crossmember to the rear bumper.
Sound is relatively quiet and unobtrusive at idle and low RPM, no drone or anything annoying at cruise, and AWSOME sound at high RPM.

Obviously 2inch wont suit everyone, but I've only got a stock standard SR20 red top with original turbo.

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'71 Datsun 1600, SR20DET red-top
Mostly OK, needs typical rust and panel work.

So my thoughts is the 2" exit actually improves the efficiency of 'sound reduction' for the hot dog.
A small amount of back pressure increases this efficiency.
I originally thought a 2.5" constriction would do it and when trying to pass the sound emissions my exhaust was originally above the legal limit. The engineer at the time wanted this rectified, so I made up a small 3" to 2.5" reducer and filled it with stainless steel scourer. This was inserted inside the 3". Low and behold, it worked and I was below the required limit.

On the dyno there was no difference in power output.

It's just not getting rid of the lower drone sounds.

My current plan of attack will be based on vibration in the cabin, and mounting of the system, followed by modification to the exhaust efficiency.
I currently have the exhaust supported at the gearbox near the cross member (bolted onto the gearbox via a custom mount).
From there it passes through the cross member (being held in a typical factory method with a rubber isolator) and then to the rear muffler which is held in place by two rubber exhaust hangars. I have tried thinning these down to minimise vibration transfer, but it didn't make that much difference.
I was always interested in reducing the air going out of the rear ventilation vents, thinking this would reduce sound wave propagation, but never got around to implementation.

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1972 Datsun 1600, S14 SR20DET Engineered (204rwkW @ 17psi.)
viewtopic.php?t=6579
#SR20Datsun @SR20Datsun

Mitsubishi Evo Muffler.
Internal flap that changes whether you have heaps of gas flow or not. It's a smart design even without the flap.

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1972 Datsun 1600, S14 SR20DET Engineered (204rwkW @ 17psi.)
viewtopic.php?t=6579
#SR20Datsun @SR20Datsun

Have you looked into Flowmaster mufflers? I'm considering using one of these (pending size limits under the body) for the Celica. Their range splits up from mild/stock to straight through loud mufflers. My experience with mufflers is that the larger the box the better and reverse flows and small diameters seem to work the most effective for low resonance.

Without plugging something I haven't used yet - it seems the Flowmaster mufflers are designed so that the exhaust flow doesn't change overall direction - as d mentioned earlier inverted cones etc.

This is what I intend to use but they're reasonably large: http://www.flowmastermufflers.com/super-50-delta/

While it may not be your solution I hope it helps. Also there is a video on the Youtubes "No more exhaust myths" which has a bit of flow-related content and simulated flow tests.

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910 S3 Bluebird - FJ20ET, E85, IRS, 5 Speed, 4.4 LSD
910 S3 TRX - RB25DET, Coilovers, S13 Front end, Power Steering
RA23 Celica - 18RG, Twin 40mm Webers, Coilovers, rolling on SSR mesh 15x7 0s.

Just in relation to the above. I've had mine braced off the gearbox x member and unsupported. Also off the chassis tail Baz style and unsupported.
Basically I've had the system fully supported and unsupported and in every configuration in between.
I think it's better when it's supported off the gearbox mount.

I've also stuffed foam inside the rear vents to try and block the noise. Did not work.

Perhaps something of a hybrid type design might work? Like this perhaps? With sound absorbing packing around the two outer ends of the perforated tube? I updated my above post to show the type I was thinking.

_________________
Ongoing Project: 1972 Datsun 510 S13 SR20Det. http://ozdat.com/forum/viewtopic.php?f=33&t=17898
New Daily: D22 Navara (The new workhorse)
Retired 12/2016: MY98 Subaru Impreza RX
Previous Car: Restored Green 1972 Datsun 510, Hot L18

Wonder if you could fit a twin loop muffler under there. Just reuses the one muffler twice

I have a twin tip 2 1/4" straight through. pretty noisy for my liking after changing all the exhaust recently.

after the butterfly trial fail, I tried covering one outlet as they are 1 3/4" each inside. big difference, 5dB!. it looks ugly as but obviously smaller means quieter.
thoughts now are to make 2 straight 1 3/4" tubes inside muffler and have an electronic butterfly on one inlet of the muffler that opens at 3k or 4k rpm perhaps so quiet most of the time through one tube but then opens both for all the power high revs should yield, which perhaps will be still quieter than now.

also it has seemed to make it more driveable at low rpm, but that's off topic...

My single tip 2 1/4'' straight through with a hotdog on my old l18 powered datto was reasonably quiet. Got a bit noisy on long trips but was bearable. It had a hot cam in it also.
Electronic silencing would work great. Unfortunately it's not legal for me!

_________________
Ongoing Project: 1972 Datsun 510 S13 SR20Det. http://ozdat.com/forum/viewtopic.php?f=33&t=17898
New Daily: D22 Navara (The new workhorse)
Retired 12/2016: MY98 Subaru Impreza RX
Previous Car: Restored Green 1972 Datsun 510, Hot L18