Free-flow Exhaust and Drone Elimination

[Bugjuic]

So after 220k km of driving and the odd bash on the exhaust system(which is very low lying on the V6), I realized I had some leaks and decided to have a look at it, since I have hit the silencer(Muffler) numerous times and then started resembling a konsertina.
Thus I sent my vehicle to Tignique in Pretoria west. First have to mention what amazing service I got from Marco Ferreira, who is also a qualified Mechanical engineer and REALLY understands pipes and flow etc... Was a pleasure having him work on my vehicle which stood out as a sore thumb, as he does a lot of racing cars as well.

After going through the system, he politely told me, it would be a better option to rather re-do the exhaust, as he had found more than 12 cracks in the pipe, and the silencer was not in a good condition.

Fast forward a few weeks. Time to get the new exhaust installed, stainless all the way, but keeping the original branches, and free-flow thereafter with resonators.

Here a picture can be seen of wrapping the exhaust manifolds with some special magic, keeping a lot of the heat away from the engine bay. Also they decided to put some special imported heat reflectors on, which really seem to do the job extremely well!

40 plus hours of labour later with no bend and pie-cuts only, purged all the way through the exhaust was now replaced and cat foolers installed. The 1GR-FE 4.0 V6 engines have 4 catalytic converters, one after each exhaust manifold and two in line before the silencer(muffler). These have already started to clog up, and the one was hit so hard (lying so low) that the cat, de-cated itself and burnt out the exhaust .

Here we can see the finished product before installation. This would have two resonators after each exhaust manifold, and one larger one where the silencer was. The beauty of this system is that the whole exhaust system is now about two inches higher than it was, but with a larger diameter pipe (2 - 2 ½ inches pipe). So no more scraping the exhaust and one less worry.

Now installed in the vehicle, looking absolutely great and all out the way!

Was a bit loud to my liking, but soon rectified by Tiqnique with a larger resonator at the back, but still wayyyy smaller than the silencer that was on there.

Took a trip to the freestate, and WOW! Increase of 1.2km/l on the open road and could honestly feel a difference in power and responsiveness!

Just one thing I never took quite into consideration. Part two:

[Bugjuic]

Part Two: Resonance:

Being a N/A engine and eliminating the silencer (Muffler) I now had a resonance at 2800RPM as well as 1400 RPM.

So here comes some theory (Manny you are now welcome to stop reading, this might hurt your brain.)
At 2800RPM, you divide that with 60 to get seconds, thus 46.667RPS(Revolutions per second). And having a 6Cylinder 4 Stroke motor, there will be three pulses per revolution (6Cylinders divided by 2). Hence your frequency at 2800RPM will be 46.667x3=140Hz.

This was also verified by a free app when driving and hearing the resonance(Drone) frequency.

Will post some of the calculations just now.

Now this will have an effect on 1400RPM and 5600RPM and these are multiples of this resonance.

So not wanting to give up my freeflow system and install a Muffler. I decided to go along with theory and build a J-Pipe with the help of the internet. Which has a lot of conflicting information, hence my write-up to confuse people even more. The J-Pipe is also part of a Helmsholt resonator elimination.

Now if we look at a standard Muffler:

There are sound waves being thrown around everywhere bouncing away, eliminating each other, being pushed through various size cavities etc, basically disturbing most amounts of frequencies which are being produced and thereby muffling the sound but also creating restrictions and back pressure, which many people argue is needed, but never in n N/A motor in my opinion. So overall very good system, but very large and very restrictive.

Here we have a schematic of a resonator, which is still a free flow but dampens the sound through sound insulation but does not restrict any flow, but does not help with the vehicle eliminating drone noise.

So I decided to build a J-Pipe, which targets the drone but keeping my free-flow.

So here is some theory again.

When the drone starts(in my instance 140Hz) I want to cancel it out using a 180 Degree offset. Now bear with me.

Phase shift.jpeg (10.08 KiB) Viewed 1409 times

The blue line here would simulate my “Drone frequency” or resonance, now if I could offset that with 180 degrees(red line). The “red” line should eliminate the “blue” line and the drone disappears.

OK, onto the next step. We will now place a pipe parallel and blocked off on one side, thus the sound wave (blue) will travel to the end of the pipe(90 degrees), and jump back (90Degrees) offsetting the sound waved by 180 degrees and eliminate the drone. Very easy. Drone should be gone.

Now just one variable left. Speed of sound at certain temperatures! This makes a huge difference! Hence, you have to determine the temperature of your exhaust(I measured mine at 220Degrees Celsius where the two pipes join in as one).

Now all that is left is to determine the J-Pipe length:

Some easy maths:

V=Speed of Sound (in meters per second)
T= Temperature in Celsius
L= Length of Pipe in (mm)
f= frequency (measured or calculated as above)

Speed of Sound(V) = 331.4+(0.6xT)
Length of Pipe(L) = (V/f)x0.25x1000

In my instance Speed of Sound was 331.4+(0.6x220) = 463.4m/s
Length of pipe would thus be = (463.4/140)x0.25x1000 = 827.5mm long
Easy, I must just build a J-Pipe of 827.5mm long and weld it to my exhaust.

For those that are still here, the 0.25 in the Length of pipe equation is a quarter of the sound wave(or the 90Degree offset) which will be the pipe length to offset the 180Degree wave.

Next up make a J-Pipe. Fit it and test. Again took it to Marco at Tignique. They indulged me in my playing around and installed the pipe for me. Here are some pictures.

And would you believe it. It works! Drone is 90% gone, think my temperature measurement was a bit out, but now the vehicle sounds exactly as I wanted it, without any drone at a specific range and much more linear.

So for those of you still reading, maybe this will help out someone one day, massive shoutout to Marco and his team, it was a fun experiment.

Will try and post the Excel sheet, so you can just fill in where needed, and it tells you the length of the pipe.

J-Pipe Calcs.xlsx

(12.46 KiB) Downloaded 43 times

[Paul#25]

Thanks for an interesting post. Extracting performance from an exhaust is a black art that I delved into when I was still racing. This was something that I had never looked into as I normally had earplugs in while driving the Lotus 7.

[jakeslouw]

Standing wave resonance is always that result from free-flow exhausts. Nice to see that you had the brains to sort it out with a resonation eleminator, most people would have just moved the silencer and then ended up chasing the resonance at a different rev.

[grips]

Thanks for a very interesting post.

Few years back when I were still into building V8 engines I found exhaust systems a and cam shafts a grey area. To solve this I got myself involved with American Engine Builders and did some learning on exhaust systems and cam shaft profiling.
Adding to droning are welds that are not smoothed and the angles the system joins.

[Mad Manny]

Thanks Peet, your posts have cured my insomnia...

[Bogaards]

Very interesting post!