Exhaust DroneSo as I said before, the exhaust drone was brutal.
The red peak (125Hz @2900RPM) is what caused all the hell, nearly 30dB above the base line. Every octave (double the frequency) you see the same peak resurge.
The green line is @4000RPM where the headers are working correctly and the drone reduces. As weird as it sounds, the car was quieter while driving hard:
"Why were you speeding son?"
"I was trying to stop my brain being rattled out of my head - it's quieter this way!"
So I left my car with a friend who I trust when I went back to Perth and with his help I had Brintech install a side resonator. Other than installing it in the wrong spot, they did everything else I asked for - including making the length adjustable. This point is very important since you can only estimate the exhaust temperature and you will certainly be wrong, when calculating the length of the resonator.
The installation made immediate differences to the volume but wasn't perfect, but running another spectral analysis I was able to observer the target frequency clearly and re-do the calculations to infer the exhaust temp in the resonator and sort out the correct length.
In this case you can see the green line is the volume after the resonator is installed. It becomes effective a little too early, wiping out nearly half of the noisiest spot, but missing the loudest peak. Doing the maths again resulted in a shorter resonator. After adjusting it again, the results are below:
It totally wiped out the cruising drone, and the cabin volume dropped from a blistering 85dB to 75dB - which is now about as loud as a stock NB MX-5. The open muffler induces such a wide array of drone frequencies that the perfect solution would be three side resonators targeting just below the peak, just above the peak and the peak drone points. However, with the brutality knocked off it I will just spend ~$100 on some insulation and line the boot and under the rear seat. Another 5dB and it will sound like stock volume inside the car. The good news though is that there isn't any exhaust volume while crusing, it isn't till you open the throttle that it gets loud again.
Below are the pictures of the side resonator with a clamp that makes it adjustable:
I have some recordings that demonstrate the differences when I get a chance.
Good news for you LHD kids Rama has started fitting LHD parts!
I'll also be doing some experiments with tapered intakes and plenum volume to see if we can squeeze a bit more out of it
And one more thing...Rama has started work on his straight replacement M42/4 LTW Flywheel. It will use an M20 clutch kit and that is it! He has offered me one to test-fit, but there is a slight problem. I have a brand new clutch and OEM flywheel (like ~1000kms ago) and I'm hesitant to pull the gearbox off again. The other issue is that I've spent more than I can reasonably at this point in time, especially if I want to do things properly. A lot of people would like to know the real world benefits of a light flywheel, as well as the driveability. I was thinking of installing it and running a before and after dyno run in 2nd gear to show how it affects transmission losses and wheel-power. I can't really afford the M20 clutch kit, installation and two more dyno runs...
So here are the questions, would anyone want to buy my slightly used clutch kit and flywheel?
Does anyone want the kind of numerical feedback you would see on a dyno?
And would anyone like to buy-in early? If Rama has some sales ahead of time, he may be a bit keen to invest a little further and assist me with doing detailed testing. I think by this point its clear that Rama isn't producing vapourware and these will be on the market soon - flywheels, ITB kits and everything else! I've been trying to prod him into giving people early access prices, so now would be a good time to show your support!
