Project Miatabusa Part 16: What’s that noise?

Project Miatabusa - Death Rattle Noise!



Project Miatabusa Part 16:


What's That Noise?

by Dave Coleman

First things first: Where the hell has the Miatabusa been?

There's a complicated string of excuses involving lots of things that have nothing to do with Miatabusas, like buying 90-year old homes, filming zombie movies and machining medical devices. (This is a complex alliance, this Miatabusa cabal…) But there was one car-related issue that contributed to this big delay: a horrible, horrible noise.

As you'll recall, when we first fired it up, the Miatabusa impressed us not only with cool vroom-vroom noises, but also with a chainsaw massacre sized oil leak and a nasty rattle noise where an oil pan bolt was tapping against the subframe. Or at least that's what we thought was making the noise…

After tearing things apart, fixing all those problems and firing it back up, we heard the same nasty rattle we thought we just fixed.

This is what it sounds like:

As you can hear, the rattle sounds different at different times. From a deep, slightly nasty background noise at idle to a sawzall-through-your-skull resonance at around 6500 rpm. The sound is dramatically worse from inside the car, where that mid-rpm buzzsaw sound is like a murder of hammers trying to bust through the floor. 

When we first heard the noise at idle, we suspected a bum timing chain tensioner. After ruling that out, we hoped it was a normal Hayabusa noise. Calling in Neel Vasaveda from Apex Speed Technology, his experienced ear, ominously, had never heard a Hayabusa engine make this noise.

When we finally drove the car and heard the deathhammer, we naturally assumed it was something hitting the bottom of the car. Hence the excitement when we found an oil pan bolt hitting the subframe. 

Back together with all the obvious problems solved, though, the noises were exactly the same. 

Maybe that wacky wraparound header has some nasty resonance? Damping it with a (gloved) hand did nothing to the sound, nor did changing the natural frequency by removing exhaust support brackets. And despite the distinct sound of stuff hitting the floor, none of the tight-fitting exhaust was actually touching the car. 

Listening to the noise from below on MD Automotive's lift it was suddenly easy to pinpoint the source of the sound to about  6 inches forward of the flywheel. Right about here:

Hayabusa powered MIata has a horrible death rattle

When the engine was out for its last round of leak and rattle repairs, rocking the flywheel back and forth made a clank noise that sounds exactly like a single note of that horrible rattle. That sound is from the backlash between the primary drive gear on the crankshaft and the driven gear on the flywheel shaft. (If you don't remember how or why we have a flywheel shaft, brush up on your Miatabusa history HERE). 

After eliminating every other easier-to-fix possibility, it is now painfully clear that the death rattle is coming from this gear. Here's what we think is happening:

Hayabusa powered MIata has a horrible death rattle 

In stock form, the primary drive gear drives the clutch basket through a sprung hub. Note the six springs buried in this clutch basket, and the three giant rivets holding the clutch basket to the sprung hub. The purpose of these springs is to isolate the transmission from the torsional oscillations of the crankshaft.

Crankshafts don't spin smoothly it turns out, but actually speed up and slow down many times during each revolution. There are three main driving forces behind these variations in speed. First, and most obvious, is because the four-stroke cycle involves a big burst of torque during the power stroke, followed by 540 degrees of not only coasting, but actually doing work (and therefore slowing down) sucking in and compressing air. Now, stacking four cylinders in a row and staggering all of their 4-stroke events by 180 degrees should even these pulses out, since one cylinder is always on the power stroke. Of course, the torque delivered during the power stoke isn't delivered in a nice even push over 180 degrees. It comes on with a bang, peaks somewhere around 15 degrees ATDC, and then tapers off. You have to stack a lot more than four of these together to add up to smooth torque delivery.

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