Mazda Gearbox chatter on Overun ?

[ScottR400D]

Your first paragraph makes an incorrect assumption. The chatter, which is very very common, can be excited by the engine but actually emanate from the diff (CWP). Its frequency roughly corresponds with engine speed but it’s the diff.

I think most of us find it easy to manage by adjusting the throttle slightly or changing gear. 

 

 
 

Edited May 13 by ScottR400D

[Drakman]

Could also be the clutch throwout bearing and or mount. perhaps the clutch fork, if your car has one.  Input shaft bearing?  What gear box oil are you using, is it at the correct level?

[Geoff Brown]

With a hot car, in neutral the idler gear does chatter in both Type 9 & Caterham 6 speed. Depressing the clutch kills the noise. Also adjusting tick over to 1000rpm or a tad more creates silence. Not sure that the noise is same in Mazda box ?

 

[ScottR400D]

54 minutes ago, Geoff Brown said:

With a hot car, in neutral the idler gear does chatter in both Type 9 & Caterham 6 speed. Depressing the clutch kills the noise. Also adjusting tick over to 1000rpm or a tad more creates silence. Not sure that the noise is same in Mazda box ?

 

I don’t think we’re talking about that Geoff. This is the distinct chatter a lot of us get on light overrun almost regardless of engine/box combination and it happens in different gears. 
I’ve discussed at length with both RRT and SPC and it seems accepted that it’s the CWP though RRT will say it might be something to do with the engine tilt while Steve Perks is convinced it’s to do with light flywheels and during pulses. I go with the latter. 
As I said, it’s easy to manage and I don’t think many see it as a real issue. 

[Geoff Brown]

16 hours ago, ScottR400D said:

I don’t think we’re talking about that Geoff. This is the distinct chatter a lot of us get on light overrun almost regardless of engine/box combination and it happens in different gears. 
I’ve discussed at length with both RRT and SPC and it seems accepted that it’s the CWP though RRT will say it might be something to do with the engine tilt while Steve Perks is convinced it’s to do with light flywheels and during pulses. I go with the latter. 
As I said, it’s easy to manage and I don’t think many see it as a real issue. 

I can say hand on heart that I have never experienced the chatter discussed, Titan or new Tracsport. My LSD CWP has always produced a distinctive noise at certain RPM on the over run but definitely not a chattering noise. If I introduce throttle it disappears but does not necessarily return at exactly the same speed or rpm. The noise is more akin to a large marble running around a corrugated tube for a short while. Well that is as near as one can get to an explanation ! 

[ScottR400D]

“The noise is more akin to a large marble running around a corrugated tube for a short while.”

That sounds like a chatter to me and exactly what we’re talking about!

As you say, slight change in throttle it goes and will return in different gears at different rpm depending on loading but is easy to manage  

 

[JAL73]

The problem with these noises is everyones verbal description varies. One mans chatter is another mans gnashing.

My over run noise always appears in exactly the same engine rpm band, never higher or lower.  It is not easy to manage as I often find myself coming off the throttle inside this rev band during normal road driving which forces an unnatural gear change to avoid it, which can be tedious. My noise is also accompanied by vibration. I think the vibration is the cause and the 'chatter' is the sympton. 

I agree that this noise is common place but some cars seem to have it worse than others. In my limted experience, it does seem to be worse on cars with lightened flywheels and maybe also worse on cars that are tracked more(?). I dont think it has anything to do with the Titan LSD per se, as I've heard people with open diffs complain about it too. i.e the CWP is potentially the soundboard.

The strongest argument I have for thinking its originating in the engine and not the gearbox, prop or diff is that the noise stops instantaneously when the drive train is disengaged from the engine by pressing the clutch. Simply knocking the car into neutral and simulating engine braking by easing off the throttle whilst coasting and braking does not cure it. Given the torsion damper springs in the friction plate is the only bit of engineering designed to attenuate the transmission of engine vibrations to the input shaft I do wonder how effective they are are lightened flywheel cars.    

I originally thought my flywheel and clutchcover might have a slight rotational imbalance but if that were true the source vibrations would get worse under acceleration with climbing revs but thats not my experience, my vibrations actually occur with falling revs on over run. My sense is to resolve the source of the resonance/ vibration rather than mask the symptomatic chatter of the drive train. It is exasperating and intriguing in equal measure but Im sure a clever NVH engineer could do it. 

Edited May 15 by JAL73