Chatter Noise Under Load

[matticus]

Hi All,

I was driving my R33 home from work today and noticed when accelerating from 80 - 100 in 5th under heavy load there was a chatter coming from the engine bay somewhere. Not too sure what it is but I did notice that my A/F ratio is running pretty rich (13 - 13.5)

I took off the pod recently and put back the stock airbox however I think the SAFC was tuned with the pod on.

Any thoughts on what the sound might be?

[StealthX]

13-13.5 certainly isn't rich buddy!

That's lean as fark.

11/10/9 is rich, the lower the number the less air there is for a given amount of fuel.

[matticus]

13-13.5 certainly isn't rich buddy!

I thought 14.7 was stoich and below this would be rich.

[mad082]

14.7 is stoich, but you only use that for cruise. when it comes to putting load on in a turbo car then you want things to be 12 or less (anything higher than about 12.5 at high boost and rpm is going to have a short life), however at 80 to 120kmh in 5th gear you are pretty low in the revs so 13.5 probably wouldn't be too far off the mark.

what did the noise actually sound like? was is like a marble in a tin can? if so then that is most likely pinging, which is bad. however if it starts doing it more often in other gears and rev ranges then you may have a broken manifold stud and the sound is the gasket vibrating.

if the sound was more like it was misfiring then it's either your coils or spark plugs

[matticus]

Cheers mate,

Ill check it again during lunch and give a better description.

Ill also get someone to tell me the A/F ratio during high boost...I really dont want to be screwing the engine.

[GTR-N1]

It may be pinging as Marc said. If so, you can top up your fuel with extra toluene (methylbenzene) just to check.

Alternatively, octane booster can be used; but 1x bottle will make little difference. 2x bottles are needed to check.

If pinging ceases, then your state of tune will need to be rectified.

[matticus]

thanks for the replies guys,

when driving home on some nice road it didn't really sound like marbles in a can. oddly enough the only sort of sound I can compare it to was two pieces of paper hitting each other really quickly. It also isn't a constant noise.

Ill put some octane booster in over the weekend to see if it helps.

Might also go through the engine bay to see if anything is loose.

[mad082]

check the exhaust manifold studs to see if any are missing. it may just be the manifold gasket if it is a fluttery sound

[gtrpommie]

Yeh manafold that's what happens to my 32 but it had cracked needed to be replaced does it happen on boost in any other gear?

[matticus]

I got it to happen in third but it wasn't as prominent as it is in 5th. Ill check out the studs today.

Is it possible that it was a bad batch of fuel?

[mad082]

bad batch of fuel would sound like misfiring and would happen whenever you floor it

[matticus]

Well I finally got one of the NVH guys at work out over lunch to listen to the chatter.

Found out that the sound of coming from the gearbox/gear stick.

When I am accelerating in 5th gear and push the gear stick up and to the left the chatter sound becomes worse but when the gear stick is pulled slightly downward it goes away.

We were thinking it may be gear boot related. Has anyone else had this issue before?

Cheers

[jrm]

Is it completely gone in 4th?

There are bearings (cluster bearings?) in the gearbox that can get pretty noisy in all gears except 4th (where there is no load on them)

[matticus]

The sound isn't always there. ill check to see if it in 4th.

In terms of fixing, I know I need to change the clutch soon, what else would need to be done?