[Diy_efi] some pictures...

[Mike]

At 11:41 PM 7/4/06, you wrote:
>>It *is* correct, these things spin at rather high rates, 80,000 to 120,000 rpm
>>and really do need to be balanced after re-assembly or they will howl and
>>can oscillate and effectively explode at high revs but not necessarily straight
>>away - it depends how long, what load etc.
>
>Aha, you sound like someone who actually knows about this from experience :)

Currently driving a 3L Nissan engine with turbo, taken a few apart after various failures,
plus know of two people in last 10 years who skipped balancing !, plus seen many on various
user groups and visited two turbo service shops, plus few mechanics and
collection of  anecdotes for last 20 years or so...

>this is a _very_ tiny turbo, just a 3cm compressor wheel (and the turbine is smaller still!)

In that case, to develop much boost it would need to turn somewhat faster, maybe 170,000 to 200,000 rpms

>it does howl very strongly, and always has (the new one is a little quieter but not much). I know many do spin at 80,000+ RPM but this one doesnt *sound* like its anywhere close to that - the pitch of the thing is perhaps around middle-C to an octave higher or so. (of course, this could be a harmonic or something)

Ear cant discriminate anything useful beyond about 16,000 rpm, at that rate
for a small turbo it wouldnt develop much boost, what you are likely hearing
is low order oscillations, resonant cavity either inlet or exhaust but certainly not
anything near turbo spin rate !  Much like the F1 motors, when they spin at 19,000rpm,
you dont hear the motors at all, you hear the gearboxes and the layshafts etc etc

>>Proper balancing will ensure it
>>lasts the full service life, pot luck with no balancing and only assembly
>>means you could be anywhere in terms of dynamics and that means it
>>can fail at any time and probability suggests it will not reach a fraction
>>of its service life if you expect it to provide any power and that means
>>high revs...
>
>how is the post assembly balancing done? One thing that puzzles me is that the compressor is onlyheld on by a tiny little nut. no spline or anything, so I'd have thought theres a chance it could slip slightly anyway, which would ruin any balancing done on it.

When I had my turbo reconditioned it had a new and larger compressor wheel
added, these are already balanced at factory, so that part is removed from the equation
(mostly), the nut and its position on the shaft shouldnt vary much. The machine
is fairly sophisticated, they call them a "VST" over here,  spot magnetisation of a turbine blade
and visual mark along with vibration sensor - oh and source of compressed air to spin the blighters
up to anything up to 200,000 rpms... plus the necessary oil feed.

Note: Turbine (exhaust) is friction welded to shaft, more sensible to pre-balance
inlet side at factory as well as trim on the nut(s) when placed on the balancer.

>>Note: Even if you reassemble carefully you still must get it balanced.
>
>Question there is 'how' ?

Specialist shops do it, ring around for best price,  if in USA expect it to be much cheaper
than in Australia where we have a bit of a closed shop, ie Price collusion. Shouldnt
take more than an hour to do it properly but then you pay for machine amortisation,
see if you can include yourself observing process as interested academic in price ;-)
Last cost here (2004) for full turbo service, larger comp wheel and balancing was AUD$700.

Others can comment on the USA prices,

>>Static balance might just be ok to around 5000rpms
>
>engine RPMs or turbo? because if thats the case, Im never gonna get that high anyway :)

Turbo rpms, I was just using that as a low end useless figure which wont give you any
boost but then its pointless, sorry I was being tangential. In effect I was saying that if
you dont dynamically balance the turbo its probably only usable up to 5000 rpms, which
isnt nearly fast enough, especially if its small, to develop any boost... In other words
get it dynamically balanced. Consider that dynamically balancing a crank makes it work
more smoothly when balanced to 5000 rpms - and here you want to run a turbo at
around 120,000 or even 170,000 for a small one and wont get it balanced, trust you see the irony...



Regards from


Mike
Perth, Western Australia
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