Secondary Balance Shaft

[MJL944]

The mechanic (very well versed in 944 engines so I trust his judgement) currently fitting upgraded gear linkages to my S2 told me today that he suspects the "˜Secondary Balance Shaft is out of synch' in my car's engine.

I've read up on what the Balance Shafts do and his diagnosis would seem to tie in with why I briefly experience noisy and harsh vibration through the drive-train when slowing down from high speed "" at the point just before you need to change down into 2nd/3rd from 5th. I had the head gasket changed last autumn and this vibration has been apparent ever since that job during which the belts were removed and replaced. Various exhaust system issues that coincided with the HG change (and that have continued since) had caused me to believe the vibration was the result of an ill-fitting new middle exhaust section.

I don't want the current mechanic to look into the possible balance belts issue (as I'll probably not get my car back until 2015 if the glacial progress with the work I've given them so far is anything to go by!). My preferred option is to get the specialist that changed the HG and fitted the new exhaust system to look into the balance belts issue - but, they're 100 miles away from me.

How safe is it for my engine/mounts/exhaust etc. to drive with iffy balance shafts/belts?

The engine idles fine and isn't clattery. The only time you notice the vibration is in the slowing down scenario described above. But, having spent a fortune on the HG job I really don't want to risk damaging the engine on a long drive especially when I have a couple of good OPC options very close by (Nott'm & Leicester). With an OPC doing the job in mind - Any ideas on the likely labour time required for adjusting the Secondary Balance Shaft?

Many thanks

 

[tref]

I looked into the problems caused by lack of balance shafts when I had a problem with mine. From what I found the reason racers get away with it is because they don't spend much time idling. The primary problem with not running the balance shafts seems to be cracked oil pick-up pipe. So... I would say a long journey is fine so long as you don't drop below 80mph... ever;-)

There are several ways to get the balance shaft alignment wrong when putting it back together from doing a headgasket, so I would suggest the person looking at it now is probably right. Time to check and adjust? Unfortunately unlike just checking the tension, whilst one balance shaft is visible by taking off the top belt cover, the other isn't, so I believe you have to take the alternator and power steering belts off first. At least the balance shaft belt is on the outside so doesn't need to be disturbed. Time...? A well equipped garage that knows what they are doing I would have thought could have it done in an hour or two... Somone that isn't familiar with it will probably that time staring at the manuals trying to work out the O and U for upper and lower, and which little window they should appear in, according to which year of car, and where the alignment marks are!

 

[edh]

Mark - is your car at PH? I've seen a white one in their workshop - looks very nice. Rob said the new gear linkage is excellent.

They have had a bit of a problem with manpower - I understand Paul had a holiday to Dubai sprung on him last week, so everything pretty much stopped. Anyway he's back now.

If so get Rob to check the balance belts it'll not take him long.

 

[robdimond]

It should be a very, very easy job for a specialist, they just need to take the top and bottom timing covers off (which as tref says means taking the alternator and PAS belts off), slacken the belt tension, line everything up and put everything back together. Surely can't be more than an hours labour, would probably take me an afternoon without a ramp and spending most of that time searching for lost tools.

 

[MJL944]

Thank you gents - as ever, lots of useful information in your replies.

Ed - you guess correctly! I'm due to speak with them on Monday but I suspect I'll be taking the car back and looking elsewhere to get the issue sorted.

Would anyone strongly advise against a 100 mile trip with iffy balance shaft belts?

 

[edh]

I suspect a bit of a communication problem...I said to rob - "that's been here a while" & he told me that you were in no great hurry to get it back. Checking the balance shafts is a very quick job.

That sort of delay / lack of communication is a bit of the price you pay for low labour rates...I can live with it as I don't need my porsches for work.

 

[vitesse]

Most 4 cylinder petrol engines don't have balance shafts & get by OK--the 2 shafts are not connected to anything other than themselves with a timing belt-so just drive briskly keeping the engine spinning
& don't let the engine labour--if stuck in traffic turn it off rather than idle and/or use a touch of throttle if idling.

 

[MJL944]

Colin that's interesting & useful advise - thanks. I was stuck in a load of traffic taking it to the garage it's taken root in now so I wish I'd known your advise earlier although as I mentioned I previously put the vibration down to an ill-fitting new exhaust middle box.
Funnily enough though, the engine at idle seems fine - it really just tries to shake itself loose very briefly once you've let off the throttle from high speed and engine braking takes effect and at just the moment you would need to change down to 3rd or 2nd to keep moving there's a brief but harsh shake through the car - especially noticeable through the gear lever. I wonder if the new Porsche engine mounts (fitted at the same time as the HG job) are effecting the shake.

Ed - yes, comms are an issue over there, epitomised by the comment you received. I'm totally exasperated with the situation but will bite my tongue on a public forum. I will try my luck on Monday to see if an hour can be found to sort the belts issue.

 

[edh]

I believe there are 2 specific engine speeds where the vibration is pronounced (not idle). Google it - somewhere around 1900 and 3800?

 

[vitesse]

Ed,I wasn't being that technical-& it's a long,long time since I studied the differences between 4,6,& 8 cylinder crankshaft harmonics & the inherent better balance of the latter 2 types-however just based on common sense & my experience of our 2- 924S's-both generally idle lumpily & quickly smooth out as revs are increased-& it helps quite significantly if Bosch Superfour spark plugs are used instead of any others in my experience-obviously it helps to avoid any "rough" spots throughout the rev range.
Incidentally,both ours are set at the correct idle speed.