james_stan said:
HI Trikky
Thanks - things move slow in these parts as you can see.
I will check out vac off at high revs although I think I did try it .. btw should I set timing for 30degs max
as per 009 ?
On an allied topic I read through the dissy section in Seumes book last night and not once does he discuss mech/vac distributors only ever mech only, his favourite seemingly the 010 followed closely by the 009. It also says that on an 009 number 3 is always retarded 3-4 degs more than the others to help address the overheating issue on that pot - I never knew that. ! Of course that assumes you're not 180 degrees out..
Cheers
JS
When your checking to see if the vac is working its the throttle position thats important. The high revs are a by product of there being no load on the engine. The vac advance only advances the ignition at small throttle openings during driving. Not a tick over and not at wide throttle openings.
On a stock SVDA dissy you just set it static but these days, with old, swapped or non stock ones, its wise to check the max advance if you have a strobe and your timing marks have been checked to make sure they are accurate. You can never be sure what previous owners may have done.
It does no harm to check the dissy gives you 28 to 30 max advance at 3000 rpm vac hose disconnected.
Seumes book was written a long time ago and there are some errors in it as well as some bits of advice based purely on opinions at the time rather than on any test results. His stuff on mech dissys is an example.
There are no end of posts and discussions on the subject but it basically boils down to two simple benefits:-
The SVDA gives a small boost to advance as you open the throttle which means you dont get a flat spot when pulling away from rest. This can also be cured on 009's by running the carb with a richer mixture.
The SVDA gives extra advance at light throttle openings which improves fuel economy. For instance if you doing 45 mph on a flat road and the mechanical advance is giving 30 degrees, the vac advance adds around 4 degrees. This amount of advance is fine at light throttle but would cause pinking and engine damage at full throttle.
Seume apparently did not understand engine/combustion dynamics and just went with the flow at the time.
I have always found it most peculiar that many people seemed convinced that VW put so many unnecessary parts on their vehicles when they were selling their expensive vehicles in a very price sensitive part of the car market and were therefore obliged to build down to a price.
The slight amount of retard on the dissy lobe for cylinder No.3 was on some stock dissys as well. It was to compensate in extremis for the reduced cooling to No.3 because of the reduced flow of cooling air caused by the oil cooler. Its why there was so much attention paid to the correct position of the dissy drive when assembling an engine and why there was a notch on the dissy denoting cylinder No.1
The one lobe retard was dropped when the dog house oil cooler design came out which gave even flow to all cylinders.
