Timing Belts
- Thread starter Dan944t
- Start date
Indi9xx
New member
Flat plastic disk with a notch in it which goes behind the distributor cap, around the shaft of the camshaft nose section that the rotor arm bolts too.
Very flimsy plastic bit, the often people throw away thinking it doesn't do anything.
944.602.803.00 from Porsche main dealers, about £10 plus VAT retail (Used to be about £2.50!) They list it as a "Dust Cap" but it's main job is to separate the electrically charged ionised air from inside the distributor cap, from the free flowing air within the belt cover. It is not air tight, but it limits the amount of airflow between the two.
Also known as an Ozone cap, searches for the effects of ozone on rubber via google should bring up pictures of the familiar cracking you see on old tyre sidewalls. Ozone is a byproduct of electrically charging air molecules.
It is not only rubber, but also other materials, such as aluminium or as a rule, anything which suffers from oxidisation or degradation in an oxygen environment tend to suffer entropy being accelerated.
Interesting is the effects of ozone on combustion, which has been found to reduce the delay between ignition and combustion. So in theory, a vacuum hose routed to a hole in the distributor cap, routed to the intake manifold, could in theory reduce oxidisation of the rotor arm tip and contacts within the distributor, as well as potentially improving performance or efficiency... However, scientific tests have found the performance increases are not measurable, even though the combustion delay reduction has been observed.
Chemistry and Physics at play.. If more performance tuners studied and showed an interest Chemistry, Physics and certainly thermodynamics, more power would result..
Very flimsy plastic bit, the often people throw away thinking it doesn't do anything.
944.602.803.00 from Porsche main dealers, about £10 plus VAT retail (Used to be about £2.50!) They list it as a "Dust Cap" but it's main job is to separate the electrically charged ionised air from inside the distributor cap, from the free flowing air within the belt cover. It is not air tight, but it limits the amount of airflow between the two.
Also known as an Ozone cap, searches for the effects of ozone on rubber via google should bring up pictures of the familiar cracking you see on old tyre sidewalls. Ozone is a byproduct of electrically charging air molecules.
It is not only rubber, but also other materials, such as aluminium or as a rule, anything which suffers from oxidisation or degradation in an oxygen environment tend to suffer entropy being accelerated.
Interesting is the effects of ozone on combustion, which has been found to reduce the delay between ignition and combustion. So in theory, a vacuum hose routed to a hole in the distributor cap, routed to the intake manifold, could in theory reduce oxidisation of the rotor arm tip and contacts within the distributor, as well as potentially improving performance or efficiency... However, scientific tests have found the performance increases are not measurable, even though the combustion delay reduction has been observed.
Chemistry and Physics at play.. If more performance tuners studied and showed an interest Chemistry, Physics and certainly thermodynamics, more power would result..
vitesse
Active member
Well neither of our 924S's have them-but our original 924 Lux had one to cover the Hall effect sensor & shield it from ill effects.
As all the distributor does on a 924S/944/968 is distribute the spark to each plug in firing order,I would image the benefit of such a shield is not measureable.
As all the distributor does on a 924S/944/968 is distribute the spark to each plug in firing order,I would image the benefit of such a shield is not measureable.
Indi9xx
New member
vitesse said:
The original question that spawned the explanation was what outside influences degrade the belts. Ozone created within the distributor cap is far more corrosive than oxygen, which is far more corrosive than the air around us, especially for rubber components.
Spark performance will indeed not change if the car has or has no ionisation/ozone cap.
Indi9xx
New member
968, 944S and 944S2 have a large flange on the bottom/back of the rotor arm to close up the gap between the arm and the diameter of the hole as well as quite close tolerances between the tin plate behind it which is also used as the missing tooth for the hall sensor.
Does not need to be air tight, just to stop it being open to the belt cover, or at least have more square mm of ventilation in the cover than it has between the cover and the distributor cap, if that makes sense.
Does not need to be air tight, just to stop it being open to the belt cover, or at least have more square mm of ventilation in the cover than it has between the cover and the distributor cap, if that makes sense.
Indi9xx
New member
It's potentially not even a problem with the average annual mileage of a typical UK 944 multiplied by four or five years (or hours of exposure to a belt between belt changes) but none the less, a contributing factor that needs to be taken into account of what degrades a belt over time.
You could all decide that as your cars do not cover many miles per year that you could stretch out belt changes to 10 years, ditch the ionisation cap and all sorts of things like that, but that's cutting corners I would not want to cut.
You could all decide that as your cars do not cover many miles per year that you could stretch out belt changes to 10 years, ditch the ionisation cap and all sorts of things like that, but that's cutting corners I would not want to cut.
vitesse
Active member
Well I'm not interested in cutting corners-even though both our cars are over 30 yrs old,I have always tried to maintain them as original spec,even to replacing the little rubber bung in the camshaft belt cover where it was missing or nicked after being at a garage.
I'm well aware of the ozone problem with belts but why would Porsche miss it off on our grey car where we are only the 2nd owners & can't see why it would have gone missing at 26K miles having been serviced by a main dealer,the original supplier.
As ozone is present everywhere,there is a continual air change in the engine compartment /distributor cap-differential pressures everywhere,it's effect can only be marginal for the average owner,who listens to forum posts & abides by the advice.
I'm well aware of the ozone problem with belts but why would Porsche miss it off on our grey car where we are only the 2nd owners & can't see why it would have gone missing at 26K miles having been serviced by a main dealer,the original supplier.
As ozone is present everywhere,there is a continual air change in the engine compartment /distributor cap-differential pressures everywhere,it's effect can only be marginal for the average owner,who listens to forum posts & abides by the advice.
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