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Peter said:
This is actually myth. It has more to do with combining an overly compromised ignition advance curve with an unbalanced intake pre-heat system and a long exhaust manifold. Most of this was cured when VW used improved advance mechanisms and, subsequently, more efficient intake pre-heating.

The position of the oil cooler had virtually nothing to do with it.
Really... then why does #1 not have the same problems since it also has the longer exhaust....?

Why does doing a "Dog House" conversion with no other changes reduce head and oil temps?
 

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Mr.010 said:
.....why does #1 not have the same problems since it also has the longer exhaust....?
#1 is not as far forward as #3 plus the 3 & 4 side suffered more intake ineffeciency than 1 & 2. However, these just setup the foundation of the problem, as it was the ignition advance that provided the proverbial 'straw'.

Why does doing a "Dog House" conversion with no other changes reduce head and oil temps?
For the simple reason that the dog house system has a larger cooling capacity. It is a common mis-conception, with the pre-71 system, that either the oil cooler "blocked" the air to #3 (not-to-mention #4) or that the cooling air for #3 had to "pass through" the cooler first.
 

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Peter said:
This is actually myth. It has more to do with combining an overly compromised ignition advance curve with an unbalanced intake pre-heat system and a long exhaust manifold. Most of this was cured when VW used improved advance mechanisms and, subsequently, more efficient intake pre-heating.

The position of the oil cooler had virtually nothing to do with it.
OK I will accept what you say is correct but, lack of head cooling would not necessarily burn a valve. But how can an incorrect advance curve affect one cylinder alone? The advance curve must hit all spark plugs the same way. I can see a too retarded spark will have a tendency to send still burning charge down the open valve and burn it - but would not all be the same.

Also I would be interested in a more detailed explanation on the unbalance pre-heat. Again I see that is unbalanced, but are you saying un vapourised fuel causes the valve to burn.

The two burnt valves I have come across - one in a car I drove and another in a friends both occured (as best as we could tell) after very very long drives - like about 14 hrs solid.
 

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metric_thumbs said:
....how can an incorrect advance curve affect one cylinder alone?....
Each cylinder has a slightly different set of conditions, basically because of how the engine is laid out. The engineers tried to adjust for each condition so that they all ran as close as possible. There was a cost issue too and so there were compromises. #3 would have run slightly hotter than the others (or at least closer to the limit) but still within tolerance. When the ignition over-advanced for all cylinders, it push the one that was at the limit, over it. Even so, it wasn't a big deal as when they figured out what was happening, they just retarded the ignition to that cylinder (another compromise) causing little or no impact to the cost structure..

.....I would be interested in a more detailed explanation on the unbalance pre-heat. Again I see that is unbalanced, but are you saying un vapourised fuel causes the valve to burn......
The intake manifold pre-heat system utilised exhaust gases from #2 port, flowing from that right side and then over to the left. Now, although it may not seem so, this meant that the left hand side of the manifold (3 & 4) received less heat. This caused the left side to allow a leaner mixture through to 3 & 4 cylinders; "lean" makes more heat. In 1972 they reversed the direction. In some cases they fitted dual pre-heat for maximum efficiency.
 

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Thanks - I understand now

The leaner mixture I can undertsand burning an exhaust as the vapourisation of the fuel in the cyl head extracs a significant amount of heat and without a long overlap cam, there is not much 'fresh' air blowing past the semi-open valve.
 
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