[Gunter]

Originally Posted by Grady Clay

Mountainman has it correct.

There are two acceptable choices:
Use the heater systems as designed with the proper air flow through the heat exchangers
OR
install ‘headers’ (exhaust pipes without heat exchangers) and cover the openings from the fan shroud.

The worst possible is to leave the openings from the fan shroud open (wasting necessary cooling air) and have heat exchangers with no air flow (allowing the exhaust pipes and tin to get so hot as to ignite residual oil).

With original heat exchangers, it is also important to have the heater control ‘flapper’ valves in place as they provide design restriction to the air flow through the heat exchangers.


Even in Texas, there are days where heat (windshield defrost) is necessary.
The Factory heat exchangers are very good performance parts. Use them as intended.

There are situations where not using heat exchangers is appropriate.
Those include some racing in warm weather.
It also includes situations where the engine cooling system is stretched to the limit of capacity (117F in Houston with the A/C on).
Here, you want every single molecule of cooling air to cool the heads and cylinders.

Best,
Grady

+1

Heat is the enemy for aircooled engines, especially in hot climates.
Premature valve guide wear is one negative aspect along with a few more.
Either do correct "back-dating" adding SSI's (which you want anyway) or ensure some way of airflow with the stock set-up.

BTW: For the 964/993, it is recommended to remove the engine under tray for increased airflow helping with cooling and extending valve guide life.

It only makes sense when considering the high air temps in certain areas.
Just because the oil temps stay within limits doesn't mean that the exhaust ports on the heads don't get hot as hell without airflow.