Intake air preheating question - early cars with stock carbs

[sjbartnik]

So last night before bed I was pleasure-reading the early Type 3 Bentley, as one does, when I came across an interesting tidbit about the intake air preheating system. I searched here about it but didn't find any mention of it.

So there's a note in the Bentley about the preheater flap in the intake air system which automatically toggles between cold outside air or preheated air depending on whatever it depends on. They note however that at times when the car will encounter temperatures around 50 degrees F or less the preheater flap should be free to move. But then it said that if the car will be encountering sustained temperatures above 50 degrees F, the preheater flap must be jammed so that it does not move freely and so that it only pulls cold outside air or else there may be "pinking" (so British) from the engine.

I thought that was interesting because in the owner's manual, the only mention of the preheater flap is to say "make sure it moves freely." It makes no mention of temperature ranges or jamming it closed in hot weather.

I know in later carb'ed VWs the intake air preheater flap was thermostatically controlled but it does not seem that this is the case on the early Type 3.

So does anyone know what the story is here? Do people really go and jam their preheater flap shut in the summer? I would certainly want to do so if I knew that it was not automatically pulling only (relatively) cool outside air for the engine intake.

[ataraxia]

Mine is almost always left in the 'jammed open' position since I only drive it during warm months to keep it out of the snow. The control box has a notch where the lever rests when 'jammed open' and it's relatively easy to move in/out of position.

I've never really noticed much of a change between 'always open' and 'free to move' - even on cold mornings.

[KTPhil]

I don't know about early Type 3s, but early Bugs had this same advice like this form the '67 Owners Manual:

[sjbartnik]

Thanks guys, good tidbit to know.

[MagmaJctAz]

Uh, I think the issue is semantics.

In this regard, "fixed" does not mean it has to be "jammed" open at temps above 50F. What they mean is "fixed" as in "motionless" at temps above 50F.

That is, the thermostatic element is calibrated in such a way the flaps remain open to cooler air, regardless of engine speed.

Yet, when temps are below 50F, the flaps will be free to move with engine speed. That is, at idle/low speed, the flaps will be closed to cooler air, admitting hotter air. As engine speed increases, the increased flow of air will counteract the weights and allow cooler air it.

Essentially, the thermostatic control "inhibits" motion above 50F. Think of it as a "lock-out". Enjoy!

Regards,
Mike
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