I'm curious if anybody has done some testing of running their /4 or /6 with and without an engine lid. Or maybe regular vs. GT engine lid. Any hard numbers to say if there's a noticeable impact given the low pressure zone over the engine bay.

No effect on my car in the hot San Ramon summer. Although I think the true reason for overheating on a properly tuned 914/4 2.0L with everything in order (no vacuum leaks, timing, valve job, oil cooler not clogged, nothing stuck in the fan blades, cooling flaps working properly, etc...) is the oil pressure relief system.

I’m not convinced that the GT engine cover was done for cooling. I think that it was done to provide sufficient air for the carbs and cooling fan.

Ive driven around with my deck lid open for carb tuning ease and did notice a reduction in time to achieve operating temp. Accidentally left it open and experienced the same thing.
I have 2 GT lids... I should do an actual test.

In the early 1970's, I lived in Germany and drove a 914/6. The muffler rusted out and I replaced it with a 911 muffler. The rear valence would not fit because of the different angle of the 911 exhaust pipe so I left it off. The car ran noticeably cooler without the rear valence.

We have seen 914s that ran cooler with no engine lid. I think someone here had done some testing, but I don't remember who or when any more? (Hmm, hopefully I'm not getting that confused with McMark's testing of with/without thermostat flaps.)

It's certainly not a fix for running too hot, but it does seem to help a little.

--DD

Nice tip for the rear valence - mine is currently off with the 2-out /6 sports muffler - perhaps I'll keep it that way.