I ordered a 1970 442 from the factory and I am sure there was no screen under the hood. Was this a different year or an after market hood?

 

The screen was only used on the OAI (fiberglass) hoods. It provides electromagnetic shielding, reducing the amount of interference (from the ignition system) from leaving the engine compartment.


The factory steel hoods had no screen, as the solid sheet of steel provides the same function (and better).

 

Is there specs on one or is it available somewhere? I have a glass hood and it does not have a screen.

 

Yeah? I'm dying to hear your explanation of how the metal liner of the OAI enhanced this. Radio noise? Oh, there's a steel cowl that blocks EMI and a distributor noise suppressor capacitor. C'mon, you can come up with something more creative than this.

 

I'm sure if you got in touch with Troy Thornton, he would sell you one of the reproduction screens they put on their re-tooled W25 hoods.

 

Are you doubting my Oldsmobile knowledge or my electrical knowledge?


There's a page in the 1970 Chassis Service Manual which pictures all the various features added to the car to suppress RFI. The underhood screen is one of them.


As for how it works, the screen is of course electrically bonded (by welds) to the steel frame of the OAI hood. The steel frame is electrically bonded to the rest of the body through not only the hinges but also some funky clips at the cowl. How does a hole-y screen stop electromagnetic waves? That screen looks like solid steel to any waves longer than a certain length (determined by the hole diameter). Very short waves (like X-rays) pass right through, but the lower-frequency stuff, down in the AM and FM band, gets blocked, drained to "ground" (the body).

 

Faraday cage principle, just like the screen in your microwave door.