why deck lid vents

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PJMS
Posts: 296
Joined: Wed Oct 22, 2003 11:09 pm

Re: why deck lid vents

Post by PJMS » Sat Jun 06, 2020 4:51 pm

The vented deck lid was first introduced for the sedan models with 1600cc engines whether with the old cooling system or the doghouse system. This was simply because the engine sucked more air through the carb, and that suction force is much stronger than that provided by the cooling fan. So, if air supply is short (not enough venting), the engine intake will win the battle. Another way to look at it is that, under these conditions, the air pressure/density drops; and low density air absorbs less heat. In addition, because of the aerodynamics of the car, the air pressure at the rear of the car will drop the faster you go. This is why you will find that the overheating occurs only at the higher road speeds. The added deck lid vents, providing they are the correct type, remove the restriction of air to the engine compartment so the fan is no longer losing air to the engine. Note that only a year after VW introduced the doghouse system, they increase the vented area on the deck lid as the doghouse system consumed more air, causing additional low-pressure scenarios. These are the general points as there is a bit more detail to this; as in most things.

aussiebug
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Re: why deck lid vents

Post by aussiebug » Sat Jun 06, 2020 6:39 pm

Checking to see if you engine has the doghouse Fan shroud is easy. The doghouse is in the left side of the shroud, between the shroud and the firewall. If the shroud is flat there, it's the old style in-shroud cooling system. If there is a box shape on the shroud and a rectangular duct running forward to the front breast plate, it's the doghouse cooling shroud.

The old style cooling system was right at its limits with the 1970 1600 single port engine, and VW added the two-slot cabriolet vented lid to try to improve airflow into the fan a little.

For any engine greater than 1600cc, you really should have both the doghouse cooling system and it's larger (fatter) fan, plus either the two-slot or four-slot engine lid, or you engine WILL run hot at higher speeds.

My experience was similar to FJ above. A 68 with a 71 1600 twin port engine and doghouse Cooling system with the solid 68 engine lid always ran hot at speeds over 80kmh/50mph.

With a spare 2-slot 1970 engine lid fitted, that overheating problem disappeared immediately.
Regards
Rob
Rob and Dave's aircooled VW pages
Repairs and Maintenance for the home mechanic
www.vw-resource.com

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dstar5000
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Re: why deck lid vents

Post by dstar5000 » Thu Feb 11, 2021 8:03 pm

I LOVED THE DECK-LID THAT I MADE.......BUT I STILL WENT TO THE JUNK YARD AND PULLED A JETTA GRILL AND CUT IT IN....
2316CC, 45 DELLS AND A CAM THAT NEEDED 10:1 CR WITH PORSCHE 12 BLADE...
IT NEEDED A LITTLE EXTRA COOLING!


DON
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‎"Let me say it as simply as I can: transparency and the rule of law will be the touchstones
of this presidency,".. Barack Obama January 21, 2009, 30 minutes before he signed the law
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dstar5000
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Re: why deck lid vents

Post by dstar5000 » Wed Feb 17, 2021 10:12 pm

THIS CAR IS IN POLAND NOW AS FAR AS I KNOW......
‎"Let me say it as simply as I can: transparency and the rule of law will be the touchstones
of this presidency,".. Barack Obama January 21, 2009, 30 minutes before he signed the law
sealing all his personal information....

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