why deck lid vents

[PJMS]

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]

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.

[dstar5000]

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

[dstar5000]

THIS CAR IS IN POLAND NOW AS FAR AS I KNOW......

[bnam]

The decklid with the most (always open) vent area for German production beetles was the 70-71 convertible decklid which had 4 vents in 7,7,7,7 configuration. In '72 both sedan and convertible got 4 vent decklids, but they were in 7,6,6,7 configuration - with 2 less slots than the previous decklid. Why did VW reduce the number of slots?

I know that later on they added a thermostat controlled vent under the license plate which increased the vent area (making those later lids the ones with the most total vent area).

[Bruce2]

Then in about 75 the Thermo controlled rectangular flap under the licence plate was introduced.

This was only installed in some cars for very limited markets. That assembly is extremely rare in North America.

[PJMS]

It is an important point to remember that 'vents in the deck lid' began with the convertible model. This was because the modifications to the body, and the position of the lowered convertible roof, meant that the vents below the rear window had to be excluded. The regular version of the car did not require any deck lid venting until much later, and simply adopted the lid used on the convertible when it did. The earlier versions of these vents were in two banks, either side of the rib in the centre of the lid and when that 'rib' was taken out the gap between the vents remained similarly wide and allowed for the angled positioning of the model/make name plate often placed on the lid. Then in either 1969 or 1970 (can't quite remember) they introduced, on the convertible model, the '28-vent' lid (4 banks of 7), as mentioned in the post above. This was not used on the standard model Beetles until the 1971 model year, but as the 26-vent version; leading to the question why?

I do not think there was an official explanation for this (though normally there would have been) but I can think of two possible reasons. From the technical point of view, it is possible that the lid, specifically designed for the convertible, was a little too well vented for the standard models and may have caused carb icing under certain conditions. The removal of the two vents may have been enough bring that under control. The subsequent introduction of the thermostat-controlled flaps under the license light hood indicated how sensitive the engine was to venting - These plastic flaps were used mainly on the fuel injection models and, interestingly, late model convertibles. The other reason, and I say this with all seriousness, is to better accommodate the model name plate on the lid. The 4 banks of vents left no space at the centre, as previously, and the top edge of the name plates were almost on the lower vent of the centre-left bank. The solution to this would be to remove one vent from each of the two centre banks (for visual balance). VW would have tested the new arrangement and so long as it had no negative impact on the running of both the standard and convertible models, it could proceed.

It could have been a combination of both, but the latter seems very 'VW' to me. Anyone who has access to the full changes list for the 1971 model year might find the official explanation, though sometimes that 'explanation' also needs explaining!

[-Nate]

Hi ;

Older long retired VW Mechanic here, I was just hipped to this interesting site .

Additional deck lid vents was an "M" code for vehicles sold in Desert climes way back when as well as on convertibles .

Look at an older parts book, the 'M' codes are in the front .

In any case, the insistence of keeping the upper and lower air flow separate is critical no matter what engine you run .

If you think otherwise, start your Bug and let it idle, place a piece of paper over any portion of the intake vents and it'll be held in place by the massive air flow even at idle speeds .

All those niggly little rubber bits and lower sheet metal tins are *very* important to long engine life .

-Nate