why deck lid vents

The VW Beetle. Everything about bugs!

Moderator: Marc

weldvair
Posts: 191
Joined: Sun Apr 20, 2014 6:24 pm
Location: st cloud fl

why deck lid vents

Post by weldvair » Wed Sep 10, 2014 3:39 pm

It looks like the engine cooling air comes from under the engine so please explain
the concern over adding more air fresh to the engine compartment......is it just to
get more air for the carb?

Don
retired mech engr living in central florida

User avatar
trbugman
Posts: 119
Joined: Mon Apr 01, 2002 3:01 am
Location: SoCal

Re: why deck lid vents

Post by trbugman » Wed Sep 10, 2014 6:09 pm

Unless you have a type 3 or 4 the cooling air comes from the top and exits to bottom rear.
The intake for the air cooling is in front of the alternator/generator, between the fan housing and firewall.
2333(103x70) Type 4 DTM MS3+X on 73 Karmann Ghia ignition only with LS2 truck coils firing in sequential, fuel next.

User avatar
FJCamper
Moderator
Posts: 2670
Joined: Wed Nov 14, 2007 5:19 pm
Location: Birmingham AL

Re: why deck lid vents

Post by FJCamper » Thu Sep 11, 2014 11:05 am

Hi Don,

To expand on TRBugMan's explanation, for Bugs, Ghias, and Busses with Type 1 engines, the cooling air enters the engine though the front of the fan housing and exits under the engine.

The air being sucked into the fan housing is coming from the engine compartment, from the Bug's vents just below the rear window, from the engine decklid if on a Ghia, and from the side body vents if on a bus.

VW had to add extra engine lid vents to the Bug when the dual port head engines came out because of the extra engine heat.

The problem, Bug, Ghia, or Bus, is the engine compartment seals wear away over time and hot air from under the engine can enter the engine compartment get sucked into the fan. Overheating is almost guaranteed.

FJC

modernbeat
Posts: 15
Joined: Sun Jul 13, 2014 9:13 am
Location: Dallas, TX

Re: why deck lid vents

Post by modernbeat » Thu Sep 11, 2014 12:38 pm

Maybe a few illustrations will help.

Image

Image
Jason McDaniel

weldvair
Posts: 191
Joined: Sun Apr 20, 2014 6:24 pm
Location: st cloud fl

Re: why deck lid vents

Post by weldvair » Wed Oct 01, 2014 2:01 pm

What great replies.....Don
retired mech engr living in central florida

User avatar
Max Welton
Posts: 2436
Joined: Mon Jun 03, 2002 2:01 am
Location: Black Forest, Colorado
Contact:

Re: why deck lid vents

Post by Max Welton » Wed Oct 01, 2014 4:15 pm

weldvair wrote:It looks like the engine cooling air comes from under the engine
I'm guessing that if you are looking at your car and asking this question, you may be missing the parts that are supposed to keep air from entering the engine bay from below.

That would be bad.

Max

User avatar
Buggin_74
Posts: 690
Joined: Fri Sep 01, 2000 2:01 am
Location: NSW Australia

Re: why deck lid vents

Post by Buggin_74 » Thu Oct 02, 2014 4:31 am

FJCamper wrote: VW had to add extra engine lid vents to the Bug when the dual port head engines came out because of the extra engine heat.
It was because dual port engines with the doghouse oil cooler have a larger fan than single port which draws in more air than just the vents below the window can provide.

Even after the first year 2 sets of vents wasnt adequate and they went to 4 vents and a few years later even an extra thermostatically operated vent under the license plate light.

The metal rain tray that sat under the decklid of 70 models was also discontinued when dual ports started as it obstructed the air flow.
I've seen a lot of people fit those to later cars scared of a little water on their engine.

A lot of people mess up German engineers work when they put dual port engines in early cars with no decklid vents, stuck in the mindset that its not bursting into flames so it can't be hot
1974 Germanlook 1303 Suba-Beetle
Subaru EJ25 Boost R 17", 4 Wheel discs, Topline suspension and A/C

User avatar
Marc
Moderator
Posts: 23722
Joined: Thu May 23, 2002 2:01 am
Contact:

Re: why deck lid vents

Post by Marc » Thu Oct 02, 2014 9:14 am

The decklid louvers first appeared on the Sedan in 1970, before the larger fan - but considering the disaster which was the `68/`69 H5 1500SP, they were probably trying to err on the side of caution - especially since the displacement was being increased. Shortly after the larger fan was introduced on the dualport engine, the Sedan rear window size was increased and even more decklid vent louvering was added. These changes didn't occur in lockstep (stockpiles of old parts were used up first) and the fact that the Convertible - with no louvers at all under the window - in most cases received the same lid as the Sedan indicates that it wasn't considered to be a critical problem...but the trend over time was to more and more air inlet area.

Max raises a good question, if it looks to a mechanical engineer that the cooling air is entering from below something must be missing. There should be no path for air from the "dirty" side to enter the engine compartment (other than the warmair pickup hose to the aircleaner).

Ol'fogasaurus
Posts: 13099
Joined: Tue Nov 14, 2006 1:17 am
Location: Just north of Seattle

Re: why deck lid vents

Post by Ol'fogasaurus » Thu Oct 02, 2014 11:36 am

Sticky?

User avatar
volksbugly
Posts: 613
Joined: Thu Sep 16, 2010 7:09 am

Re: why deck lid vents

Post by volksbugly » Sun Nov 30, 2014 7:54 pm

Buggin_74 wrote: It was because dual port engines with the doghouse oil cooler have a larger fan than single port which draws in more air than just the vents below the window can provide.

Even after the first year 2 sets of vents wasnt adequate and they went to 4 vents and a few years later even an extra thermostatically operated vent under the license plate light.

The metal rain tray that sat under the decklid of 70 models was also discontinued when dual ports started as it obstructed the air flow.
I've seen a lot of people fit those to later cars scared of a little water on their engine.

A lot of people mess up German engineers work when they put dual port engines in early cars with no decklid vents, stuck in the mindset that its not bursting into flames so it can't be hot
Do you have the documentation to back this up?

I'm really looking hard into this because I have a turbo tail installed.
https://picasaweb.google.com/1078300117 ... directlink

I plan to mount oil coolers in the tail and have fans blowing outward. I was under the impression that the decklid vents were for cooling purposes of radiated heat with the larger displacement and had minimal to do with air intake for cooling.

My 1967 Bug with no vents in the decklid had a 1641cc in it. It was getting so hot it was vapor locking. I put a decklid stand off kit on it, the kind that stands the latch up a few inches, and then it ran much cooler and no vapor locking.

User avatar
Max Welton
Posts: 2436
Joined: Mon Jun 03, 2002 2:01 am
Location: Black Forest, Colorado
Contact:

Re: why deck lid vents

Post by Max Welton » Sun Nov 30, 2014 10:21 pm

volksbugly wrote:I plan to mount oil coolers in the tail and have fans blowing outward. I was under the impression that the decklid vents were for cooling purposes of radiated heat with the larger displacement and had minimal to do with air intake for cooling.
In the stock configuration at least, air is drawn in through the vents and is expelled out the bottom. Your fans will be pulling air away from the cooling system.

Max

User avatar
Piledriver
Moderator
Posts: 21717
Joined: Sat Feb 16, 2002 3:01 am
Location: Van Alstyne, Texas

Re: why deck lid vents

Post by Piledriver » Sun Nov 30, 2014 11:22 pm

volksbugly wrote: Do you have the documentation to back this up?

Pretty well known history of modifications from the factory, and the fact the decklid standoffs help tends to prove the point.
I also found they stood-off hood seemed to act ~like a spoiler, increasing high speed stability.

You can easily duct some of the air from the whaletail into the engine compartment.
I, for one, regularly embrace our new robot overlords, as I am the guy fixing the robots...

Gulfvet
Posts: 2
Joined: Sat Jan 30, 2016 4:32 pm

Re: why deck lid vents

Post by Gulfvet » Thu Mar 31, 2016 9:48 pm

The Deck lid vents were first used on the Convertible. No room to put vents in the normal place

User avatar
Marc
Moderator
Posts: 23722
Joined: Thu May 23, 2002 2:01 am
Contact:

Re: why deck lid vents

Post by Marc » Sun Jul 17, 2016 6:42 pm

volksbugly wrote:
Buggin_74 wrote: It was because dual port engines with the doghouse oil cooler have a larger fan than single port which draws in more air than just the vents below the window can provide...
Actually the louvered decklid was adopted on the Sedan in 1970, last year of the singleport engine with the narrower fan. Apparently the engineers felt that the 1600cc displacement alone warranted increased air supply. In practice, you'll find that a stock 1500SP gets along just fine with a non-louvered hood but a 1600 (SP or DP) will run hot on the highway, so it looks like they were right. No problem around town at sub-45MPH speeds, but on the highway you need to provide more air to anything bigger than a stock 1500SP, regardless of which fan it has.

aussiebug
Posts: 1510
Joined: Wed Mar 07, 2001 3:01 am
Location: Adelaide Australia

Re: why deck lid vents

Post by aussiebug » Sun Aug 14, 2016 7:07 am

My experience exactly Marc

I have owned my 1970 Euro spec 1500 swing axle bug since new. The engine lid is the stock 1970 "two slot" version. It does not overheat, even in Australia's tropics. The fan on these blows 22cf/sec at 4000 engine rpm.

I also have a 68 which has a 1600tp installed by a PO. Solid deck lid. It has the larger doghouse fan of course -25cf/sec at 4000 engine rpm. It ALWAYS overheated when running above 80kmh/50mph (winter or summer) until I installed a spare "two slot" engine lid. It would then run to about 120kmh before starting to overheat. I don't drive it that fast so have not bothered with a "4-slot" lid, but that would cure the problem at any speed.

The cars were getting heavier and so needed more jp for the same speed and acceleration (1808 lb for the 68/69 manuals, about 1900 for the USA 1970 bug and over 2000lb for the 71 convertible), so needed to get rid of more heat.

It would appear that the solid lid was ok for the 53hp 1500, marginal for the 57hp 1600sp in 1970 so the slotted cabrio lid was added. That lid was again marginal when the larger fan and 60hp engine came in 1971, so from 72 onwards all cars (even the 1300 twin ports used outside the USA) got the 4-slot lids. Then in about 75 the Thermo controlled rectangular flap under the licence plate was introduced. All in an effort to get as much cooling air into the fan as possible.
Regards
Rob
Rob and Dave's aircooled VW pages
Repairs and Maintenance for the home mechanic
www.vw-resource.com

Post Reply