why deck lid vents

[ProMetalShop]

Lots of great information ! !

[Clackety Clack]

Yeah, I thoroughly enjoyed this thread. Is there a book out there that walks you through in detail Porsches design of the beetle step by step. For example why there are vents in the deck lid, why he chose this over that etc....love to read it if it exists.

[tattooed_pariah]

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.

Holy ancient thread batman!

My 1972 beetle had a rain tray when I bought it back in 2002.. but I was younger and dumber back then, and didn't think I'd ever be living outside of California, so I took it out and threw it away..

Fast forward to about 3 weekends ago.. I live in Maryland now and we had some really bad rain storms.. While sitting parked for the weekend, I got about 3/4 a cup of water in #1 cylinder.. my only speculation is rain came in the louvers, soaked into the air filters (dual IDFs) and pooled in the air filter bases before dripping down the intakes under the velocity stacks (no gasket under the stacks..)

I went ahead and drilled some tiny drain holes int he bases, and put gaskets under the stacks and also bought a new rain tray.

I put the tray in last night, and driving to work this morning, CHT's were about 20-30 degrees F hotter than without the rain tray. so it DEFINITELY obstructs airflow.. but I don't want to get stranded with a flooded engine, so I'm gonna keep it and see what I can do to bring in more airflow.. right now I'm thinking of the standoff that raises the bottom of the decklid further from the apron..

[FJCamper]

Hi Pariah,

You need Porsche style "rain hats" for the Webers.

The stock air filters do absorb water like crazy when they can. Get yourself a couple of enameled loaf pans that, fitted upside down, cover the top half of the filter. Find a way to bolt them down so they are suspended over the filter for air flow.

It works.

FJC

[Ol'fogasaurus]

Hi Pariah,

You need Porsche style "rain hats" for the Webers.

The stock air filters do absorb water like crazy when they can. Get yourself a couple of enameled loaf pans that, fitted upside down, cover the top half of the filter. Find a way to bolt them down so they are suspended over the filter for air flow.

It works.

FJC

Now that is do-it-yourself enginuity! Impressed FJ

Lee

[Piledriver]

Bakers Secret loaf pans rock.

[saggs]

Good thread, with all this being said do you think I should add some fresh air in through the front firewall tin of my T4 powered 56 oval? There isn't a lot of slots on this model. Havent tried propping the decklid open at the top or bottom yet to see if there's a difference? My CHT is around 350 at 65-70 mph.

[aussiebug]

350cht is about as hot as you'd want to get.

The early bugs with solid lids and doghouse engines WILL run short of cooling at at higher speeds as the larger fan just can't pull the required amount of cooling air through the under window slots. Try cracking the engine lid open at the bottom and see what happens to your temps. This will (I expect) demonstrate the need for extra cooling slots. A cabrio lid, a standoff licence plate with a slightly smaller hole behind it, or slots grafted into your lid. Any of those will help.

[saggs]

good info, I will try propping the lid open this weekend.

[tattooed_pariah]

I haven't done much.. I don't leave the house too often, mostly just daily driving to/from work, and it seems like mid afternoon (1530ish) cruising at about 70-80mph, #2 is getting to about 315F, #3 gets to about 290F and #1 and #4 go between 250 and 270F, so I'm concerned, but not overly concerned..

and if you're wondering why #2 is my hottest cylinder, my best guess is because I don't have the forward (bellhousing) cooling tin, or the rear-most cooling tin in place right now, I need to buy new ones and make em fit.. so I'm guessing the exhaust heat is hitting #2 the most because that's where my header merges and the muffler begins..

[Bruce.m]

What!

Running an aircooled engine without a full set of well fitting cooling tinwear causes overheating.

[Piledriver]

If the fan can suck hot stale cooling air or off the exhaust, it will.
Do't let id idle for any extended period.
You should be in great shape once fully sealed, can you adapt the stock thermostat flaps?

[PJMS]

To get a more accurate understanding of why deck lid vents were introduced on the sedan models, look first at their introduction with the 1600sp. This engine shared the same cooling specs as the 1500sp so the engine itself was not going to run any hotter. However, the engine did suck more air through its intake and this effectively robbed air from the cooling fan (intake is more powerful), or rather reduced air pressure making the air less able to absorb heat. So, add vents to keep the air pressure up! The problem simply got worse with the higher capacity doghouse systems, so the vent area was increased. A similar drop in pressure happens at high vehicle speeds, due to the way the air passes over the car; so engine temps will go up with speed.

By the way, the rain guards were discontinued, not because they restricted air flow but, because it was getting too crowded in the engine bay. To make up for its removal, VW water-proofed the electrical system; was way cheaper too!

[aussiebug]

Can't agree with the above.

1. The 1500 engine produced 53hp

2 the 1600sp engine produced 57hp. That's 4 hp more so definitely more heat.

3. This is the same fan as was originally fitted to the 1300 engine with 44 hp.

So even at 53hp it was close to its limits for cooling capacity.

This fan produces 21 cf/sec or 35600L/min at 4000rpm

The 1500 engine consumes 3000L/min through the carb - about 8% of the airflow which goes through the fan.

The 1600 engine consumes 3200L/min through the carb - an insignificant increase.

The slots added in to the engine lid in 1970 reduced any restriction in the fan air inlet

It's the flow through the fan which is critical, not the flow through the carb.

[SCOTTRODS]

I say it's both... so, considering everyone has an opinion but none actually engineered the cars... I have yet to see engineering data (Although both of the above posts have great points).

If you have an engine that is literally pulling almost ALL of the available air to cool AND aspirate the engine, then you up the size of anything from the oil cooler to the carburetor throat, you begin to introduce variables that can be dealt with in a variety of ways. Working on Small engines daily I see the results of both issues, and neither have a Classic VW attached. If you restrict the available flow of air to either the cooling system, or the Carb, you create bigger heat issues... In the VW they share the same space and available air for cooling and Running. If you restrict air to the running (carb) you always increase heat int the exhaust by leaving a richer running engine, which will increase exhaust temps... And Likewise, restrict the cooling air inlet, you decrease the cooling systems ability to remove heat... both at the same time. This tells me both of you are completely right.

And... Just because you disagree with something doesn't;t give you the high road. Yes... your data looks better on paper, but it doesn't;t give the whole picture, just the data to support your claim.

I have talked to old VW techs that would agree with each of the two positions, and more. You also have to figure in a couple of other issues like heat created and from where it comes from... bigger engines do produce higher temps. In any situation higher temps are created, you do need to remove more heat... period. If you plan on running more horsepower, it creates more heat... BUUUUT... it also requires more breathable air to operate at peak efficiencies... which VW was really good at riding the line with these two things... and I think they did a Great job at a particularly low price point.

So my point is, the flow through the fan AND the carb are both contributors to the need for more vents in the deck lids over time. Can't have one without the other.