why deck lid vents

[PJMS]

Scottrods makes good points, but I am not sure that both 'opinions' can be entirely correct. I have always thought VW's rationalizaion process almost more interesting than the subject matter. Though in this case it is not a big deal, it is important to more accurately follow the rational so as to not compound any errors later on.

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

My contention here is the assumption/belief that the vents were added because the bigger engine ran hotter. It is not true that, just because the engine produced 4hp more there is "definitely more heat" as there are other factors that could counter that effect. It also does not follow that since the fan was the same as the 1300sp that it was already at its limit with the 1500sp.

VW built the 1500sp and the 1600sp with the same capacity cooling systems, so that has to tell us that the bigger engine was not producing anymore heat, or at least not enough to have any negative impact. Even if it was running too hot, extra vents would not make any difference unless it increased the density of the air in the engine compartment (ram air). So, if the cooling system is the same, and adequate, it should then operate under a solid lid with no problems. The fact that it does not (at least in some condditions) means that the air pressure must have dropped and the only way that could happen is through the 7% increase in air sucked through the intake. Really the only thing at the "limit" was the lid itself!

[aussiebug]

I think the issue of weight has been forgotten. The 1300 bug of the mid 60s weighed 1675lb. The 68 1500 bug weighed 1800lb. The 70 IRS bug weighed over 1900lb and the cabrio vetsion over 2000lbs.

Increasing the weight requires more power (heat) for the same speed do to increased rolling resistance.

So sorry, but the 1600 engine WILL make more heat for the same speed in a stock bug, and it's got to remove than heat with the 1300 sized fan and NO increase in engine finning.

The slotted engine lid would certainly reduce any inlet air resistance.

[aussiebug]

I think the issue of weight has been forgotten. The 1300 bug of the mid 60s weighed 1675lb. The 68 1500 bug weighed 1800lb. The 70 IRS bug weighed over 1900lb and the cabrio vetsion over 2000lbs.

Increasing the weight requires more power (heat) for the same speed do to increased rolling resistance.

So sorry, but the 1600 engine WILL make more heat for the same speed in a stock bug, and it's got to remove than heat with the 1300 sized fan and NO increase in engine finning.

The slotted engine lid would certainly reduce any inlet air resistance.

[PJMS]

...So sorry, but the 1600 engine WILL make more heat for the same speed in a stock bug, and it's got to remove than heat with the 1300 sized fan and NO increase in engine finning......The slotted engine lid would certainly reduce any inlet air resistance.

OK. I am not in the position to look up the info now, but I would hazard a guess that the 1300sp used a smaller diameter crank pulley which would mean more CFM for the 1500sp, and so that may not be a good comparison. If you are right, that the 1600sp ran hotter than the 1500sp (and you may be), then why did VW not up the cooling to suit? Put it another way: if the two engines were being bench-tested (no cooling air restriction) and the 1600sp was running too hot, what would be the cure for that? You couldn't un-restrict any more air. If it did run hotter, then VW determined it was not significant enough to up the cooling (unless we have missed something with the crank pulleys on these two units). However, once the engine was placed in its true living quarters, they determined that extra vents were needed. Only the carb and the fan required this air and the only thing sucking more air, than previously, was the carb.

Interestingly, VW never used the 1600sp in European Type 1's, continuing with the 1500sp instead. These 1970 model 1500sp also had the extra louvre's in the lid. I always assumed this was to simplify production, but this did not make complete sense as the solid lid was still being used for 1200 and 1300 models. Maybe the use of the vented lid on the 1500sp was for 'model differentiation', or perhaps it provided an improvement to temperature control. If the latter is true, it would suggest that the 1500sp had already been operating below the air pressure limit and the increase in cc's made it worse.

[aussiebug]

The crank pulley remained the same. You might remember that the VW engine DID have occasional problem with breaking no3 exhaust valve. This valve ran hotter than the others as it had the most convoluted air path, and prior to the twin port engines with the stand-off oil cooler, the left cylinders got their cooling with pre-warmed air.

The 68 and 69 1500s had solid engine lids; but the 1970 1500s (like mine in Australia), and the 1600sp USA cars ALL got the slotted Cabrio lid. I think VW realized they had a worsening heat problem and the Cabrio lid was a stop-gap until they finished developing the twin port engine with the doghouse fan shroud. Design for this started in 69 and took about a year, at the then huge cost of about $US1 million. Then when they put the doghouse 1600 with its fatter fan - putting out 25cf (up from 21cf/sec)
in the 1971 cars. They found they still had a slight cooling problem so added the four sets of slots in 1972 and later years, even to the 1300 twin port cars. For info, both the 1200 engine, and the 1300 twin port engine, remained popular options outside the USA. Remember too that the 34pict/3 carb (introduced with the doghouse 1600) was trying to run the engine a bit leaner for emissions reasons, and this, together with the increasing weight of the cars, increased the heat output (hp needed) of the engine.

It's a fascinating process to look at the car starting out at 985cc and 25 hp, then progressing through 60 years of production to a 1600 60hp (65 hp in some versions) engine with substantially the same engine arrangement. That's a HUGE proportional increase, and attests to the quality of the engineering that they were able to come up with progressive solutions over that length of time, without changing the layout of the car.

[Ol'fogasaurus]

Sorry for butting in but this is an interesting discussion. Sometimes I wonder if the vents are not so much for cool air in but hot air out. The shape of the rear of the bug and pix of "cotton tails" that I have seen do show some turbulence back there.

Louvers on old cars hood sides and hot rods on the top of the hoods were more for escaping hot air than cool air for intake for engine use; e.g., a scoop. One of the reasons for the direction of the opening of the louvers facing towards the rear drawing air out than facing to the front like a scoop.

He who must not be named.

[aussiebug]

Nope, definitely for more fresh air in. The engine is divided upperside from underside by the breastplate tinware, and the cooling fan and carb draw their air from above the engine, so the under-window slots and the engine lid slots are all about getting enough cooling air and carb air in. Yes, the fact that the air had to reverse direction to be drawn into the cooling air slots was certainly a problem - much more so in later cars with an 80mph (about 135kmh) top speed than the first bugs with a 65 mph (105kmh) top speed.

In fact that emphasizes the importance of having all the tinware in place to prevent any mixing of the hot used cooling air under the car and the fresh cooling air above. All cars have a high pressure area under the car and a lower pressure area above and behind the car. Any gaps in the tinware encouraged hot used cooling air to "leak" upwards into the upper engine area so your cooling fan sucked hot air into the fan for cooling - overheating is thetefore almost inevitable.

[Ol'fogasaurus]

I made a post but deleted it as I had goofed and turned things around . Air comes in the back by the window vents then is (supposedly) sucked up by the fan and driven the though the engine so the heated air is blown out the bottom (hopefully anyway). The vents on the engine cover lid is more problematic especially with airflow around the body.

Buggies and rails don't think that way as we have to deal with a fully open (hopefully) situation. Turboing and such can cloud the situation depending on how things are arranged and/or packed into tight areas. Kit cars can be more problematic like bugs than buggies.

[volksbugly]

Air goes in the top vents, then air goes out the vents on the deck lid once a certain speed is reached. Its a Venturi effect, just rember the slope of the decklid and the low pressure vs high pressure areas.

[Ol'fogasaurus]

Air goes in the top vents, then air goes out the vents on the deck lid once a certain speed is reached. Its a Venturi effect, just rember the slope of the decklid and the low pressure vs high pressure areas.

Assuming you aren't getting any curl to the wind like a pick up truck does.

wind-effects-australia[1].jpg

For example

[PJMS]

To follow on from Rob's comments. Though the positioning of the vents at the back of the car meant that air could theoretically be sucked put of the engine compartment, this was overwhelmed by the low pressure created inside the engine compartment by the carb and cooling fan. The design of the vents also insured the least resistance to that low pressure inside. This is one of the reasons why these engines get hotter the faster you go; due to the dropping low pressure behind the car.

[Piledriver]

I always attributed it to drag going up the square of the speed...

[aussiebug]

Air goes in the top vents, then air goes out the vents on the deck lid once a certain speed is reached. Its a Venturi effect, just rember the slope of the decklid and the low pressure vs high pressure areas.

Fraid not. If that was the case, cabrios which ONLY have deck lid vents would have no air flowing into the fan or the carb.

The slotted lids are definitely to allow more air into the upper engine area, mostly for the fan (which got fatter for more airflow with the doghouse engines) but also for the carb. The fan on doghouse engines pulls 43,000litres per minute at around 4000 rpm. That's a LOT of air. The carb on a 1600 engine is drawing in about 3200 litres per minute at the same rpm. If the deck lid vents were drawing air OUT of the engine bay, where would all that air requirement be coming from???

Re the comments on drag. Wheel/mechanical drag is roughly linear - double the speed and that drag roughly doubles. But as piledriver said, air drag increases with the square of the speed, so double the speed results in 4 times the drag.

Think of this - the original 1940 bug could do 105kmh (about 63mph) on just 25 hp (985cc engine). But the 1970 1500 bug could manage only 135kmh (80 mph) on 53hp, and the 72 1600 bug got 83mph on 60 hp (non emission AD series engine). 25hp gets you 63 mpg and 35 extra hp gets you just 20mph more. THATS the effect of air drag. The extra weight of the bug is part of it of course, but the effect is small compared to the areodynamic drag.

The engine gets hot at higher speeds mainly due to this increased hp requirement. The 1300, 1500 and 1600 all have the same fin area, which Bob Hoover (remember him?) determined could remove only about 45hp worth of heat continuously, so if you use more than that, the temperature rises. The 1700-2000cc type 4 engine, in comparison, has about 30% more finning area, and those engines can make a lot more continuous hp without overheating.

[jedavisss8]

I know this thread is a couple years old, but I'm new to the forum.
I have a '70 bug Autostick which has a 1300 block with dual port heads and 34pict3 carb, stock exhaust. Obviously a rebuild some time in it's past. Odometer shows 74k, but it's blowing a little oil smoke. I'm looking to rebuild up to 1776. It has the two vent deck lid, but I assume not the doghouse shroud and larger oil cooler.
Here's the question: given the larger displacement and more heat, should I (1) replace the shroud with the doghouse, and if I do, do I need to replace any of the rest of the tin, or, (2) replace the deck lid with the later 4 vent lid, or, (3) get and external oil cooler, or any combination of the three. I've never liked the look of the standoff lid.
Replies appreciated.

[FJCamper]

I had a '68 Bug that dropped a valve in 1970. H series case. Nonvented engine lid.

I replaced it with a single port head 1600 VW dealership factory rebuilt engine. The 1600 ran hot. After half an hour of interstate cruising, the oil light would flicker but once I was on a side street until the engine cooled it was OK. I had one of those dipstick thermomoters at the time which used the oil warning light to warm of overheating. If I remember right, it triggered at 240 degrees F or over.

I installed a vented engine lid and the oil temp problems imediately went away. I was just learning aerodynamics at the time and realized from the example of the the Porsche 550 Spyder that the cooling air was coming in from the rear (!) although to the eye that seems wrong.

FJC