Oil coolers--What Berg says! m

For what it's worth, here's my opinion:

If you had a 1776 engine, I would think it's fine to go back to the original OEM cooling system, but yours has been modified so much you'll need to find a new, complete, fan shroud, the cooling thermostat and its' mounting bracket and push rod, a new oil cooler tower and maybe a few other pieces of engine tin if they were left out.

BTW: From your description of what was in there to cool the oil, I don't think it would have worked very well at all.

For an 1835 engine, you're on that hairy edge of being able to cool it with a stock OEM cooling setup, and that's where people generally move to a full-flowed case with a fan-assisted external cooler, in addition to the original, stock, 1971-or-later cooling tower/fan shroud.  

My 2,110 would run around 220F at 90F degrees ambient on a turnpike with a very complete stock, 1973 OEM fan system.  I thought that was a tad toasty, so I full-flowed the case and added a DeRale 16-pass cooler and that brought me down to 190-200F under the same conditions. 

As an alternative to the drilling/tapping of the case for full flow (see my article in the knowledge base), there are a few oil pump covers being sold that allow some percentage of the oil pump output to go out to an external filter/cooler and then back to the oil pump.  Not as good as a true, full-flow set-up, but this type of system is effective to some degree and might be just the ticket for an 1835 which doesn't have the cooling requirements of a 1915 or larger engine.

So, bottom line:  The absolute BEST thing to do is pull the engine and have it modified for full-flow, then add an external filter and cooler and be done with it.  That's not what Gene would do, but Gene was dealing with VW sedans and Ghias which had significantly more air flow to the engine compartment than our cars do.  We're telling you what has been shown to work in Speedster replicas.

The next best thing would be to tap off the oil flow with a pump cover designed to do that and add the external filter and cooler as above.  Not as effective, but should work well enough to cool your engine.

Hope this helps. 

I've been playing with these engines on and off since 1974 and have built several engines along the way, as well as paying attention to what other guys are doing and being successful (and sometimes not so successful) with; not trying to toot my own horn, but just telling you my experience. As I said in the other thread, run the doghouse cooler, shroud and flaps. The full flow filter, of course is a great addition, since these engines don't have any adequate filtering. If the engine is in proper tune (including spark plugs of the right heat range) , jetted correctly, compression ratio within reasonable limits for the combo, and there is proper airflow through the engine compartment (these things are aircooled afterall!), your 1835 should survive quite nicely without an add-on cooler. 

Note the words highlited, as this is more important than most people realize. When you have your car up and running, even if it seems to run within proper temperature parameters around town but overheats on the highway in warmer weather, try driving around for a little while with the engine lid propped open (one of the guys used a chunk of pool noodle) for a little bit, and if the oil (and head) temps come down at all there isn't enough airflow THROUGH the engine compartment. I know, your car has lots of air access into the engine compartment, but it needs to flow through and remove the heat radiating off of the engine as well, so the air going into the carburetors and fan (to cool the engine) is at or close to the ambient temperature around the car and not entering the fan preheated (where it will remove less heat). And colder air into the carbs will make more hp, too.

Most of the time (there are situations where it is beneficial) putting an extra cooler on a stock stroke (think < than 2 liters) engine isn't needed, hot oil is a symptom for something else wrong and over cooling the oil is a bandaid, not a cure. Now oil temps stay within proper parameters, but head temps could be above what they should be. The engine will run like this for a while, but when it dies prematurely (and you'll be thinking, "what the hell happened, it was running so well?") it will probably strand you in the middle of nowhere.

A lot of what Gene wrote still stands today, although people have proved his thoughts on compression ratios wrong. His c.r. ideas were formulated in the mid-late '70's when gas was going through major reformulations, and unfortunately never continued his research into what worked on the street with engines of different power levels and camshaft durations.

In the late '70's we could all still buy Sunoco 260 at 104 Octane and most of the "regular" pump gas was 93 Octane or something.  Some of us lucky ones lived near airports and could buy 110 Octane for Friday night "cruising" with our Mustang-killers. 

Yeah.....Back when pump gas had "Cojones"!!

At the other extreme, for you San Diego/Tijuana guys, was Mexican "PEMEX" gas at roughly 67 Octane (and, I swear, refined from pure tap water).

I pretend to be an engine guy, so I'll give it a go.

If you have a decent set-up, and follow ALL of Gene Berg's advice, your engine will cool well and last a long time, period. The thing is, Gene's recommendations leave a LOT of power on the table, and assume daily-driver use.

An 1835 has 92 mm cylinders. Cima/Mahle 92s dropped into the same registers (case and head openings) as 90.5s, so the cylinder walls were thin. Really, really thin. Too thin for reliability, no matter what was done for cooling.

The thinner a cylinder wall gets, the more likely it is to distort (out of round) as it gets hot. Stock cylinders are thick. 90.5s are slightly thinner, but still pretty thick- that's why a lot of guys like 1776s and 2110s: they use 90.5 cylinders. 94s drop into their own (bigger) register. They are thinner, but not by a lot. Using 94s means you better do enough to keep temperatures under control- do it right, and you'll reap big rewards; do it wrong, and you'll pay the price. Cima/Mahle 92s are a bridge too far. They're too thin for reliability, no matter what your cooling system is... unless you run 7:1 compression.

But... here's the wild card. A few years back, an Asian company named AA started producing 92s that drop into the 94 register. They are thick. Really, really thick. Thicker than stock thick, which makes them the cat's meow for reliability. You can run some nice, high compression ratios with "thick-wall 92s", and still get good ring seal, especially with Mahle pistons and rings. Jim Ignacio went darned near 100 K mi on his thick-wall 1835 in LA heat with a 36 hp dog-hour shroud.

So, what do you need? It really depends on what you've got. 

I'm not an engine guy, and don't play one on TV.

But, after two years, two engines, and two coolers, I've learned this. If you do install an external oil cooler, mount it in the wheel well, or just about any place other than in the compartment in front of the engine compartment - the space you can see into if you look through the hole in the firewall.

This is the place where VS and other builders mount their external coolers because it is convenient. There's lot's of room there and no chance of rubbing against wheels or anything else. But, don't do it.

It's a closed space on three sides, and heat from the cooler collects there, making the cooler less and less efficient as things heat up. But worse, that hot air gets sucked into the engine compartment by the engine fan, making THAT less and less efficient, too.

Any cooler works best when in a stream of cool, flowing air. If you go to the trouble and expense of installing one, give it a fighting chance to do its job.

"...and colder air going into the carbs will make more hp too" (so sayith ALB)

    Has anyone devised a scoop from in front of the rear wheels sucking in ambient air through the side firewall and directed to the carb air filters?  Perhaps a Venturi accelerated air flow configuration?

    Louvers theoretically do a good job of evacuating hot air because of the negative air flow pressure over the lid and the rear of the car. But the entire lid propped open about two inches really sucks that heated air out!

    There's an aesthetic down side to cruising about though with the engine lid ajar...so what if an unobtrusive evacuation vent (4x9"?) were installed, out of sight, behind the license plate?    

Henry does this on the 911 powered cars.

As far as the wheel-well air-intakes. I've considered this more than once, but always get hung up on sucking water spray driving in the rain (and I have driven in biblical-grade rain).

So venting out the heat seems to be as important and bring in fresh air. Anyone thought about a bilge pump encased neatly on the right side of the compartment with an exhaust outward. Seems to me if you have a way to suck hot air out it would naturally draw fresh air in??? Maybe duct it upward and centered with a flex heat hose under the lid so the heat is drawn from both sides. Or, get rid of the license plate and run a cool looking port with a chrome grill straight out the back--I'd still think a bilge pump thermostatically controlled would be the cat's ass.

I read about the AA jugs--sounds like a winter alternative as I plan on breaking it all down as I continue going through the new toy.

I think he means a "Bilge Blower" rather than a pump.

Wheel well fresh air intakes for induction or brake cooling has been used by racers for decades and many of the long distance international races often run in down pours with no issues of rain spray entering carbureration. Think about it functioning hood scoops that draw fresh air right onto carbs seen on many muscle cars etc. do not have problems. The Cobra has a functioning hood scoop that draws fresh air to the Webers, I've driven that car several times in heavy rains and never had a problem. Also many cars, even newer ones, draw fresh air through hoses to the induction directly through the grilles. Some stock and aftermarket air cleaners are built/designed with fresh air hoses drawing from the grilles.

If the fresh air induction hose for a Speedster is screened at the inlet end to prevent any debris from entering, mounted low enough to allow fresh air in and the hose has a bend in it, which it would on a Speedster, on the way up to the outlet point I seriously do not think there'd be a problem of moisture intrusion on the carb/carbs.

I've had "bilge" blowers on a couple of boats.  They don't move a whole lot of air - like, maybe 150 cfm.  The good news is that they're 12V DC, but that's about it.  I really can't see them evacuating or blowing in to the engine compartment.  It would compete with the engine fan and lose big time.

Carl wrote:  "so what if an unobtrusive evacuation vent (4x9"?) were installed, out of sight, behind the license plate? "

Henry Reisner uses that trick (I think Merklin does, too) and it seems reasonably effective, but instead, how about a grill just above the engine cover as Alan built into his last Speedster build?  I know he did it because he deleted the engine cover grill, but I think it would make a whole lot of sense in addition to the engine cover grill.

If that wouldn't cure high running temp problems on a Speedster replica, nothing would.

In school I did poorly in science and math...but would have excelled if 'common sense and skepticism' had been offered as a subject.

Unless the volume of induced air into the engine compartment replenishes (equals) the volume of expelled air, that space will remain hot!... The doghouse fan is merely recirculating hot air. A constant supply of cooler air is required to establish such a flow... which means vented ambient air in has to equal air out.

An opening behind the doghouse provides between 25 to 36sq" which can't provide the volume to match the potential exhausted air of the single grill with 112sq"

I've seen photos of massive cc engines in bugs in which the entire fire wall is perforated with rows of louvers...It seems like common sense to me!     

You're right, Bill, those louvered aluminum firewall covers are just that. Carl gets it though; you can't have airflow removing radiant heat from the shroud, cylinder covers and top of the case if the air coming in can't get out. 

I have been forced to learn about air movement, since my SAS Subi engine overheated as designed.  I'm still learning.  In a closed compartment, such as the VW engine bay, a low pressure exit is as important as a high pressure entrance, since it the pressure DIFFERENCE that's important.  A sealed engine, however, has a high pressure air inlet with a corresponding high pressure build up in the compartment itself, even with minor blowby from seals, tins, etc.  If you have 5 psi positive air pressure at the inlet to the engine, and 5 psi internal pressure, air doesn't move.

A Dwyer magnehelic gauge measures from 0 to 1" of water pressure.  Gauges are fairly cheap at about $150 new.  Prior to deciding the best places to open up a firewall or engine cover, measurements should be taken to determine where the greatest pressure differences exist as the car sits, i.e., before adding vents.  

Surprisingly, we really don't realize much about air pressures, since they remain invisible.  By using a magnehilic gauge, which shows small pressure differences, you can see where the lowest pressure is on the outside of the engine lid, and where the highest pressure is in the engine bay itself.  That area of greatest difference is where you would place a scoop/vent/air exit. Moving the air inlet hose on the gauge a few inches to a foot shows that air pressures can change dramatically in a short distance.  Vents should be positioned to take advantage of the exact spot that has the largest pressure differential.  

BTW, most of this info is counter-intuitive, or, at least it seems so to me.  When you look at a hood on a front-engine car, for example, the air pressure across that hood, i.e., front to back and right to left, has extreme differences in air pressure, from negative .1 to +.4" of water.  No wind tunnel needed, just the magnehelic gauge.  The front of the hood has negative pressure, with pressure increasing as you move toward the windshield.  That's why muscle car guys struggle with hood scoops, just like we do with engine lid louvers.

Pressure differentials, and our inability to understand them, is what leads to inlet vents that are actually outlet vents, and vice versa.  In my case, I need to know where to place heat exchangers for best efficiency.  Mine will be used for engine water cooling, A/W intercoolers, and oil coolers.  Knowing pressure differentials prior to placement helps for proper placement.  I will then use ducting before the HE's and fan shrouds to help air exchange.

There are also ways to lower the pressure under the car to assist in increasing pressure differentials, through the use of undertrays.  We'd like to suck that hot air out of the engine compartment, and funnel it under the car and out the rear.  

It's easy to find research papers that discuss all of the above, but rarely provide how-to directions or the means to measure and prove an increase in cooling capacity.  Bottom line is that, if the pressures inside and outside the engine compartment are the same, air doesn't move.  Naturally, fans are used in both water & air-cooled engines, but fans are more effective if we improve pressure differentials in our engine compartments.  The air needs a low pressure exit point.

Admittedly, air pressures that affect a vehicle change with vehicle speed.  Air pressure at the front of a heat exchanger is not enough to force air through the fins unless there is a lower pressure at the rear.  An engine compartment can fill with air, then lower the pressure differential so much that the HE quits working efficiently, unless the air has a low pressure place to go.  

It's a subject that's pretty interesting and complicated.  I'm not a science whiz, so it is taking me longer to learn than someone with a science background.  If one person mastered the aerodynamics of the speedster, he could set up an assembly line at Carlisle to place the pressure gauge at various spots on the inside and outside of the engine lid, check differential pressures by road testing, and recommend the necessary louvers/vents/scoops needed to lower engine temps.  This is one of the many areas on line that has LOTS of information, with only a tiny fraction applicable to our replicas. 

Well... almost! Except for the "recirculated air" part.

There's no possibility of recirculating air inside the engine compartment, unless you've got no engine sealing tins. The engine has a big 'ol fan, and it's blowing straight down, and out of the engine compartment. The entire engine compartment (topside) is a giant negative pressure zone, and has to be for anything to work. Air flowing "through" the engine compartment doesn't go in the front and out the back. It comes in every crack and crevice, and gets sucked down the intakes or pushed out the bottom of the engine.

It's impossible to expel more air that you take in, no matter how big a fan you have. You are only going to move as much air as the fan and carbs will pull through the intake opening at a given static pressure. If your engine compartment had no louvers, and was completely sealed- we wouldn't move any air, but we wouldn't be re-circulating anything.

The problem with speedsters is that the louvers are in a negative pressure zone, so the fan (and the carbs) have to suck that much harder to overcome it and entice the air into the engine compartment. A hole behind the licence plate is also in a negative pressure zone, as is pretty much anything besides a fender mounted scoop. However, any opening into clean air helps, as at least you don't have to overcome so great a restriction through a smallish opening. I'm not sure what's going on in the space between the cabin and the firewall, but it's not clean/cool air, and I haven't noticed anything being any better with 2 giant holes there. 

The fan is also fighting with the carburetors, which are flowing as much air as the cylinders will hold during 500- 3000 suck, squish, bang, blow cycles every minute. The combination of the 2 is a lot of air.

So, make no mistake- your louvers are moving a LOT of air, and in the right direction (in). It's just not enough. The faster you go, the greater the static pressure you have to overcome to pull air into the engine compartment, and the greater your carburators' hunger for air becomes. It's great that the fan is engine driven so that it's speed increases as RPMs increase, or you'd have no chance at all of getting air over your heads. Popping the hood clearly creates some turbulence that must negate some of the negative pressure behind the car- the effect is certainly more than the increase in the opening.

The miracle is that this works at all.

"Popping the hood clearly creates some turbulence that must negate some of the negative pressure behind the car" 

It's the turbulence that creates negative pressure...It's what sucks air out of the engine compartment. I't's what 'lifts' aircraft off the ground. The faster you go the more turbulence is created. The faster you go the more air it 'sucks' out...the more it lifts an aircraft until velocity minimizes the need for lift to keep it from falling out of the sky.

Turbulence simply doesn't "negate negative  pressure"

...It increases it.

A long time ago I taped strands of fluffy yarn to the grill of my speedster. The faster I went the straighter the strands stood up reaching for positive pressure in the air flow above them. I reached a speed in which everything light weight and loose in the cockpit, including my baseball cap, danced right out! It wasn't wind velocity that 'blew' them up and out...it was increased negative pressure that 'lifted' them up and out.    

I only know enough to be dangerous, but I think Carl has it right.  Low or negative pressure is what we want behind our cars.  That helps create the pressure differential needed to make air move out of the engine compartment.  Even a slight "rake", as we used to say way back when, with the rear of the car slightly elevated, helps to draw air under and behind the car.  Low pressure on the outside, high pressure on the inside = lots of air movement and more efficient cooling.  You can find color diagrams on the net of air flow over car bodies, with different colors showing high to low pressure.  All very interesting stuff to me, strange as it may seem.

How cool would it be to be able to bring our cars to a Porsche wind tunnel with experienced engineers to map out the air flow patterns.  The correct front spoiler, air splitter, and under tray for maximizing low pressure under and around the car, proper fender vents to the engine, man, you'd have a real rocket with low head and oil temps, and great down force.

Look- I'm just a guy with a high-school education, but you're right, Jim- this stuff is interesting.

You're both correct. Turbulence creates negative pressure, but it also adds buffeting. And yes, you're correct- stuff wants to come out of a convertible at speed (once you hit the real negative pressure zone behind the front seats). I had a friend in HS that tried to urinate standing backwards on the back-seat of an Impala convertible, and ended up taking a yellow shower.

There is also a pocket of still air behind your windshield, which is ahead of your turbulent zone. The higher the obstruction, the greater the turbulence- but the wider that still-air pocket is. I'm suspecting this small pocket is where the engine gets it's air with the hood popped, but what do I know?

The fact remains that the air your engine uses comes from somewhere, and it's not readily apparent where. Your fan (and carbs) have to be able to overcome negative pressure and move air top to bottom, or the engine would burn up. There's no other explanation. Your engine uses an enormous amount of air.

I know it's popular to believe the air comes from the firewall, but my IM came from Vancouver with a solid firewall. When I opened up 2 8" holes a few years back, the engine actually ran hotter- not a lot, but appreciably.

It runs cooler (by at least 15* of oil temp in the desert) with the lid popped. Perhaps the air is being scooped by the leading edge of the deck-lid, but I doubt it. Terry Nuckels has noted that he runs 10* cooler with a suitcase on his luggage rack. I have to think a disruption of the laminar flow over the back of the car has something to do with it.

Here you go, Jim: 

Back in the sixties, I raced a Fiat-Abarth 750, it had two brackets to hold open the deck open to exit hot air (it was water cooled, but the radiator was mounted in the rear next to the engine)(?) it also created down force.

Carl;

The science is the same, when racing sailboats many years ago, we put yarn in several  points on the leading edge of sails. This way we could trim the sail to get maximum lift from the sail and the wind.  When the yarn was horizontal up and down the leading edge, we knew the boat was trimmed up properly.

Over the years, I've probably Googled every 356 permutation possible to see photos like Bob posted above.  One in particular still makes me smile.  A guy had an authentic Speedster to which he attached a HUGE scoop upward and forward on the deck lid.  The scoop was a slightly cut down 15-gallon trash can, looking like it just came off the shelf at K-Mart.  The owner had an authentic SEG, but the Grey Pouponers looking on seemed horrified.

Well reading all the posts are very interesting and some very good insights and logic. Now for my two sense worth. The comments regarding berg and compression ratio is very valid. Those tests or facts relate to a vw bug body. The issue with these plastic cars is airflow in the engine compartment. Think about it. The motor is like a big air pump. The combinaction of the motor bore & Stroke, head design/casting, oiling system, and compression ratio along with cam design all play a huge part on how the motor will run. In addition using a dog house shroud with the vanes or a aftermarket porsche copy makes a big difference on the cooling. The factory design is very efficient. But many people don't seal the cooler to the shroud very well and it loses its effectiveness. Now the porsche fan just blows a lot more air than a factory set up which keeps the heads, barrels and case cool. Now where am I going? If you open your deck lid and look at your engine you see two air cleaners which require a certain amount of air flow to operate(cool air, the better) and there is the engine fan Which requires a large volume air. The air inlet on your deck lid is believed to be the source of fresh air. The air going into the engine compartment is challenged by the heat generated in the compartment and being contained. Your carbs and fan begin to suck that hot air into your air pump(engine). The efficiently of the engine is reduced and it begins to run warmer. 

now how do you get fresh air into the engine compartment? This is the ongoing debate. Terry's comment regarding the luggage rack proves it somehow diverts the air flow into the vent opening Compared to a car without one Which the air flows over the back of the car and acts as vacuum at the vent grille and draws air out. (if you drive down the road with little ribbons tied to the grille they are not sucked into the engine compartment but are standing upward. Therefore you must duct fresh air into the engine compartment from elsewhere.

the motor is going to generate heat. It's all a matter how you manage it. Large sumps and oil coolers prolong the time you reach that "hot" point. The trick is to get it to operating temp and maintain it! Cool air flow is the ticket.  

getting back to the engine, a well known vw guru (RIP) had a trick of cooling the engine heads. I use to do the same sort thing when racing vws and I did it by mounting a spray bar in the valve cover to spray oil to cool the heads and Ray would do by drilling a very small hole in the pushrod to cool the heads. The counter issue was the oil temps increased but the head temps dropped.

i may have confused some of you. But bottom line is fresh cool air. 

I agree with Stan 100%. Back in the day, in a bug, Berg was on point. Today's fuel and higher compression CAN coexist. I'm running 9.8:1 and 32 degrees total advance at full load. for what, almost 38,000 miles?

The proof is in the pudding, I have a 911 fan which is not supposed to work as well. Also have an external 96 plate cooler, full flow, and oil and electric fan thermostats. In a 2165 type1 in a Spyder. Oil runs 180, CHT 270 or less, max 300 in traffic. Lots of work to get it to function reliably and keep cool, but I'm there and I've been there for a good long time now, over 20,000 miles.

Note the fans in place of the shroud

No way that's going to live long on the street. Must be a drag car?

     An alternative explanation as to why Terry is running cooler with luggage strapped to his rack is that the suitcase protruding into the already low pressure bubble of disturbed air is acting like extended 'slats' on an air wing...creating a much larger low pressure bubble and therefore a great deal more lift.

    It's unlikely that the turbulent low pressure bubble (established over the entire car from the windshield back) can reformulate itself into a smooth laminar flow within the short distance of 12, 14 inches, between the back of the cowl and the beginning of the grill, to force high pressure ambient air into the compartment. 

    Maybe Terry can end this speculative thread by attaching a few strands of yarn to the grill in front of  -and along the sides-  of his grill as he winds (blasts?) his way into Tahoe!

    There's one thing about engine cooling configurations that I simply don't understand. I've been reluctant to ask because I'm possibly the only SOC's that doesn't know the answer...and I really hate broadcasting my ignorance!!! (I'm tempted to pull out the Ouija Board and try contacting Ferry directly for the definitive explanation)...

    Why is it... that Spyders can operate without engine tins while the same air cooled engine in a Speedster requires such tins within a tightly enclosed compartment?    

Damn good question, that.

The same applies to Dune Buggies with the engine hanging out in free space.

  Neatly cutting out behind the lic plate, run some edging around the opening to finish it off, off sett the tag will do wonders for cooling, my standard M/O when ever possible.

Alan; Thanks great idea. Um, though I don't understand off set the tag?  Do you mean license. plate? and "off set" , make it proud of the hole with, say rubber washers?

Yes,  rubber washer similar to shock rubbers.... When you cut it out be sure to leave " tabs" in the fiberglass to accept the tag bolts.

Carl, must be the general shape and/or the extremely cavernous open air under the rear clamshell. But I don't have a clue, it just works......

Alan, a nice idea for you: After you open the license plate area and trim it off, drill the 4 holes for the plate and put in some aluminum 1/4-20 rivnuts. Then get some nylon spacers/standoffs to give some room for the air to go in.

I did all that except for the air hole on mine, and I also bent the plate to follow the Spyder contour. Keeps the plate off the paint and looks nicely finished.

I do understand opening up the license plate area, but how far do you stand-off the plate with the nylon spacers?

Very interesting method of removing hot air to supplement the deck lid grille.

A US license plate is what, 6" X 12"? The hole you make should be say 4" x 10", so 40 sq. inches. So if the plate is 6" x 12", then the circumference is 36", so a 1" space is 36 sq. inches, close to the size of the opening you'll make. It's just math........

The tin is there to keep hot air that has been dumped across the cylinders and oil cooler, and more importantly radiating up from the exhaust from being re-circulated into the engine again. That's all it's there for.

The cooling fan is not small- and contrary to how it looks, it's actually a pretty sophisticated design. It's a backward inclined fan, capable of overcoming huge static pressure differentials. Other types of fans move more air with less horsepower, but I think it's telling that Dr. Porsche used this kind for the VW Type 1 and 356. They self-stall, and they can overcome static pressure differentials of up to 20". A backward inclined fan can suck harder than the negative pressure zone behind the car, and still move air from the top side (what we see as the engine compartment) to the bottom side where it is expelled out the back. This is no small amount of air we are talking about- the fan takes up to 20 hp (by some dyno observations) to drive. It moves a lot of air across some pretty crazy static pressure differentials.

The carburators also require a lot of air. I have a 2276, which means that over two and a quarter liters of air move through my carbs at half the engine RPMs. So as I'm driving down the road at 3500 RPM, my carbs are sucking 2276 ccs of air 1750 times a minute. That's almost 4 million ccs of air every minute, or 140 cubic ft of air. And that's for the engine at 3500 RPM. My engine is capable of turning 2x that much. That's as much volume as a good sized bathroom. Every minute the carbs suck all that air down and throw it out the exhaust. Perhaps a better picture is to think of the volume of your exhaust. That exhaust came from somewhere- the vast bulk of it was sucked down your carbs.

That's a freak-load of air being sucked down your carbs. The combination of that air, plus the air your cooling fan is moving, is a huge amount of air. All that air has to come form somewhere. The idea that holes in the topside of the car let air out just fly in the face of those air requirements. It really does have to come from somewhere.

So the nylon spacers are one-inch long and the plate stands-off one inch, correct.  The open area is an easy cutout.

I searched the interweb, looking for any testing/data/information on airflow/air intake on an air cooled VW.  So far I haven't come across any actual tested results.  No hard data.

Surely, someone has done some airflow testing on a high performance air cooled VW.