Air Cooled engine in action

Going by many names, such as opposed, boxer, and flat, the engine configuration famous to such cars as the Porsche 911 has had a storied run throughout the decades.

The hallmark of the opposed engine is the fact that the engine has two cylinder banks and each piston, directly horizontally opposed from another cylinder, does not share a crank pin with the opposing piston. The latter characteristic is essential to the engine being considered opposed; often times people confuse the boxer engine with the 180 degree V engine family, which is similar, yet attaches both pistons to the same crank pin.

Gaining the name ‘boxer’ from the appearance of two boxers going at each other from the synchronously reciprocating motions of the pistons, the opposed engine was first patented in 1896 by Karl Benz. Almost as old as the automobile itself, the opposed engine design has certainly had its run throughout the decades. Perhaps one of the earliest and most prolific uses was on the VW air cooled flat four, which became the staple of the Volkswagen Type 1(Beetle) and remained in production for almost 70 years before being discontinued; however, its appearance in road cars went on the wane much earlier, beginning in the mid 90s. Porsche has also been reputed for using flat engines, particularly on the 911 and succeeding model variants. For a time, some of the most prolific boxer engines were also air cooled, though this is no longer the case; Porsche shipped the last air cooled 911(badged as the 993) in 1998, and the air cooled Beetle ceased with the introduction of the New Beetle in 1997. Beyond Porsche, Subaru has also made extensive use of boxer engines, with the Impreza being a well known example. Ferrari experimented with the flat engine design in the 1980s, with the most prolific example being the Ferrari Testarossa, which sported a flat twelve.

Boxer engines derive several performance advantages inherent to their design, since the center of gravity on an opposed engine is much lower to the ground compared to an in-line or V design. This allows for better lateral acceleration. In addition, the benefits to handling are enormous. Better turning/cornering capability, decreased roll, and better grip of the roadway all come natural to the flat engine over its alternatives, due to its lower profile. The boxer engine also has a leg up in terms of balance; due to the fact that its pistons are directly opposed to each other, firing motions can cancel entirely. Consequently, the flat four is ideally balanced over the in-line four, due to the fact that second order vibrations can cancel; the in-line four is seen as imbalanced, hence the need for balancers and dampening mounts. Boxer engines also generally have better cooling system functionality after start-up, since, due to the horizontal nature of its profile, oil and coolant remains more evenly dispersed throughout, rather than sinking down as happens in in-line or V designs. Lower profile also makes power transmission more even, as the engine is on a plane closer to the rest of the drive-train.

vw-flat-fourDespite all of the opposed engine’s benefits in terms of performance, however, it does have a few marked disadvantages which have largely precluded its rise over the decades. The flat configuration always catches flak for the fact that the wide profile makes the engine significantly harder to work on. With each cylinder head right up against the side of the engine bay, a simple task like swapping out spark plugs can become a protracted and arduous process. In addition to irking those who wrench on these cars, it also increases maintenance cost for the average owner when it does come time to have the car in the shop. But the added costs of a boxer engine don’t stop there. They generally require more parts and components, due to the fact that having two cylinder heads is innate to the flat design. This, in many cases, doubles the number of head components, valve-train components, and cooling jackets. The added cost builds up, making the boxer engine much more expensive to produce. Obviously weight considerations come into play here, though they don’t present a huge obstacle. Many boxer engines are constrained to mid engine designs, due to the additional size requirements imparted by its wide profile; this is not always the case, with smaller boxers generally exempt.

Some folks will tell you that the boxer engine is the best thing to ever happen to motoring, while others will contend it isn’t worth spitting on- just depends who you ask. Regardless of which of these two groups is right, one can’t help but notice that the engine has always been more of a niche technology. Where does this leave the engine’s fate in today’s automotive world, where things are changing so rapidly? Probably not anywhere different than it’s always been. Sure, no new manufacturers are adopting the configuration, but those who currently make them remain committed to the design. Subaru recently unveiled a new line of smaller, more fuel efficient boxers, designed to meet the challenge of a world where fuel economy and efficiency mean everything to so many. Porsche also isn’t abandoning its flat six tradition. In fact, the German manufacturer recently announced that it was investing in a brand new line of flat engines, including both four and six cylinder mills. The folks in Stuttgart are even rumored to be working on a flat eight to grace one of their upcoming models. And of course, BMW remains committed to its line of opposed engines employed in its motorcycles. Expect the flat engine to soldier on, much like it has for decades. It might not be the next big thing anytime soon, but it’ll be around.

  • Which way does the cooling air travel through the shroud? Am I right to think that the air is sucked by the fan through the opening on the other side of the shroud by the firewall area?

Yes. The fan sucks air in through the large opening at the back of the shroud.

  • And then it is pushed by the fan down and over the heads and cylinder fins and exits under the back of the car?

Yes. Later models use a “dog house” style fan shroud, which has a bulging extension at the back for the oil cooler (hence the name “dog house”) this extension gets it’s share of cooling air from the fan and uses it to cool the oil cooler.. the warmed up, used air exits through a small channel, and is routed under the car next to the transmission.Earlier shrouds had the oil cooler mounted inside the shroud, but it was found to cause overheating when the factory stepped up the size of the engine to 1600cc. The warmed-up air from the oil cooler on those shrouds was used to cool cylinders #3 and #4, but it was no longer “cool” because of the heat from the oil cooler. So especially the #3 cylinder had a tendency to overheat in extreme conditions.

  • And in that case what is the purpose of the hoses that attach to the nozzles on either side of the shroud?

Those provide fresh air for the heat exchangers (aluminum casting around the exhaust pipes, wrapped in sheetmetal). The air heats up inside the heat exchangers and is then pushed inside the car by the pressure created by the fan in the fan shroud.To maximise engine cooling in the summer, many people block off these hose outlets in the shroud. If you remove the hoses, you must block the outlets AND the respective holes down in the engine tin, otherwise the fan will suck in very hot air from the exhaust pipe area under the tin.

Sealing off the heater pipe outlets in the fan shroud will increase the air pressure inside the fan shroud a little and would result in slight increase in airflow to the cylinders (see our article on Solving Overheating Problems for better ways to increase cooling).

BUT – VW designed the heat exchangers to have a small continuous flow of air through them (which is spilled out through small slots at the front (front is front of car) of the heat exchangers when the cabin heaters are turned off) – for several reasons.

  1. It results in less heat-soak from the very hot exhaust headers which run past the rocker covers on the way to the muffler. This radiant heat from the exhaust headers would increase the temp of the oil in the rockers covers a little but the flow of cooling air around the header pipes reduce the temp the rocker covers nearby “see”, and
  2. When there is any moisture in the air it can get trapped between the header pipe and the heat exchanger outer cover and increase the likelihood of rust. Running a continous small stream of air through the heat exchangers prevents a build-up of moisture inside the heat exchangers.
  • Is the cooling air actually picked up from under the car by where the transmission is and goes up between the shroud and firewall into the fan opening?

No. It is drawn in through the vents below the back window, as long as the engine bay seals are in good shape. Cool air in the top of the engine, hot air out the bottom.That’s where you got a bit off track. The engine bay is sealed to prevent air from under the engine reaching the fan, because the air under the engine is very HOT. The fan gets it’s air through the vent holes on the engine lid, and under the rear window. Fresh air comes in from there, and hot air exits from under the car.

*Some* warm air is sucked in to the air filter via a pre-heated air hose, identical to the heater hoses that connect to both sides of the fan shroud. The air filter has a thermostat controlled flap that allows warm air to be sucked into the carburator from under the right side cylinders, when it’s cold enough outside. This helps prevent carb and intake icing.

  • If that is the case isn’t that rather ineffective? Wouldn’t it be better to have actual ducting pick up the cold air from the outside and feed it directly into the shroud opening instead of relying on the air somehow squeezing it’s way up between the shroud and firewall and into the fan opening?

That’s what it does. There is actually a small high pressure vortex that forms over/behind the back window at speed. Getting sufficient cool air in the vents is no problem. Also note that the rear engine tin prevents any air from being drawn up from underneath/in front of the engine.

  • Also where does the air that feeds the carburetor come from? The few magazine articles that I’ve read tell you to make sure your engine compartment is sealed properly so no hot air gets in.

Again, the engine compartment is sealed from the BOTTOM. Warm air from the #2 cylinder head is provided to the carburetor from the pre-heat hose described above.

  • Does the carburetor air come through the row of louvers just above the decklid?

Yes — the same louvers that feed the fan. Later models had louvers in the decklid too, when the engine size grew to 1500 and 1600cc. (Forget about using a non-louvered decklid with a souped-up street engine unless you provide more fresh air to the carbs and fan from someplace else.)

  • But if the engine compartment is sealed, then after feeding the carburetor how does that air get out?

Like I said, the engine compartment is sealed only at the bottom, so the hot air from under the engine doesn’t get in. Only the needed amount of air is sucked in (except when you have the above-mentioned high power engine in there — then there’s not enough air supply, and you get HEAT!) The air that is fed into the carburetor gets mixed with fuel and burned in the cylinders where it becomes a combustion product (CO, CO2) and is exhausted through the tail pipes.

  • And finally, I’ve read that you should keep your seals in good shape (sparkplug boots and all other rubber seals) so the hot air doesn’t recirculate into the engine compartment.

Exactly. Especially the large rubber seal that goes between the engine tin and car body, it surrounds the entire engine. That seal is critical for engine cooling.