Porsche's new 918 Spyder is an out-and-out performer, a perfectionist company's vision of a no-compromises-made supercar. Ultimately, Porsche's aim was to build an impressive machine, and it's an understatement to say that they have succeeded. They didn't cut corners, they couldn't have. Everything about this car was carefully calculated before it was put into production. So why do the exhaust pipes exit through the top of the car instead of the under the rear bumper as is the norm?
When you're tasked with building a supercar from the ground-up, you have to start with a blank sheet of paper. You can't bring any preconceived notions to the table, or you'll just end up rehashing the same old stuff. Porsche's engineers, when faced with a blank sheet, said 'The 918 will perform better if we can route the exhaust out the top', then they figured out how to make it possible.
The top-pipe exhaust exit on the 918 Spyder was more about function vs. form. Benefits include weight reduction, thermodynamic air channeling, reduced back pressure, and improved aerodynamics.
1. Weight reduction
The 918's exhaust system, because it is mounted on top of the engine and exits straight up, is much shorter than a standard exhaust that would need to exit out the rear. This allows the system to be made from much less material, and take up less space. The exhaust itself is crafted from an extremely thin-walled alloy steel/nickel called "Inconel"; a material that is much more heat resistant than titanium. In fact, Porsche has said that a titanium exhaust would actually be heavier in this application because it would need much thicker walls to withstand the 850+ degree exhaust temperatures created by the car's high-rev V8 engine. In total, the exhaust system only weighs a paltry 29 Kilograms (about 64 lbs).
2. Reduced Back Pressure
While some back pressure is key to a well tuned engine, you generally need to decrease back pressure as the revs increase. And boy does the 918 have revs! In an effort to provide a very free-flowing exhaust system, Porsche reduced the size of the muffler used on the 918's 4.6 liter V8 to only 24 liters. By comparison, the 991 GT3's 3.8 liter flat six has to carry around a 48 liter muffler. Tuned not only for performance, but also for aural quality, the 918's sonorous exhaust note sounds close to perfect to our ears. Besides, with a shorter system and less back pressure, you occasionally witness some pretty amazing blue flames popping out on the overrun. That's just plain cool.
3. Improved Aerodynamics
A side-benefit to the top pipe exhaust is a smoother aerodynamically flowing full-body under tray. Without clunky pipes, converters, or mufflers hanging down below the engine, the 918 engineers were able to bring the Porsche's aerodynamics into sharper focus. With no exhaust tips exiting through the rear of the car, Porsche was able to extend the rear diffuser down to meet with that under-body tray to again provide greater aerodynamics.
4. Thermodynamic Air Channeling
When you are dealing with a car that has batteries, electric motors, regenerative braking, and kinetic energy recovery, heat is your absolute worst enemy, and must be avoided as much as possible. Wherever heat is unavoidable, the exhaust for instance, it's necessary to minimize its effect on the Porsche's retention of heat. This is, perhaps, the number one benefit of having a top pipe style exhaust system.
In order to minimize heat the most, Porsche designed the engine with the 'hot side', the exhaust, exiting toward the 'vee' of the engine, and the intakes on the outside. We've already stated that the exhaust can exceed 850 degrees in temperature, so it would make sense to get that heat up and out of the engine compartment as quickly as possible.
In conjunction with the top pipe exhaust design, Porsche developed a system that keeps the lithium batteries within their optimal temperature range (about 70 degrees to about 110 degrees). Without this top pipe exhaust, that battery cooling system would have to work much harder, greatly reducing the overall energy efficiency of the car.
So there you have it; four fantastic reasons why Porsche bucked tradition and installed the only top-exiting exhaust system that has ever been seen in a road going car. You know that if Porsche refuses to compromise on its exhaust system, then surely they took the task of engineering every piece of this car extraordinarily seriously. The 918 isn't just a halo model, it's the ethos that goes into every Porsche made.
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