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Is the Porsche 918 the first production car with a top mount/exiting exhaust system?

This article points out that by routing the exhaust up, Porsche was able to equip a completely flat underbelly and longer diffuser, which has significant benefits to cooling and aerodynamics. It also says the shorter exhaust tubing will reduce back pressure and weight, but then again that weight is now located above the center of gravity.

I'm most interested in how the hot exhaust gasses effect to the aerodynamics of the car. Does hot air increase/decrease laminar flow? Does it somehow create a barrier and act as a spoiler for the cooler ambient air coming off the roof? Would the varying exhaust temperatures change anything?

How would it benefit FI engines, and does the future hold an MR2 Turbo with a similar system!? (puh-LEASE, Toyota).

Either way, I think we can all agree it looks pretty %$&?!#@ cool shooting blue flames over the engine bay.

  • Looking forward to the responses in this one thanks for digging this out of your head. ;-) – DucatiKiller Feb 24 '16 at 0:30
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    I don't know if you saw what @Zaid was dealing about the M5, but it has a top mounted exhaust as well. It has nothing to do with why the 918 Spyder has it. it uses It so it can cross the exhaust tubes over between the two turbos and allows everything to run smoother. Also, I'm pretty sure the new F1 engines are running a top mounted exhaust with the advent of their regenerative systems. – Pᴀᴜʟsᴛᴇʀ2 Feb 24 '16 at 0:49
  • I don't think the article is saying the hot exhaust gasses help the aerodynamics. It is just saying that the car now has a smoother underneath without exhaust pipes which therfore improves aerodynamics. I may have just missed something though. – HandyHowie Feb 24 '16 at 8:28
  • @ᴘᴀᴜʟsᴛᴇʀ2 I did not see that about the M5. It does sound like an efficient way to route the turbo piping, I'm surprised they were able to pull that off in an FR layout though. Thanks for the info! – MooseLucifer Feb 24 '16 at 14:30
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I certainly don't claim to have any special inside knowledge as to why Porsche engineers decided to route the exhaust that way, but I think that we can analyze the possibilities and trade-offs from an engineering view and perhaps see how one might come to such a conclusion.

The possible options

I think we can safely eliminate any front-facing exhaust configuration, so that leaves three choices one could imagine for routing the exhaust:

  1. out the sides,
  2. under the car or
  3. out the top

Each has its costs and benefits, but let's look at the considerations that may have led Porsche engineers to their conclusion. (Note that I'm lumping "straight out the back" as with the McLaren MP4-12C in with "under the car" in most of the following discussion, since most points would apply to either.)

Eliminating heat

First, it's both a high horsepower sportscar and a hybrid. The sportscar part means that the exhaust temperatures tend to the higher end of the spectrum and the hybrid part means that keeping the batteries cool enough is an important consideration. So getting the heat away from the batteries (and the other electrical systems) is an important goal. Any of the configurations might be employed successfully in routing heat away from the batteries, and away from the engine, but the entire length of the exhaust pipe will be very hot and radiate heat, so that dictates that the batteries almost have to be forward of the engine. In fact, the 918's batteries are between the driver's seat and the mid-mounted engine.

Weight savings

It is probably obvious that making a car lighter is a good way to both make it faster and to make it consume less fuel (both of which were design objectives for this vehicle.) Generally, weight should be eliminated where practical and even expensive things become "practical" for an $800K sportscar! Keeping the exhaust pipes shorter saves weight. This would likely be a point in favor of either top or side exhaust versus bottom exhaust.

Weight distribution

It's important to pay attention to where the weight is located in a high performance vehicle. Weight that can't be eliminated is generally preferred to be closer to the ground to lower the center of gravity of the entire vehicle to allow for better handling characteristics. Batteries, engine, electric motors and transmission are all heavy and are mounted as low as practical given other considerations. This makes bottom routed exhaust not impossible but perhaps more difficult to route than side or top exhaust.

Aerodynamics

The bottom of a racecar (and the 918) is smooth to allow for a continuous and unobstructed flow of air beneath it. Keeping the air flowing smoothly until it's already behind the car helps it go faster by reducing drag. The entire undertray of the 918 can be smooth all the way back to the diffuser at the rear of the car in part because there are no holes for exhaust ports.

If you follow Formula 1 racing, you may know about the "blown diffuser" used to great effect in the 2011 Red Bull RB7. The exhaust gasses were used as an aerodynamic element by having the exhaust flow over the rear diffuser. This was very effective until it was banned in 2011. So in theory, could the 918 have had the exhaust flow beneath the car through the diffuser and have an aerodynamic advantage? Maybe, but the real magic of the Red Bull car was that air flow and an engine mapping allowing the engine to continue to blow air over the diffuser even when the driver lifted off the throttle. Lacking that, as soon as the driver lifts, the aerodynamics of the car would have abruptly changed, which is not good. It may be that Porsche considered but rejected this because the system would have been more complex as a result and would not have been effective at other-than-racing speeds. Score another point in favor of either side or top exhaust. Having the exhaust above the engine does indeed mount it above the center of gravity for the vehicle, but the exhaust was also engineered to be as small and light as possible, mitigating the negative effect of mounting it high.

Awesome sound effects

Porsche takes some care in ensuring a visceral driving experience which includes the sound of the engine. Even Porsche models that are a bit more affordable include features such as the "Sound Symposer" which is a design feature with the sole purpose of transmitting the right engine sound to the driver's ears. Definite score for the exhaust out the top on this point.

Efficient top exhaust

Most of the points above favor either side or top exhaust. In fact, most V8 engines have exhaust headers on the side, so the natural shortest path for exhaust would be to the sides. However, the 918's engine has an unusual (unique?) feature compared to most V8 engines -- the exhaust headers originate within the V instead of outside it. That makes the "straight up" choice a clear winner in terms of shortest path.

  • Nice deconstruction and logic. – DucatiKiller Feb 24 '16 at 4:41
  • Thanks for including the RB7 info! I look forward to researching that further. I'll post back if it gives me any new questions/ideas as to how the exhaust gasses might aid the 918s aero package. – MooseLucifer Feb 24 '16 at 14:25

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