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Old 3 Jun 2004, 00:38 (Ref:992106)   #1
Homer Simpson
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Sound and Porsche's air-cooled engines

Has there been an analysis into the distinctive sound emmited by the old air-cooled engines in Porsche road cars? I know it is a complex mixture of sounds (fan, belts, exhaust, etc), but is there any one single component that dictates the very unique sound of these engines? I remember reading about 2-stroke kart engines, and there was some mention of the cooling fins being resonated as a reason for their (very loud) engine tones. Was this a factor with the Porsche engines?
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Old 3 Jun 2004, 04:59 (Ref:992204)   #2
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I think you are right, some part of the sound must be due to the fins (a metallic ringing noise), overlaid by boxer thrum and fan roar.

Certainly the 996 has a less distinctive sound and has lost some of the high frequency noise (still sounds hot though).
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Old 4 Jun 2004, 12:13 (Ref:993732)   #3
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Not sure if this answers your question (or just tells you something that you already know) but...

The water jacket on a water-cooled engine does attenuate (quieten) the sound considerably. Not only does it quieten the motor overall, but it affects different frequencies to a greater or lesser degree. So in the first instance, that's one of the key factors for a unique "air-cooled" sound
Installation (think of the engine bay as an echo chamber) and the relative noise of the exhaust and other ancilliaries also affect perceptions (I think of the 500cc F3s, where the exhaust noise is so loud it drows most of the air-cooled sound).
I've not heard of resonating fins, but it sounds reasonable.
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Old 5 Jun 2004, 09:06 (Ref:994489)   #4
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Water jacket! Of course, I didn't even think about that. The attenuation (or lack thereof) of certain frequencies would be enough play change the characteristics of the sounds emitted.
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Old 9 Aug 2005, 00:00 (Ref:1375384)   #5
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theres nothing like a delicious engine sound.
anyone know for sure about the resonating cooling fins ?
probably need very high frequency to get a metal plate of such little width to vibrate.
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Old 10 Aug 2005, 17:24 (Ref:1379136)   #6
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How do you find the frequncey of a engine vibrating?

I wouldnt think its just from the RPM, nor RPM times number of cylinders.

There is all the parts in the head too, to think about.

Any ideas.
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Old 10 Aug 2005, 21:58 (Ref:1379373)   #7
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The other thing you have to consider is that it is a boxer engine, and they have their own characteristic sound. Think of the difference, and more over the resonance of a V6 vs a straight 6. The V always sounds raspy, where the straight sounds silky, The Ferrari Boxer engines, vs their V12's. The boxers always had an edge, or probably more up to date - the sound of a scoobydoo impreza/legacy vs an inline 4.

But yes, the sound of a Porsche, flat 6, air cooled, is still, nice.... not as nice as a big V8 though..
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Old 11 Aug 2005, 04:50 (Ref:1379546)   #8
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Quote:
Originally Posted by richard_sykes
How do you find the frequncey of a engine vibrating?
Not mathematically that's for sure... depends on what part of the engine you are talking about too. Hugely complicated issue

racing59 - I think the "boxer" sound is more due to firing order and cylinder pairing. In a similar way a flat-plane crank V8 (eg Lotus Esprit or many racing engines) don't sound the same as "normal" V8's
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Old 11 Aug 2005, 04:58 (Ref:1379549)   #9
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I think the sound comes from the engine configuration as stated previously...

Compare the Flat 4 of the 356's and that of a WRX...
Similar sound, but the WRX's is dulled (probably by the water jacket)

On a side note, The "air" cooling in a misnomer...They are "oil" cooled, and the oil is cooled by the air.
As the water is cooled by the air in "water" cooled engines.

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Old 11 Aug 2005, 12:40 (Ref:1379756)   #10
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several things I believe make them nice sounding. One is the firing order and also that fact a flat 6 fires in opposition ie balanced. Also, Powers of 3 (Eg 6, 12) create harmonics that sound nice to the human ear.
Ever heard a 3 cylinder diahatsu with a broken exhaust? sounds not bad either!
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Old 11 Aug 2005, 22:55 (Ref:1380235)   #11
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Quote:
Originally Posted by DKGandBH
On a side note, The "air" cooling in a misnomer...They are "oil" cooled, and the oil is cooled by the air.
As the water is cooled by the air in "water" cooled engines.
do you mean there are huge oil jackets around the cylinder?
maybe with a jacket oil sirculation pump ?

in that case an air cooled engine would be even quieter than a water cooled one as oil is a "softer" fluid and has a higher tendency to dampen vibrations.
sound waves even moves at a lower velocity in oil.
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Old 11 Aug 2005, 23:25 (Ref:1380254)   #12
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Yes I don't understand that statement either, I only had one flat 6 Porche engine apart and that was a 916 in the 70's and that was definitely air cooled in the same way as a flat 4 VeeDub engine.
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Old 11 Aug 2005, 23:33 (Ref:1380261)   #13
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That comes from the local Volkswagen specialist I worked for when I was on vacation from University...

Yes there is an aspect of direct air cooling (that is what the fins are for in the heads obviously), but the Oil also draws heat, and then is cooled by the air, and I was taking that to the extreme.

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Old 11 Aug 2005, 23:36 (Ref:1380262)   #14
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Engine noise

On a Porsche 911 air cooled engine cooling is a combination of air being blown over the cylinders and head fins by the engine driven fan, and by the circulating engine oil removing heat from the engine and returning it via the oil cooler to the tank (dry sump) the oil acting as a medium to transfer the heat away from the engine components. One of the reasons I believe that Porsche went to water cooling was to quieten the motor sound to comply with noise control legislation in some countries, ie Switzerland
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Old 11 Aug 2005, 23:58 (Ref:1380273)   #15
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Yes but surely the same is true of a water cooled engine as the oil and associated coolers do their part to aid cooling as well.
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