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Thread: Questions About Flat Engines

  1. #1
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    Questions About Flat Engines

    The little information that I have read about flat engines, specifically Porsche's flat-6 lead me to wonder why it is used at all. Other than it being a tradition in Porsche cars, I have read that flat-6's are larger than a similar displacement V-6 as they occupy a lot of horizontal room. From some sources I have also heard that flat engines are less rigid than V's. The only good thing going for flat engines seems to be that they have a low centre of gravity.

    I am far from an engine expert, but I was wondering what exactly are the advantages of a flat engine, if there are any.

    Furthermore, does anyone know why Porsche seem so adamant to stick to a rear mounted flat-6 in the 911 as opposed to a mid-engined flat-8 or even V-8 as some Porsche engineers have expressed interest in to better distribute weight? Is the flat-6 rear engine layout used only out of tradition?

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    Smoother and more compact from what I have heard...

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    Quote Originally Posted by The_Canuck View Post
    Smoother and more compact from what I have heard...
    I heard that they took up MORE volume!

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    Quote Originally Posted by Kitdy View Post
    I heard that they took up MORE volume!
    hmm, strange...

    lower center of gravity though...

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    They have much less vibration, which makes it able to put out power easily, since the pistons practically cancel out each other's vibration. It seems to be a very good reason, I'm surprised it's used on so few cars.

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    The vibration is only in on one plane - horizontal, but V-8's are just as balanced so I have heard if they are angled at 90 degrees (pi/2 radians for math nerds ). I'm trying to be the devil's advocate here, but maybe one of the engineer type guys will clean this confusion up.

    I believe the reason that the engine was in the back for Porsches is either because the original VW Beetle had a rear engine and it was designed by Ferdinand Porsche, and Ferry Porsche (his son) put the eninge in the rear for the 356 as it could fit there due to it's small size in volume. However, the flat engined approach it seems, meant that the engine was very wide - this necessitated it being at the back of the car where it would fit width wise. This continued on with the 911. Correct me if I am wrong.

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    Flat 6 engines do take up more horizontal room, but they're very short length-wise since they're essentially as short as a straight 3. Also, since they can be mounted so low, a lot more stuff can be fit in the engine bay (turbochargers, superchargers, intercoolers, etc.). A shorter engine can also be mounted closer to the middle of the car, resulting in better balance.
    Last edited by Sweeney921; 03-01-2007 at 02:53 PM.

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    i dont want to talk too much about secondary vibrations etc but heres a simple guide:
    http://www.autozine.org/technical_sc...ne/smooth2.htm

    Manufacturers achieve cost control through engine modularisation. e.g a manufacturer will stick to its current configeration for varying volumes. eg the merc v6 is 90 deg because it comes from the v8 manufacture line. Many small car manufacturers stick to inline 4's because it allows safety and space allowance plus generic engine platforming. The extra cost of a second cylinder head is enough to put a lot of people off unfortunately. You must also consider the ease of the transverse i4 packaging into a Fwd transverse gearbox layout, the mainstay of mass produced euro-boxes. Boxer engines have to drive a longitudonal crankshaft going against the all round benefit of a regualar inline transverse fwd setup.
    autozine.org

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    It is actually to do with weight and rigidity/strength. A boxer engine's crankcase is very strong for the weight. I have a 4 cylinder 2.2 litre boxer engine here that has an engine case weight of 4.5kg. There is also the lower centre of gravity point as well but not as beneficial as the weight and strength. V engines usually fit better in a vehicle. They are very compact but in practice are heavier than a boxer because they require material between the banks of cylinders to keep them ridged. On a boxer engine the force on the crankshaft bearings is applied from both sides evenly, whereas an inline or V engine it is applied mainly down on the crank from on side. So boxer engines usually have more reliable and longer life main bearings (or smaller depending on the required life). A boxer engine's crank also requires less counterbalancing reducing weight.
    Last edited by revetec; 03-01-2007 at 03:23 PM.

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    Ah, I had seen that website before but I had to read more carefully what was causing the second order vibrations. I suppose those second order vibrations are still existant in 90 degree V engines as well then?

    If this is true, then isn't the best possible configuration for pure performance then a flat engine or am I missing something? What about the rigidity of the engine that I mentioned above? Am I correct there?

    EDIT: The specific case I read about was in R&T or C&D's article about the RS Spyder that said they went with a V8 engine because it has more rigidity than say a F6 or F8. I can understand that rigidity can be beneficial, but also can't rigidity cause more stress on the engine?
    Last edited by Kitdy; 03-01-2007 at 03:22 PM. Reason: forgot something

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    V engines are very rigid but the sacrifice is weight. Engine block rigidity is important. This is why a lot of late model V engines have side bolted main bearing caps, to increase the assembly's rigidity.

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    It is not the case of the porsche or other normal street car, but in F1 they quit using boxer engines because it has low torsional and flexion rigidity to work as a stressed member of the chassis. it can handle it own vibrations well but when you use it to transmit suspension and, in general, chassis forces it is very weak.
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    Ferrari used it in their testorossa i believe to make room for a gearbox and mount it lower. but arn't they bad because lying flat the air and fuel occupy the bottom of the cylinder it lies on as opposed to fallin on the piston then getting pushed. cloging the valves
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    Quote Originally Posted by coolieman1220 View Post
    Ferrari used it in their testorossa i believe to make room for a gearbox and mount it lower. but arn't they bad because lying flat the air and fuel occupy the bottom of the cylinder it lies on as opposed to fallin on the piston then getting pushed. cloging the valves
    The flat-12 Ferrari engines aren't proper boxer engines, but rather 180º V12 engines.
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    Quote Originally Posted by mikelzapi3 View Post
    It is not the case of the porsche or other normal street car, but in F1 they quit using boxer engines because it has low torsional and flexion rigidity to work as a stressed member of the chassis. it can handle it own vibrations well but when you use it to transmit suspension and, in general, chassis forces it is very weak.
    So in race situations, the V provides more rigidity than does the boxer? What exactly is torsional and flexion rigidity anyways?

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