Re: Bore v. Stroke (Last Long Post I Promise)

["Dr. Bob" <rmalinka@spec.net>]

Mark,

In general, VW's longer stroke engines generate more torque and should
breath better. Coupled with 5 valve technology giving cylinder head better
swirl characteristics for combustion of fuel, however, this engine design
doesn't adapt as well for high RPM's.  
Short stroke engines of course have higher potential RPM in the design. I
notice you didn't post the torque ratings?  If your looking at engine design
parametrics both HP and torque should be compared. One thing you will notice
in comparing various european manufacturers:
MB likes big bores, BMW has shorter stroke engines ie. higher RPM potential
in design,
VW/Audi has long stroke engines with higher proportional torque however
lower RPM potential 
in design.  I had an article around here somewhere which covers engine
design parametrics  in detail but I can't find it right this minute.  

Dr. Bob



At 08:01 AM 12/4/98 -0800, you wrote:
>Wow I am posting like a MADDOG!  Okay guys  In doing the research for that
>little post I made I came across another oddity.  The fact that, for the
>most part, most high performance engine manufacturers (Porsche, Ferrari,
>BMW, etc. etc.) use engines that have a larger bore than stroke (except
>the M3 engine for some reason).  Now I know the theoretical advantage of
>this (exposing more piston surface area to allow quicker, more even
>burning); ,  my question is what is the advantage of using the OPPOSITE;
>i.e. having the stroke bigger than the bore.  All VW engines have this,
>including the 1.8T.  For comparison here are some different strokes and
>bores for different engines:
>
>Car		Cylinders	HP	Liters	HP/Liter    Bore/Stroke
>
>Mazda Protg	Four		122	1.8	67.78	     83mmx85mm
>Mosler Raptor	Eight		446	6.3	70.79	     102.4x95.3*
>Passat		Four-Turbo	150	1.8	83.33	    81x86.4
>Crown Vic	Eight-Super	355	4.6	77.17	     91.4x91.4
>Nissan R390	Eight-Tturbo	550	3.5	157.14(!)    85x77*
>Chrysler 300M	Six		253	3.5	72.79	     96x81*
>Porsche 911	Six		296	3.4	87.06	     96x78*
>Honda VTEC V6	Six		210	3.0	70	     89x93
>BMW M3 	Six		240	3.2	75.00	     86.4x89.6
>Ferrari Dino	Six		180	2.0	90	     86x57*
>VW VR6	Six		172	2.8	61.43	     81x90
>My VR6		Six		189	2.8	67.50	     81x90
>
>Okay enough.  Anyway, it is interesting to notice that there seems to be a
>pretty even split between cars with bores bigger and cars with strokes
>bigger.  It is also interesting to notice that the VR6 really isn't that
>impressive in the HP/L category. Plus,  the VR6 seems to have an low bore
>compared to the other sixes.  Stroke seems to be on par with other cars of
>the same size.  One really interesting this is that the Honda VTEC, which
>is able to spin REALLY fast, has a very long stroke!  Kind of disproves my
>theories in the previous post (although they use Nikasil liners in the
>cylinder)  Any tradeoff?   If you want to see a excel spreadsheet
>comparison of pretty much every engine in the US market let me know.
>What's the point of all this?  Well what is the damn advantage of the two.
>Is bigger stroke or bigger bore better?  Someone of you out there MUST
>know.	
>
>
>
>Okay, I will make this my last long winded post for awhileDiscuss amongst
>yourselves.
>
>
>Mark "In a Talkative Mood" Radelow.	
>
>
>Ps.  You notice that the 1.8T an VR6 share the same bore?  I wonder if
>this is a coincidence.
>
>
>
>
>
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