[Vwdiesel] Exausts & back pressure

LBaird119 at aol.com LBaird119 at aol.com
Sat Apr 29 16:21:33 EDT 2006

In a message dated 4/28/2006 5:22:12 AM Pacific Standard Time, 
toensing at theunion.net writes:

> It is my understanding that some cars will  operate more efficiently with 
> some back pressure &others will operate more efficiently with out any back 
> pressure. I once had a 2 cylinder Panhard that only got 30 MPG which essentially 
> had  a straight pipe &straight thru  muffler. I was told if I would get the 
> factory Panhard exhaust system that had the proper back pressure engineered 
> in I would get 40+ MPG,

  Not diesels though.  The more free the better.  On a gas engine nearly 
all valving has overlap, when the intake and exhaust are open at the same 
time, especially on two cycles where the incoming charge "chases out" 
the exhaust.  The correct back pressure keeps the fresh charge from getting 
sucked out the exhaust before the valve/port closes.  
  Diesel's have no fuel in the intake charge so a little loss of fresh air 
results in less available air so less power given the same amount of fuel 
(black smoke).  Don't smoke and economy should remain about the same.
  While a perfect back pressure would give the optimum, a bit out the 
exhaust ensures less non-combustable exhaust left in the chamber as 
well for better combustion.  (then some engineer got the bright idea to 
INTRODUCE exhaust into the intake charge of diesels!)  Grrrrrrr.
  Valve timing seems to have less overlap than a gasser too.

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