diesel valve springs

[drew@dyermaker.cs.uoguelph.ca (drew)]

Be careful not to confuse displacement with compression ratio.  The VW IDI
(indirect injection diesel) engines have a high (even by diesel standards)
compression ratio of about 23:1 (compare 16.5-17:1 for a direct injection
diesel, and most industrial indirect injection diesels)  This high
compression ratio sacrifices pumping efficiency for ease of starting -
something VW felt was important for an automotive diesel application.  

The minute size of the compression chamber, which is made up of the
precombustion chamber (insert in the head) and a relatively small space at
the top of the piston, is repsonsible for squeezing the 397cc of air (for
a 1.6 NA) to make it hot enough (this is the key to the compression
ignition) to ignite the injected diesel fuel (injected at 2000+psi.)  You
still have to get the same volume of air (theoretically) into that
cylinder as you would air/fuel mixture on a 1.6L gas engine at WOT.  In
reality you'll have less intake restriction than you would on a 1.6L gas
engine at WOT, so you'll get more efficient cylinder filling (on a 1.6L NA
VW diesel, the air filter is bolted to the air intake runners...)

Compression ratio is NOT related to valve lift, diameter etc., but rather
how much the intake charge (or air in the case of a diesel) is squeezed at
TDC.  The valves had better be closed when that piston is on the way up,
or else you're loosing precious air out of the cylinder, and in a diesel,
that's a bad thing.

The cam has to maximize lift and duration and minimize overlap in order to
support these requirements (maximum cylinder filling plus high compression
ratio.)

Hope this helps clear things up,


Drew MacPherson - '84 Wolfsburg Edition Scirocco TurboDiesel





On Fri, 5 Jul 2002, Dave Ewing wrote:

> At the same time you need to consider that it's not just how much air the
> motor is sucking in, but the combustion chamber size and with a diesel it is
> relatively nonexistant.  The combustion chamber on a diesel is the thickness
> of the headgasket and the shortness of the rods since there is NO combustion
> chamber in the head (flat) and the pistons are flat also.  So the cam does
> not have to open the valve very much for the engine to have high
> compression.
> 
> That said, refering back to the original question that started this thread,
> it does make sense that there are stock springs somewhere that out performs
> the stock springs that come in our engines, just have to find them!!  My
> reasoning on this is purely common sensical in that the comparisons between
> Kent/Shrick and say Autotech/TT are so different.  Autotech/TT are buying
> springs that are stock in another vehicle where Kent/Shrick make their own.
> Even though the outcome may be similar (and Autotech/TT might actually move
> more and make more money)  but the observation is still that they couldn't
> sell them as cheap as they do if they were custom springs like Shrick/Kent
> springs are.  So, they must be coming from somewhere and it could very well
> be a VW diesel motor.  What we need to do is buy one of each and compare!!
> If they are the same, I will advertising HD springs on Ebay SOON!!!
> 
> Dave
> 
> >
> > Considering that a diesel is COMPLETELY reliant on getting as much air in
> > the cylinder per stroke, it would not surprise me in the least that the
> > cam profiles were agressive.  Remeber folks, these are CI engines, not SI.
> >
> > Eric
> >
> >
> > On Thu, 4 Jul 2002, Kevin Collins wrote:
> >
> > > Ron Wood once told me that Techtonics "HD" valve springs *used* to
> simply be
> > > OEM diesel springs.  That was BEFORE they got a batch of horrendously
> bad
> > > ones from some other supplier..  :0
> > >
> > > --
> > > Kevin Collins
> > > Huntington Beach, CA
> > > '86.5 16V 2.0
> > > '02 GTI 337
> > >
> > > _______________________________________________
> > > Scirocco-l mailing list
> > > Scirocco-l@scirocco.org
> > > http://neubayern.net/mailman/listinfo/scirocco-l
> > >
> >
> > --
> > Eric
> > Northglenn, CO
> >
> > 76 Honda CB400F Super Sport (400cc, 4 cyl, OHC 8v, 10k RPM redline *grin*)
> > 80 Scirocco (DD duty, AGAIN)
> > 81 Scirocco S (2.0 16v swap and complete restoration in holding pattern)
> > 81 Rabbit Truck LX Diesel (Waiting for BAE Turbo Install)
> > 91 Cabriolet (2.0 Crossflow 8v swap and mechanical rebuild in progress)
> > 00 Golf GL TDI (FOR SALE!!!! Email for details)
> >
> > "For Aquarians, the physics of loopholes is a perfectly valid science."
> > -Kelli Fox
> >
> >
> > _______________________________________________
> > Scirocco-l mailing list
> > Scirocco-l@scirocco.org
> > http://neubayern.net/mailman/listinfo/scirocco-l
> 
> 
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