[Vwdiesel] Injector break pressure efficiency?

James Hansen jhsg at sasktel.net
Mon Mar 21 00:13:05 EST 2005

I thought i had replied... sorry.
More is better the older the engine is, or the lower the compression.
Noisier running, much much better cold starts, but power? academically, yes,
but with the seat of pants dyno I own, not discernable. My 89 td Jetta was
running at 170 at the last before I sold it.
Could there be something to that timing belt thing... I have shorn the teeth
from a belt on the crank pulley about 10000km before it's duty cycle was up.
Coincidence?  Extra  36 pounds on the camplate would not be terribly
significant on loading of the crank sprocket, double that of the load on the
injection pump sprocket, but still in foot pounds, hardly more than an extra
foot pound or so since the force is actually taken up mostly by the thrust
bearing of the pump. Over the duty cyce of the belt, I doubt it makes much
difference, and should still be within the belt capacity.

-----Original Message-----
From: vwdiesel-bounces at vwfans.com [mailto:vwdiesel-bounces at vwfans.com]On
Behalf Of Mark Shepherd
Sent: Saturday, March 19, 2005 5:31 PM
To: vwdiesel at vwfans.com
Subject: [Vwdiesel] Injector break pressure efficiency?

 I posted this recently but got no reply from this site

" ...This leads me to my current dilemma...I'm considering
my injector break pressures from 155bar.
Will this lead to better efficiency due to a finer spray?
What happens to the life of a pump when it has to hit the
fuel with that extra force. Camplate wear increase? Shaft
bearing wear increase. Timing belt strain?

Or does everything operate well within limits?..."

But here's the response from Loren's mate:

"Yes, it will put more stress on your pump and timing belt.
However, it works fine for a turbo-diesel, so... I wouldn't
say it is overly-abusive on any of these components. You can
actually figure out how much more pressure it puts on the

155bar-130bar = 25bar extra hydraulic pressure = 370 extra
psi hydraulic pressure

plunger surface area = (9mm diameter / 2) ^ 2 * PI
= .0986 square inches

added force on camplate from changing to 155bar injectors
= 36 extra pounds pressure on camplate during injection

And to put it in perspective, the pressure on camplate
required to develop 130 bar hydraulic pressure would be 187

You can imagine how that extra 36 pounds camplate pressure
causes more force to turn the injection pump, causing more
dynamic loading on the timing belt, etc. But the thing is,
it seems to work fine on turbo-diesels, so there is no
reason it wouldn't work OK on a naturally aspirated diesel.
I've tried running 155bar injectors on a naturally aspirated
application and it works fine but really didn't seem to give
any benefits as far as I could tell. Power and fuel economy
didn't seem to be effected by upping the breaking pressure.
So whatever efficiency was gained by improve atomization of
the spray was apparently offset by the extra power sapped of
the crank to push the extra fuel pressure. The extra
breaking pressure did noticeably seem to make the idle more
shakey though. I think naturally aspirated diesels have a
slightly smoother idle as of result of their lower fuel
injection pressure.

Replacing an old worn out injector(s) that squirts a stream
of fuel with a fresh one that shoots a nice finely atomized
"cone" of fuel spray (regardless of breaking pressure), is
effective in improving combustion efficiency, resulting in
less idle "roughness", idle speed being higher, speed of RPM
drop when off-throttle not being as abrupt as before, less
smoking, improved fuel economy and torque.

A general rule of thumb I've noticed seem to apply after
installing new injectors is: if the idle speed increases,
then either the fuel quantity rate or the combustion
efficiency has increased."
Jake Russell
'81 VW Rabbit GTD Autocrosser
1.6lTD, SCCA FSP Class

Of course I'm already at 155 bar so I'll re-ask wrt 175 bar

Quantum TD

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