From: floyd@eng.umd.edu (Mike Hughes)
Subject: Re: Fuel octane, Carbon, pinging, etc
Date: Fri, 10 Apr 92 15:42:40 GMT
Organization: University of Maryland
Cc: floyd@eng.umd.edu (Mike Hughes)

In article <1992Apr10.075304.26037@tcom.stc.co.uk> sunil@tcom.stc.co.uk (Sunil Nagpal) writes:
>
>All this talk on high octane and stuff has prompted me to ask the nets
>advice on my VW Scirroco GTI.  
>
>The problem is pinking (pinging ??), on acceleration.  This
>only occurrs after the car has reached normal operating temperature.
>
>I took the car to get tuned, and the mechanic told me that the tuning
>was already spot on, and reckoned that I had excessive carbon build up 
>in the head.
>
>Anyway, I've got a few questions about this:-
>
>1) Is bad gas a cause of carbon build up
>2) Can I get rid of the carbon without pulling the head,
>(I've heard of a product called Redex over here in the UK, anyone used
>it ?)
>3) Is switching to a higher octane gas, (I'm currently using
>Superplus unleaded 97 octane) likely to cure the pinging ?
>
>As always, thanks for any help.
>
>	Sunil
>
>-- 
>"I used to be indecisive but         | Sunil Nagpal <sunil@tcom.stc.co.uk> 
>now I'm not so sure..." 	     | Dept 30770 BNR Europe, 
>            Matt Johnson             | Oakleigh Rd South, London N11 1HB,  

[James adds...]

Article 36777 of rec.autos.tech:
From: jws@cica4.mlb.semi.harris.com (James W. Swonger)

 You might reduce pinging by running at a lower water temp. This may affect
efficiency slightly but I don't think by much. If you remove the head to
do valves or remove stubborn carbon, consider reducing compression ratio
by using a slightly thicker aftermarket head gasket. (This -will- reduce
performance slightly). Maybe being over there closer to VW, you can have an
easier time getting modified ignition parts - you might benefit from a
lower static or centrifugal advance and more vacuum advance, so you can
back off further under low vacuum conditions (of course if you are
computer controlled my advice is worth two bits).
--
##########################################################################
#Irresponsible rantings of the author alone. Any resemblance to persons  #
#living or dead then yer bummin. May cause drowsiness. Alcohol may inten-#
#sify this effect. Pay no attention to the man behind the curtain. Billy!#

	Watch out...changing your thermostat results in a bit more than 'slight
efficiency loss' [NOT a direct quote].  My references state:

Operating temp		40F	100	140	160	180
-------------------------------------------------------------
Cylinder wear 		.008in	.002	.001	.0005	.0003

Fuel consumption (Gal/hr) 3.8	3.5	3.2	2.9	2.8

Power (horses) 		26	27.2	28.5	29	29.5

	If you think about shimming the head to reduce compression...think 
again.  I'm not sure if this engine is OHC, but I imagine it is.  Shiming the
head will change the distance between the crank pully and cam pully.  The 
timing belt will need to be streched a bit, resulting in advanced valve timing.
If this enging has pushrods, the distance between the cam and rockers will be
increased, possibly resulting in valve lash out so far that you can't adjust
them.

	As for the original posters questions...I'm not sure if bad gas can
cause carbon build-up, but I know that the wrong operating temperature (too 
cold) or incorrect fuel mixture can.  The main source of carbon build-up in the
combustion chamber is oil consumption.  Oil in the mixture also lowers the
octane, further increasing the chances of preignition and detonation.  You're
using 97 octane?!?!  If available, higher should help.  I have had good results
removing carbon build-up by pouring a special top engine cleaner down the carb
with the engine racing at about 3500 RPM's.  This evidently quenches the car-
bon, causing it to crack and flake off.  It seems pretty scarry, but it "Works
for me."

-Mike

:Suicidal Tendencies=====> He's a carrier of death a stork in reverse,        :
: =8^) Mike Hughes  =8^( : he blesses you with sickness cause love is a curse.:
:Mail---floyd@eng.umd.edu: The arsenic in your Koolaid, the bomb in your mail,:
:Voice--(301) 340-0922   : he disappears in motion but leaves a bloody trail. :
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