[Diy_efi] Timing Advance Curve?

[Mike]

At 01:18 PM 22/12/2002 -0000, you wrote:
>My comment was partly based on some stuff I read ages ago mostly on aviation
>fuel and gave a history of fuel design from 1st world war onwards, there
>have been some very informative replies since then covering race fuel which
>have made me wonder about the absolute validity of my post.

Well as I think most have discerned from all those that posted in
respect of there not being an automatic relationship (I used the
word causal - which in hindsight is not quite accurate), between
octane and speed of burn that it can be contrived either way then
this raises cognitive issues to rationalise what is/could be
going on under a variety of situations.

>With reference to pump gas, please explian why it is normal to run more
>advanced ignition timing on a high octane fuel, it suggests to me
>that the process of igniting and burning the fuel is slower than with the
>low octane offering.

This is an aspect of the rationale that is not extracted from 
certain givens, the major being that auto companies design/use
an engine for a particular octane +- a factor - primarily for
reliability. They dont want warranty returns on damaged engine
parts within the warranty period *and* to a slightly lesser
degree for periods after the warranty so as not to gain a critical
mass of dissent amoungst the buying public. Having said that,
there is some margin for variation that owners and tuners can
'mess with' therefore there is some range by which 'amendments'
can be made to aftermarket tuning of these primarily production
engines - the tolerances are of course a bit tighter at the
higher end of the market but nonetheless the owner/tuner has
option to make adjustments as they see fit...

I dont really know what you mean by 'normal' in your about
context, as most of the people I know run turbos as I do, and
timing we do not tend to advance at all, we are looking to 
retard a tad if anything so we can run higher boost. The
inference you make re burning fuel may well be encompassed
by the spread of tolerance allowed for in automanufacturers
re para above re warranty.

To elaborate. If I just stayed with the bog standard 7.5psi
boost for my engine then I might well be able to push the
base timing a little to say 20 BTDC and stay with the same
ULP. But I want a little more power so I run 10.5 psi
and use premium but stay with the base timing of 15 BTDC, or
even retard a little to 12 BTDC giving me that comfort zone.

Bear in mind the point at which max power occurs in the
stroke is subject to some tolerance for the reasons
given in my para above, and this very tolerance may well
be the issue as detonation is reduced to allow the higher
advance but the point at which max power occurs shifts by
the same amount but the gain is disproportionate.

Think of it as two bell shaped curves of distribution that
intersect as in the case of set theory, one bell shaped
curve is power output from ignition advance before detonation
(or max before that occurence)
and the other bell shaped curve is say power peak at point
from ignition (or something equivalent but words escape me
for the moment). The width of these bell shaped curves is
sufficient such that manufacturers have leeway to maximise
reliability and shift their intersection with economic imperative,
we ie owners/tuners have more directed discrimination ;)

Thats it for now, I realise I've been a bit active on this
group and need to get some work done,

rgds

mike


>If I can find the article or any other information I will post it.
>
>Hugh
>
>
>
>
>
>----- Original Message -----
From: "Mike" <erazmus at iinet.net.au>
>To: <diy_efi at diy-efi.org>
>Sent: Friday, December 20, 2002 12:12 PM
>Subject: Re: [Diy_efi] Timing Advance Curve?
>
>
>> Hugh,
>>
>> Please quote the authoritative source of your information
>> that "octane has everything to do with speed of burn" ?
>>
>> rgds
>>
>> Mike
>>
>>
>>
>> At 10:15 AM 19/12/2002 -0000, you wrote:
>> >
>> >Mike wrote "Isnt octane a measure of the fuels ability to resist knocking
>> >" - yes
>> >
>> >This is at its most basic level achieved by altering the fuel to burn
>more
>> >predictably and slowly by removing the most volatile components or
>modifying
>> >the way that the most volatile components burn with additives.
>> >
>> >In short octane has everything to do with the speed of the burn.
>> >
>> >High octane fuels do not necessarily give the most power. Brian Geddes
>> >posted a couple of days ago that his Miata did not knock regardless of
>> >ignition timing. In this situation, it might be worth trying a lower
>octane
>> >fuel which when optimised for timing and fuel mixture may give a higher
>> >cylinder pressure through the faster burn and therefore slightly more
>power
>> >even though the calorific values of the high and low octane fuels are
>> >similar.
>> >
>> >Do not confuse high octane with high energy as unless it is a racing
>fuel,
>> >the two do not normally go hand in hand.
>> >
>> >Hugh
>> >
>> >----- Original Message -----
>> >From: "Mike" <erazmus at iinet.net.au>
>> >To: <diy_efi at diy-efi.org>
>> >Sent: Thursday, December 19, 2002 5:25 AM
>> >Subject: RE: [Diy_efi] Timing Advance Curve?
>> >
>> >
>> >> At 11:23 AM 18/12/2002 -0800, you wrote:
>> >> >--- "Geddes, Brian J" <brian.j.geddes at intel.com>
>> >> >wrote:
>> >> >Well, if you are talking in that sense, simply up the
>> >> >octane to cover the extra compression.  However, you
>> >> >can run into a point of diminishing returns as far as
>> >> >power output at higher rpms with the slower burn...
>> >>
>> >> Huh ?
>> >>
>> >> WHat has octane to do with the speed of the fuel burn ?
>> >>
>> >> Isnt octane a measure of the fuels ability to resist knocking ?
>> >>
>> >> Mike
>> >>
>> >>
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>> >
>> >
>> >
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