My understanding of why cars ping when the throttle is wide open is that as the
VE increases (as Trevor points out) the amount of air/gas mixture being
compressed in the cylinder goes up, resulting in an increase in pressure and
temperature at the top of the compression stroke.
At some temperature and pressure combination the mixture starts to explode, or
"detonate" when ignited (rather than burning with a flame front), or to ignite
from the compression heating alone before the spark which soon leads to
detonation. Octane in the gasoline resists this compression ignition, so it
keeps the engine from pinging.
The full throttle effect is especially true at low RPMs because (with a mild
cam) the engine has more time to take in a full charge of air (further
increasing the VE).
Vizard's book on Tuning BL's A Series Engine goes through these aspects of the
A-Series engines in much detail.
Bert Kleinschmidt
-----Original Message-----
From: Trevor Boicey [SMTP:tboicey@brit.ca]
Sent: Thursday, June 25, 1998 9:27 PM
To: Les Myer
Cc: spridgets@autox.team.net
Subject: Re: voodoo
Les Myer wrote:
> I think the flame travel varies upon a number of factors, instead of being
> constant. That is why engines ping when you step on the gas, even though
> the rpm is the same.
...when the throttle is open, the engine's effective VE goes up
so you get more "stuff" in the cylinder.
This is probably what makes the flame front travel slower,
the increased pressure.
--
Trevor Boicey, Ottawa, Canada.
tboicey@brit.ca, http://www.brit.ca/~tboicey/
[ Seeking some miscellaneous MG parts, see the list on the web page... ]
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