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Date: Tue, 09 Nov 1999 03:33:31 -0500
From: albert jenab <jenab_at_his.com>
Subject: Re: [E36M3] J.C. reprogamming questions
At 09:56 PM 11/8/99 -0700, you wrote:
>>2) Why is it more of a problem to use regular unleaded with the Conforti
>>program than it is with the stock settings? Won't the knock sensors
>>protect the engine either way? Does the Conforti program disable the
>>knock sensor for some reason? Will you destroy your engine if you must
>>out of necessity run a tank of regular unleaded through it?
> It's no more of a "problem" except that you should never "out of
>necessity" have to
> do it.. this is after all, America.. and pretty much every gas station
>on the planet
> has at least midgrade unleaded.
> Knock sensors can only dial out so much spark from a motor.. if the fuel
> is that bad, engine damage can result regardless of the program running
> many knock sensors you have.
Correct me if I'm wrong here. But it takes way more retard to stop
knocking than the amount of additional advance that caused it. This is
called "hysteresis of knock" and is caused by high cylinder temps which
need to drop before normal combustion can reoccur. So suppose you were
running say 38 degrees of advance at some rpm, and bad fuel means knock
would start at 36 degrees. It may take four times or more (8+) retard in
degrees to stop knocking. If your chip has bumped timing to say 40 and you
needed 36, well, then you need as much as 16 degrees or more to handle the
bad gas. You'll quickly run out of range of the knock system capabilities.
There is also an obvious performance issue here. Even if you don't melt
your engine, a car running with knock sensor kicking in will lose
significant HP, from the retarded timing and from poor combustion. Dyno
runs on cars that were taken while KS was engaged were down as much as 10%.
Plus the curves were really jagged.
Knock sensors are safety systems only. This is apparently news to some
merchants selling add-on systems as "performance mods".
On a related note, super/turbocharged cars need way less advance under
boost, not more. Combustion times decrease as a function of increased
intake charge pressures. This is why we had vacuum advance distributors on
pre-electronic ignition cars. Part-throttle (longer combustion event),
more advance than full throttle (shorter combustion event) to achieve peak
cylinder pressures at the proper moment after TDC. Add some boost and you
need even less advance. This has nothing to do with charge temps or A/F
ratio, though these are important too. Bad gas + boost might mean you are
20 degrees off optimal, needing (gasp) much more to stop knocking. Ouch!
So if I were buying a forced air kit for an M3, I'd ask some hard questions
about how fuel management and ignition timing mods are dealt with. You can
see how it would be really easy to get way beyond the KS system's
capability under boost unless the chip had some serious re-mapping done to
it to handle the extra air and fuel. Not much margin for error.
FWIW, I found all this out trying to get my 9 psi supercharged &
water-injected Miata running right. And I wondered why the add-on knock
sensor sucked. The solution is to back off idle timing to the point that
under full boost, there is no engagement of the knock system. This
compromises low-end and non-boost performance, but gets timing closer to
right for WOT operation. The Miata does not have a removable chip and only
base idle timing is adjustable. The next step is a fully programmable
aftermarket ECU, but that costs serious $$.
Live and learn I guess.
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