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Date: Tue, 09 Nov 1999 03:33:31 -0500
To: e36m3_at_Mailing-List.net
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
>you use
> is that bad, engine damage can result regardless of the program running
>or how
> many knock sensors you have.
>
> Jim

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.
-Al
95 M3