(Knock detection methods) Re: Maximum advance

[Shannen Durphey]

Axel Rietschin wrote:

> 
> With a copper pipe bolted on the head and connected to a "stethoscope" on
> the other end, across a fully soundproof wall, you will hear even isolated,
> spurious knock bursts and be able to back off timing very quickly. For this
> process to work well you need to stabilize the engine at every load/rpm
> points and adjust them individually, and you need silence and concentration.

As you mention further in the post, a knock sensor connected to an amplifier and
a good set of headphones will reveal huge amounts of engine noise, knock
included.  It's a good experience, IMO, for every average tuner to drive around
and listen to one of these babies.  Even if you're not chasing knock, it's good
to listen to what the knock sensor normally hears.  For the guys that like to
set timing almost at knock, it's a great experience.

Shannen

> 
> Knock is very random in nature and you may only have one burst of 10 or 15
> out of 200 or 300 burns that pings, I'm not sure how you'll catch and
> display that in real time with a PC, but your ears definitely will, every
> time, provided you are in a silent environment.
In a car and with good headphones you'll hear a fair amount of noise also.

> 
> When advancing timing small steps at a time you will feel an ever so subtle
> change in the absolutely chaotic internal engine noise that will tell you
> knock is just about to show his ugly face and soon thereafter you'll hear an
> erratic burst of noises somewhat similar to what you hear when you lit a
> match, only much shorter, slightly standing out of the huge background
> noise. When you hear that, quickly back off 2-3 degrees and you are done
> with that map point. If you back off timing but still hear more matches
> lighting up, reach that big red pushbutton (the one saying "emergency stop")
> as fast as you can but chances are it is already too late.
> 
> When I mapped my engine we were up to three in the booth, the person driving
> the dyno, myself on the laptop and the third person with his hand on the red
> button. We had one expensive piezo sensor connected to an oscilloscope
> (useless), one standard Bosch knock sensor connected to a Nagra tape
> recorder with me on the headphones and the copper pipe for the dyno
> operator; the only words allowed were "OK" to move to the next rpm site and
> "Stop" to back off immediately.
> 
> We spent about 5 minutes on every row on the map including maximum load,
> releasing the brake in small steps to stabilize the engine at every rpm
> sites on the current row; it took me about 10-12 seconds per site to reach
> the premise of knock and back off two degrees or so; we let the engine idle
> at 3000 rpm for 2-3 minutes between rows to allow the turbo to cool down
> from bright yellow to dark red; the ECU was taking care of the mixture
> details using its own NTK sensor, precisely tracking the target lambda set
> in the appropriate map (we did a rough fuel map before using a very
> conservative ignition map).
> 
> The whole process took several hours and take my word for it, it takes a lot
> of nerves to play with the knock limit at high load for 5 minutes flat at a
> time with the whole exhaust header and turbo yellow-white, with up to 1100C
> on the temp dial toward the end of the rpm range. The ECU was connected to
> all the usual engine sensors and we had additional sensors for almost
> everything, pre and post intercooler temp, per cylinder exhaust temp, oil
> pressure, fuel pressure, oil and water temp in and out of the engine, oil,
> water and fuel flow, manifold pressure, crankcase pressure and another WB
> O2 - nearly 20 extra sensors  - with computer controlled normal range, alarm
> and emergency stop values for every of them (the dyno would shut down
> automatically if one of the value went out of the allowed range) - I
> describe all this just to emphasize you can't really do a proper ignition
> mapping job on a chassis dyno, much less on the road, and that you need more
> than a plug sensor, no matter how good, to do it right.
> 
> Also I don't want to discourage anyone, but the only ECUs I know that are
> capable of using those pressure sensors for closed loop ignition are using a
> dedicated DSP chip just to process the sensor's signal, and they _are_ the
> ECUs, I mean they already knows where the crank is and when they fired the
> cylinder they are monitoring. I believe the signal is extremely noisy and
> somewhat hard to interpret properly in real time. The software has so many
> adjustable parameters in the knock strategy section it makes me wonder how
> long it takes to adapt it effectively to a particular engine. Note that I'm
> not against progress (I believe I was the first to post references to the
> NTK/NGK spark plug sensors here a while ago) it is also well known that Saab
> and others pioneered innovative ways of sensing knock but for most of us,
> the old way is still by far the best IMHO.
> 
> --Axel
> 
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