[Diy_efi] Ignition Timing

hugh at sol.co.uk hugh
Thu Jul 28 10:59:05 UTC 2005


As a guide for your ignition timing, I find that 24? to 28? advance at .5
bar of boost is a normal and 14? to 18? at 1.5 bar boost and 8? to 12? for
2 bar boost is normal on my engine.

I have used straight output from the standard NTK knock sensors graphed on
the screen of my laptop and found the output to be pretty hard to work with.

You will find a spectral analysis of an engine with detonation at the
following link http://www.sol.co.uk/h/hugh/Knock/Knock%20output.jpg which
shows you that the knock events occuring at just above 6 kHz are lower in
volume (darker colour )than the general engine noise at 4kHz, which means
you will not easily hear them without some form of notch filter.

There are a number of knock boxes around, but the manufacturers do not tell
you much about the way that they work. Some have no filtering and are not
going to help you much.

Hugh

I do have an EMS that's capable of using my stock knock sensor.  I'm
actually pretty impressed with its knock response that I am able to
setup.  

I think I already have my answer.  Thank you so much for your input, all
of you!  

One more question, more along the lines of technique.  Do any of you use
knock boxes?  Do you like them, or do you just prefer to use the stock
knock sensor's output displayed on the screen against known engine
noise?   

-----Original Message-----
From: diy_efi-bounces at diy-efi.org [mailto:diy_efi-bounces at diy-efi.org]
On Behalf Of Adam Wade
Sent: Wednesday, July 27, 2005 5:15 PM
To: diy_efi at diy-efi.org
Subject: RE: [Diy_efi] Ignition Timing

--- "Becker, Damon (Damon)" <damonb at avaya.com> wrote:

> I suppose I'm advocating relying on the knock sensor as little as 
> possible.

If your engine is knock-limited, then you want to detect knock, either
through an acoustic/piezo sensor or ion sensing.  It's one hell of a lot
easier than trying to make a map to account for all possible conditions
on a turbo'd vehicle.

> I realize the OEMs do this, but my aftermarket EMS might not be 
> capable of doing the same things as the OEM setup.

I'd seriously consider finding one that could take input from a knock
sensor, if you're using an engine that is knock-limited.

> Besides, doesn't that amount to a band aide on a poorly-done mapping 
> job?

Huhwha?  Knock limiting is designed to get the most power possible with
low emissions, and is done almost universally on car engines today.  You
can't make any more power from the engine that running at the knock
limit, unless you add water injection or change the compression ratio
(or lower the boost, or use a bigger intercooler).

> As for the compensation maps, setting a map means holding all else 
> constant and making measurements relative to that map.  Since that's 
> difficult or impossible, you have to rely on theory and give a buffer.

If you're using a knock-limited engine, expect your "conservative" map
to lose a lot of power over running on the verge of knock.  I wouldn't
begin to be able to suggest anything theory-wise that would allow a
computational-based (yet accurate) knock limit advance curve for an
automotive turbocharged app.

> So if the theory says 1 degree for ever 20 *C of intake charge, then 
> give it 2 degrees and call it good?

What about boost pressure?  Coolant temperature? 
You're going to have a great number of mpas all being used together for
the same calculations (or a number of multi-dimensional matrices
combining several of the variables).  I would be pretty amazed if you
could get good power and stay out of knock usion theory-based maps alone
on a knock-limited turbo vehicle.  It's much easier to do on a vehicle
that isn't knock-limited, as you're not having to try and estimate where
knock will occur; you're shooting for MBT.

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