Quote:
Originally Posted by NoDOHC
Please expound on what you see on the graph that indicates preignition...
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There's multiple stages of preignition...
We do know that preignition = "uncontrolled combustion due to uncontrolled ignition events", right?
There's an old racer adage about "lean is mean" - you can lean it to the point that it'll produce power, but...
At this point, BMEP starts to spike - this is why the power starts to gain a little.
This is a pretty ragged and very narrow range though.
The dyno might or might not show this, depending on the dyno (and dyno set-up) - it could manifest itself with a more "ragged" looking graph.
(Be careful about blaming preignition on your ragged dyno graph - there's a LOT of other factors that can do the same thing, for example: insufficient spark due to bad plugs, wires, coils, under voltage, bad ground, etc. or bad fuel delivery due to fluctuations in fuel rail pressure, dirty fuel injectors, bad ground, dying fuel pump, dirty fuel filter(s), fuel temp too hot, etc.)
EGT's will start to spike also, but only experienced tuners will know that it's too hot.
You will almost never hear this with the human ear.
Keep pushing it past this point, and power will start to fall off on the dyno.
This due to the uncontrolled combustion - the engine is fighting itself to harmoniously push the crank (well, in our case, it's the eccentric shaft) in the right direction to put the power (technically, torque) down to the wheels.
EGT's will significantly increase at this point.
You may or may not hear this.
Keep pushing it even further...
You'll start to induce ignition misfire cause either, 1) mixture is too lean to ignite properly, or 2) ignition timing is too advanced and way outside the power producing range.
EGT's at this point will drop due to the fact that it's a misfire - you're not getting proper ignition and combustion, i.e. BMEP plummets.
Obviously, something it very wrong at this point, cause the engine just does not sound good at all.
Depending on the dyno model, you can get all kinds of weird lines.
My recent experience with Dynapack shows spikes - I'll try and grab some pics, as we're going to retune a customer car next Sunday.
Side note...
As dangerous preignition and detonation are (to the rotary engine), the absolute more critical point in the graph is at PEAK TORQUE.
When you make your runs on the dyno, keep an eye out on when torque peaks at what RPM range - you will see a general idea of the torque graph / peak over multiple runs.
Peak torque = peak BMEP, and having the engine detonate at this point is almost instant engine failure.
Keep this in mind when adjusting your maps in those ranges.
-Ted