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How to find TDC on a ported engine?

Old 01-23-06, 02:49 PM
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How to find TDC on a ported engine?

Ok, maybe this question is totally useless... but I'm reading one of the Haynes books on engine management but off course it's totally piston engine inspired.

Now the first par of the book is how to understand compression/Ignition and finding a starting point for these items.

So is TDC ALWAYS the point where the rotor reaches maximum compression or is it the spot where it has the maximum filling of the compression chamber?

If so, you will change this spot by porting an engine and therefore you will change the spot where you have the center of intake/outlet overlap. Which will affect the position where you need to start from where you will set the ignition and decide to advance or retard ignition timing...

Am I still understanding correctly here? I'm reading this in English and I'm Europe. I understand English very good but sometimes the tech info makes me doubt.
Also, transferring tech info from piston to rotary is still a bit of a grey zone for me.

Thanks
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Old 01-23-06, 03:02 PM
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the simple answer... max compression
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Old 01-23-06, 03:06 PM
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OK thanks... I'll read that part again :-)
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Old 01-23-06, 04:45 PM
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The E-shaft keyway should be at 9 o´clock
TDC on a rotary is the point is where the compression cycle ends and exhaust cycle starts.
Ergo where it has, and i quote "Max compression"
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Old 01-24-06, 02:57 AM
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OK I see, thanks Kim.. Just trying to relate the piston engine theories to the rotary
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Old 01-25-06, 03:09 PM
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No. Porting will NOT change the location of "Top Dead Center." This is a purely mechanical point. Porting may change the ideal ignition advance, but it will not change top dead center.

-=Russ=-
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Old 01-30-06, 02:14 PM
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and how can that be determined? is it something you need to test by putting the rotor on the exc. shaft and measure at what degrees the port opens and closes again and count the total degrees the port is opened?

Still, I'm having a hard time how to determine a good ignition map. Can it be done by a formula or is more a matter of feeling and powerband preference of the tuner/driver?
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Old 01-30-06, 06:27 PM
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at TDC compression the rotor face is basically flat against the spark plug side of the rotor housing. and the actual tdc is, like kim says, when the eshaft front keyway is at 9 o clock. if you have a motor apart, its easier to see than to explain
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Old 01-31-06, 10:53 AM
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I have my motor apart and I'm aware of the point you are refering to.

The thing is that I'm about to rebuild my motor with some changes (streetport, non seq turbo's, external OMP,...) and my ECU is a PFC. so I'm trying to get some insight in the logic or non-logic of how to determine what the new ign.settings, fuel settings and other settings I need to look out for. Either if it's based on formulas or pure feeling, I don't know.

Most engine management books tell you what the reason is to change ignition forward or backwards but they never mention if that change is based on an exact or as-close-as-I-can-get-guess equation.

I'm really in the dark here and eager to understand
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