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Ignition advances


rusty_wingnut
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I am having some problems working out the correct advance curve for an older engine that has had a few modifications:

-higher compression than stock

-higher octane fuel from the era it was built

-Better carburettor (probably not a factor to consider)

I have figures for the original advance curve, but they may not be suitable due to the above factors. I don't want to jump straight in and have my distributor modified to suit without knowing what is ideal.

Is there anyway of working out what this engine's advance curve should be?

(£5 says someone will mention megasquirt!)

Any help would be useful chaps! :)

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Universal knock sensor (AKA microphone glued to block) + headphones and tune it by ear, lots of stuff about this sort of thing on the MS forums (there, I mentioned it, where do I claim my fiver? :P )

Although you can apply science to it, the problem is you don't *really* know with any degree of accuracy most of the factors about the engine so would end up having to fine-tune it by ear/seat of pants anyway, or just settle for something "better but on the cautious side".

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A Rough way (unless you can find info from T'internet)

find someone with same engine

Remove advance pipe

Run engine with strobe light rev engine in 500 rpm increments, and note Advance, this then gives you

a base line that you can build advance into re MAP etc,

As a said - a rough way - but worked on a werid engine we had to get running on squirt with not much help at hand

then a rolling road trip, which cost a lot, and the changes we not as dramtic as I feared ! :ph34r:

Nige

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Higher compression makes the mix burn faster, therefore you need less ign advance than standard.

However the more modern fuels have a higher octane (can cope with more advance) than the original ignition curve was designed for, and the original curve was probably 'conservative' to cope with whatever was available... so, since the engine is fundamentally the same design, the original curve and settings will probably be not far off for your 'uprated' engine.

If it 'pinks' under load, then there is too much advance and you need to back off the advance by a couple of degrees.

You can only get an accurate recurve if you get it done on a rolling road. Everything else is 'best guess', but for the reasons above, I think the standard curve, is a good place to start.

Regards, Diff.

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Done "properly" it's quite involved and just making it run so that it doesn't "pink" is less than half the story.

With the engine on a Dyno or rolling road you fix the conditions, throttle (usually wide open) and the dyno keeps the speed constant.

Measuring torque you plot torque against ignition advance for each of a range of engine speeds.

Ok with that so far? then you'd imagine a curve with a humped back, you'd be right.

Near the top where the curve flattens out you have what's called "minimum advance for best torque" (MBT) ie the best power with the least advance, least strain, least piston heat etc.

At the other end of the plateau you get whats called NMBT or not minimum advance for best torque. The ideal timing window is between the 2 and as close to MBT as possible.

Then there is the knock margin. At some point on the curve you get auto-ignition of the fuel because the pressure in the cylinder reaches a peak high enough to make the fuel go off on it's own. Fuel takes roughly 1/300th of a second to burn and although it's desireable for the last bit of unburnt fuel (end gas) to self-ignite too much causes damage.

So you back of perhaps 3-5 degrees or so from the knock point and that's your know margin.

Plot MBT, NMBT and knock margin on a curve and that indicates the best advance curve, keeping below the knock margin and using MBT as much as possible.

A traditional dizzy will have 2 bob weights and 2 springs, one of which is slotted so that one spring handles the inital advance and then the second joins in to control advance at higher speeds.

You get an advance curve with a dog leg in it where the 2nd spring comes in. If the bob weights act on a cam to make them progressive then the 2 parts of the advance curve will non-linear ie 2 curves joined with a bend.

Still with me? well that's how mechanical advance is calculated. Vac advance is done in a similar fasion to account for things like carburation and valve timing, as such it's done with a variety of throttle openings. The vacuum unit has only 1 spring which is roughly linear, the throttle body conrols how much inlet vacuum is generated and how much is passed to the vac advance.

To do any of the above without access to a rolling road is almost inpossible, perhaps the closest you can get is timing in-gear acceleration for a number of different static advances but it won't allow you to modify the advance curve, just move it.

So as a very rough guide:

Higher C:R will increase peak pressures and often requires the ignition to be retarded.

Better fuel, higher octane will allow more ignition adavnce, so one counteracts the other.

A bigger carb may reduce the vacuum at part load, so you may (bit of a guess) get less vac advance at low revs with wide open throttle, if the engine sounds "hollow" this is the cause.

Where a megajolt helps is the ability to program the advance curve without a stripdown, the ability to cope with flat spots or pinking without retarding the entire curve and off course a better spark.

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