Ignition advance, peak torque, what's going on?

[carbonlife]

I'm trying to get a mechanical advance distributor back to manufacturer's spec. There are 2 possible specs from the manufacturer - quite similar overall - the rest of the engine spec is identical (apart from carb jetting) but one curve reaches maximum advance at 6000 rpm, the other at 6800 rpm - on an engine which produces maximum torque at 5000 rpm. Any suggestions as to why that might be?

The 2 original spec advance curves can be seen here in red and blue.

http://www.senstyle.com/image001.gif

The red curve is for cars that had to meet emissions specs and the steep advance at low rpm for the blue curve is indeed correct - the advance for this curve is linear and jumps 10 degrees by use of a switch when the throttle passes a few degrees of opening.

You can see my attempts to get closer to the original curve - clearly the secondary advance springs need replcing as the gradient of the secondary slope advances too quickly (I'm trying to get hold of more springs) and I should be able to get a few degrees more overall advance...... but I'd also be very interested to hear what might be going on at around 5000 rpm..... those 2 local peaks on the pink and yellow curves are not measurement errors - no matter how the springs are shimmed or how many times I check this, there is a local peak at somewhere near 5000 rpm. Since this is around peak torque, it's kind of important not to be over advanced here, and that's just where it jumps ahead......

My theories (guesses) are that the hall effect ignition is triggering from the trailing edge of the pulse and for some reason suddenly triggers from the leading edge....... Seems very unlikely although I haven't tried to record what's going on yet....... Any other ideas? And any reason why the standard advance curve should keep going so far beyond peak torque? The non-emissions, linear advance curve engines which keep advancing to 6800 and produce max power at 7000 are reputed to produce more power.

 

[enginebob]

The drop at 5000 is probably because that area is worn some allowing the shaft to back up some. And the choppy effect is either due to roughness of surface finish or vibrations feeding into the system. You need a bit of lube on the sliding memebers and they need a good finish. I don't think you will get if perfect. Reminds me of the good ole Corvette fuel injection days, advance wise.

 

[PFM]

Carbonlife,

I'm with Bob on the why part of your question. As for spark advance, any spring / weight advance mech is a compromise at best. Is this for a street car? With or without smog? Where I am going is to an aftermarket ignition with a programable curve, you lock out the worn distributor, and dial in a good no compromise curve. Best done on the dyno, but can be done without as well. I have seen very good gains in power and drive ability with such a swap.

Enjoy,

PFM

 

[carbonlife]

Thanks very much for the responses guys. Very reasonable ideas based on my previous post but having been looking at this thing for a while, I don't think that's what's happening.

To give you a little more info, have a look at these pics of the mechanism:

http://www.senstyle.com/dzzy/attach/advanceassy.jpg

http://www.senstyle.com/dzzy/attach/longside.jpg

The advance springs (ordinary compression coil springs) are contained inside the guide indicated by the red arrow. There's no measurable wear on the shaft and I don't see how, with this kind of mechanism, the curve can suddenly advance and then back up a few hundred rpm later........

As far as the choppiness is concerned, well some of this is due to the way the graph is plotted in excel - and I'll put a little down to measurement error too. I think the chopiness is a red herring - the mechanism is beautifully clean and greased and moves very smoothly when actuated by hand - and there's almost no play. Even on the slides and the "top hats" the manufacturer's machining marks are clearly visible and the very little play that exists is simply taken up at low rpm. I could well be wrong but I can't see how it could do otherwise. I do think the jump at 5000 rpm is a real effect though...... any further ideas?

As to your specific suggestions:

Bob:

I think you have a good point when you say that vibrations could be part of the problem....... Possibly that's contributing to what's happening at 5000 rpm. Any guesses as to what the vibrations could be doing here?

PFM - it's for a street car, no smog (never had any)..... Sure, mechanical ignition advance is going to be a compromise - and I agree that a fully mapped system delivers big benefits - almost as much on its own as going to fully a mapped injection and ignition system in my experience - at least from a driveability point of view. Still, I'm avoiding going that route on this particular car......

Are you not surprised at how much (standard) advance there is beyond peak torque? Or do you think this is no so exceptional? Continuing ignition advance to 200 rpm short of max power seems unusual to me - any inferences about what this might imply about the compromise? Or again, is this more common practice than I think it is?

 

[enginebob]

Carbonlife, It doesn't look like the ramps could cause it, your correct there. I was thinking old time delco style weights etc. Also I wasn't thinking electronic ignition either. That has way increased the variables. The vibration I was thinking was crank harmonic feed back. But it all could be an electrical problem also. It could be something designed into the circuitry, or a fault, don't know. If the advance unit is checking out okay. Then most likely it's electronic in nature. Like an rc time constant dropping off or something. ie kinda detunning at that frequency (speed).