AFter playing with the idea of digital control of the spark curve, I have decided to back away from that until I get a clearer idea of what I want to do there. For now, I need to keep the centrifigal timing out lower in the RPMs. Right now it starts in around 800 rpm. The engine has early preignition under boost between 1500, and 2000 RPM with the present curve, unless I set the timing back to TDC. I would like to keep the centrifigal from coming in below 1500. Any experience out there with the springs, and how to set it up to accomplish that? I think this engine would like only 25 or so total with the turbo boost, and would like it to start coming on at 14-1500, and be all in be 2500. I can take it to a local shop to set up, but thought someone might know the formula to achieve that.?

I am also trying to figure out how to hook up the vacuum advance so it sees ported vacuum instead of manifold. The Buick TB I am using has only manifold vacuum ports, and getting the engine to idle stably is presenting a problem. The problem is, that if the RPM falters at idle for any reason (like the timing jumping around because the centrifical advance comes in too low), then the vacuum drops, the timing falls back 'cause the vacuum advance sees less vacuum , and then the engine goes into a death spiral and dies. It doesn't happen often, but once is enough to make you want to fix it. Is there consensus of opinion that tapping into the TB right above the blades will yield the same vacuum figures that are below it once the throttle is opened? How big a hole should there be? I can buy any sized vacuum nipple I want.

Can anybody out there help me with this? Doc, you must have recurved hundreds of these old distributors over the years. I went to the local speed shop, who specializes in Chevys, and was pretty much ignored by the owner. I stood there for fifteen minutes while he chatted about nothing to a customer who was already done shopping, before I left in a huff. He appologized as I was leaving, but I was really burned up by his rudeness, and lack of interest or any sense of normal customer service, and told him I no longer had time and left. I really don;t want to go back to see this jerk, but he is the only one I know who has a distributor machine. Most of the speed shops these days are installing digital control for spark curve, and distributor machines are getting to be passe. How much advance is built into the centrifigal portion of the stock slant distributor? I want it to be 15 degrees, with 10 initial, totalling 25. I also want it to start at 1500 RPM, so the advance all comes in between 1500, and 2500. Am I stuck with either going back to this PITA or finding another local shop, or can I figure this out on my own?

You can figure this out yourself and you don't really need a distributor machine.
(Do you have another SL6 car you can test distributor curves on?)

It sounds like you will need a short slot plate (8X) or may-be a "three spring" set-up, here is what I would try: (a 3 spring curve)

Keep the light spring you have and set it to come in right off idle. (sounds like you already have this set)

Take a smaller diameter / stiffer spring with a short end loop and string it through the center of the current light spring. Set this second spring so it starts working after 4-5 degrees of "off idle" centrifical advance.
Last, get a real heavy spring with a short loop that starts working at 13-14 degrees / 2500 RPM. (bottoms-out in it's loop) The real stiff spring will only give 3-4 additional degrees of advance as you buzz the engine up to 5000+ RPM.

This suggested 3 spring set-up will give you a little "shot" of quick advance right off idle, then slow the curve way down as you climb RPM, with the heavy spring acting as the limiter. (not the slotted plate)

You can use a running engine as your "distributor machine" if you have a timing light and mark-off the damper at 15, 25 and 30 degrees. (or get a retard type timing light)

Truth is that a distributor machine is the "slick" way to get the curve dialed-in with-out having to rev-up your engine a bunch as you look at the curve.
Do you have a few extra distributors to work with and steal springs out of?
DD

this may be to backwoods for such a fancy set-up but would a brass valve like the small ones used for fridge ice machines and some barb fittings place inline on the vac hose to regulate the suck?

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Thanks Doc. You have given me exactly the start i need on this. I think I want to eliminate the early quick shot of advance, as that is where the turbo boost and low RPM are creating preignition. With initial timing set at 10 BTDC, travelling down the highway at 1500 RPM, if I leave it in 4th gear, (5 speed tranny), and floor it, boost build right away at 1500 RPM, but so does preignition: big time. If I time it at TDC (0), then this goes away, but then the engine will not idle well. I can see the timing bouncing around as the centrifigal plates bounce in and out at it';s current setting around 900 RPM.

If I slow the idle down, the timing stabiilizes, but then, with the vacuum advance drawing from manifold vacuum (the only choice on the factory Buick TB), the engine will go into a death spiral and die if the rpm drop a 100 RPM for any reason. Vacuum drops off with rpm drop, timing retards slightly, RPM drops a little further, timing retards a little further , etc, and then finally the engine just quits. Of course this just takes a second. But the second half of the puzzle is to get the vacuum advance hooked up above the throttle plates. I have done that already, actually, but have not been able to check out the new port, and see if it is actually drawing vaccum at cruise throttle plate openings. Thanks for your help. I need to prove the this silly Chevy guy that us Mopar guys stick together, and don;t need to tolerate their rudeness.

The turbo boost adds another variable to the advance curve that is difficult to deal with. Mechanical advance alone does not work well with boost because it is fixed to engine RPm, not load, that is why todays turbo engines use computer controlled timing.

The early turbo set-ups used a vacuum retard on the distributor. The mechanical advance curve was set for non-boost condition and as boost came-up, the "retard pod" pulled advance out.

Not having any advance right off idle will give you "flat" throttle responce until the boost comes in, it will be a juggling act to get good off idle performance without the mid throttle pinging.
DD

Thanks again Doc. Megasquirt is now offering an upgrade they call Megasquirt II which has a spark curve. I guess I should sigh, and go for that. I was resisting getting into another digital device that I won;t understand for awhile. Learning always invloves feeling stupid for awhile. MSD does sell a boost retard which will work with the MSD box I have. That would be a simpler fix, but maybe not the best. The MSD master boost retard, or what ever it is called, is more expensive than the MS II upgrade. So dollars speak in favor of the digital spark curve.

DV, you are a prime candidate for a MSII. Once it is set up, the advance settings you want are as simple as typing the values in a box. I don't know how much you've read up on the MSII, but it's not really another digital device. It's a plug-in replacement chip for your current CPU in the MS you have. Looks like it's going to be around $75.

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It's a Slant thing. Even I don't understand.

1974 Duster, EFI /6 soon to be turbo...

Get that Monkay! Get that nasty thing!!

Sorry for dragging up such an old thread but i wanted to check something.
Does Doc mean putting another spring inside the light spring i have a arrow pointing at in the photo ?



If so how do i set it so it starts working after 4-5 degrees of "off idle" centrifical advance ?

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do you have a selection of distributor springs to work with?

if not BigSlantSixFan had them in sets at one time,,

the springs included instructions on using various springs to set an advance curve,, I had that, but lost it in a basement water event,,, that is another story,,

a couple of other options are,
1) lock out or get a locked ( lean burn) distributor and a programmable ignition control box to control the ignition curve

2) contact DusterIdiot directly, he could probably recommend a curve and set up for your application.

setting a curve for a boosted engine always involves some risk, so be conservative.

Thanks for that.
I still have 2 yellow and 2 black springs from a bigslantsixfan kit but am unsure which curve i should be looking at for a boosted engine ,
I will contact DusterIdiot to see what he can recommend.
I wanted to try something like this before i go spending money on a box .

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Yes, that's what he is talking about.

Thanks for that.
Does anyone know where to get the smaller diameter springs Doc is talking about ?

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First of all a big thanks to Dart270 and DusterIdiot for their help on getting my curve sorted for my supercharged slanty .
I am getting close but would like to try a couple more curves .
I have attached a photo of the graph that comes with bigslantsixfan's spring kit
Can someone explain the curves that have only one coloured spring in them .
That is Y BL G
Does that mean you only put one spring on one weight and the other weight doesn't have any spring on it ?

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I was getting the same symptoms as Dartvader with my engine (supercharged)dying every now and then.
Tried a few different curves but it still did it every now and then .
I couldn't get boost in first and second without working through a flat spot .
This was with vacuum connected to the inlet manifold and disconnected.
After reading about vacuum canisters i decided to adjust mine so the spring had less tension.
I turned it full CW and backed it out half a turn .
My vacuum canister has 8.5R stamped on the arm .
Connected the vacuum line back up to the inlet manifold so it can act as retard when boost comes on .
Amazing what a difference this has made , now as soon as you hit the throttle you can see the bypass valve snapping shut and no hesitation .
My curve comes in at around 900 rpm and is finished by around 1800 rpm .
I have 20 mechanical advance in the dizzy and have initial set at 5.
Slanty idles great and cruises great at 100 kmh .
Now when changing gears boost comes on straight away .
Thought this might help someone .

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