Distributor Phasing Question

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BillGrissom

BillGrissom

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There have been discussions (and arguments) about how the distributor rotor moves when mechanical and vacuum advance are applied. The understanding is that mechanical advance does not change where the rotor is (rel. to post) when the spark fires. That is because the weights change the angle between the rotor and drive shaft. Vacuum advance does shift rotor position, but is mild enough (15 deg max) that the rotor tip stays close enough to the post to not cause a phasing problem.

My question is what happens if you lock out the mechanical advance and use an electronic controller (like MSD) to apply the rpm advance? It seems that could lead to a severe rotor mis-phase, since then the rotor would shift >30 deg. The spark might then jump to the wrong post. Below is from an RB electronic distributor currently on ebay:

"The mechanical advance has been 'LOCKED' (tack welded) for use with and to be controlled by after market digital ignition systems that allow for adjustable ignition timing curves."

My guess is you might get it to work for drag racing only, if you adjust it so the phasing is close at high rpm (not easy), then just never try to idle the engine. Most big cam engines barely idle anyway.
 
Bill:

The way this works is by locking the distributor, but aligning the rotor for the midpoint of the advance curve. If you have say 45 degrees of total advance, the tip of the rotor should line up with the post @ about 22 degrees BTDC.

Then the trigger is rotated to fire @ 45 degrees BTDC. Not the distributor, just the trigger in relation to the reluctor.

The ignition box can't advance the timing. All it can do is delay the signal to the coil - retarding the timing. So the mechanics of it are actually set up to provide 45 degrees of timing, With the electronics actually removing timing.

The rotor is phased to the midpoint of the curve, and the distributor is locked. Since the phase angle is half the crank angle, that means the tip of the rotor is never more than 10 to 12 degrees one side or the other from the post.

B.
 
bohica2xo
Thanks for the excellent description. That is the way I figured it must work, by splitting the difference. One must be smart to set it up correctly, such as use a distributor cap with a cut-out to view the rotor with a timing light (I have done).

The reason I am thinking about this is that I plan to eventually use an 8-pin HEI module (GM 87-95), which allows electronic advance, controlled by a Holley Commander 950. I am contemplating keeping the mechanical weights for optimal phasing and just add to it electronically. If the weights don't stick much they ought to be smooth and predictable, and leaving the weights in makes it simple to swap back to standard mode.
 
805moparkid said:
oh boy... here comes 67 again lol
Click to expand...

I don't have a problem with this

Bill it seems to me that much of the problem with Mopar distributors (spark scatter, in-accurate timing, etc) is in the overall SYSTEM of the whole dist. and the drive.

So to get things tightened up, using a crank trigger would be the cat's meow.

I would also think it should be easy to find a lean burn dist. and use that.

But remember, the dist. only turns HALF the speed (and half the distance) of the crank. This means that a 20-24* typical performance advance curve AT THE CRANK, only results in a rotor swing of HALF that, or 10-12* of rotor travel.
 
If you go to electronic timing, then also do coil on plug or waste fire coils. Crank trigger the ignition, and one pulse from distributor for TDC sync. Then no need for rotor.
 
The rotor is phased quadralaterally to the second phase shift position. As such, the plasma bearing molecules remain unchanged. However, during quantum flux capacitance, the irridium core properties transpose into irregularities called quantum phaseous anomolies.
 
The mechanical advance changes the rotor position and the vacuum changes the timing by moving the breaker plate. tmm
 
StrokerScamp said:
The rotor is phased quadralaterally to the second phase shift position. As such, the plasma bearing molecules remain unchanged. However, during quantum flux capacitance, the irridium core properties transpose into irregularities called quantum phaseous anomolies.
Click to expand...

Ya....what he said!
 
StrokerScamp said:
The rotor is phased quadralaterally to the second phase shift position. As such, the plasma bearing molecules remain unchanged. However, during quantum flux capacitance, the irridium core properties transpose into irregularities called quantum phaseous anomolies.
Click to expand...

You've been watching too damn much of this..................


[ame="http://www.youtube.com/watch?v=Ac7G7xOG2Ag&feature=related"]"Turbo Encabulator" the Original - YouTube[/ame]

[ame="http://www.youtube.com/watch?v=lBp5ag6SJH4"]Allen-Bradley Retro Entabulator - YouTube[/ame]
 
KitCarlson said:
If you go to electronic timing, then also do coil on plug or waste fire coils. Crank trigger the ignition, and one pulse from distributor for TDC sync. Then no need for rotor.
Click to expand...
That is the best idea and what all current engines use, as far as I know. Indeed, I already installed a 36-1 trigger wheel on my 65 Dart 273 crank pulley to later use a Ford EDIS ignitor.

However, I don't read that the Commander 950 can command EDIS timing. It can control earlier "computer distributors" - Ford TFI and GM 7/8 pin HEI. The later w/ interface cables came with my Commander 950. I will probably try that on my 383 since I don't see an easy way to install a trigger wheel on the crank pulley. Sparking off the distributor is probably good enough if the cam chain is tight. It was used until 1995 in GM trucks. The Commander can also react to a knock signal, like from a GM ESC module.
 
The rotorless system I suggested would be home brew. Several years back, I developed an engine management system for an Dodge Omni 2.2 turbo. I can use two of them for a V8. The distributor timing signal could be done with the 2.2L hall sensor and a modified 2.2L tab wheel. The crank sensor just four tabs. I am considering using the flex plate. While some say more tabs are needed for resolution, there are tricks to tab design and firmware that resolve resolution inaccuracy.

I normally would be jumping in and doing the project, however my Barracuda is very stock. If I start changing there will be no end. That includes changing to EFI at the same time.
 
KitCarlson,

You sound like a hard-core designer. The problem for me is ideas are many and life is short. I am lucky to occasionally be allowed to develop new designs at my day job, but aerospace instead of auto.

I agree that using just the distributor for spark timing, but in a distributorless system is the best balance for retrofitting old engines. I expect someone will come out with a turn-key system soon. Retrofitting a crank sensor is not trivial, given the varying components and pulleys on even the same engine type (ex. SB Mopar). Adding a cam sensor to avoid "wasted spark mode" makes it even harder. If the spark controller ultimately increases spark until knock is sensed, wear in the timing chain hardly matters, so why not just use the distributor directly?

Re resolution, even 36-1 crank wheel systems use a digital timer to interpolate between teeth, so doing similar in the distributor is reasonable. I don't know how many teeth in the new Hemi's "toner ring", but doubt >36. Still, I don't see how one can avoid customizing the distributor pickup since distributorless requires a TDC marker, so why not add more teeth for better resolution at the same time?

Back to my original post, everyone above concurs that the rotor won't move too much to not align with the post, even without advance weights. It works in both production engines (GM "computer" distributor) and many after-market racing distributors. Interestingly, I ran across a slant six distributor cap on e-bay described as special, with "wide contacts", which might work better. I also found posts on www.slantsix.org about new rotors with a short tip that causes mis-fires, even without a rotor phasing problem. Distributorless takes care of all those concerns.

For now, I will keep the weights and vacuum advance and let the computer add tweaks to that. That makes it easy to swap back to no computer control if stranded on the road.
 
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