Joined: Sat Aug 19 2006, 05:03PM
Location: Ontario, Canada
Posts: 2919
I finally got around to pulling the dist from my hardtop on Sunday and setting it up with the help of my distributor stroboscope. My plan was to convert it to use constant vacuum source, not ported as Mopars generally are setup for. Most engines, especially performance engines, should run smoother and idle cooler with more advance. You can't use too much initial advance when starting though, or you'll have hard hot-starts because the engine will buck and fight the starter when it fires. The factory setting is a compromise.
Most Mopar vacuum advance cans are not setup properly for vacuum at idle. They pull too much advance and come in at too high vacuum, so will be only partly-engaged at idle and your idle timing and RPM will wander. For a proper setup, the vac advance needs to be all-in at a lower vac than your engine produces at in-gear idle, so your idle will be stable. I happen to have an adjustable can that pulls 16* at the crank. My engine has 13* vac at in-gear idle. With the adjustment screw fully clockwise, the can is all-in at 12*, which is marginal for my engine.
I setup for 10* initial, plus 16* vac for 26* total at idle. I set mechanical for 24* for a total of 34* WOT and 50* cruise. The only thing I didn't like is that my mechanical starts advancing at 1150 RPM and is all-in at 2300 RPM. I would like to see it all-in closer to 2800 RPM, but I have no other Mallory springs right now, and it's been fine like that since I installed this distributor, so I figured I'd try it as-is.
Idle wanders around a bit. I found that I could actually get it to idle better with a bit more advance by turning the distributor, putting the initial+vac advance up to 30*, then resetting the carb idle settings of course. I put it back to 26* for a test drive because I didn't want too much advance at WOT. Will try more later, since I may need to reduce the mechanical travel as well.
Took the car out for a HARD drive. I was very happy. !drive No pinging and it seemed to pull even harder from a lower speed than it did before. Previously I had been running 17* initial and 17* mechanical, all-in at 2000 RPM, and there used to be a large bog if I hammered it from low RPM. Increasing the accelerator pump nozzle size helped but didn't totally fix the bog, but it is much improved now.
When I stopped and idled for a bit, coolant temp didn't seem to climb as it used to. !thumb That was one of my main reasons for this experiment. So far so good.
Next steps are to increase the idle timing so initial+vac = 30* and get a Mallory spring kit so I can tweak the mechanical curve. <span class='smallblacktext'>[ Edited ]</span>
Joined: Sat Aug 19 2006, 05:03PM
Location: Ontario, Canada
Posts: 2919
FYI, to adjust mechanical advance on a Mallory distributor without using their "timing sticks" I use drill bits: For every 1* of advance, you need about 0.010" slot width. Figure out desired degrees, the resulting slot width, then round DOWN to the nearest drill bit size. grab two identical drill bits of the determined size, stick these in the slots, and retighten the torx adjustment screws. All done!
This method has a bit of error on the high side, which is why you always round DOWN when selecting drill bit size. <span class='smallblacktext'>[ Edited Mon Jun 28 2010, 11:03AM ]</span>
Joined: Thu May 01 2008, 11:15AM
Location: Chicago,IL
Posts: 2868
Interesting, so you now have an inverted timing curve, lots of timing at idle and then it backs out when you get into it and will come back when you hit cruise. That's one way to do it if you have a radical cam and want to stay a sleeper, makes for a buttery smooth idle with so much timing!
Joined: Fri May 18 2007, 04:59PM
Location: Vegas
Posts: 82
Basically what you are describing is that you have recurved your distributor.
Using manifold vacuum will certainly help smooth out a low vacuum cam, but no matter if manifold, or ported vacuum, they are the same at anything other than idle.
If your cam was smaller, you would probably find "pinging" at initiall acceleration with manifold vacuum. That's why the factories designed ported.
Ford used a great system of a temperature controlled vacuum "tree" which would switch from ported to manifold if the engine temperature was higher that ideal.
I only point this out so that you don't come to a conclusion that one is better than the other during your experimentation, as ported is usually better if you are running a cam that has decent vacuum.
Joined: Sat Aug 19 2006, 05:03PM
Location: Ontario, Canada
Posts: 2919
Q-ship wrote ... Basically what you are describing is that you have recurved your distributor.
Obviously, that's what a distributor stroboscope is for. !nana
Q-ship wrote ... Using manifold vacuum will certainly help smooth out a low vacuum cam, but no matter if manifold, or ported vacuum, they are the same at anything other than idle.
Yes that is true. You'd be surprised at how many supposedly knowledgeable people would argue otherwise though. I even did a test with two vacuum gauges plumbed into my car while driving to prove it, and posted the results in a thread here on the Dock.
As I said already, the primary reason for this experiment was to keep engine temp in-check when cruising around town and stopped, ie:idling. That the engine now seems to respond better when I hammer the throttle leads to some new questions. !think
Q-ship wrote ... If your cam was smaller, you would probably find "pinging" at initiall acceleration with manifold vacuum. That's why the factories designed ported.
I don't know about that. The main reason I can think for pinging on initial acceleration would be because the advance isn't dropping back fast enough because the vacuum advance module can only react so fast.
I believe that all pre-emissions Chevies (pre-1968?) were setup to run constant vacuum, not ported. One of my inspirations for this experiment was a series of technical articles by a Chevy tuning guru on distributor setup and why ported vacuum is an aberration forced upon engines by the EPA and/or CARB. When I started reading this stuff I was initially confused, since Mopars have used ported vacuum since before emissions controls.
Q-ship wrote ... Ford used a great system of a temperature controlled vacuum "tree" which would switch from ported to manifold if the engine temperature was higher that ideal.
That sounds like a bandaid for the early emissions control systems. It also makes me think I'm on the right track with my experiment. I have a technical book published by Hot Rod in 1971 with an article on emissions control systems, and it discusses this temperature-controlled vacuum control valve. Later on they did all kinds of wacky things to change the behaviour of the ignition timing in the name of reducing emissions.
Q-ship wrote ... I only point this out so that you don't come to a conclusion that one is better than the other during your experimentation, as ported is usually better if you are running a cam that has decent vacuum.
Maybe so. I suspect many engines can benefit from more initial timing than what they are setup for, they just won't start that way when hot without kicking back. I know some Dockers have problems with their engines running hot at idle. Maybe they need to pay more attention to their initial timing, and maybe using constant vacuum is a solution for some of them, but it has to be setup properly. I'm sharing my experiment in progress so others may benefit. Heck, maybe some day I'll even get a big sack of cash to setup a dizzy for someone else. !dance
I did find one shortcoming when I took the car out tonight, although part of it is because my electric choke comes off too soon and drops the engine entirely off high-idle. When the engine is cold, it idles low and rough because there's not quite enough vacuum to keep the vacuum advance pegged. I had to two-foot the gas and brakes a bit while backing out of the garage. Once everything was heat-soaked it ran and idled well, and I've been able to cut my idle down by about 100RPM.
Another thing I thought odd: I looked at the instructions for the Mallory spring kit today, and the chart showing timing curves with different spring combinations. Mine is a real Mallory dizzy, not the Mopar-branded one which has a notoriously bad setup out of the box. The stock setup with the purple and brown spring should be perfect for me with the purple spring coming-in at 15* advance and mechanical all-in of 24* at 2900 RPM so I shouldn't even need a spring kit. However I measured that the timing is actually all-in at 2300. Just looking at the weights, I can see that the purple spring isn't even doing anything when the mechanical is fully advanced at 24*. Either I have a defectively-made purple spring or that spring perch is was bent when I got the dizzy.
Joined: Sat Aug 19 2006, 05:03PM
Location: Ontario, Canada
Posts: 2919
Here are my latest observations:
I set the choke to stay on a bit longer and that has helped when the engine is cold because it keeps the idle up a bit longer.
Coolant temp still climbs to 200F eventually, maybe takes longer than before but could just be a cooler day when I was paying close attention to that. I have noticed that the shutdown run-on problem the engine sometimes had has completely disappeared, so the combustion chambers and spark plugs are running cooler.
From a seat-of-the-pants feel, torque from stopped or slow rolling start is same as always or a bit improved with light to medium throttle. With heavy throttle to WOT however, it feels like it is down a bit. Makes sense, since the timing effectively drops to 10* at low RPM once the vacuum disappears.
A ported vacuum setup may be preferable for a street/strip car because of this, or a vacuum advance that pulls even less than 16* so more initial timing could be dialed-in.
