Distributor Curve Science Simplified

[seattle smitty]

Relieved to hear that what I thought I knew about the subject was reasonably close!! But such knowledge as I possess is general, and since you're an ignition specialist, I'm asking for particulars.

Some guys here will want to disable the EGR system, for whatever reason. Personally, with age I have become willing to accept most of the "smog crap" (emissions control systems) on non-racing machinery. However, there are a couple of reasons I might take out the EGR. First, if you build a raised-compression street motor and control detonation with water injection, a FULL-TIME water injection system has the secondary effect of cooling the combustion event, rendering the EGR system superfluous. Second, in a few motors, the factory EGR system has been found to be a chronic maintenance headache. This is the case with the 2.0L G63B engine in my '87 Dodge Colt Vista, and I intend to ditch the EGR.

With engines such as the classic Detroit sixes and V-8s of the 1970s and '80s, you can find pre-smog versions from the 1960s that have distributors calibrated for non-EGR parameters. Maybe the advance weights, maybe the springs, maybe the vacuum advance motor, or any combination of these might be different from the parts in a smog-motor distributor. So when we disable the EGR in a smog-motor, instead of trying to adjust out the resulting engine ping/knock by retarding the initial static timing of the smog distributor (or disconnecting the vacuum advancer as a temporary emergency band-aid), the right solution is to recalibrate the distributor with pre-smogger parts and specs.

Am I right so far?

Unfortunately, some of the engines we deal with are new enough to have been smog-motors from the start, and there are no pre-EGR parts for the distributor. So here it is, Steve: Q. If I brought my EGR-less Vista into your shop asking that you modify the distributor for the most efficient NON-EGR advance curve and vacuum advance specs, how do you address this problem? (With the distributor customized in this way, I can road-test my way to somewhere near the right initial timing).

No doubt there are a lot of variables that make it impossible to offer a one-size-fits-all recommendation here. The wierd hemi-head on my Vista surely reacts differently to ignition mods than the flat Heron head on the 1600cc Kent motor in my '79 Ford Fiesta. So what I'm asking for is your suggestion to "Change this part first, then try a series of these parts, followed by . . . , etc., etc.."

Thanks, Steve.

[Mwilson]

I missed this entire section somehow! Thanks I'll be reading this evening.

[paulo]

Mr Davis Your article gives me the courage to recurve but I need some help.In ´99 I built a Pontiac 350 for a friend with custom 10:1 Ross pistons and too big a camshaft (292* Pro 3000 Fed Mog),stock heads,rods,crank.Being that it is a ´71street car,I ordered a new HEI with proper wires and plugs.All around me here in Brasil told me to disconnect vacuum advance,which I disagreed.I adjusted it to pull only 6* (ported) since all we had back then was 91 oct gasoline.The result was no good and I had to time it at no more than 8* static or detonation would heavily occur even at light throttle.Then Petrobras blessed us with a 95 oct gasoline and it allowed me to time it to 12*,which generated a higher idle vacuum of 16" hg and a smoother running engine.My centrifugal curve now is this:

750 rpm (idle) 12*
1000 15*
1500 22*
2000 30*
2400 34* (max)

It is borther line into detonation and I want to switch the vacuum from ported to full manifold.What do you think?

Additional info.: 3,90 gears,normal 15" tires,automatic,3000 pounds and a unique gasoline that carries 25% of ethanol by law.

Thanks Paulo

[SteveDavis]

Originally Posted by paulo

Mr Davis Your article gives me the courage to recurve but I need some help.In ´99 I built a Pontiac 350 for a friend with custom 10:1 Ross pistons and too big a camshaft (292* Pro 3000 Fed Mog),stock heads,rods,crank.Being that it is a ´71street car,I ordered a new HEI with proper wires and plugs.All around me here in Brasil told me to disconnect vacuum advance,which I disagreed.I adjusted it to pull only 6* (ported) since all we had back then was 91 oct gasoline.The result was no good and I had to time it at no more than 8* static or detonation would heavily occur even at light throttle.Then Petrobras blessed us with a 95 oct gasoline and it allowed me to time it to 12*,which generated a higher idle vacuum of 16" hg and a smoother running engine.My centrifugal curve now is this:

750 rpm (idle) 12*
1000 15*
1500 22*
2000 30*
2400 34* (max)

It is borther line into detonation and I want to switch the vacuum from ported to full manifold.What do you think?

Additional info.: 3,90 gears,normal 15" tires,automatic,3000 pounds and a unique gasoline that carries 25% of ethanol by law.

Thanks Paulo

I suggest you try one "stronger" advance spring in order to delay full advance to 3000 rpm. This should help get rid of your detonation.

[SteveDavis]

Originally Posted by SteveDavis

I suggest you try one "stronger" advance spring in order to delay full advance to 3000 rpm. This should help get rid of your detonation.

Try direct vacuum first, because this will give you extra vacuum advance at idle, keeping your spark plugs cleaner over time; however, some motors tend to idle too rough or too fast hooked up direct, in which case, you should run ported vacuum.

[hsutton]

Steve, My question is performance related, i'm wondering if your Mini-VIP can be wired into a system using a "MSD crank trigger" without complications. I'm told that part of the "control system" for the unit controls a momentary output pulse to the coil in order to fire the plugs and i'm wondering if it can be arranged so the increased 18V input to the coil will have enough time to work properly with the crank trigger? What are the secondary voltage levels with and without the Mini-VIP inline? How about the effects on a nitrous shootout car?

[SteveDavis]

Originally Posted by hsutton

Steve, My question is performance related, i'm wondering if your Mini-VIP can be wired into a system using a "MSD crank trigger" without complications. I'm told that part of the "control system" for the unit controls a momentary output pulse to the coil in order to fire the plugs and i'm wondering if it can be arranged so the increased 18V input to the coil will have enough time to work properly with the crank trigger? What are the secondary voltage levels with and without the Mini-VIP inline? How about the effects on a nitrous shootout car?

Most of the MSD products can withstand 18 volts. They have a chart in their catalog that shows corresponding part numbers and operating voltage.

The higher cylinder pressure caused by the nitrous needs all the spark it can get.

There will be no adverse effects on the crank trigger. Just running the 18 volts to the positive coil connection will suffice. We desiigned the 18 volt Mini-VIP so that the 18 volts flows only to the desired device.

[Horsewidower]

You note a difference in burn times and the necessity for different advance curves when switching between leaded and unleaded fuels. What about between gas and ethanol or methanol?

[SteveDavis]

Originally Posted by Horsewidower

You note a difference in burn times and the necessity for different advance curves when switching between leaded and unleaded fuels. What about between gas and ethanol or methanol?

Methanol and ethanol can use "quicker" advance curves. That is, total advance can be achieved at lower rpms, without causing detonation.

[olNick]

Hi Steve,

Really cool article. Right off the bat you say

"The whole concept of ignition advance is centered on charge burn time and burn time can be broken down into two categories. These are ignition delay and the bulk burn rate. The goal we are trying to achieve is to get peak cylinder pressure to occur at about 15 degrees past TDC on the power stroke. If we make that goal then the three main issues of torque, hp and fuel economy will, from the ignition stand point at least, be maximized."

So if one were to measure peak cylinder pressures v.s rpm, would the main goal be to find "the" advance curve that keeps this pressure at 15* or so (the specific engines' need). What else would need to be considered, e.g varying loads?

Maybe I'm stating the obvious, but hey, I'm a garage sweeper ;-)

regards,

nick