Timing for electronic ignition upgrade

[Cudamike56]

Good day all. The 340 in my 68 Cuda formula S had been converted to Mopar electronic ignition, orange box, prior to my acquiring and restoring it. I had read somewhere when converting from the old dual point dist. to electronic, the initial timing setting needed to be advanced from the factory 5' BTDC, which is where it is now. Has anyone else heard of this idea? If so, does anyone know what the timing should now be set at? I have not touched the engine other than attempting to repair the never ending rear main drip, so not sure what may have been done inside. Compression is fairly good at 145 PSI. My previous experience with 340's was as snappy engines reasonably quick off the line. This engine is quite slow to wind up and wondered if it might be running somewhat retarded.

 

[RustyRatRod]

I've never heard that one. I would just experiment around with it. Give it as much initial timing as it wants without detonation. Stock engines can usually take somewhere between 12 and 20 initial but you need to limit the total mechanical advance "somehow" to about 34-36 degrees.

 

[toolmanmike]

Good day all. The 340 in my 68 Cuda formula S had been converted to Mopar electronic ignition, orange box, prior to my acquiring and restoring it. I had read somewhere when converting from the old dual point dist. to electronic, the initial timing setting needed to be advanced from the factory 5' BTDC, which is where it is now. Has anyone else heard of this idea? If so, does anyone know what the timing should now be set at? I have not touched the engine other than attempting to repair the never ending rear main drip, so not sure what may have been done inside. Compression is fairly good at 145 PSI. My previous experience with 340's was as snappy engines reasonably quick off the line. This engine is quite slow to wind up and wondered if it might be running somewhat retarded.

It might need a quicker advance curve. There's many threads about that here in the site in the ignition forums or use the search bar. If you aren't handy with that sort of thing, Halifaxhops here on the site is a expert on distributors. He has a dist. machine and can fix you up.

 

[Bewy]

The timing would not change going from points to electronic, as in the elec version a transistor does the switching instead of mechanical points.

I would change the initial to 12*. 5* is pretty low. 145 Psi is not great, but could be due to worn rings, low static CR, worn cam or longer duration cam. Idle rpm? Is it smooth? What are the cam specs?

 

[Cudamike56]

All good input, guys. Thanks.
Bewy, as I mentioned I have no idea what is inside the engine so I don't know what the cam specs would be. Once its warmed up, it idles a little "loapey" at 650 RPM but nothing drastic. I'll try advancing the initial timing until it starts to detonate, then back off a degree or two, and see if it makes a difference. If it doesn't, I'll have a local speed shop check out the distributor on their machine. Thanks again.

 

[Mattax]

I had read somewhere when converting from the old dual point dist. to electronic, the initial timing setting needed to be advanced from the factory 5' BTDC, which is where it is now. Has anyone else heard of this idea?

Its false. There area least a couple of dozen different advance curves that came from the factory on LA engines with ECUs. Aftermarket is adds an additional variable.

If so, does anyone know what the timing should now be set at?

Not really. 5*BTDC at 650 rpm for the automatics was to reduce HC emmissions while idling. To use the factory inition, the engine must have the matching distributor. The factory distributor advanced quickly from idle rpm, so by 950 rpm it had the same timing as it would have if there was no concern about emmissions.

I'll try advancing the initial timing until it starts to detonate, then back off a degree or two, and see if it makes a difference.

Right is better. Maximum it will tolerate for starting does not identify maximum power. Your engine was not spec'd at 5*BTDC for improved starting, but for emissions. Factory non-smogged high performance LA engines will provide the strongest (and most fuel efficient) idle between 10 and 14* BTDC. With your automagic you can test this by placing the selector into D and noting the drop in rpm. The strongest idle at a given rpm, will be the adjustments (timing, fuel mix) that can tun the transmission converter and pump with the least drop of rpm.

If it doesn't, I'll have a local speed shop check out the distributor on their machine. Thanks again.

Getting the distrributor advance information will be immensly helpful. You can do it on the car, but its easier to do it on the machine.

 

[Mattax]

Timing should be in the range of the the gray if one was to use an initial around 11*BTC at 650 rpm

An explanation for 10*-12* non-clean air package initial is here

Its possible your magnetic pickup distributor came from Mopar Performance. In which case it already has an advace curve in it very similar to the gray curve.

 

[Cudamike56]

Thanks for the background/rationale for the 5' BTDC spec., Mattax. I'll move forward with the timing adjustment and adjust the carb as needed.

 

[Cudamike56]

Just saw the graph and your previous post it originated from. Good demonstration of the CAP vs Non-CAP performance.
With the RPM now set at 650, am I to understand I'll want to increase the idle to 900 as I set the timing to 12' as per the graph? If so, I'll presumably need to adjust the mixture as well.

 

[67Dart273]

"Bump the timing"

NOT TRUE. (Unless the old one was set retarded LOL)

The one thing to remember when playing with timing is what is mechanical TOTAL timing? This is when you have the foot to the floor, which dumps all vacuum advance, and the result is initial + mechanical. So when you "bump up" the initial, you also move TOTAL and THAT number is what can damage your engine the most

As someone said, you may need to recurve

REMEMBER THIS ALWAYS. Federal emissions laws started in 68, and got worse every year, and started in CA in 66. So ANY car, 68 or later, has a longer slower advance curve. By say, 72-73 it was quite bad. THAT IS UNLESS you buy a Mopar performance kit or distributor (which has an adjustable curve). ANY stock breakerless Mopar dist. is going to have a smog slow, long advance curve

 

[Mattax]

With the RPM now set at 650, am I to understand I'll want to increase the idle to 900 as I set the timing to 12' as per the graph? If so, I'll presumably need to adjust the mixture as well.

950 rpm is approximately where the timing for CAP distributor becomes the same as the non-smog distributor.
In other words, the engine and operator will not notice any difference to speak of above 950 rpm.
Below 950 rpm the '68 CAP will have less unburned fuel and CO coming out the exhaust in trade for a little less torque.

Do not increase the idle speed with the idle speed screw (throttle stop) unless you have to.
One point of plotting the timing versus rpm is to see when the advance begins.
For example with the distributor for the '68 manual transmission, the specs show advance could begin between 700 and 1000 rpm, and the automatic between 650 and 950 rpm.

Now, lets say you set timing at 950 rpm with the original distributor for an automatic.

That might be OK IF the primary spring had a lot of tension resulting in an advance began at the very latest considered acceptable.

But what if the spring tension was average or a little weaker, allowing advance to begin immediately above idle rpm?

In that case, the advance follows the top curve, so by 950 rpm, 7* have been added to the initial.

In other words, the entire timing curve is shifted 7* down, and the base timing is actually 2*ATDC.

This one reason why initial timing must always be measured at consistance rpm,
Its best to measure the initial at an rpm below the start of timing advance, so a small difference in rpm makes no difference in timing. However with hot rodded engines and mystery distributor guts, this isn't aways possible.

 

[Mattax]

longer slower advance curve.

Long FAST curve.
At least through the early 70s, the initial portion of the advance was very fast, and very long, then the secondary curve was similar to pre-smog.

An extreme example of the difference is the 273 Federal versus California Manual trans with Clean Air Package in 1967.

Idle rpm spec also increased because they couldn't run well at the lower rpms with retarded timing and leaner idle mixtures.

 

[Mattax]

I set the timing to 12' as per the graph?

Here's another way to look at it. IF the distributor is an older Chrysler built Mopar Perfomance distributor, it will have a timing curve something like this one I measured. The advance is similar to a non-smog 273.

An initial around 12* (still at 650 rpm) would result in timing for the entire rpm range that will produce good power and work well with vacuum advance. Its basically what a non-smog 340 would have recieved if such a unicorn was ever produced.


You may be asking, what happens if a long fast CAP advance is set around 12* BTC, 650 rpm?
Lets take the same factory distributor.

The answer is, depending on fuel, altitude and engine, in this example it would probably be OK or on the edge.
Except with vacuum advance and part throttle it will probably let you know its unhappy when applying light to moderate throttle between 2000 and 3000 rpm. It will ping a bit, and there may bea dead spot in the throttle, or 'trailer hitching" on the highway. Even if its OK cold, it won't be when fully heat soaked. At least that's been my experience.

If so, I'll presumably need to adjust the mixture as well.

Adjustments of timing within a reasonable range shouldn't need much fuel adjustment. Moer than a degree or two, or any adjustment to the throttle stop position, then yes, readjust mixture for best power.
If the engine has the original carb, check the manual. I can't recall off the top of my head which carbs had restricted fuel mix adjusment to a fairly narrow range. Some had caps or plugs on the fuel mix screws, and an adjuster on the air bleed. The idea was to allow fine tuning to meet a specified lean AFR using a CO meter. It also prevented the techs from easily attaining richer non smog idle mixes.

 

[oldkimmer]

For 1 thing no 340s ever idled at 650 rpm. 900 rpm for them all. Kim