Need some help with my timing curve.
- Thread starter 75slant6
- Start date
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My 2 cents..
Looking at the Chrysler EFI timing, and tying that in with, smaller chamber, 'swirl' port, emissions oriented truck application (higher loads but burn as lean where possible).
I would NOT bump up the initial because it will move the entire curve up, and increasing timing from roughly 2000 rpm up could cause issues. More so if your going to do a high gear WOT through the top of the rpm.
Do it as is, then look at the spark plugs for signs of detonation you didn't hear. Of course if you hear it, back off the throttle. I don't know if the heads cracking relates to loads or not. If so, then obviously more caution needs to be observed since that won't provide warning signs like the pistons will (blowing bits of aluminum off).
IF the secondary spring can be brought in sooner, then the initial can be advanced the same number degrees.
You may have another option as well....
Keep in mind that base timing curve is for maximum load, which is usually wide open throttle or near wide open throttle.
Fuel milage will be mostly in that plus the vacuum advance.
Easiest is with a vacuum gage and vacuum pump. Apply vacuum until the advance mechanism begins to move, note the vacuum. Apply more vacuum until it stops moving. Note the vacuum. Next time you have the distributor open see if the arm is marked with distributor degrees.
The other thing to learn is the vacuum at idle (in gear if automatic) and if possible vacuum at cruise.
With a relatively lean burning idle as the stock EFI 5.9, the burn is slower.
You're running a carb, and aiming for power efficiency, so a couple things to try:
1) Adjusting the idle mixture a bit richer. Use the traditional method. Start on the rich side, and turn the idle mix in 1/8 turn at a time until the rpm and/or vacuum drops, then turn back richer 1/8 or 1/4 turn. I'd go 1/4 since you're doing this in park or neutral.
Test by placing in gear if automatic. RPM should not drop more than 100 rpm. Less is better - it shows the engine is making enough power to overcome the resistance of the transmission pump, etc.
2) Knowing the vacuum advance and the engine vac at idle, may be able to use manifold vacuum at idle to increase the timing advance. Probably won't want to do this with aftermarket cam and heads, but for the stock engine, it looks like thats effectively what the factory was doing. Of course they had to meet emissions - but the whole magnum setup was designed to do that and still make power. Idle rpm was probably a bit higher than a mid 60s LA engine and thats why we see the timing advance begining at 800 or 1000 rpm.
TT5.9mag knows a ton more than me about these engines and I'm just speculating based on the principles so take it for what it is.
That's what serious drag racers do, or used to do. Cut the engine clean after a pass. This isolates the impact of WOT on the plugs.
Be safe!
TT5.9mag
Two atmospheres are better than one
I’d like to clarify one thing quickly. The wot run and plug read will give you a good idea about wot fueling and if the peak total timing number is where the engine wants it. It is really hard to read a plug for idle and/or cruise fueling and timing especially with vac advance. But the concern (where damage mostly will occur) is heavy load at rpm and that’s what the plug read will tell you.
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