Timing Curve

midnight340 said
1) Yes, Mattax, there is the issue of what "part throttle" means, huh? :D

Cruise too! For some guys think it means tooling along at 25 mph...like cruisin for girls in town. :D

Just took a half turn out of the rod tree setting, went for a drive, and at about 1/3 throttle it is now going 1/2 to 1 point leaner on the A/F till about 2/3 throttle, then it's getting richer... the 13.5 or so. So we're probably good there.

:thumbsup: Definately in the ballpark now.

I opened the air door just a wee bit more. Is that the right way to go on the TQ??? Seems like more air with bit wider opening would lean it, but still not sure how the door works next to the spray bars. Didn't change much. Still pretty rich at WOT. The motor does seem to like it, but if I can lean it a bit without having to change the jets for now I'd try that.

The air valve controls when and how much additional air-fuel mixture to provide. The secondary jets control how rich or lean that additional air will be.

2) As to the curve, right now the MSD has one heavy and one light silver spring in it. Their next quicker curve uses two blue light springs the same weight, but doesn't show it coming in earlier, just steeper. The step after that is to use one silver light and a blue light spring, so I may try that, see if it helps the bottom end.

There should be a way to keep the same springs but change the initial tension. The Chrysler built advances have ecentric mounted spring perches which can be rotated. Prestolite and Mallory advances require bending the spring perch slightly inward to reduce tension.

(I forgot to mention it is a 9:1 motor (actual, cc’d) so (likely) can use more total with vac. adv. than for higher compression motors, correct? And with lower compr. I've read that going higher than 50 is OK???)

At idle speeds the low compression and high overlap will typically produce an exhaust diluted slower burning mixture. So at 800 rpm it makes sense that the engine likes 18* BTDC. Once the rpm starts getting up into a range its working then I wouldn't assume its any slower or less efficient of a flame front than a stock version of the engine. A lot of factors come into play, heads, compression, squish, cam, fuel...

That's why I figure if you're looking for starting points, go with the ones published by Direct Connection/Mopar Performance. They probably had more than a gut feeling that those numbers work.

I thought it was pretty interesting that the factory timing curves I looked at add up to roughly the same 50* BTC in the highway crusing rpms (3000 ish).