Setting an MSD advance curve for the motorhome 440

[Mosin]

In anticipation of getting my dizzy back from MSD warranty repair I have been pondering whether or not the factory MSD advance curve could be improved upon for the Winny 440 (as opposed to the Chargers they have in mind for most customers) but I don't know enough about "timing theory" if you will, to know which way to go with the advance springs and stop bushings. I usually cruise on the highway at a maximum 3200-3300RPM, with rare stretches at 3500, and split seconds up to ~3800+RPM... like the peak of passing on a hill. As the MSD distributors come configured from the factory (with two heavy silver springs), all of the mechaical advance is not in until 4000 RPM. Thus, If I understand this correctly, I have rarely, if ever, achieved the total timing of ~34-36 deg when cruising on the highway. Should I be? In other words, at what RPM do I want my total advance in by if my usual cruising RPM are below 3500 RPM?

Furthermore, with the stock (blue) advance stop bushing, the MSD dizzys only provide 20 deg. of total advance. As such, I have had to set the initial timing at ~14 to achieve ~34-36 total timing (which I don't think I'm gettin anyway, as per the above). Would there be any reason to swap out the 20 degree blue bushing for the 25 deg. silver or 28 deg. red so I can set the initial timing closer to the stock ~8 degrees?

TIA for any ideas or tips!

Dave

[DaveVA78Chieftain]

For reference:
Stock electronic ignition curve is as follows:
(To get total advance, all values are additive initial+centrifugal+vacuum)

440-3

Initial	Centrifugal	Vacuum
7.5 +/- 2.5 degrees	650 PM - 0.5 to 4.0 degrees 950 RPM - 6.0 to 8.0 degrees 2500 RPM - 10.5 to 12.5 degrees	10.5" - 1.0 to 4.0 degrees 15.5" - 8.0 to 10.5 degrees

Plot the stock centrifugal advance curve (they do not include initial so use as listed) on the MSD distributor documentation curve and work from there.  As far as the 34-36 total advance, subtract the initial 7.5 (34-7.5=26.5 max advance) from that when working with the MSD curves.   Your trying to account for the fact that the MSD distributor does not have vacuum advance.  The stock Dodge distributor used vacuum to adjust timing for engine loading and emissions.

[Mosin]

Thanks for the stock curve info!    I neglected to mention that the MSD Ready-to-Run dist for the 440 does have a vacuum advance (which connects to the ported side of the carb and adds up to 10 deg @ 15 lbs. of vacuum) because I thought it wasn't related to the total timing. I need to study up more b/c clearly it is a part of the overall curve.

Is the 12.5 degrees centrifugal advance @ 2500 RPM on a stock Mopar/440 dist. the maximum centrifugal advance it provides, or does it advance further between, say, 3K and 4K? On a hill climb under load there is little, if any vacuum, so if the stock initial timing is 7.5, and total timing needs to max out around 34, wouldn't this imply that the stock dizzy provides ~26.5 in cent advance? If so, do we know at what RPM it is all in? I'm trying to get a ballpark of the stock curve in the RPM range that I cruise (~3200-3300RPM) to figure out whether I would benefit by bringing in more advance, earlier. Thanks again!

Dave

[DaveVA78Chieftain]

You almost need a dynometer to get the curve just right.
A vacuum advance system reduces timing at Wide open throttle and under heavy engine load.
That is reflected in Figure 1 for the 8367 MSD Distributor.
Read this wiki about ignition timing http://en.wikipedia.org/wiki/Ignition_timing.  Your system would be a  mechanical ignition system (centrifugal/vacuum advance), not a computer controlled one like used with modern fuel injection systems.
Here is another description of ignition timing principles http://www.chevyhiperformance.com/howto/97438/index.html.  Even though it is about a chevy, the principle is the same.
A Dodge version http://www.sweptline.com/tech/engine2.html
or Google http://www.google.com/search?hl=en&num=100&q=Dodge+ignition+advance+curve

I only posted the information from the Dodge chassis manual (electrical section).  I do not know if they listed max amount of centrifugal advance.
The MSD spring selection curves only reflect centrifugal advance (no initial or vacuum advance included)

Dave

[Mosin]

That Chevyperformance timing link was especially informative re. the combined advance of cent and vac during non-WOT driving (thanks!) but I'm still finding one issue elusive that will probably require experimentation and/or a dyno to figure out. I think I'm probably in the ~6-8lb range when cruising at 3200 RPM, so I'm getting a little vacuum advance at that point, but according to the MSD charts, at that RPM I'm getting about 16 degrees of mechanical. That brings me into the 30s for advance at cruising speed, with some headroom, which I think (?) is a good thing. In my process of diagnosing the misfire that was going on I located a dyno shop that can handle Pandora (Westech in EL Cajon, Ca) so I just might kick in the $150 for three runs after I get the dist back.

And speaking of my dist (~$400 MSD 8387 in for warranty repair) I found out today that they are backlogged "13 working days" which basically means a month under the prevailing, incessant Curse of Pandora.  We have a trip for a friend's wedding coming up much sooner that we can not miss, and Pandora is a critical part of the logistics (i.e. our comfort and privacy). Thus, horror-of-horrors, I think I am actually going to have to lay out the ~$275 for an MSD 8546 from Summit (pro-billet without the vacuum advance or ready-to-run brain that I'll spark from the 6AL). I could call MSD on Monday and beg/b*tch for faster service, but that's not my style, and I'm sure the other people who need their ignition stuff fixed are equally as needy and POed. At least I'll have an extra MSD dist to go with my extra MSD coil!     

[DaveVA78Chieftain]

Why do that?  :-\  Simply go get a used stock Dodge Distributor for a few dollars from a junk yard.  Hook it up IAW page 19 of the 6420 Installation manual (From your other post, I think thats the MSD unit you have http://www.msdignition.com/instructions/Products/Ignitions/6420.pdf?terms=6420).
Thats the Chrysler Ignitions, Wiring a Chrysler electronic ignition using magnetic pickup drawing.

A lot less expensive than another MSD distributor.  Careful though, you might discover the stock dissy works just as well. 

Dave

[Mosin]

Thanks Dave. That got me vexing on what a stupid waste of cash another MSD would be... don't have the original dizzy and no time for a junkyard search and rebuild so....I hemmed and hawed and hemmed some more and... found my backup distributor should MSD fail to pull through with my #8387 waranty repair in time for our next short trip:

(Wysco billet faux MSD ready-to-run HEI knock-off for the 440... $65 delivered via eBay)

Despite the positive feedback/reviews of the seller and nothing out of the ordinary in searching for testimonials on these knock-offs (Wysco is the same as Procomp) I hope I don't actually have to install and use it!! Should MSD lag on my warranty repair, however, I will give it a 500-mile r/t and then consign it to the rescue parts drawer.

[Mosin]

Update: I received my MSD distributor back from warranty repair first, so I didn't bother trying out the knock-off. I immediately switched out the stock blue 20 deg. stop bushing with the 25 deg. silver, set the initial timing at ~10 degand then tried a few different advance curves by changing out the springs. I definitely got the best response from the two light blue springs (a mid-range curve on the MSD charts) with no detonation/pinging, but was experimenting on a cool night. Until I can try them out on a hot day when I can run witht the doghouse off I decided to back down to one heavy silver and one light blue (the second stage curve after stock). When I finally get it dialed in right I'll post the results. I'm still considering the dyno, since I'm still experiencing the same misfire issue (!) but the piggy bank says next month at the earliest. Thanks again for the responses.  ^RokOn^

Dave/Mosin

PS Re. the reliability of the chinese-made MSD ready-to-run copycats (under brand names Wysco and Procomp) I see that 440source is now carrying them ($75) which is encouraging. Certainly a worthy spare to have on hand! At least that is my current rationalization...