Advance curve prestolite Chrysler 318

A safe number for a 318 is 35*, for old 318 with Iron heads we are talking about Total Timing.
The amount received once the distributor is fully advanced.
The curve you show above is not giving much timing.
With a total mechanical of 14* you would need an initial timing setting of 21* to reach the total advance number of 35*. I think that is too much initial advance for good performance.
Each engine configuration can vary slightly and if you add a performance cam you will need more base timing. Does your engine have a performance camshaft?
With a 318 modern information says you want full timing all in by 3,500 RPM. This means your distributor mechanical advance should be fully advanced by 3,500.
In a boat you always want a strong idle, dropping a boat in gear fully loads the engine and we all know an engine that stalls at the boat ramp is no fun and not safe.
Most say 10-12* initial works well for a 318 but I dont know this engine so you will need to establish what initial timing works for your engine in your boat for a starting point because a strong idle is so important.
You may find 5* BTDC works or you may find 15* BTDC works best, I would try a few settings and see what the boat likes best at idle for initial timing and build from there. Once you think you have a good initial timing setting test it a little because sometimes what works well at idle initially may be hard to start after a shut off.
Once you determine a good initial setting do the math and set an advance curve that provides 35* at 3,500.   It should stop advancing at that point.
The springs you choose will determine at what RPM the advance comes in.
I think a steady advance from idle to 3,500 max woud give the best performance.
If you bring it in too fast in a boat you will have pinging or detonation issues which rob power. So if you have an initial timing of 10* and max of 35* at 3500 RPM your distributor would need to be limited to giving 25* mechanical advance. 10* initial + 25 Mechanical = 35* total.
There may be a Guro out there that can hand you a timing map that gives great performance in these old 318 marine engines but if not you will need to do a little trial and error to get your perfect curve.
Vacuum advance is never used in boats, in boats even in cruise mode there is significant load on the engine so vacuum advance would not work and is never added.

67 ski nat wrote: Had my original prestolite ibm-4109 on Chrysler 318 run on an old school distributor machine (looks like a 60-70’s record player with strobe lights around edges) Wanted to see what oem distributor in the boat was set at for advanced curve to match with new pertronix springs and compare Boat ran fine so knowing was good base start. 1500-2500 @ 5 degrees 2500-3000 @ 10 degrees 3000 - 3500 @ 14 degrees max Looks like the bronze springs in pertronix d241800 kit is match, the next step up are the silver springs, 10 degrees more at same rpm (too much??)

You might think this is a silly question, but are those numbers showing "distributor degrees "or "engine/crankshaft degrees" of advance?

Typically something like an old Sun distributor machine tells you "distributor degrees" which have to be multiplied by 2 to get "engine/crankshaft degrees"

Also, I don't know why you have numbers that appear to ratchet their way up

You should be able to plot a curve with advance at every 500 or so rpm easily enough and with weights and springs controlling the advance, the advance at 1500 degrees isn't gonna be the same as at 2500 like you show for example.

If you're not sure on distributor vs engine/crankshaft degrees, put the new Pertronix on the machine and plot it's curve and you'll know if you have to double the machine readings

I seem to think you have a LA/LM 318 engine and the manual in the reference section gives 2.5 degrees BTDC for an idle setting.

The manual for the older Polyhead M318 gives a spec of 12.5 degrees

Just checking to see what you're using for a reference   

So the big question is are you reading"distributor degrees" or "engine/crankshaft degrees" ?

KENO wrote: Just to clarify a little If you have 2.5 degrees initial at idle (LM 318 spec) and you double your 14 degrees max you'll have a distributor that has 28 degrees of crankshaft/engine advance added to your 2.5 initial for a total advance of 30.5 degrees which is right about where Chrysler probably wanted you to be for low octane regular gas on a heavily loaded engine in a boat. The advance limiting slots in the distributor advance plate usually would limit you to about 28 degrees on a Prestolite from that time period. Over the years they changed, and the slots were shorter but for you that doesn't matter So ........like I said earlier, verify whether your numbers are distributor or engine/crankshaft degrees. Your 14 at full advance makes me think you're reading distributor degrees on the machine

You lost me here Ken, I have seen the engine and distributor RPM referred to when putting a distributor on a machine and confusing people but engine and distributor DEGREES are only different as they relate to rpm and I don't think he could be getting it wrong to get his 14 to really be 28.   On a typical 4 stroke the distributor is only turning half as fast as the engine. So if the timing would be all in at 3500 engine rpm the distributor running at 1750 on a machine should have all the advance.   But even if you screw that up and run the distributor at 3500 rpm you will still only have all the advance?    

Ok my machinist says his distributor machine shows both engine and distributor rpm and those low numbers are already doubled .
I will accept his numbers for now
But will time initial to 2.5. See if she starts easy smooth
With ‘silver’ medium springs that shipped in pertronix (5 to 24 advance mechanical)

If you time it with a dial back timing light or adjustable timing light you will see exactly what is going on with the engine running. These timing lights really help dial in total timing in your running engine.

You are blowing my mind Ken - here I was thinking I was pretty good at turning a distributor a few degrees and getting the timing I wanted to workout but really I have only been turning it half as far as I was trying to and getting lucky

Echo joe in ny. That explanation of rotor degree measurement s borderline brilliant , actually really help.
Unfortunately I have cheapy light may need upgrade
I will say opening the doghouse on the lake and fiddling with distributor and advance really not my favorite
I set pertronix in hole immediately after old was.out to avoid confusion so it is set. Hindsight should taken him both to compare.
His machine had James Bond type knobs
; 4,6,8 cyl
Gm/Chevy/ford/aux
Stroke
Volts 6/12

Thanks much for all your support. I’m kinda feeling comfortable starting it Saturday

Well pertronix has good reputation for this Chrysler marine d2418 I understand they did research too and silver springs good stock advance. The copper and bronze maybe for more racy and a de-tune option. Hopefully in good hands
I like your approach makes sense and professional
Yes a fancy light is in order, however book does says 2.5 degrees,(not 12.5 )
You like 8-10. To start still ?
If the motor starts and runs great I will ‘edit’ my previous post and delete word borderline

Thanks Duane

Hi power out in north Santa Rosa now. 50 mph winds. Fire close by I’m being evacuated
What’s important is I got off work early Friday. Hooked up fake lake and turned key
She fired right up. Let it warm and she sat down at original idle before prestolite extraction. Super happy she runs again
Thank you
Hindsight: get baseline initial timing with prestolite before removing
Have discussions on forum before removing

I will get fancy light and see where I’m timed at before real lake test under load