Help! with ignition timing settings, please

[johng]

Trevor, I think you are reading Lars papers correctly and you should limit the amount of vacuum advance (although Oneball doesn't seem to think that this is particularly important).
As you say to reduce the amount of vacuum advance you need to limit the movement of the pin. Here is how I've done this with an aluminium plate on top of the attachment bar. In this photo I've put a vacuum on the can and it is fully retracted.

 

[Roscobbc]

Years ago the 'supposed' smart move when installing a hot cam was to junk the vacuum advance components, probably because that was done with race cars and there were zero concerns about (relative) economy. Todays thinking is different and vacuum advance is considered a good thing (if one can get it 'dialed-in' correctly.

 

[Oneball]

Trevor, I think you are reading Lars papers correctly and you should limit the amount of vacuum advance (although Oneball doesn't seem to think that this is particularly important).
As you say to reduce the amount of vacuum advance you need to limit the movement of the pin. Here is how I've done this with an aluminium plate on top of the attachment bar. In this photo I've put a vacuum on the can and it is fully retracted.

View attachment 15794

I didn’t say you shouldn’t limit max advance. I said I didn’t know what the max should be.

 

[johng]

No offence intended Tim

 

[Forrest Gump]

The stop plate I've got in my HEI dizzy, with four positions giving different limits. I think I'm set on +14 degrees

 

[Oneball]

No offence intended Tim

None taken. I was saying I agree

 

[trevbeadle]

Forrest, that looks like an aftermarket kit? Already on or fitted by you?

 

[Forrest Gump]

Forrest, that looks like an aftermarket kit? Already on or fitted by you?

Yes I put it on. It’s MSD part 84281 and was probably overpriced for what it is. My distributor is an MSD copy of the GM HEI so can’t completely guarantee this part would fit your distributor, but likely would. Or make key-hole shaped plate to do the same job!

 

[trevbeadle]

I think as Forrest and johng say, I will experiment with a plate and see what figures I'm coming up with. I think I'll aim for half what I'm presently getting,, 12 to 14, which will give me 22/24 initial plus vacuum at Idle. Adding the 23 I'm currently getting centrifugal, the most should not exceed 48.

 

[Roscobbc]

I checked back earlier this afternoon on some timing notes I made about 15 years ago after doing a custom advance curve on my then basically stock L36 390 hp 4 speed Vette. This was before I changed the engine to a 'stroker' 489 (interestingly this has a 21/22 degree initial advance with only 15 degrees centrifugal (distributor) advance - which comes-in at 2800 rpm.
Using the stock distributor I simply replaced the stock avance springs and weights with those from a bob weight and spring kit (possibly Moroso).
These kits usually give you a choice of three pairs of spring and bob weights and allow you to 'tailor' the advance curve.
My engine was stock other than small tube headers, mufflers, a Holley 750 double pumper and Edelbrock Torker 11 single plane inlet manifold and MSD ignition.
With its relatively high ratio 3.08 rear end and close ratio Muncie M21 using too much advance could potentially create pinking (detonation) issues if an automatic transmission was use. Purely due to engine size a big block should be more tolerant to higher amounts of ignition advance - correspondingly a smaller capacity engine (327/350 SBC) perhaps less tolerant of extreme advance sttings.
Reading-up on some USA forum they tend to make big deals of getting the advance-in early to improve performance and potential economy (and I have done this on other V8 in the past). I installed the lightest springs and the heaviest bob weights to get the centrifugal advance-in as early as possible.
With the initial advance set at a very conservative 7 degrees on the balancer and a total centrifugal of 28 degrees it would give a total of 35 degrees (with vacuum advance 'can' disconnected).
I plotted the advance curve for the following rpm's. As you will see 1000 rpm upwards really brought-in the advance very early, flattening-out at about 1500 rpm, with full advance achieved at 2000 rpm. Considering that even 'performance' advance curves are generally 'all-in' at about 3000 rpm one could perhaps be concerned obout overdoing things.......however this really livened the engine-up, with much improved responses in pick-up.........and importantly absolutely zero pinking.
550 rpm - 0 degrees advance
850 rpm - 8 degrees advance
950 rpm - 10 degrees advance
1000 rpm - 12 degrees advance
1100 rpm - 20 degrees advance
1250 rpm - 24 degrees advance
1750 rpm - 27 degrees advance
2000 rpm - 28 degrees advance
Not suggesting the above will work for anyone elses engines......just showing how it worked on my engine combination.

 

[Forrest Gump]

Interesting that there is a big jump in the advance from 1000 to 1100 rpm

 

[Roscobbc]

Interesting that there is a big jump in the advance from 1000 to 1100 rpm

I wondered about that at the time.......

 

[Roscobbc]

I found the L36 extremely tolerant in terms of ignition settings. However the investment of a MSD box and good quality HT leads improved engine performance below 1000 rpm significantly.