Total advance curve on a late SC motor?

[cstreit]

Gang,

As I was looking at ignition systems, I remember hearing that with twin plug systems, you probably want to end up with less overall advance than a single plug.

I can't seem to find a good total advance curve for the late SC motors. THe factory manuals give timing settings, but with a programmable system, the true advance curve is set, rather than the non-vaccum setup.

Does anyone know what the true advance curve on an SC motor is?

[tsuter]

The 1978 3.0SC has a vacuum advance distributor. It starts advancing at 1000RPM and then will advance again at 5000 RPM. Because I have a custom turbo, I have limited both of the advances to 10 degrees mechanical. That provides 20 total degrees with the initial timing at 5 BTDC for a total timing of 25 degrees at 6000RPM. The distributor advance curve can be modified by removing the little plastic oval shaped cover on the side of the distributor housing. You will have to rotate the distributor shaft. Inside you will see two pieces of steel 180 degrees apart that are unsupported at the top. These are the limiters for the two centrifugal advances. They can be bent in at their tops in equal amounts to achieve the total mechanical advance you desire - 20 degrees in my case. You need a distributor machine for this modification. Not sure how the distributor may have changed for later models but you should be able to limit your total advance mechanically if you want and still use the vacuum advance that is built in.

[cstreit]

Anyone got this information for a STOCK SC motor?

[cstreit]

Buehler? Buehler?

[stlrj]

Total advance for an SC is not much different than a 2.7 or 3.2 and the factory allows anywhere from 32 to 38 degrees btdc. If you go twin plugs, you'll need to reduce that by about 6 degrees.

Regs,

Joe

[MotoSook]

Chris, I have the advance curve (graph) for an 81 SC from my manual. I'm trying to figure out how to get it posted on the BBS, but I might have to e-mail it. I think it is what you are looking for.

[cstreit]

Souk,

Maybe you can just bring it Saturday? The one I have for the SC only shows partial advance to 4000 RPM....

[MotoSook]

We may be looking at the same thing Chris. I have the factory Workshop Manual for 72-81 (I think).

I'll bring it with me.

[POPS]

I have been looking for this info for a long time. E-mail or BBS would be great. I have a 79 SC and want to have my distributor re-curved and get rid of the vacuum advance. Thank you very very much. John

[emcon5]

Here is the advance curve from a 1980, from the factory manual.

It isn't very easy to read, but the bottom numbers are 500 to 3500, and are distributor shaft speed, so X2 for engine RPM. The numbers up the side are 5° ATDC on the bottom to 15° BTDC at the top in 5° steps, in advance at the distributor shaft. I guess you would double that for total advance. Not sure why there are 2 lines.


The text says for centrifugal advance, with vac disconnected timing should be between 15-20° at 3000 rpm and 19-25° at 6000rpm.

It is also very irritating to note that my new used factory manuals are missing some pages, for the 1981- 83 advance curves. Dammit.

The vac advance chart probably isn't helpfull, it lists advance° vs Vac in mbar. max advance, again at the dist shaft is ~3.5°.

Tom

[MotoSook]

John,

Vacuum advance on a 78-79 looks to be only about 3 deg +/- 1 deg. 26 deg +/- 2 at 6000RPm, as you know. So why recurve the dizzy! Just disable the vacuum and advance it to 10 degrees at idle, and you should get 30-35 degrees full advance, just about. That should be safe. I think mine is something like 12 degrees at idle, and it seems safe. This is the "prevailing thinking" as someone put it. It makes a noticeable difference.

My vacuum behaves funny (probably poor vacuum , another story) so I disabled it. Purely mechanical works fine for me and some others on this BBS.

Others may have a difference experience.

[MotoSook]

Damn Tom...I've looked at those charts so many times without picking out the X-axis description....dumb_ass me!

Two lines I assume to be the max/min at the RPM.

[MotoSook]

Strange, the curve for the 78 gooes to 4000RPM, does that mean my motor can spin to 8000RPM? and at 3000X2 (6000RPM at Motor), the total mechanical advance is 22 deg (+/- 1deg) add the vacuum advance (now 6 deg +/-2 according to the new thinking), I get 28 deg +/- 3 deg.

Seems right. Is it? Say it is so...

[MotoSook]

Somewhere I have a 78SC chart with mbar converted to the units on my vacuum gauge. I have a conversion program that I can send you Tim (not a biggie conversion).



I'm stuck on this dang topic now. I need closure!

[RoninLB]

Originally posted by stlrj
If you go twin plugs, you'll need to reduce that by about 6 degrees.

yep.. about 6deg.. I think a 5-8deg reduction is what I once read..

[POPS]

Souk I set my advance at 19 at idle to get 35. 35 happens at about 4000 My little car seams somewhat dead till about 4000 ++ I have used 2 timeing lights ( one new ) to get this information. Thanks for the new information John

[cstreit]

Ohh... I just got the part that shaft speed is half RPM... Duh... I couldn't figure out why they only went to 3500...

[cstreit]

WTF,,, How come the text doesn't match the chart?

[emcon5]

Originally posted by cstreit
WTF,,, How come the text doesn't match the chart?

This has me stumped as well. The only thing I can think is because the advance on the chart is at the distributor shaft, it would also be doubled at the crank because the crank turns faster. Even then the numbers don't really match up.

Doubling the 1500 dist rpm number of roughly 7.25° would give you 14.5° slightly under the low spec for of 15-20° @ 3000 engine RPM. The 6000 engine RPM number is even more off, 18° vs 19-25°.

This can't be right.

This might need it's own thread.

Tom

[cstreit]

Cripes!

All I wanna know is what to set my electromotive for... Is that so wrong?