Timing advance
 

Hi everybody!
It's me again from Italy! A lot of rain these days so I had time for get lost in some technical thoughts…
Timing advance…
I have a 1969 L36 engine and the factory indications give 4° advance at 800 rpm (manual transmission). Actually I hear that the most part of the people prefer have more advance in these engines, like 8 or 10°.
I really can't understand why…: in the past the petrol had more octane so more anti-knock effect than actually; consequently the timinig advance should be less today…
What do you think?

The stock timing curve had to suit (ie: survive the warranty period) all the octane/weather/climate/altitude conditions that the thousands of cars might see, so the timing was purposely less-advanced/retarded compared to the optimum/proper timing curves for each engine/environment. Just put the timing curve in that makes your engine happy/responsive for your octane and climate/altitude situation.

Use manifold vacuum for vac advance and set initial at ~12* and go from there.

L36. Big block. They like about 36 max centrifugal timing at 3000. Don't have a dial back light? Put the timing mark on balancer at 10 degrees btdc mark on timing tab. Paint a line on balancer at 0 mark on tab. Put that Mark at 10 degrees btdc. Paint another Mark at 0. Now you have 0. 10. 20 marks on balancer. When you see 20 mark at 16 on tab with timing light, you are at 36 btdc. Without vacuum at 3000. Drop to idle. You should see 0 mark around 8 to 16. Hook up vacuum. You should see 20 mark around 10 or so. rev with vacuum hooked up. You should see 20 mark on balancer run off past the 16 mark on tab.

Your timing at idle is the least important setting. You need enough advanced timing for power-economy. You need little enough advance under load that you don't burn pistons.

Read this and then email Lars for his timing instructions.

https://www.corvetteforum.com/forums...long-post.html

Thanks to everybody

I disagree. I would bet that 99% of Corvettes spend more time at idle than they do at WOT. Proper low RPM/load timing is vital for good idle quality and keeping the engine "cool" at a stoplight. Unless it's a racing engine, proper idle timing is not something to treat lightly.

Actually Derek is correct. Of the four IGN Timing events, initial IS the least important.

Total Timing w/o Vac
Total Timing w/ Vac
Mechanical Advance & degrees of limit.
Initial Timing

There is an old saying that still holds true today. Set your total at 36 and let the initial fall where it may.
And your statement about W.O.T makes no sense. Your Total Timing is configured for anything above 2200-3000 RPMs. Not just W.O.T. And "that" mid range RPM is where 99% of Corvettes spend 99% of their time.

There are not "four" timing events. There is only one, and that is the crank angle that the spark actually happens in the cylinder. There may be multiple mechanical components in our antiques that coordinate to determine where that one timing event is (in crankshaft degrees), but it is one event, period.

If you want to take this topic deep, I'm all ears.

Post #7 covers it perfectly.

The idle timing will end up being whatever it is: 36 deg (or so) minus the total mechanical advance, all-in at 3000 RPM. The one thing to check is that the advance springs hold the distributor to zero mechanical advance at your idle speed.

Unless you are an expert witness in an Alabama courtroom, you will never need to know the idle advance.

the thing with the wide open throttle Advance, is it has to stop at a certain point. If you're running 42 or 46 degrees of Advance at wide open throttle you're going to be burning up your Pistons. That's why that went is the most crucial timing event. It's not that it has to be high enough, which it does it has to not be too high or really bad things are going to happen.

Me personally....I would set total timing to 36 degrees and let the initial fall where it falls........anywhere between 12-16 degrees on a stock distributor.
Stock GM was like 30 degrees total........you will not believe the difference 6 degrees makes on total.

Jebby

Semantics aside, what is technically wrong with setting the timing the way half a dozen posters have now suggested?

Paraphrasing (the full answer is still in Post #7, especially the part about emailing Lars)

Make sure everything else is in proper working order (distributor advance moves freely, set gear lash, etc).
Ensure all-in mechanical advance at 3000 rpm (lighter springs in the distributor)
Set full mechanical advance to 36 degrees btdc (vacuum advance disconnected)
Let the initial, or idle, or whatever you want to call it advance fall where it may (go ahead and check)
Use manifold vacuum on an advance can limited to about 12 (+/-) degrees of advance
Check for pinging, adjust idle speed, make sure that you don't have mechanical advance at idle (or else you need stiffer springs)

Isn't that what everyone is saying? Does it matter the degree of importance of the initial advance, if it's not a variable you can adjust with this method?

I sure think so! I used to use the "fall where it may logic" but that led to less than optimal timing curve. So, I followed a method to achieve optimal timing advance that begins with getting the optima initial advance to start with. That was attained using a vacuum gauge in combination with adjusting the idle speed screw and rotating the dist'r. So for my engine the initial advance was ~20*. My mech'l advance was 22* for a typical GM hei. Well that's 44* total which was way too much especially with aluminium heads. Add in another ~16* vacuum advance and you now have 60* advance at cruising speed! Think it'll ping? So to achieve a nice curve without adjusting my optimal initial timing I needed to reduce my mech'l and vacuum advance to get total timing at ~32* @ 3000 rpm and about 44* at cruise (max for my engine to eliminate ping at cruise for 91 octane). I could have just retarded the initial but that takes away performance. So I bought a electronic programmable dist'r that allows me to easily and quickly change my timing curves for near optimum performance short of tuning on a dyno. The other way without a programmable dist'r was to use advance spring combinations and restrict the mech'l advance mechanism. Then deal with limiting vacuum advance as well as the vacuum to bring it in and when. So, not so simple a task as to adjust total to 3X* at 3000 rpm and let the "initial fall where it may" and expect this to be an optimal curve. Sure it works for the most part for an unmodded engine and have at it. But for those with high cr and long duration cams you are sacrificing power by not properly optimising the timing curve.