When I got this motor 10 years ago Mark's paperwork recommended no more than 36 degrees, all in at 2000 including 5 vacuum. This is a 520 hp/560 torque version. The car idles best with a lot of timing which is around 30 including initial and vac. I guess the best option for me would be to limit the mechanical so i don't get too much total I + M + V?
It seems to run better with more than 36 but i don't want to do any damage. Am I missing something in that total doesn't include vacuum? What are ideal advance curves?
I'm confused. Mark if you could comment I'd appreciate it.
I think you can experiment to get the results you want, just be careful to stay away from detonation. You could try more initial timing, just make sure it doesn't knock when you let out the clutch.
initial and mechanical are added for the total timing;
vacuum advance is only there when there is some vacuum,
like idle and cruise;
under hard acceleration the vacuum advance goes away...
so most of us set our initial to 15*-22* and then limit the mechanical advance to target the total advance number of 34*, 36*, 38*, etc...
the vacuum advance does not get added to these figures as it is not always there...
I do understand how and when vacuum advance works. I was confused as to "total timing " meaning maximum possible timing. So if 45* maximum possible is ok then I'm good. Please correct me if I'm still getting the concept wrong.
Brian, as explained above total timing is measured using initial plus mechanical. Vacuum advance goes away when the engine is loaded, and only comes into play at idle and light engine loads. So timing at cruise speeds with a light load can be much higher than the 34-36 degrees you get from initial plus mechanical. And that's a good thing. Dave Ray's vacuum advance modifications are worth doing, and will help you optimize your advance curve, and avoid detonation.
OP: you'll still want what we're referring to as "total timing" to be around 34-36 degrees. As you get higher, you can beat up your engine more....as my chasis dyno guy likes to say.
Mark is suggesting that you can keep pushing this figure higher until detonation starts to occur and then back off. Absolutely correct.
But please don't think that "total" timing (as defined above) of 45-50 is a good idea. It's likely too much.
I'd say that you better have good reason or be very conscious of going over 36 degrees.
Engines mostly idle better the more initial advance you throw at them; it doesnt mean it is best.
Brian, " It runs very good at 18 I 12 V and 15 M. Never get any knock/pinging." that 18/15 is close to where you need to be at. = 33 total. It might want a few more, +3 to 36 to make a bit more power. NOT INCLUDING VACUUM as its not on at WOT. If you do bump th einitial lead to 20/21, I would also cut the vacuum back.
"Please describe how the vacuum advance is set up, and what vacuum type is being used?"
What vacuum used, manifold, or ported?
45 degrees isn't excessive when the engine is on no load, as in coasting, and that would add yur vacuum advance degrees to the total of I + M.
Where we want the vacuum advance to go away is acceleration, that is for the mechanical curve to do. And, we want it to supplement the initial timing, to get between 22 and 24 IDLE degrees of timing.
Sounds like you don't know as much about vacuum advance setup and use as you might believe you do.
do the work, get the results. Dave's stuff is right on.
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Related Threads
?
?
?
?
?
Team Chevelle
5.1M posts
115.6K members
Since 1998
A forum community dedicated to Chevrolet Chevelle owners and enthusiasts. Come join the discussion about restorations, builds, performance, modifications, classifieds, troubleshooting, maintenance, and more!