Trying to get everything optimal in my RS. Its a sbc 400+HP/400+TQ engine.

Engine seems to really prefer tons of timing! Running an HEI setup, Summit/MSD box, NGK shorty UR5's gapped@.035-.040. Moroso wires, MSD cap & coil, new curve kit with 1 medium spring and 1 heavy spring, Accell adjustable Vac adv can.

Base timing is 16* BTDC
Base timing w/vac adv is 25*
Timing @ 3K RPM is 22*


Not really sure where to go...turn down the vac adv all the way & advance the dist to compensate some, change the springs back to med-med or change them to med-light??

before I started working on this it drove decent, didnt feel like it was all there for all that the car has & for how light weight it is. Crusing in 4th around 1600, if i downshifted or stepped into it and the trans downshifted, i got the "hammer banging around under the hood" bad (det) until i shifted back into 4th. I know its obviously detonation and is a timing issue and getting the carb dead on is my real goal but without having the timing taking care of all the carb tuning in the world wont do any good.

Ive read the Lars Grimsrud, John Hinckley & Damon Nickles/Mike Ervin pieces. All VERY good, and i find myself re-reading those pieces as more and more sinks in but I am open to alittle more specific input from anyone.

 

Where does it run the best?

("Best" being some combination of throttle response, fuel efficiency, resistance to spark knock, max power, etc.)

THAT'S where THE ENGINE wants the ignition timing to be. Be assured, THE ENGINE does not whup out some "light" and look at some "mark" and then go find some "book" to look up some "spec" in and see if the "light" agrees with the "spec", and then decide it's going to run great or crappy accordingly. No. THE ENGINE will tell you in no uncertain terms when you have it where THE ENGINE wants it. Listen to THE ENGINE and don't worry about "number", THE ENGINE couldn't give a greasy beer s*** about "number", all it cares is whether it likes the time in its cycle that the spark occurs.

Sounds to me like you're on the right track. Only thing I see wrong with what you posted is that the curve looks REALLY REALLY SLOW to come in; I'd like to see FULL advance (in your case, from 16° all the way to the entire 35°) by 3000 RPM or a bit earlier.

If the kit you have is the one I think it is, try one med and one light spring. See how you like it.

One problem with HEIs (or any other dist) is that the shape of the curve on the cam the weights work against, and the shape of the arms on the weights, is what makes the curve. A common hack from days gone by is to take the weights and heat em up red-hot, and bend em inward (tighten the angle, so to speak) to make the part that meets the cam have more effect faster as the weight part moves out. Sometimes you have to grind the cam or the weights to get the curve limit and shape like you want. Don't be afraid to experiment, the weight kit is cheeeeeeeeeeeeeeep enough to not worry about destroying and drawing another off the shelf. (yes I used to keep several on hand for tuning purposes...)

 

I can tell you that I've timed motors exactly how Sofa has told me to do and they are always more powerful feeling.
You want the advance kit for the lightest springs and bushings to advance as quickly as possible. Bring your motor to 3000 to 3500 rpm, set timing at 35 degrees. Rev it further to make sure it's not advancing any more.
Now idle it down and shoot the timing. The base timing generally doesn't matter if you do it this way.

I've had the base fall at 14-16 degrees and I've had it as high as 22ish doing it this way. The high base timing setup was very hard to crank when the motor was warm.


This way will get you close to a ballpark figure. Obviously you can tweak and listen as you drive.

 

Yup, all of us that get the most out of these old motors do it this way

 

Thanks for the replies everyone!

Sofa, I honestly don't know where it runs "The best" as I would get det part throttle. I know that ever since i switched to manifold vac though it always seemed to bun better but I never had adjusted the vac adv. Its embarassing, Ive wrenched on cars doing all kinds of stuff, built my first sbc back in '09(that my buddy runs in a gutted Toyota mud truck) I just never really delved into How vac adv works, when its engaged and when the engine doesnt use it, until recently.

I guess (weather permitting) I'll try to get the car out this weekend. Seems my confusion was in the area of attaining the "max 34-36*" & the whole "let it fall (timing at idle) wherever" thing. So as of rightnow i've gone one medium spring and one light spring in now. I'll get the car up to op temp, disconnect the vac adv like normal throttle the engine & hold it @ 3K and turn the dist til i see 34-36* lock the dist down, turn the vac down so its advance & advance rate is minimal (again the accell adj vac adv seems to be adjustable in w/both criteria, unlike the summit one i had and all the others, at least it seemed that way when i hooked it up to my vac pump and was watching the pump gauge and the amount the arm would move) and give it a test drive and will have to keep adding vac adv till I get det then back off a turn & try again. On the dyno sheet it made its numbers above w/ 35*

 

Here's another paper on timing that you may find useful.

http://classicinlines.com/spark.asp

 

Good info but my brain hurts now...

 

That brain hurt is good for you.
I use the Crane vacuum advance kit with includes an adjustable can for the point at which the timing is initiated vs manifold vacuum and the amount of advance that's applied. It's especially useful for tuning out the detonation you're hearing a part throttle. If you don't have the limiter plate then the vacuum can you have will be adjustable only in one aspect or the other. Some cans adjust the amount of advance and others adjust the set point.





Use a handheld vacuum pump (like the type that's commonly referred to as a brake bleeder) and a vacuum gauge and tailor the curve in the driveway. Combine that with a portable tachometer and you can build the curve to where the engine runs the best.
My part throttle timing is close to 50 degrees depending on my cruising speed.
16 initial+16 vacuum advance (all in at 9" vac)+18 mechanical (all in at 3000 rpm).

EDIT: Looking over your last post I see that you've got the procedure figured out.
Some go so far as to take the pump into the car and manipulate the vacuum as they drive to determine the point as which the maximum can be applied before detonation.