Ok..... Background on timing is here: http://www.thirdgen.org/techboard/te...misfiring.html (HELP: Misfiring) SO i know this is the third thread ive put up for timing, but i no longer need to know how to time with a new cam, and my car is not misfiring, so i thought id start over.... Been busy with school, so i only got to tweak the car "properly" today. Ive been setting the timing over and over to see what it likes, but today i set it with good parts. My Accel adjustable vacuum canister was not being consistent, so i returned it, and bought a crane kit, with the limiter plate. I went to race the car with 36" of mechanical timing (locked out) and it had LOADS of torque, but hardly any horsepower up top. I usually shift at 6500 rpm, and it was squealing the tires in shifts, but when my bro gave it a try and drove "nicely" only shifting at 5000 rpm, he gained half a second, and 5mph...?(15.0 flat at 93mph) So i set it back to 34* today like i had it before, when i ran a best of 97mph. The car LOVES the timing now, its nice and smooth, HOWEVER; i put in the crane adjustable vacuum canister and the limiter plate. Since the instructions are uselessly vague, i idid some research and found; 1) You install with the "notches" of the limiter plate facing INTO the distributor on an HEI (contrary to instructions). 2) You turn fully clockwise with allen key on the canister, and then out (counterclock...) 9 turns. which brings it all in by 2800rpm. 3) Adjust the base timing AFTER the canister install, because it advances it by 2* per notch. 4) Install the limiter plate with MAXIMUM advance locked out, and keep adjusting notches at a time (2* advance each) until the car EDIT: starts to knock. MAXIMUM locked out should provide 12* of advance roughly. Did all this, and my limiter plate is at the MAXIMUM locked out setting, but when i was timing the car, i revved up to make sure i only have 12* of advance, but i have roughly twice that..... What gives? The car was knocking on part throttle on the drive home only under load. But purrs any other time. Any ideas? Im going to check tommorow to make sure the limiter plate didnt "slip" onto no lockout, but im pretty sure i installed it correctly. Was a pain to hold it in place while i tightened it down.

Vacuum advance comes in with manifold vacuum, not engine speed. You're not bringing it all in by 2800 RPM, and blipping the throttle is going to reduce the vacuum and give you less vacuum advance, not more. If you want to know what the actual vacuum advance curve looks like, you'll need to spin it up on a distributor machine, or use a vacuum pump and a timing light.

yes i am aware of that.

The throttle wasnt blipped, it was slowly raised, and so was the engine vacuum. I have about 14 inHg of vacuum at idle, and it will go up to about 25 when when slowly brought up in rpms. I realize the "all in" is incorrect terminology, but thats about the rpm where the vacuum is highest, without load, when brought up slowly.

My concern was, if i use the crane LIMITER, to limit it to 12* of vacuum, shouldnt it NOT surpass that mark? I was told the factory vacuum advance gives about 20-24* of advance, as the limiter sits, its limiting the canister to about half of its travel, by pushing it forward.

When the revs go up (along with vacuum) the tab showed i was roughly twice the distance away from the 12* advanced mark (from 34 degrees). How is that possible?

I may have to get some sort of a vacuum pump to play with this, i just know why its going past 12* and its causing knocking.

How do you guys set it to 12-16* of vacuum advance? and check it?

EDIT:
Worth mentioning, i am running the vacuum signal from the ported line on a holley 4150 carb. I tried manifold vacuum, and the idle seemed to get worse, but that was also with the ACCEL canister. I will give it another try and see what happens.

Unplug vac advance, check timing, plug it into manifold vac, check timing. Difference is how much you gain. That's amount, the only thing effecting that is the limiting tab. Just make sure you have enough vacuum at idle to fully bring in all your timing. Make sure the can is set "loose" enough that your idle vacuum is enough.
That's what I did, my mech advance doesn't come in until around 1500RPM, so if need be, when doing this test you can bring your idle speed back down, to be sure mech isn't coming in yet.

What balancer do you have? aftermarket, fully marked? or did you put on a timing tape?

sounds like you're on the right track, and you're doing it right. 14" at idle? good for you! That's awesome!

Applying a known amount of vacuum with a pump would help you figure out what the curve looked like, but it's not completely necessary. You can do it all by trial and error without worring about exactly when the advance comes in.

What Apeiron said. You leave it at idle and supply vacuum to the canister with a hand vacuum pump (or just use your mouth and a long vacuum hose) while shooting the timing with your gun.

If you've got the vacuum advance cam set to it's limit (least amount of vacuum advance possible, or as much locked out as possible- however you prefer to say it) it should show about 12* difference between 0 vacuum and fully retracted (high vacuum applied).

You're seeing the combined effects of the centrifugal and vacuum advance doing it by just revving up the engine.

Remember vacuum advance is IN ADDITION TO the initial + centrifugal systems.

Here's a pretty typical performance advance curve:

12* initial + 20* max centrifugal = 32* 'total' timing. Add another 12* of vacuum advance on top of that and you're at 44* maximum possible advance.

OK, so can some explain to me how the timing looks from part throttle to full throttle? For example, on a stock engine, i guess the vacuum advance is in full swing at idle, and slowly backs out, then the centrifugal advance takes over... etc etc... I dont understand completely how the vacuum advance behaves.

I have a stock balancer (i think, since i dont know when it was replaced, if ever). I am using "timing tape" that i created to do the timing. So its probably +/- 1* or so. My balancer circumference is 54cm exacly, 54/360=0.15. 34* of timing = 34*0.15 cm, etc etc...

I have no centrifugal advance. Its locked at 34*. This lets the car idle fairly nicely, but can be hard to start if cold. When i still had weights and springs in there, it needed atleast 22-24* of initial to idle "nicely", but would ping once the weights kicked in. I havent had enough $$$ to recurve the distributor by limiting the mechanical advance, so instead, ive locked it out.

The only things contributing to the timing are the 34* locked out + whatever vacuum advance i am getting. I will have to check it tommorow, cuz im stuck at school all day today. So at idle, or part throttle under load when vacuum is highest, i am getting like 34*+ x* of timing. But under load it pings.

This will be my recurved distributor comes spring time, when i have some more $.
http://store.summitracing.com/partde...5&autoview=sku

But in the mean time i have what ive mentioned.

When the throttle is closed, there's a thin charge in the cylinder that takes longer to burn completely. When you're accelerating at WOT, the cylinder is fully charged, combustion rates are higher, and you don't want the extra advance. The vacuum advance adds extra advance while the manifold vacuum is high under low load to give extra burn time. At WOT the manifold vacuum is essentially zero so there's no vacuum advance adding extra time.

At idle the distributor can be connected to a full-time vacuum port, which will have vacuum advance at idle, or it can be connected to a timed vacuum port which will have no vacuum advance until the throttle is opened slightly. Usually engines with larger cams like having full-time vacuum advance.

If it's pinging under part-throttle load (ie. under mild acceleration), you need to reduce the amount of vacuum advance and/or the rate.