I have my vac. adv. connected to full manifold vacuum. 14 degrees initial and 36 total. My vac. adv. is adjustable but I don't know what I should be shooting for when adjusting it. Is there some timing or vacuum number I should be shooting for or am I supposed to dyno the car to get results? I just don't know why I should/would be adjusting the vac. adv.

Without knowing specifics on your engine....
14* initial and 36* total sounds pretty good.

At what RPM is the 36* advance fully in?
(Your weights and springs control the curve.)

You'll have to forgive me because I can't remember everything exactly. I haven't messed with the car since October and spring is coming so I'm getting it ready. If my memory serves me, 36 came in at 3500. This is before ever adjusting the vac. adv....My question is.. what exactly am I changing when adjusting the vacuum advance? How is this helping or hurting and how can I tell?

Shawn,

Who makes the vacuum advance can? I think I have the instructions laying around for an Accel unit on my HEI.

The vacuum advance bumps the timing at cruise. The instructions say to turn the adjustment screw completely counter-clockwise. After that advance in 2 turn clockwise increments until a surge or ping is heard at cruise rpm. Readjust back ccw until the surge/ ping is cured.

Shane

Some cansiters only have adjustment for the rate of advance (how much vacuum it takes to pull all the vacuum advance in). That you usually adjust to something similar to how much vacuum your engine is generall capable of pulling. On a street engine with a mild cam, having it all in by maybe 12-13" of vacuum is pretty reasonable. On something a little more radical that never pulls that much vacuum, having it all in by 8-9" might be better. FOR SURE, you'll want to be certain that it's all in at a vacuum level that is less than your idle vacuum, if you're attaching it to a full manifold vacuum source. If it's fluctuating somewhere in the middle of it's range at idle you'll NEVER get a stable idle from the motor. And you do this check IN GEAR if you've got an automatic- when it's idling under load from the torque converter.

Beyond those general guidelines it's a try-it-and-see thing. If you're getting pinging at part throttle you need to tighten it up so it takes more vacuum to kick it in fully. You want it all-in as quickly as possible, short of getting part throttle pinging. And don't play on the ragged edge. Better to lose a smidge of part throttle performance than to be borderline detonating at part throttle- it heats up the chambers and pistons unnecessarily and can cause pinging when you go quickly to WOT where you didn't have any problems before using the vacuum advance.

Some canisters also have adjustment for the total AMOUNT of vacuum advance. This is very helpful. On your motor you won't need or want more than about 10-12* of maximum vacuum advance. Once you have your initial and centrifugal set for maximum WOT power (32-38* total- typical settings for a SBC) it doesn't take much more vacuum advance on top of that for optimum part throttle performance. Stock canisters often supply as much as 20-26* of maximum vacuum advance- that's WAY WAY too much once you've dialed in your initial and centrifugal for best WOT performance. It won't matter how tight you set the advance rate in the canister if the max. vacuum advance is that high- it WILL cause part throttle detonation anyway.

Just some ideas to get you "in the ballpark" for a good running combo.

I have spent hours trying different things out with my distributor timing. It's almost always time well spent. A hand held vacuum pump/gague and a dial back timing light are all you need to do a pretty damned good job of it in your driveway.

It's alot of road testing to get the vac advance dailed in.

On my car I have higher cyl pressure, pushing the limits of pump 93 fuel. I have to run 34* total in at 3,000 rpm, initial timing of 14*, then I had to limit my vac advance to add only 8*. If any more than that I pinged at part throttle.

Now on the other end of the scale. My dad's engine with 9.1:1 compression and twin T3 turbos. He has to run 28* total, 9 initial to keep it from pinging under full throttle. With the vac advance adjusted to bring in 10* it was a dog at part throttle and even jerking. He added 6* more vac advance by adjusting the can and helped ALOT.

My can is from proform and my dad's is a pro comp HEI. Both of ours changes the timing degrees when adjusting the can.

It's just alot of testing to see what your engine combo likes/wants.

The Accel adjusts the same as night rider described. The curve table shows that the max advance can be changed from as little as 3-4 degrees clear up to 21-22 degrees. And it reaches max advance at only about 7-9 inches of vacuum.

Shane

So you guys are saying I should keep my 14* initial and 36* total and then add as much vacuum advance as possible without getting pinging problems?
I can do that. My dist. is a MSD clone, so I'll have to see whether i have to turn the allen wrench clockwise or counter to increase advance. Is there a way with the timing light? I do remember tinkering with it an having the idle get rough when going counterclockwise. I am still a little unfamiliar with Damons first paragraph about having it all in below my idle vacuum and testing it out in gear. I can swap out cams, heads, valve train, etc... but when it comes to vac. adv. I need a good lesson.

Yes, you want to keep you initial and centrifugal setup where it is for maximum full throttle scoot. You make the vacuum advance work on top of whatever that is.

HOW TO:

Hook up your vacuum gague to a spare full manifold vacuum port. Warm up the engine to normal temp, drop it in gear and let it idle at whatever your normal idle speed is WITH THE VACUUM ADVANCE CONNECTED. Check your idle manifold vacuum reading and write it down.

Shut the car off, pop the distributor cap and hook up your hand vacuum pump directly to the vacuum advance canister. SLOWLY pump it up while you watch the arm on the vacuum advance canister. When it JUST TOPS OUT (hits the max advance stop) check your vacuum reading. That's how much vacuum it takes to pull your vacuum advance all the way in- write that down, too. You might want to check this several times and pay very close attention in your observations. Make sure you can get repeatable results to be sure you're getting an accurate reading.

Make sure your idle vacuum is MORE THAN the amount of vacuum it takes to bring the vacuum advance all the way in.

If you got, for instance, 13" manifold vacuum but the can only takes, say, 11" to be all-in, you're OK. If it's the other way around, you need to loosen the can up so it's all in at a point less than your manifold vacuum.

If you CAN'T get the canister significantly lower than the idle vacuum (most can't go below about 8-9" vacuum for full advance) you can't use a full manifold vacuum port- you'll never be able to hold a stable idle.

As with all types of tuning, this is an itterative process. If you change your idle speed, your cam, compression, other timing settings or any number of other variables that could affect the engine setup you should go back and check all this again.

Very informative... thank you very much!!! One last question: After I shut the car off, do I hook up the vacuum gauge in-line with the hand pump (T-valve?)

Do you have your carb connected to full manifold vacuum? I've read many articles that swear by it. Cooler engine with more throttle response.

No, you disconnect the vacuum line from the carb and hook it up to the hand vacuum pump so you can apply and hold vacuum to it while you watch the arm move.

In my current engine, yes, I have it hooked up to manifold vacuum but it's a low compression blower motor and not comparable to more typical N/A setups. Many street engines with modest compression can be hooked to full manifold vacuum and successfully tuned. Some can not- depends on the combo. Many guys with big cams don't have enough vacuum to work the canister effectively at idle and they often hook it to a ported vacuum source. Also, high compression with a small cam is another common scenario where it sometimes can't be tuned to work acceptably with full manifold vacuum.

Tuning the vacuum advance is definitely in the category of "try it and see."

I understand that I should I hook up the hand pump to the vac advance but where do I hook up the vacuum gauge? Does is it stay connected to full manifold vacuum? Or does the hand pump have a gauge in it?

The hand vacuum pump has a gague built into it. You can pick one up at Sears for about $20.

Oh I see. Cool. Thanks for the info!
Just for fun... and to save a trip to Sears.. do you think the previous idea of having a vacuum gauge hooked in line (by a "T" connector) to the hand pump would work? Just curious.... no big deal...

I have a 228/235 cam - .530lift -106LSA with 10:1 CR. It makes about 10"vacuum, if i remember correctly. I really love the sound of the rougher idle when connected to the timed port but the car seems to run cooler on the full port.

Well you need the hand pump to apply vacuum to the canister manually. Every hand pump I've seen has a gague built into it so I guess you could T-in another gague if you really wanted. But it definitely can't still be hooked to the vacuum port on the carb when you do this or all the vacuum will shoot instantly through the port in the carb and you won't be able to draw any vacuum down on the canister.

Damon,

You said some canisters can adjust the rate of vacuum and some can also adjust the total. Here is the pic of mine (the one on the right) I assume it only adjusts the rate? My car pulls in about 10" of vacuum. I assume adjusting my canister to be all in at about 9" would be good.... Correct?

http://img.photobucket.com/albums/v243/shawntmartin/HEIDistributors.jpg

If you're going to hook it up to a full manifold vacuum source, then yes, something under 10" would be good. Remember that after you hook it up that way there's going to be a fair bit more advance at idle and you might end up pulling more than the 10" of vacuum you've got now. The more advance you put in, the higher the vacuum will be (up to the point where it's over-advanced and beginning to fight itself). It's a process you often do several rounds though before you figure out what the optimal settings are- change one thing and it affects all the others. Not massively so, but to some extent timing, vacuum, idle speed and idle mixture are all inter-dependent.

"Remember that after you hook it up that way there's going to be a fair bit more advance at idle and you might end up pulling more than the 10" of vacuum you've got now."

And that means what? Thats not being sarcastic... I just really don't know how that effects performance. I'm a vac adv. idiot.

I've got a question.

If you hook the vac advance up to manifold vacuum and not ported vacuum, do you set the timing with the vac advance hooked up or disconnected?

Disconnect the vac. adv. and plug it. Plug the carb too.

Always check intial advance with the vacuum advance DISCONNECTED and plugged to prevent a vacuum leak. Then plug in the vacuum advance after you set your initial timing.

More timing will increase your idle speed (up to a point) and that raises the vacuum level under the carb. When you set initial timing (vacuum advance disconnected) your idle will be, say 700 RPMs. Then plug the vacuum advance in and it'll pop up to maybe 800. You'll then back out the idle screw to bring it back down to 700 again. But there will be more vacuum present under the carb than before. You're idling the motor at the same speed as before but with less throttle opening. The engine is "sucking" against the restriction of the carb harder and so you get more vacuum. You'll see. You'll get a feel for it very quickly once you start playing with it a little.

Oh, I've already done all of that.. and 10" that I posted earlier is the final result after setting the timing and re-setting the idle. So you are basically saying ( I hope): check the manifold vacuum reading AFTER setting up the timing and idle... then set the vac. adv. so that it is all in below the manifold vac. reading.

Correct?

Yeah, that's basically it.

Lots of good info there Damon, Ill have to save this thread..I think I might have beat up a rod bearing due to over adv. timing..Mine has the adjustable can but I never messed with it....Part of my problem is that my car is so loud I dont think I hear the detonation...
Hulk

Damon, i don't have a hand pump yet but what I did for fun is checked the idle vacuum in gear(10") and then I sucked on the vacuum gauge to feel what it takes to suck 10" with just my lungs. I then popped the top of the Dist, hooked up a hose to vac. adv. and sucked with my lungs again. i couldn't even get the arm to move and was sucking as hard as I can. So it definately isn't right. My question is, if I can't get this to lower past 10", then can i buy another vac adv, instead of using the timed port?

Also, if I have to use the timed port, what would I set the vac, adv. to?

UPDATE: Crap, I was turning the vacuum adv. screw and I heard a POP! and now the allen wrench goes all of the way in the vac. adv. Im assuming I can buy another for a MSD clone dist.