Jester

I'm gonna take the position against typical wisdom for the sake of argument. No..it's not. Even on a street car.

You have to remember that vac canisters are on OEM distributers that are set up very much differently from the way most of ours are. A typical OEM setup has about 6 degrees base, heavy springs and tight wieghts good for about 18 degrees of mechanical advance..give or take. you get a whopping 24 degrees total, all in by about 5000 RPM for example. Now thats not nearly enough. You need more added in all across the board, which is why they added vacuum advance. It fit the bill for a nice emissions friendly curve very well, but obviously we know the power benefits from a more aggressive curve.

Now the way I have my distributer setup is a little different. Because it's a boosted application, I didn't even bother starting to setup vacuum advance. I run 38 degrees total, all in by about 3000. My base is about 16 degrees, with 22 degrees of mechanical from a typical $5 aftermarket recurve kit. I think that even if this was a mild street NA setup, that curve would be about perfect. I cruise at about 3000-3500 on the freeway, so full advance. Plenty to keep the plugs clean on extended cruise. Even if I still had less gear, a 2000 RPM cruise should yield an advance somewhere in the high 20's...should still be enough to keep the plugs clean. Plus I don't think that 16 degrees is too much base to cause too much starter wear...even tho I do run a mini starter anyway for other reasons.

So I'm gonna say for the sake of argument that even an adjustable vacuum can is unnessecary. It can just be eliminated. Get rid of the guessing of exactly how much you are or aren't getting since there is relly no way to tell without running it on a dyno. What do yo guys think?

Damon

You'll still improve mileage and part throttle performance by using vacuum advance. You don't want anything near a factory 22-26* canister, however. You want less than half of that if you have really optimized your initial and centrifugal advance for best WOT power. If you do an significant street driving it's well worth it in my expereince.

F-BIRD'88

Your N/A motor will still want the same or near same 40 to 50 degrees of total combined advance (initial+ mechanical +vacuum) at part throttle, hi-vac cruise. Especially with a big cam. Over time your motor will last longer too.
If your supercharged motor is under some boost while cruising
than you may not need as much or any vac advance at cruise.

All you really need to do is restrict the travel of the linkage rod of a stock 20/25 deg canistor to around 10/15 deg to work with your more agressive hopped up timing curve.
When you get it all dialed in right, you'll have tuned out a lot of the disadvantages of using a big cammed performance motor on the street.
Retaining and fine-tuning vacuum advance to match your
particular modified street engine combination is....
Worth the effort.

IHI

iTrader: (1)

I agree with Jester on this one, for our like minded purpose of performance. Yes I drive mine on the street as a toy, not a must have for the daily grind, so I can get away with a few things that alot of the members here would not try on their vehicle for endurance sake. The factory set-up's has been proven over time to be a long term (higher mileage without replacement) deal whether it's fuel pumps, intakes, induction, etc..., sure you can get mod parts to last long if driven "normally" but who the hell hops up cars to drive like grandma? In my opinion that's why modded cars don't last as long-they're purpose built and driven for that purpose. So-I like to go fast, could give a rat's *** about mileage, and durablity cause broken parts mean better new ones, by having the distributors set-up like Jester and mine, you have a ignition system that is spun balanced to higher rpms than we'll ever take it, advance coming in exactly when needed, plus you don't have to look at that ugly can and ANOTHER frickin hose.

ChrisFormula355

On my firebird I don't have the vacuum connected because the motor would detonate under light acceleration since the factory canister added such a rediculous amount of advance, and I don't really drive my firebird all that much, so why do I need it if when I'm racing the car there is no vacuum advance anyways?? For that matter you do'nt really need a curve. I went one step beyond the "vacuum canister delete" and actually locked out my distributor on my grand prix. I have an MSD boost control timing retard that I use to start the car, then up it 15 degrees to 31 total (31 is the most I can run on pump gas with the 425 heads). Works like a charm and you never have to wonder when all your advance is coming in. I'd say on a somewhat radical street car (400+HP) you really don't need or want a vacuum advance, however mechanical advance is probably still a good idea if you have starting problems.

Jester-you have a supercharger?? Damn, when did you win the lottery?

jamesbern

Well, since I have tried both with and without vaccuum advance on my 383, I figured I would add my opinion. Without vaccuum advance off idle performance was noticably lacking. I was running 34 total and if I recall about 16 initial which worked well also, but I did notice significant improvement after I installed the adjustable vaccuum canister. Also another factor is which port you use for your vaccuum advance.

IHI

iTrader: (1)

How big was your stall, and at what rpm was the advance coming in at compared to stall speed. When you say lacking, are you talking about pulling away from a stop sign like a normal person does or ***** out mashing the gas and low performance. If you were mashing I would say the stall wasn't flashing quite high enough, or you had the advance coming in too late. Right now I'm all in at 3000rpm, but I got screwed on my convertor supposed to have been 2800, but flashes at 1500 if I'm lucky (ebay deal) plus I'm running girly 3.42 out back. My short times and E.T.'s show it too @ 1.90/14.1 Once I get the rest of the combo working together the missing canister should by all rights be an improvement-for a set-up like mine that is-NOT a daily driver which alot of these guys have.

Dragula

I know that mine was detonating like an SOB at light throttle in overdrive. In order to not detonate, I had to drop it to third and unlock the convertor.
Since its not a daily driver either.......bye-bye vacuum advance

jamesbern

Either way, regular takeoff or just nailing it. My converter is 2000 stall behind a 350, but I am getting about 2600 out of it flash and about 2300 out of a brake stall. My cam comes in at 1500 and it seems to match very well. I'll give you this, it was very hard to tune with the vaccuum canister and not have pinging at cruise speed, but it can be done. This is just my opinion, I know others will vary because no two cars or engines like the same thing.

F-BIRD'88

You can restrict the amount of vac advance by removing the
vac adv canister from the distributor and bending up a small
metal "clip" like the one in the pic that will clip into the linkage slot
on the attaching arm. If you need more or less vac adv after trial and error, just bend up a narrower or wider "clip"
When you get it just right, epozy it or solder it in.
The amount of advance is stamped on the arm like in the pic.
With your new more aggressive timing curve you'll only need about 10deg of vac advance instead of the stock 20 or 25.

Not that hard.

NTChrist

I agree with F-BIRD'88. I believe that having 2 forms of advance (vac. and mech. together) you can have a more versatile advance system.
Adjustable vac. canisters are available from ignition aftermarket companies and would help, as F-BIRD'88 says, tune for an aggressive cam.

JPrevost

A good friend of mine just sold his 69 camaro. He drove it everyday as a daily driver with a 370hp 327 with no vac advance. He should have hooked up the vac advance and he knows it. For performance, no, you don't need vac but for the street why WOULDN'T you want to tune it? It's a simple and effective way to keep the car running at it's best. Besides, you slam your foot down the manifold pressure goes to atmosphere and you won't (at least you shouldn't be) running any vac advance, just mechanical.

Dan83

Hooking up the vacuum advance to a constant vacuum source can let you run your initial timing bit lower to help the engine turn over easier and will help smooth out the idle of an engine with a large cam. That is the only difference I noticed when I finnally hooked mine up.