Vacuum advance

-
Yea I read so much stuff on here I forgot alot of the stuff by the time I need to use it and then have to go back and reeread it lol. Thanks again bud I will go back and read all that again.
By now you realize that it is just more than the vacuum advance that is effecting the tuning of your engine the timing curve, the fuel, cam and the converter they all work as a team to provide performance. If one player is not right it can/will effect the other players and performance will suffer.
Dont worry about trying to absorb all the info that has been provided it will all make sense with time.
When you do your tuning try to make one change at a time and record the results.
Good luck and have some fun.
 
Adjustable vac adv units.
From maaaaany years of experience with various brands.
[1] The Crane unit is probably the best known. It is also the stupidest design. Along with Accel [ & I think Mr. Gasket ], there is a spacer inside the spring. If you wind the the wrench in, it locks up! You get no VA. With these, I uncrimp the can & remove the spacer. The best adj VA units I have found come with Chinese dists

[2] VA has two adjustments. Rate, adjusted by using the wrench to change spring tension. And Total or amount of VA added. This requires some form of stop or limiter to limit plunger travel. I have found that in the majority of cases, the total amount will need to be limited. Too much VA [ ported or manifold sourced ] usually manifests it self as light surging &/or slight misfire. The concept of VA then gets blamed [ 'didn't work for me' ] when the real problem is that it was not dialled in properly.

Couple of general comments.

[3] The Crane instructions say to connect to manifold vacuum.

[4] Some of you are probably aware of, or even use, Progression Ignition dists, with models to suit Chrysler engines. Page 4 of the instructions say to connect to manifold vacuum. In upper case letters it says, 'DO NOT connect to ported vacuum'.
 
Adjustable vac adv units.
From maaaaany years of experience with various brands.
[1] The Crane unit is probably the best known. It is also the stupidest design. Along with Accel [ & I think Mr. Gasket ], there is a spacer inside the spring. If you wind the the wrench in, it locks up! You get no VA. With these, I uncrimp the can & remove the spacer. The best adj VA units I have found come with Chinese dists

[2] VA has two adjustments. Rate, adjusted by using the wrench to change spring tension. And Total or amount of VA added. This requires some form of stop or limiter to limit plunger travel. I have found that in the majority of cases, the total amount will need to be limited. Too much VA [ ported or manifold sourced ] usually manifests it self as light surging &/or slight misfire. The concept of VA then gets blamed [ 'didn't work for me' ] when the real problem is that it was not dialled in properly.

Couple of general comments.

[3] The Crane instructions say to connect to manifold vacuum.

[4] Some of you are probably aware of, or even use, Progression Ignition dists, with models to suit Chrysler engines. Page 4 of the instructions say to connect to manifold vacuum. In upper case letters it says, 'DO NOT connect to ported vacuum'.

All that's well and good but we've gotten way off track here. We're discussing a close to stock, very mild engine. As such, he needs the vacuum can as set up at the factory for best results.
 
To Dartfreak 75.

The only difference between PVA & MVA is that MVA is operating at idle, PVA is not.
I suspect your engine will benefit from MVA. Easy to tell. Hook it up! If idle rpm increases & idle is smoother, then the engine likes it. It may not want all of the available advance that the VA can can deliver, so a stop would need to be fabricated to limit the total VA.
The difference between a locked dist providing, say, 34* at idle & MVA giving 34* is that the engine is always going to see 34* with a locked dist, which might produce pinging. MVA is load sensitive, so the 34* will reduce to a lower number under load.

You would be surprised how much extra idling timing even stock engines like.
Example: Pontiacs idled with 26*, 6* init + 20* MVA. These were high comp engines [ 10.75 ] with mild cams. Lower CR, more cam duration, needs more idle/cruise timing.
What is different between a Pontiac & a Mopar? They are made of cast iron & ingest fuel & air.
I just saw this post thanks for sharing. That makes more sense than anything iv read before and I completely agree. I was using 318willruns method of timing setting, basically he just keeps bumping up his timing at idle till the idle no longer increases or it stops idling smooth. Then backs it off 2 to 4 degrees which landed me at 20 degrees. I got all the way up to 24 degrees before it stopped increasing. And it seemed to run really good there. So using MVA I could set my initial at lets say 10* intial and using mva id be back at 24* during idle. Until the engine is under load. I guess im just gonna have to play around with it different ways and see what my engine likes. I appreciate all the help I have a much better understanding of the difference now!
 
beware the dreaded idle tip-in hesitation that takes extra pump-shot to overcome, and maybe you run out of shot 1/2 second later.....
I gotta repeat;
with an automatic, who cares about the timing below stall..... even 10 degrees short will not be missed.
But if you hit stall at WOT, with even 1* too much, over time, you are probably gonna detonate the sparkplugs to pieces. And those ceramic bits are about three times as hard as steel..... 2200rpm is one revolution every .0273 seconds, which on a V8 is 4 firings. When you hear the detonation, IF you hear it, it might take you .5 second to lift off the throttle. In that time, your engine has fired 73 times................. where are the bits, and what are they doing? Jus saying.....

At 2200 and cruising, your engine might want 56* of timing, for best fuel-economy. If you run 12 mechanical at idle, and an additional 12 in the can, on manifold vacuum, what happens at 2200 and cruising?
I'll tell you;
Your max Power-Timing with a lo-compression 318 might be 36* at 3400rpm. To get to 36 from 12, is 24 degrees. And if your curve begins at 1000, then you have 2400 degrees to get there. This maths out to 1.0 degree per 100 rpm.
Therefore, at 2200 the mechanical timing will be 12initial+12 mechanical =24. And in this case, the Vcan is bringing 12* to the table so 24+12=36 at 2200 and cruising.
But your engine might want 56*!
so you are 20* short, and no wonder the thing is a pig on gas.
What I mean is, the window of best timing in a running engine is about 25 to 28 degrees ATDC. If you fire it 20degrees late, the piston is at 45 or more degrees ATDC. So now, the already low-density A/F charge, which was compressed to the max at TDC, is rapidly losing pressure as it speeds down to the bottom, even tho it is on fire!. So the peak pressure potential is never reached. Therefore, to make the minimum horsepower to cruise on, you have to open the throttle further.................. to burn more gas, to compensate for that portion which is still burning in the exhaust system!
How lame is that!
Now
Lets put 22* in the Vcan by modifying the stops. And our target is 56*, so we need Idle plus mechanical = 56 less 22= 34@2200 and cruising
At idle, using manifold vacuum, all 22 will be in. And all 22 will stay in until manifold vacuum drops below the set-point of the springing.
If you manage to mod your D-cam, to 24*, then at 1* per 100, at 2200 will you will get 12* mechanical, plus the 22 in the can = 34, but you need 56 so you gotta set the Idle timing to 56 less 34=22.
But, at idle, with the timing set to 22 and the V-can on manifold vacuum, that would be 22+22=44* of idle-timing. So............. Ima thinking that's nuts.
So instead, you hook your V-can to ported vacuum and it doesn't start coming in until say 1600 at light throttle, but by 2200 light-throttle, as in cruising; it's all in. That's how you get to 56*@2200 for cruising.

I'm not saying your particular engine wants 56*@2200. So you might have to make some changes. But the game-plan is the same, namely, when striving for fuel-economy, most carburated engines I have worked on are sadly lacking in cruise-timing.
But even more sad is that, at Part-Throttle, the timing is almost as bad.
You think the way to fuel-economy is a 2.2 liter engine, Direct-Injected, IVVT, and computer controlled timing? Your car takes a specific amount of horsepower to cruise at a particular speed. in the neighborhood of 35 to 40. The car does not care if it has one cylinder or 16 cylinders, it only cares that it has the exact amount of power to go the speed you want to go, at the rpm you want to go at. And it's NET power, so the more cylinders you have, the more of the crank power goes to friction. But that is a minor deal. The biggest deal is the rpm. and the next deal is ignition-timing. The slower you run it, the harder it is for that ancient distributor to give it the timing it needs.
Since you don't have a computer controlling the timing, it is the tuners job to try and hit as many of the bases as possible, as often as possible. Even the factory sacrificed cruise-timing..... because they had to. If you give your ancient 318 (or other SBM) the cruise-timing it needs, it will, in steady-state running, rival the modern computer-controlled car. It has to; it takes a certain amount of power to cruise 65mph, which takes a specific amount of fuel, and EFI is NOT the be-all/end-all that the hype says it is. Yeah sure, injectors are a lil better atomizers. So they will likely always be a lil better at cruise-economy........... but;
have you ever noticed that from the early 70s to today, economy has only jumped the a small amount that it has? There is still no 100 MPG EFI system out there; even with engine size having been halved. If you look at the average economy of any similarly weight and frontal-area to our A-body cars, what do you see? I see 2.4liter cars doing ~25mpg on the hiway, I mean besides the heavy hitter cars. How many 80s Fifth Avenue cars, weighing nearly 4000 pounds, running in-the-basement-Scr 5.2liter 2valve push-rod engines on carbs , didn't get that or nearly so. With 3-gear loc-up transmissions no less!
The difference is, with computer-controlled ignition systems, those EFI cars can run the timing on the ragged edge of detonation. While us dinosaur-guys have to work with what we got, and most of the time, cruise-timing is at the bottom of the priority list.
If your V-can is of modest capacity, yeah, you can run it on manifold vacuum; but with the modest capacity, you can never hope to achieve impressive cruise fuel-economy, NOR will the PartThrottle timing ever be even close, so you will ALWAYS be driving deeper into the throttle than you should/could be, so economy suffers.
If you simultaneously have a low-pressure engine, well it's no wonder those engines get a bad reputation.
And heaven help you if you have a manual trans; you loose the Torque-Multiplication in the TC, and you loose the stall, and worst of all, the tires are married to the engine, at low rpm almost all the time.
 
Last edited:
beware the dreaded idle tip-in hesitation that takes extra pump-shot to overcome, and maybe you run out of shot 1/2 second later.....
I gotta repeat;
with an automatic, who cares about the timing below stall..... even 10 degrees short will not be missed.
But if you hit stall at WOT, with even 1* too much, over time, you are probably gonna detonate the plugs to pieces. And those ceramic bits are about three times as hard as steel..... 2200rpm is one revolution every .0273 seconds, which on a V8 is 4 firings. When you hear the detonation, IF you hear it, it might take you .5 second to lift off the throttle. In that time, your engine has fired 73 times................. where are the bits, and what are they doing?
At 2200 and cruising, your engine might want 56* of timing, for best fuel-economy.
If you run 12 mechanical at idle, and an additional 12 in the can, what happens at 2200 and cruising?
I'll tell you
Your max Power-Timing with a lo-compression 318 might be 36* at 3400rpm. To get to 36 from 12 is 24 degrees. And if your curve begins at 1000, then you have 2400 degrees to get there. This maths out to 1.0 degree per 100 rpm.
Therefore, at 2200 the mechanical timing will be 12+12=24. And in this case, the Vcan is bringing 12 to the table so 24+12=36 at 2200 and cruising.
But your engine might want 56*!
so you are 20* short, and no wonder the thing is a pig on gas.
I don't understand what you mean by hitting stall? What does the stall have do with timing or detonation?
 
What does the stall have do with timing or detonation?
You said it; For power, the engine doesn't care about any timing below the stall-rpm.But just trying to give the engine the timing it wants at stall is a very big deal .
Everybody is so concerned about power-timing, IMO, they lose sight of all the engines other timing requirements.
Lets say your engine is ok with 36* at 3400 rpm for Power Timing. and lets say you are running the typical 2.76s with say 205/75-14 tires at 26.1 tall which is a roll-out of 82 inches. And let's go with a typical 2200 stall at say 15% slip (first gear). Ok the first time you hit 3400 rpm, the speed will be ~34 mph. So until that moment, your Power-Timing has probably not been right. But hang on;
On a typical trip ; How often are you at 34 mph with the hammer down in first gear? And for how long will you be there?
Lets say it's Saturday night, 5pm and you will be cruising until 10Pm, so that is 5 hours/300minutes/18,000 seconds. And you will stomp it about once every 10 minutes for 1.5 seconds, the time it takes to be speeding. Oh wait.... you are already at 34 mph in second or third, in a 35 mph zone. OK whatever, you nail it. so every 10 minutes is 30 times during the evening and so 30 x1.5=45 seconds at WOT, so (18,000less 45)/18000= 99.75 percent of the time, you will not be at WOT. But most guys will insist on running the PowerTiming up to the max.

Now consider the stall speed. Say 2200. During the evening, nearly every time you take off, on a Saturday night, the engine is gonna get up on the stall, so that's where the timing needs to be perfect; or at least as close to perfect as you can get it, else your engine will seem/be lazy. Which is irrespective of the rear gear. Of course the smaller the rear gear, the lazier it will be.
IMO, max-timing below stall, is only important to a guy who doesn't understand the bottom end tuning, and trys to use max idle-timing as a crutch for something else.
In my experience 12 to 16 degrees of Idle-Timing, at about 1000ft elevation, with decent cylinder-pressure, will allow you to get your WOT stall-timing pretty close. Closed chamber Alloy heads will typically not need as much as iron open-chambers.
For a guy with a street hi-compression alloy-headed 360, I challenge you to feel the difference between 32 and 36 degrees of Power-Timing. With typical gearing, say 3.23s, you will only pass thru the power peak once on the way to 65 mph. With 3.91s you might get there a second time..
However, I doubt the tires will have stopped spinning so....... just try and measure the ET difference. lol.

So once again,
IMO,
for a streeter,
with low speed performance in mind,
getting the timing right at stall should take precedence over WOT Power-Timing so long as the Power-Timing does not get into detonation. If you can hit both, hey, that's good on you.
Unless you have a stroker, then who cares lol.
That's my opinion and I'm sticking to it, lol.
 
Last edited:
Did you get the timing sorted out, the standard Mopar performance distributor curve should be real close.
There's fair amount of mumbo jumbo going on in this thread. lol
 
Did you get the timing sorted out, the standard Mopar performance distributor curve should be real close.
There's fair amount of mumbo jumbo going on in this thread. lol
Mumbo jumbo
Is that anything like Yada Yada?
 
For any Canadians reading this, don't forget to convert the timing degrees to Celsius and the vacuum inches to centimeters... :canada:
 
Did you get the timing sorted out, the standard Mopar performance distributor curve should be real close.
There's fair amount of mumbo jumbo going on in this thread. lol

I couldn't agree more. lol
 
Did you get the timing sorted out, the standard Mopar performance distributor curve should be real close.
There's fair amount of mumbo jumbo going on in this thread. lol
Not yet I haven't even had a chance to work on it any yet.
 
52939E0B-17B0-41EB-B2AB-47377902B6FC.png
 

This right here ^^^^^ is how you tune timing.

These are the same instructions I've been preachin for years STRAIGHT out of the MP engine book and everybody wants to listen to some bullshit about manifold vacuum. It makes me wanna beat my head against the wall till I pass away.
 
Note the above testing procedures shown in posts 67-69 for dialling in vac adv will produce exactly the same results whether the VA was connected to ported or man vacuum. It makes no difference. Man vac cleans up the idle & tip in, ported doesn't operate at idle.
 
Note the above testing procedures shown in posts 67-69 for dialling in vac adv will produce exactly the same results whether the VA was connected to ported or man vacuum. It makes no difference. Man vac cleans up the idle & tip in, ported doesn't operate at idle.

Maybe. But you should also note that you live in a completely different country and are advising someone on the other side of the globe. Australia had different cars, engines and tunes as American cars. You need to think about that. Your assertion that GM used manifold vacuum a lot is incorrect on this side of the world. There were very few car that did that here and I cannot think of one Chrysler product that did. Kevin has a very mild build and will benefit 100% from ported manifold vacuum.

And here's the beauty of it. It's his car. He can do it however he wants to. He can even change it up if he so desires. I think we all get your point. You prefer manifold vacuum. But your assertion that they will both yield the same results incorrect.
 
So did somebody finally get through? I'm glad you believe somebody for a change.
I never didn't believe you said to run ported from the beginning and I said thanks I haven't questioned that. Other ppl have chimed in and made good points and I have a better understanding of the whole process now! Im the type of person I don't want to just know how to do it want to know why I'm doing it that way and what will happen if I don't do it that way. Please don't take offense if I keep asking questions I always heed everything you say you have never lead me wrong im just the type of person a simple how to answer just isn't enough. And that's how you learn and expand your capabilities. It's easy to learn that 2+2=4 and memorized that but what happens when you get 3+1? Its still 4 your just getting there im a different way. But I never once 2nd guessed you or your advice! Sorry if I made you feel that I did.
 
I never didn't believe you said to run ported from the beginning and I said thanks I haven't questioned that. Other ppl have chimed in and made good points and I have a better understanding of the whole process now! Im the type of person I don't want to just know how to do it want to know why I'm doing it that way and what will happen if I don't do it that way. Please don't take offense if I keep asking questions I always heed everything you say you have never lead me wrong im just the type of person a simple how to answer just isn't enough. And that's how you learn and expand your capabilities. It's easy to learn that 2+2=4 and memorized that but what happens when you get 3+1? Its still 4 your just getting there im a different way. But I never once 2nd guessed you or your advice! Sorry if I made you feel that I did.

Naw it's no biggie. It's just that several people made the correct suggestion for your build and this yahoo caint keep his trap outta where it doesn't belong. He's just slap wrong about the timing being the same for each and that's just wrong. It's bad information and here's how.

Initial or idle timing with ported vacuum is let's say....like you have now "around" 20 degrees. Let's call it 20 just to make it simple.

Now, remove the vacuum advance from ported and hook it to manifold. Now your initial timing is 20, PLUS "whatever" the vacuum can pulls in....let's call that 12 degrees. So now your initial or idle timing is 32 degrees. Does that sound right? Does it sound the same? No, because it's not. So you have to back that 12 off of the distributor (initial) timing to get it back to 20. Get it? Now, unplug the manifold vacuum from the vacuum can and what do you have? You have 12 less than that 20 or 8 degrees BTDC. There's no way in hell those two timing curves will be the same in this country or his.

You can run manifold vacuum. You can run ported vacuum. You can run no vacuum. But the best way, IMO for YOUR build, since it's really not too far from stock is ported. It also does something no one has mentioned. It gives you a certain amount of engine braking due to the vacuum falling off when you take your foot off the gas. That can make a difference in mileage right there.....and right now, gas ain't cheap. With it on manifold vacuum, when you lift your foot, vacuum goes up and pulls in total vacuum advance, and reduces that amount of engine breaking. It makes a very noticeable difference.

Ported vacuum also cleans up part throttle combustion, reduces NOX gasses and picks up mileage and gives a little extra advance at cruise. There's no down side. This slant 6 I just put in my car has 6 Hg at idle. 14 at cruise, because it has a pretty sizable flat tappet solid cam. I can tell you it benefits greatly from being on ported vacuum and I know yours will too. But my advice is try it both ways. But in order to get initial timing right hooked to manifold vacuum, do not disconnect it when setting timing, because you WANT the vacuum operational at idle. Set it the same 20 degrees initial as you would with the can disconnected from ported. You can plot the two curves yourself and see the difference. Some people like it better on manifold vacuum. You may and if you do, it won't hurt a thing.....but it ain't the same as ported. lol
 
RRR,
Many of your statements in post 72 are incorrect.
The first one is that M & P vac adv will not yield the same results, as described in the posts 67-69 I noted, is wrong. Period!
Also, Australia did NOT have different engines to the US. All GM cars that came to this country, Chev, Buick, Pontiac etc had US built engines.........& most had MVA until 1968.
There were a few locally designed/built engines. In GM, the inline 6, 253 & 308 V8s.
Mopar had the inline Hemi 6 cyl. All other Mopar engines were fully imported & built in the US. Ford had a locally designed 2V head for the Cleveland V8, but all the other V8s were fully imported like GM & Mother.
While I might live on the 'other side of the globe', we breath the same air & use the same gasoline as cars in the US.
 
-
Back
Top