To Vacuum Advance or Not to Vacuum Advance

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cudajames

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Ok, getting close to completing my engine swap - final pieces

The details: 408 stroker - 10.5 to 1 Compression, Holley 750, Comp xe285hl cam. Built 904 with a 3500 Stall

The distributor is a MSD 8388

Usage - Street bruiser (will see the drag strip and some autocross) - don't really care about gas mileage

I can not really find anything - just that Vacuum Advance helps fuel economy. And some comments that performance engines usually do not run vacuum advance

On my previous motor (not very strong - just over cam'd) - I took it for dyno tuning - the place removed the Vacuum horn right away. Stating it was a limiting factor

Hence I am a little confused. What are the pros/cons to run vacuum advance on a performance engine

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Keep it simple. Drive it on the street? Run a vacuum advance connected to ported vacuum.
 
The engine will respond better and run much cleaner on the street with vacuum hooked up.
 
Keep it simple. Drive it on the street? Run a vacuum advance connected to ported vacuum.
Butting in here...why ported vacuum vs full manifold vacuum? I've always thought that you should connect to full manifold vacuum.
 
Butting in here...why ported vacuum vs full manifold vacuum? I've always thought that you should connect to full manifold vacuum.
You don't want it to pull at a idle. When you give part throttle gas it then advances the ignition timing.
 
*I'll choose to be different. I don't run a vacuum advance on any of my street cars with after market cams. Never found a benefit...…..
 
Any street driven engine will benefit from vacuum advance. Usually ported is best. However the further he engine is from stock, the more you will have to work with the centrifical and vacuum advance curves.
 
there is an easy way to prove yes or no
Get yourself a dash-mounted, dial-back, timing box.. and start experimenting.
Stick cars respond well to Vcans because with big cams and plenty of compression, it is easy to run too much advance at low mph, and then the engine get's jumpy, because of the very powerful power-pulses. Of course that jumpiness gets into the chassis, and then into your right foot and right-quick the car is doing donkey-kicks.
But a 3500 Tc will suck all that up, so you can run a lotta mechanical advance with that. And that leads to the Vcan being less important... on the street, maybe.
A good convertor will still pull pretty good below it's rated stall, and some combos will like a lotta timing... that you cannot get with the mechanical only. The way you can tell the engine likes it is because it pulls harder with less throttle, and the engine is smoother.
The thing about the engine is this; the pressure spike from the expanding gasses needs to be delivered to the crank at about 15* ATDC, for optimum transfer and fuel useage. And every rpm and load setting requires a different ignition start-point to make that happen; and it is impossible to achieve on mechanical only.Impossible.
The closer to optimum, the less fuel will be required; because firstly the engine is not fighting itself and secondly less fuel is going out the tailpipe either unburned, or with still a lot of energy in it.
I know with that 285 cam, your powerstroke is already very short probably less than 100*, so trying to zero-in on this will be tough, BUT it still behooves you to try, cuz the engine will be much happier. and using less fuel.
In your case, if running iron heads, your pressure is gonna be almost extreme for pumpgas so IMO, I doubt you could use the Vcan very effectively with that 3500TC.
But with aluminum heads you're waaay under pressured so Have at it; get yourself the biggest Vcan you can find or just mod yours up to 20* or so. It will not be too much on the spark-port

But in either case, you may have to, or dare I say; be able to, take some mechanical timing out, if she seems a lil jumpy on what she has now.
The timing curve in the D is there strictly to satisfy WOT timing; it will NEVER be right at any other load setting. Making it right, or getting as close to it as possible, is the job of the Vcan. You the tuner get to marry the curves to satisfy as many points as possible
Happy HotRodding
 
Well, with this thread and additional reading - "how to build horsepower" by David Vizard. For anything street the benefits far at weigh . . . I went back and add the vacuum advance to my disti

I am running the same 285 cam

Heck if I can average 10 mpg - that is a huge win

Motor all dressed up (thanks Ringo/phoenix) and ironing out bugs/loose hoses and dialing it in

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I did a lot or reading on this and recently added vacuum advance to my 68 340 (actually 372 CID) Cuda. I run it off of the manifold vacuum and am running a pretty good sized cam that creates low manifold vacuum (252/257 @ .050) at idle. The slight increase in advance at idle (if any) allows you to close the throttle plates and square up the idle transfer slots on a Holley carb (if you are running one) and helps clean up the idle. Once off idle and you are accelerating there isn’t really any manifold vacuum so it doesn’t add to the total advance. When cruising or decelerating you get the benefit of additional (vacuum) advance which cleans up emissions and improves fuel efficiency.

I can say that it has made my car much nicer to drive on the street.

Of course you need to set your mechanical advance curve (lightweight mr gasket springs in mine) set your total mechanical advance (I welded up the slots in my stock type electronic distributor after the initial was set to limit the total mechanical advance) and adjust the vacuum can as needed.

Just sharing what worked for me, lots of folks also have luck with a ported vacuum source.
 
Ok, getting close to completing my engine swap - final pieces

The details: 408 stroker - 10.5 to 1 Compression, Holley 750, Comp xe285hl cam. Built 904 with a 3500 Stall

The distributor is a MSD 8388

Usage - Street bruiser (will see the drag strip and some autocross) - don't really care about gas mileage

I can not really find anything - just that Vacuum Advance helps fuel economy. And some comments that performance engines usually do not run vacuum advance

On my previous motor (not very strong - just over cam'd) - I took it for dyno tuning - the place removed the Vacuum horn right away. Stating it was a limiting factor

Hence I am a little confused. What are the pros/cons to run vacuum advance on a performance engine

View attachment 1715360788
The real benefit with a proper vacuum advance is improvement in driveability.
If your engine produces enough vacuum it would be to your advantage to apply a vacuum advance.
It adds another layer of tuneability for your performance engine.
 
Your motor will live longer with vacuum advance hooked up.
 
I can not really find anything - just that Vacuum Advance helps fuel economy.
And economy usually correlates to efficiency.
In racing that can mean less fuel has to be carried, or maybe one less pit stop on a longer race. So its not just for street cars. But it does add another level of tuning.
And some comments that performance engines usually do not run vacuum advance
Drag racing it is certainly not needed. The whole race is run at wide open throttle.
When developing a tune for a modified engine, or even desmogging a stock one, first get the initial and mechanical advance figured out. So generally no vac advance until thats worked out. Nor on the dyno (unless you're doing part throttle tuning).
Butting in here...why ported vacuum vs full manifold vacuum? I've always thought that you should connect to full manifold vacuum.
It depends.
The purpose of vacuum advance: provide more lead time for lower density mixtures.
The elegance of using ported is then vacuum advance correlates directly with lower fuel-air density.
This leaves mechanical advance to correlate with rpm, and changes in burn rate related to rpm.
And when racing (where you care about timing at 4000 rpm and above) sometimes mechanical advance can also offset time lost in the electronics, etc

Sometimes a distributor can't be adjusted to get the initial timing wanted. And sometimes in these situations manifold vacuum can be set to provide it.
Sometimes on emmissions equiped engines, manifold vacuum was mixed in with ported vacuum during warmup. My AMC era jeep has this, called a non-linear valve. It also has a 'heavy duty' coolant temperature switch that adds switches to manifold vacuum if coolant gets too hot.

The above comments about mixture density makes sense only if you know that pre-emmissions era idle mixtures were relatively rich. The mixtures get leaner the more the throttle is open until some point near wide open, when relatively rich mixtures are needed.
 
Even drag racing, it doesn't amount to a hill of beans if you have vacuum advance hooked up and operating, since it is ineffective under hard acceleration. The biggest benefit to me is how it "cleans up" the combustion at part throttle cruise. That's what helps pick up MPG a little bit. Sometimes, depending on the total combination, it's a pretty noticeable amount.
 
I meant to mention one issue with using manifold vacuum. If the vacuum is a bit unstable, then the vacuum advance may be moving around as well. Especially with an automatic transmission, this can be annoying. It only happens when the vacuum moves the advance in the same range as the manifold vacuum at idle. But having been through that, thought it was worth mentioning.

Also sometimes people today have a hard time believing rich idle mixtures were the most efficient.
Here's a snip from a 1967 Chrysler Master Tech booklet on fuel mixtures and timing.
upload_2019-8-7_12-57-31.png

Later emissions set ups sometimes went leaner, sometimes richer. All depending on what they were trying to do. They played a lot of games with both mixtures and timing as they focused on different stuff and added items like cats and air injection.
 
The distributor is a MSD 8388
A possible challenge with using vacuum advance on the MSD distributor may be slowing down the mechanical advance enough.
Lets say the mechanical advance adds 15* by 2500 rpm, and the intial is 20*. So timing at 2500 rpm is 35* BTC
And lets say the car cruising at 45 mph is at 2500 rpm, and the engine is pulling 16"Hg.
If there's a vacuum advance is fully in at 12 "Hg, and it adds 15*, then timing will be 50* while cruising at 45 mph or above.
Squeezing the throttle for an uphill or mild acceleration at 45 or 50 mph very likely will ping.
Only short trips might not happen. On longer runs where the engine gets fully heat soaked, its more likely.
If it does, need to slow the advance.
Most Chrysler distributor curves have two stages. The high performance engines picked up combustion efficiency very quickly off idle, and so got long slow advance curves above 16 or 1800 rpm.
Not all advance mechanisms lend themselves to two stage, and nor is it always needed. With your setup, if you run enough initial, two stage may not be needed.
 
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Here's another thought;
Say you had timing all in by 2800, and V-can hooked to full-time vacuum. What happens when you slam the throttle shut at some higher than all-in rpm, and the car now drives the engine with it's closed throttle,(compression braking) and the timing is say55 or more degrees?
Meh, maybe nothing.
I didn't want to find out, so I'm on the spark-port.
 
Here's another thought;
Say you had timing all in by 2800, and V-can hooked to full-time vacuum. What happens when you slam the throttle shut at some higher than all-in rpm, and the car now drives the engine with it's closed throttle,(compression braking) and the timing is say55 or more degrees?
Meh, maybe nothing.
I didn't want to find out, so I'm on the spark-port.
AJ I like your insight.
My iron head 340 roller engine is all in 36* at 2500rpm the vacuum advance adds 10* for a total of 46* at cruz speed. I have tried to add more but am limited by 10 inches of vacuum and the advance adjustment is maxed out. Connected to full manifold vacuum.
I have made numerous 6000rpm+ pulls on the road with no issues. I dont know what it would do at 50*+, lets not find out.
Keep up the good posts
 
No issues with the vacuum canister hooked to manifold vacuum on mine and I’m running on California’s finest 91 octane. Just need to make sure your tune is in order.

Like I said, some like ported vacuum, some like manifold vacuum.
 
You can mod your Vcan for more degrees by removing a buncha material from the stops. I started with a 9* arm and ground enough off to get 22*. Another member,TrailBlazer got 24*. My small-cammed 367 (230/237/110* ), comes up on the vacuum pretty quick, and I have the Vcan set to bring it in as fast as it is able, and that is working pretty darn nice for my combo.( manual trans).
I cruise at 65=2240* ; on 28 mechanical, plus 22* in the can, and up to ~8 with the dial-back timing device. That comes to 50 to 58*, with aluminum heads at 207*F minimum coolant temperature.
 
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AJ I like your insight.
My iron head 340 roller engine is all in 36* at 2500rpm the vacuum advance adds 10* for a total of 46* at cruz speed. I have tried to add more but am limited by 10 inches of vacuum and the advance adjustment is maxed out. Connected to full manifold vacuum.
I have made numerous 6000rpm+ pulls on the road with no issues. I dont know what it would do at 50*+, lets not find out.
Keep up the good posts

When you say you are limited by 10” of vacuum do you mean your max vacuum is 10” ?
The lowest can I have seen is an 8.5. Which means it gives you 17” of crank advance. So I doubt you need to file your for more advance
 
When you say you are limited by 10” of vacuum do you mean your max vacuum is 10” ?
The lowest can I have seen is an 8.5. Which means it gives you 17” of crank advance. So I doubt you need to file your for more advance
10 inches at 850rpm idle, it does go up with no load rpm
The advance I have is adjustable, 46* verified with a dial back timing light, is the most I can get out of the advance adjustment.
 
10 inches at 850rpm idle, it does go up with no load rpm
The advance I have is adjustable, 46* verified with a dial back timing light, is the most I can get out of the advance adjustment.

You may already know all this but just in case...

Canisters come in varying degrees of advance capabilties and different starting/ending points of vacuum inches.
You can change when the advance starts by adjusting it with an allen wrench. It will not change when full advance is reached or total advance. So if you crank it out counterclockwise it wont start advancing till 15” ( random #) but it will be all in by 18”... so it just shortens the window ...
The arm is usually marked for the total degrees of capabilities ... 8.5 means 17* of advance when reading the dampner.
I have heard of cans that are not adjustable but I think it is due to a smaller allen screw being used in some.

There are also different advance plates and they are marked in 1/2 crank degrees as well... I dont know why some slots are angled though ? It might allow more advance without having a super long slot or it might be a leverage feature ?
 
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