Manifold Vacuum Experiment

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RRR,
I'm not arguing about it. I am agreeing with your findings, none of which surprise me as I have been doing this for decades.....& I know the positive outcome.
Others want to change known facts.

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RRR,
It is the correct link but I think it needs to be typed in manually. It has been on the web for years & has been seen on various forums.
 
Let me add to this thread. I can't give numbers as I did this about a year ago as a "crutch" to make my car drivable. 64 valiant 170 slant 3sp stick. 9.5 - 1 CR, mildest Mopar performance cam, electronic dist (don't know the curve) Holley 1920 carb single 2 1/4 inch exh. Needed a lot of initial timing to get a decent idle, but engine would ping badly when accelerating (ported vac advance). As a band aid I connected the vac to manifold vac. and retarded the initial/total by rotating the distributer. This limited the total and stopped the pinging, but allowed more timing at idle so it wouldn't stall or need the carb opened too far. I can't add any more to this as the car is laid up for body work.
 
Bewy said:
Post #64 might be able to do quotes....but does not understand ign requirements.
And the idiotic statements.
Smoother idle as a result of MVA might not show up on a dyno curve, but the engine can sure feel the difference....& so can the driver.
And it is not just me that that gives advice on MVA [ maybe they do not do quotes either ], but people much smarter than the the poster of post #64.
Maybe this will count as a quote; [ Yes, I know already posted, but not read by post #64 ]

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#1. How much idling do you f**kin' do?

#2. Explain why you don't just turn the distributor for good idle, swap some springs, and run NO vacuum advance like so many hotrodders have done forever?

#3. Some of those articles (from what I can read, they're microscopic on my PC screen they way they're posted) say you want retard just off idle, but my question refers back to number 1 when I ask, why are you tuning your motor for a period of time when you're not using it?

If you REALLY want that much control over your timing, switch to EFI and be done with it. Chrysler tried every mechanical trick under the sun in the late '70's to do what you're trying to do (albeit for different gains), and eventually....computer control.

I ain't arguing any more. Can lead the blind to water....can't make 'em drink.
RustyRatRod said:
Hay yall, all I'm doing is posting what I did and what the outcome was. If yall don't like that, just scroll on by, because I'm not arguing, I'm not debating. All I'm doing is reporting on findings. Wanna argue it up? Please start your own thread. That's not what this is for. Thanks.

So, last night, I decided to revisit Vixen and try this experiment on her. So first thing I did was get the timing light out and verify where I had the timing set, even though I already knew. Or so I thought.

Looking at the balancer with the light. I saw no mark on the balancer. Then, just before I took the light off, here comes the mark rotating around counter clockwise as the engine was idling. lol So I reached over and goosed it a little. The outer ring on the balancer almost came off the front of the balancer hub. So I yanked the radiator and fan and pulled the balancer. When I just LAID it on the work table, the inertia ring just FELL off. LOL How in the world it didn't fly off is anybody's guess and there's no telling how long it's been like that.

So I found out two things. I need another balancer and I don't have one, and I need an adjustable vacuum advance for a slant 6. Do any of yall know if somebody makes one? S I'm on the hunt for a balancer and an adjustable vacuum advance that I'm not sure exists. lol
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See? See what manifold vacuum advance did to your motor? I told ya bad stuff was gonna happen!!! (At least YOU know I'm kidding about that)
 
jos51700 said:
#1. How much idling do you f**kin' do?

#2. Explain why you don't just turn the distributor for good idle, swap some springs, and run NO vacuum advance like so many hotrodders have done forever?

#3. Some of those articles (from what I can read, they're microscopic on my PC screen they way they're posted) say you want retard just off idle, but my question refers back to number 1 when I ask, why are you tuning your motor for a period of time when you're not using it?

If you REALLY want that much control over your timing, switch to EFI and be done with it. Chrysler tried every mechanical trick under the sun in the late '70's to do what you're trying to do (albeit for different gains), and eventually....computer control.

I ain't arguing any more. Can lead the blind to water....can't make 'em drink.

See? See what manifold vacuum advance did to your motor? I told ya bad stuff was gonna happen!!! (At least YOU know I'm kidding about that)
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LOL Yeah I know. Especially since I hadn't swapped Vixen over to MVA. So you don't like vacuum advance.....and that's cool. If we were all the same, what a boring world it'd be.
 
RustyRatRod said:
LOL Yeah I know. Especially since I hadn't swapped Vixen over to MVA. So you don't like vacuum advance.....and that's cool. If we were all the same, what a boring world it'd be.
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If we were all the same as me, at least we'd all be right

That's sarcasm, by the way. ;-)
 
Bewy said:
RRR,
I'm not arguing about it. I am agreeing with your findings, none of which surprise me as I have been doing this for decades.....& I know the positive outcome.
Others want to change known facts.

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Click to expand...
I know you're not. I appreciate all the info.
 
'How much idling do I f**kin' do?' Like every body else, as little as possible. But driving circumstances dictate that idling is & will be necessary at times.

Here is a another statement from Jos51700 that is 100000% wrong.
'If you want spark advance during wot, you're an idiot'. Post #64.
The only idiot is the person who wrote this & obviously doesn't understand ign.

You DO want spark adv during WOT. Here's why. You could go to WOT from idle, from, say, cruising at 2000 rpm....or anywhere in the rpm that the engine is capable of.

The mixture takes time to complete the burn process & exert maximum force on the pistons.
It has been determined that 14-20* after TDC provides the best mechanical advantage for max force to be applied to the pistons. It is not TDC like many think.
At 5000 rpm, there is only half as much time for the burn to complete compared to 2500 rpm.
That means as rpm increases, the ign advance needs to start earlier [ more crank degrees BTDC ] so that the burn is completed for optimum mech advantage.
Another way to look at it: say dyno testing showed max HP was made with 36* of advance.
At 5000 rpm, it takes half the time for the crank to rotate 36* than it does at 2500. So that is the centri curve starts with low advance & increases with rpm...so the mixture has the time to burn.

The vac adv drops to zero or close to zero at WOT. If you had the factory 12* BTDC as initial timing......& no advance curve....you would have 12* still at 5000 rpm....& be way down on power.
 
I'll say this and it's indisputable. If there's only ONE reason to use manifold vacuum advance on the advance can, it's so you can run extra advance at idle without lugging against the starter. Since there's no vacuum on the can until the engine starts, it would be smart for bonified race engines with 12:1 and over to run a vacuum advance with manifold vacuum on the can. I've done it, in fact and it works quite well. Every bit as good as an ignition kill switch. Since the extra advance at idle has good benefits, especially on a lower compression engine, there seems to be no down side.
 
@RustyRatRod thank you for posting results on this (oddly) controversial topic. I for one don't understand why this topic gets so heated, it's almost as bad as what oil to run or Edelbrock vs. Holley carbs. A lot of this hobby is about experimentation and making things work for your specific setup. If MVA works for you, great. If not, don't use it! It's that simple and it doesn't hurt to at least try it out.

I got the exact same results as you tuning my D200 using MVA. It has a stock 1972 360 2-bbl (stock cam and heads, 8:1 or slightly more compression) with 4-bbl intake and carb (Eddy 1406), Pertronix CD ignition box and Hedman longtube headers. I played with the static idle and mechanical advance to give the best WOT and over-half-throttle performance then hooked up the VA to manifold vacuum. Had some slight pinging pulling up long hills in top gear so I adjusted the VA can down about 1/4-turn at a time until the pinging went away. IIRC the VA can pulls in a max of 16 degrees and at idle with it hooked up the engine has about 24 degrees advance, with it unhooked it's around 8 degrees. It's just like you describe, hardly needs any throttle opening at all to keep it cruising down the road or to accelerate from a stop and idles buttery-smooth.

The 360 in my Duster is also lower compression, around 9:1 with stock Magnum pistons, Edelbrock LA heads with the machined chambers for pop-up 340 pistons (no quench) and .027" Cometic head gaskets. It likes a lot of timing and I found through trial and error that MVA helps with low-speed light-throttle driving especially just off idle. I believe a while back with my previous 360 that was 10.5:1 with KB zero-deck flat tops and iron Magnum heads it didn't like extra timing with MVA thanks to the decent quench and higher compression but I also was more naive back then to the intricacies of ignition advance so I didn't spend much time dialing it in "just right".

Some engines like it, some don't. You'll never know if you don't try!
 
I revisited it on my Valiant, Vixen. I couldn't get the allen wrench into the can properly, because of the angle, so I just pulled the distributor and adjusted it all the way clockwise. I think it's gonna rattle though. We'll see. My new harmonic balancer should be here by the end of the week and I'll get her back together.
 
I gave the truck some new sparkum plugs this mornin. Gapped um at .045. Remember, I'm running the GM big cap sho nuff HEI and it's pretty hot. I also did a compression test. Although I pulled and replaced one plug at the time, so I did not remove all the plugs so the engine would spin faster, nor did I block open the throttle. Yeah, I'm that lazy. Got 130-135 across the board. I just "guess" with all the plugs removed so the engine would spin faster and the throttle blocked open "by the book" it might see 140 "or so". Really not terrible for a low compression engine and lumpy hydraulic cam. I'll get some on the road vacuum readings soon. I haven't forgotten.
 
Just to beat on this topic a bit further.

Looking at the physics/mechanics of a vacuum canister.

*The canister in theory works in the opposite effect of a compressor where the engine creates the vacuum within an enclosed area.

*The larger the canister the more volume to draw from/to although the Hg's will never exceed what the source is capable of producing.

*Multiple smaller canisters daisy chained would have the similar effect.

*The purpose of the canister is to prolong the desired vacuum effect for the desired duration of time or lapse.

Ex..Vacuum canisters work on boosted brake systems because they provide resource and reserved negative pressure to keep the booster diaphragm engaged for the requested duration when engine vacuum diminishes. Acceleration then suddenly slamming the brakes.

So, my question is: How do you best determine what advance curve you need based on vacuum volume? Trial and error?

Please forgive me if none of this makes any sense, different part of the brain waking up.
 
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69FBCuda said:
Just to beat on this topic a bit further.

Looking at the physics/mechanics of a vacuum canister.

*The canister in theory works in the opposite effect of a compressor where the engine creates the vacuum within an enclosed area.

*The larger the canister the more volume to draw from/to although the Hg's will never exceed what the source is capable of producing.

*Multiple smaller canisters daisy chained would have the similar effect.

*The purpose of the canister is to prolong the desired vacuum effect for the desired duration of time or lapse.

Ex..Vacuum canisters work on boosted brake systems because they provide resource and reserved negative pressure to keep the booster diaphragm engaged for the requested duration when engine vacuum diminishes. Acceleration then suddenly slamming the brakes.

So, my question is: How do you best determine what advance curve you need based on vacuum volume? Trial and error?

Please forgive me if none of this makes any sense, different part of the brain waking up.
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Are you asking how quickly does the vacuum canister change timing relative to the engines change in vacuum?
 
If I understand correctly........

Loose spring in can (CW adjustment less spring tension)comes on quicker and leaves slower with variations in manifold vacuum.
Tight spring in can (CCW adjustment more spring tension) comes on slower and leaves quicker with variations in manifold vacuum.

The adjustment is made so that it comes on quick enough for starts and good idle, but leaves quick enough so that the engine doesn't ping under light or heavy throttle operation.
 
The vacuum advance canister diaphragm acts against a spring. Adjusting the canister increases or decreases spring tension so the diaphragm moves a different distance for a given amount of vacuum being applied. When vacuum is released it only takes as long as the spring can push the diaphragm back which is essentially instant.

If the engine is pinging under light load because of too much vacuum advance, the spring is adjusted to increase pressure against the vacuum canister diaphragm so there is less ignition advance at cruising engine vacuum.
 
Just to expand on posts# 90-93.
Application of vac adv [ ported or man ] has two parameters that affect engine operation:
- the rate that VA is activated or de-activated, controlled by a spring inside the canister.
- the total amount of VA that can be applied, controlled by the maximum travel of the actuator arm.
 
RustyRatRod said:
I gave the truck some new sparkum plugs this mornin. Gapped um at .045. Remember, I'm running the GM big cap sho nuff HEI and it's pretty hot. I also did a compression test. Although I pulled and replaced one plug at the time, so I did not remove all the plugs so the engine would spin faster, nor did I block open the throttle. Yeah, I'm that lazy. Got 130-135 across the board. I just "guess" with all the plugs removed so the engine would spin faster and the throttle blocked open "by the book" it might see 140 "or so". Really not terrible for a low compression engine and lumpy hydraulic cam. I'll get some on the road vacuum readings soon. I haven't forgotten.
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Your compression numbers are why you can use MVA and gain something.

If you were 195 or 200 you wouldn’t need MVA and you can use ported to clean up cruise AFR.
 
Newbomb Turk said:
Your compression numbers are why you can use MVA and gain something.

If you were 195 or 200 you wouldn’t need MVA and you can use ported to clean up cruise AFR.
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Exactly. I didn't WANT high cylinder pressure numbers in a TRUCK motor. lol

As I said, I revisited the idea with the slant 6 in Vixen. I got the new balancer today and installed it and quickly realized BOTH timing marks on it were wrong, so I had to remark it. <rolls eyes>. Anyway, after doing that, I ran a compression test. I got a nes compression gauge since the last timeI did a test on this, primarily because I was thinking the numbers I got last time were ridiculously high and I was right. Rather than the 175 plus I got across the board with the old gauge, with the new one I got right at 150 on all six. So that's actually a good thing, because I believe manifold vacuum will help here too and it seems to be. It idles cleaner just like the truck. It's much less cold natured. Has about 23 initial and with the advance on manifold vacuum has right at 40. AS SOON AS the throttle is opened, it loses manifold vacuum and you can see the timing drop out. Then as you keep slowly revving it, you can see the mechanical advance come in. I'll drive it tomorrow and see how it does. My only worry is, with so much advance at part throttle and a really lumpy cam (250@ .050 on a 108) and manual transmission that it will tend to "buck" at part throttle, but we'll see.
 
RustyRatRod said:
Exactly. I didn't WANT high cylinder pressure numbers in a TRUCK motor. lol

As I said, I revisited the idea with the slant 6 in Vixen. I got the new balancer today and installed it and quickly realized BOTH timing marks on it were wrong, so I had to remark it. <rolls eyes>. Anyway, after doing that, I ran a compression test. I got a nes compression gauge since the last timeI did a test on this, primarily because I was thinking the numbers I got last time were ridiculously high and I was right. Rather than the 175 plus I got across the board with the old gauge, with the new one I got right at 150 on all six. So that's actually a good thing, because I believe manifold vacuum will help here too and it seems to be. It idles cleaner just like the truck. It's much less cold natured. Has about 23 initial and with the advance on manifold vacuum has right at 40. AS SOON AS the throttle is opened, it loses manifold vacuum and you can see the timing drop out. Then as you keep slowly revving it, you can see the mechanical advance come in. I'll drive it tomorrow and see how it does. My only worry is, with so much advance at part throttle and a really lumpy cam (250@ .050 on a 108) and manual transmission that it will tend to "buck" at part throttle, but we'll see.
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Yup, I have no doubt it helped. MVA certainly has a place.
 
Post #95. Wrong.....again.
GM used MVA on production engines until 1968. Chevs had about 24* at idle, much of that MVA.

My 1966 GTO came with 10.75:1 comp ratio with a very mild '067' cam. 197/212* @ 050 duration. Initial timing was 6* + 20* MVA; so idled at 26*.
 
Bewy said:
Post #95. Wrong.....again.
GM used MVA on production engines until 1968. Chevs had about 24* at idle, much of that MVA.

My 1966 GTO came with 10.75:1 comp ratio with a very mild '067' cam. 197/212* @ 050 duration. Initial timing was 6* + 20* MVA; so idled at 26*.
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So? 26 isn’t that high and it’s not 40. I can do 26 with a curve and no MVA.
 
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