Vacuum advance information and myths
- Thread starter halifaxhops
- Start date
-
Rat Bastid
Dunamis Metron
I’ve seen many times your claim that if you have over 220 @ .050 you NEED more timing at idle than you can get with just initial timing. And you’ve thrown 50 out there more than once.
I know it’s a crutch because I’ve had to do it on junk I didn’t build.
You ought to get a distributor machine and do some actual testing.
RB,
Post #51 realllllllllly shows how little you know about ign. Man VA is just one method of providing ectras timing at idle, when it is needed. It is particularly useful because it is load compensating, & can be adjusted to drop off if detonation occurs. Unlike fixed timing..
Post #51 realllllllllly shows how little you know about ign. Man VA is just one method of providing ectras timing at idle, when it is needed. It is particularly useful because it is load compensating, & can be adjusted to drop off if detonation occurs. Unlike fixed timing..
RB,
Post #52. More BS. The dyno tells you how much timing the engine wants, not a dist machine.
And yes, more timing at idle with longer duration cams. Timing at idle with Chry engines is the initial timing, usually in the 8-12* range. A 220 @ 050 cam is not going to need 50* at idle & I never said such a cam would need that much. It might need 20-30* & testing will show what it does need. Man vac adv would be one method of adding the extra timing.
Post #52. More BS. The dyno tells you how much timing the engine wants, not a dist machine.
And yes, more timing at idle with longer duration cams. Timing at idle with Chry engines is the initial timing, usually in the 8-12* range. A 220 @ 050 cam is not going to need 50* at idle & I never said such a cam would need that much. It might need 20-30* & testing will show what it does need. Man vac adv would be one method of adding the extra timing.
Rat Bastid
Dunamis Metron
Yep. And the dyno isn’t the last word on a timing curve either.
BUT, you don’t need a dyno to set initial timing. What you do need is a distributor machine to change the shape of the curve and to make sure (for anything with what you think is long cam timing) you don‘t start the curve too soon.
Or you can guess.
I do the former. I have no clue what you do except stick a vacuum hose to manifold vacuum and call it good.
I‘ll say it one more time. If you NEED to add timing at idle with VA and MV you need to rethink your compression ratio, your cam timing or both.
There is far more power and drivability in those two things than you’ll get by crutching a bad combo like that.
That is not unique to Chrysler. It is a provision to address the occassions when the retarded initial timing used for emissions reduction does its job a little too well.
Retarding the initial timing puts more heat into the cylinder walls and helps insure a hotter and more complete burn of the HC. It didn't effect off idle perfromance too much because above idle the advance curve is quick to offset the retarded timing.
Compare the idle rpm of a standard setup verses the same engine of a CAP or CAS equiped engine.
There's a reason why a non-CAP engine can idle at 500 rpm and be put into gear...
Its because the fuel mixture and timing are correct for power and fuel economy with no concern about HC and CO.
Last edited:
You can often use manifold vacuum to offset the the retarded initial timing on CAP/CAS set up. Its a lot simpler for many people than correcting the advance curve.
However, since this thread is about myths, its a myth that ported vacuum has anything to do with emissions.
This is a big myth, and I beleived it too until a couple of old time tuners set me straight.
Then I started looking at stuff written and built at the time - particularly Chrysler stuff.
1959
from 1959 page 10 (Session 136) of the Chrysler MTSC.
It did not change a decade later either.
1969
from 1969 Ignition System Analysis (Session 259), Master Technician's Service Conference
Go look through shop manuals, the MTSC, at photos and surviving example and it will be interesting to find what year or years Chrysler first used a vac source above the closed idle position.
Last edited:
I would have to say MOST reading I've done on the subject(limited admittedly) says to use MVA.
Thats interesting BUT your second photo says right in there that VA isn't needed for complete combustion. That means emissions. Chrysler did use manifold vacuum on pre 68 cars. My 66 300 440 tnt only has one port and it's MV vacuum.
OK, so what I gather is that when vacuum advance is connected to PV the only time it comes into play is under light throttle cruise conditions. When vacuum advance is connected to MV it comes into play when under light throttle cruise conditions and at idle.
So how does the advanced timing at idle effect combustion, heat, and performance?
So how does the advanced timing at idle effect combustion, heat, and performance?
MV comes in right away and is reduced or eliminated as needed under load. The heat and combustion, The retarded timing at idle in a pv engine creates heat that burns the fuel more efficiently. It's does run cleaner just sitting in traffic if that is your goal and some engines with big cams and vacuum issues cannot run MV.
Performance, Some cammed engines need the advance to idle well. One of my engines runs like crap without the vacuum hooked up MV and is soggy from a dead start down the road not track. Full performance should not be affected as the vacuum goes away under hard throttle.
I'm sure I missed stuff and I'm sure to get corrected too. Go. ;)
Not sure how you got to equating complete combustion in that context (you can go look at the booklets) is only for emissions reduction. Yes it is from a 1969 MTSC, but it says nothing different than 1959.
Rat Bastid
Dunamis Metron
Excellent post and that link is absolutely phenomenal.
Getting a handle on what Shrinker is saying in that link will make getting idle and T slot tuning much easier and understandable.
Rat Bastid
Dunamis Metron
Read the link that Mattax posted above. He covers all that and more.
One interesting note is he talks about cam timing and compression ratio and why it matters.
As I’ve said, if you have to add a bunch of timing with MV/VA you have other issues.
Exactly. I said it was BIG Myth. Case in point.
It's an easy conclusion to come to when just experimenting with switching to the the manifold source using a distributor designed for a retarded initial timing - as many of us have. Its actually a better method than just advancing the initial - which leads to all sorts of problems because of the timing curves.
That's why I invited everyone to go look at the original material themselves.
When using a distributor that doesn't have a curve to compensate for retarded initial, then ported works out better - and especially with a hot cam - its more stable.
What is your initial advance at idle without VA? Cam specs @.050. carb, dual plane or single plane?
If that's working out, then go with it.
One issue that I've run into is particulalry noticible with an automatic transmission and a hot cam.
Manifold vacuum is fairly low, idle combustion is on the cold and diluted side, and the idle mix is somewhat leaner. When the transmission is placed in gear, the rpm drops and the vacuum drops. This caused the timing to drop, and the rpm dropped further....
to hell with a performance curve, let's just hook a factory pollution curve distributor to constant vacuum.
That will make a turd sorta run.
That will make a turd sorta run.
Which is a good example of why technical decisions should not be based on opinon polls.
So I like to start with a clean slate on an engine and buy the edelbrocck dist. It is adj mech and vacuum. I set the mech to the most responsive, most power I can get. Then I hook up the vacuum to the manifold port and dial in my advance to just below pinging under load.
This gets me the best throttle response and mileage and no hint of overheating. Never had an issue and my **** always runs better then the guys struggling to cool their engines. Cant go on a cruise my car over heats ect.
PV does create heat and burn cleaner. It is less friendly on a street car in my experience. I adj my carb to compensate for the adv drop and have zero bog flop issues.
I'm not sure if I'm disagreeing with you or not. PV is was for emission reasons.
Last edited:
Rat Bastid
Dunamis Metron
My current engine has a small cam and rhoads lifters to make it stock at idle. It's the tamest engine I've built. It's just a get me by engine while I complete my current build. 215 @ .050, performer, avs2 on a 273 engine. Not a great example of a perfortmance engine in any way.
yea. That's the best. People can scroll down to the examples from the 1940s if in a rush, but really worth going back and reading Shrinker in depth.
-
