Newbomb Turk
Well-Known Member
Because for that I’d do a curve that didn’t need MVA to make it idle.
I’m not sure what you gained by using MVA to get 24-25 initial.
Manifold Vacuum Advance what am I not getting?
- Thread starter 340inabbody
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Because for that I’d do a curve that didn’t need MVA to make it idle.
I’m not sure what you gained by using MVA to get 24-25 initial.
Woodys_Cuda
Ontario, Canada
I've always thought that on a street car ported vacuum is the best way to go since most driving is at lower speeds. For a street brawler or race engine manifold advance should be used. I understand cam and compression come into play when choosing.
I also think that if the carburetor is not properly tuned you'll struggle with idle timing.
Anyways here is a video that covers quite a bit of options for you guys to critique.
I also think that if the carburetor is not properly tuned you'll struggle with idle timing.
Anyways here is a video that covers quite a bit of options for you guys to critique.
Guys I guess I haven’t explained myself well. I believe I need manifold vacuum advance because it gives me full vacuum at idle and Cruz. This will allow me additional timing that my motor wants at idle 42° AND NOT RUN 54° at full advance! Ported will not do that.
See my hand drawing below.
This is what I’m trying to accomplish and I believe manifold vacuum is the only way to do it. I found top dead center three times stabbing the distributor three times blindly searching for idle quality and vacuum three times each time I get 42°. With 24° base timing I get 10Hg. At 42° space timing I get 15.Hg. My ears and eyes are telling the same story. This motor wants more advanced idle however at 42° initial base and with the curve I have in advance stop bushings. This IS MY ADVANCE curve below. This curve was selected to give me a slow advance because of detonation. My average cylinder pressures are running about 200 psi. This is a big problem. This is what I’m trying to deal with. Please see graphs and charts below.
I do realize that the vacuum signal will not be as quick as I would like to prevent detonation entirely or completely. But right now it’s all I got to work with unless you guys can think of something else.
See my hand drawing below.
This is what I’m trying to accomplish and I believe manifold vacuum is the only way to do it. I found top dead center three times stabbing the distributor three times blindly searching for idle quality and vacuum three times each time I get 42°. With 24° base timing I get 10Hg. At 42° space timing I get 15.Hg. My ears and eyes are telling the same story. This motor wants more advanced idle however at 42° initial base and with the curve I have in advance stop bushings. This IS MY ADVANCE curve below. This curve was selected to give me a slow advance because of detonation. My average cylinder pressures are running about 200 psi. This is a big problem. This is what I’m trying to deal with. Please see graphs and charts below.
I do realize that the vacuum signal will not be as quick as I would like to prevent detonation entirely or completely. But right now it’s all I got to work with unless you guys can think of something else.
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Oh and here I am with 42° base at idle in park.
Can you fill in the rpm side of your chart? Also the vacuum. It looks like you have a different curve above idle for ported and vacuum signal?
Newbomb Turk
Well-Known Member
Check my post. I think I said above idle.
The hand drawn curve is not meant to be precise but more of a concept or an example or close approximation for discussion purposes.
The total curve will be different (at idle anway) and thats my point.
The centrifugal advance portion of the curve is dictated by the springs of the distributor. B But they will and should have different curves and again thats the point.
With Manifold vacuum advance at idle I am able to achieve a higher idle advance without having too much at WOT or under acceleration causing detonation. And in my case can lower to a more retarded state if needed.
Also at t=0 seconds ie start of cranking there is little to no vacuum to speak of until just before ignition. So the cranking advance should be low aiding starting and idle capability which is another advantage of MVA. Right?
Again this is my concept and I am throwing it against the wall here. This is what I am thinking.
Does this make sense? Does anyone believe what I am saying is incorrect?
It sounds like you understand that for all practical purposes that manifold and ported vac are the same once the throttle has opened far enough to expose the ported vac port to manifold vac. In your chart that would lay them on top of each other once that happens or above idle.
If it runs better with manifold vacuum, run it.
That’s not the point at all. I think you’re missing what I’m trying to say here.. I need to use MVA in order to run a hight advance at idle………!
Newbomb Turk
Well-Known Member
Newbomb Turk
Well-Known Member
Right. And why does an engine cranking 200 psi need all that idle timing.
A hack just jacks timing into it and sends it.
A tuner will at least attempt to figure out that anomaly.
It tells me the OP has the head to be a very good tuner.
I’ve seen side-by-side comparisons of ported versus manifold gauges while driving. They don’t always do the same thing everywhere. I was surprised that there are differences. I think they’re minor though, but they are different.. But for the sake of this conversation, I’m assuming that they’re very close and that the only differences tuning.
And to be more explicit the big difference is at idle from there on out similar not identical….
I feel like that is pretty much what I said Where do we differ?
Ok would it run well without the high idle advance?
I don't believe he has 200PSI pressure. If he truly did, it would spark knock at idle, with no load at 42 BTDC. I "thought" my slant 6 was at like 195 psi, until I double checked it with a new gauge and then a friend's gauge and it's at around 155. So someties gauges are inaccurate.
That doesn't answer a thing. I asked if it ran good without the higher advance. This tells me nothing. The point YOU'RE missing is that I am an advocate of MVA. Lemmie tell you something. I've been working on and tuning on cars since 1974, I promise you "I" am not missing the point.
Newbomb Turk
Well-Known Member
Or the timing light is wrong. Or the damper moved. Or who knows?
Not sure what you mean.
Regarding compression test. I don’t know what to believe. I double checked my compression gauge to that of my pressure gauge on my spray gun set up on my compressor, which I think is pretty decent. They were identical. my guess is the gauges OK but I did only compared to at 80 psi 200 psi could be off.
Also, I did this with the engine cold. It’s just too ******* hot working in the garage over a hot engine. I also did not add any oil or anything to the cylinder. And in some instances, some of the plugs in and some of the plugs were out I don’t know if that makes a difference for this type of test.
Also, I did this with the engine cold. It’s just too ******* hot working in the garage over a hot engine. I also did not add any oil or anything to the cylinder. And in some instances, some of the plugs in and some of the plugs were out I don’t know if that makes a difference for this type of test.
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