Manifold vs. Ported - jeez, here we go
I've probably read every new post and most of the archives / tech articles over the last 4-5 years. I really, REALLY hate bringing this up, yet again... but just can't find an answer that makes sense to me.
1) Manifold vac for advance just makes sense to me. But since I want to use manifold vac as an accurate gauge of engine load - why unplug the vac advance when setting base timing?
2) Wouldn't we want base timing to be at the highest vacuum with timing in an advanced state through the Vacuum Advance (VA)? Then we hit the throttle, mixture gets rich, and manifold vac/timing retards to anticipate the richer load. Perfect timing all around.
3) Can't get my head around setting base timing to perfect with the VA unplugged, then throwing another 16* of advance on it when it's plugged in, and making it be way too advanced (31* or so at idle). If I floor it at every greenlight, I would be perfectly in time, but that's not the case.
4) If I set the VA to fall out completely when the powervalve opens (let's say 7.5 hg), then I'm still zero to 16* too advanced when the PV does open, because my mixture hasn't gotten richer other than the squirter shot for a tiny second.
I'm really, REALLY not trying to ask the same question that's been asked a million times before, and not trying to spark a debate. Have scoured the archives and have read every new post for years. Feel like a schmuck for asking. I just can't get my head around base timing in a manifold vac world.
If someone can shine the light - so grateful! I'm probably just being dumb but could use some knowledge here. Thank you all! Have (and continue) to learn so much from this site.
My take is that basing your timing on the base timing is starting on the wrong end. More important that the total timing and curve is right, the base timing ends up where it ends up as long it starts right. How much time does it spend running on the base timing? Oh..... And hook your VA to the manifold, your engine would thank you if it had a way other than working like it should.
Buford - you are totally right. I should have said the goal is to have it in at 34* to 36* mechanical (or whatever works) by 3000 rpm. Still, to get to that point you need to start somewhere, and you can have a shallow or deep curve to that point depending on your base timing.
I spend most of my time between 650 and 2000 rpm, so base timing should be important.
I still can
The Duraspark distributor can be tweaked pretty good. It has 2 slots in the shaft. Ristow has good pics in his distributor thread about swapping it 180*. This reduces timing a little, they mostly have too much mechanical on the one slot. The vacuum cans are a little much but with a tweak can be adjusted as well. You can also modify the arm to limit vacuum advance to avoid detonation(if it's a problem).
If you really want full control of timing, install TBI, lockdown the distributor and you can adjust every aspect via the bin. No more dualing curves and can tweak/tune to your hearts content with a laptop.
OK, maybe I'm being REALLY dense, and probably missed something here:
1) We set ideal base timing at 650 RPM or whatever with vacuum plugged for the highest vacuum or RPM.
2) We also adjust springs, weights, and stops to make sure that we're 34*-36* advanced at 3000 RPM, with the vacuum advance unplugged.
3) We now plug in the VA, and we're 16* over ideal timing from 650 RPM to 3000 RPM in a no-load state.
4) I'm going down the highway at 2000 rpm with not much load. I'm still 16* over perfect timing.
In this scenario, I have ideal mechanical, no-load timing all the way from 650 RPM to 3000 RPM without vacuum advance. With vacuum advance I have 16* over ideal timing with no load from 650 to 3000 RPM. If I completely FLOOR it at any point, I will be back at perfect no-load timing, but I obviously have a load and a rich mixture at this point.
If the reason to use manifold vacuum is that it retards timing when the mixture is rich because it lights off more quickly and advances it when lean, why don't we set base timing and total advance with the vac canister hooked up so we are sure we have perfect timing in a no-load state?
All the vac advance is doing is retarding timing when there is a load on the engine and the mixture is richer. So wouldn't we want timing set with vac advance in place starting from a no-load condition from idle to 3000 RPM? Then at any point along that curve timing gets retarded when there's a load.
I can just set all of this up and test it out, but really want to know the theory behind it, because it feels like I'm missing something fundamental.
Babywag, thanks for your input! I do want to go tbi at some point when I can afford it, but want to understand the fundamentals. From what I understand there are still timing adjustments driven from the TPS or MAP sensors and understanding this stuff will help.
Sorry for flogging that dead horse!
ported vacuum is zero @ idle.
manifold vacuum or ported vacuum are identical off/above idle.
with no load having xx* of vacuum advance is not a big deal especially idle or lower rpm.
under heavy load having xx* of vacuum advance can be a big deal aka detonation.
I suggest you plug in a vacuum gauge and watch it.
It is very easy to hit lower vacuum even at part throttle.
You will not have 16* vacuum advance for long using the skinny pedal, even lightly.
Any decrease in vacuum = decrease in vacuum advance.
having vacuum advance in place @ idle tends to give a snappier throttle response vs. having it on ported where cracking the throttle results in a big change in timing.
folks have been arguing about this stuff for decades.
My numbers w manifold timing
On the 86 with the new cam
at idle set initial timing at 16 with vacuum disconnected (idle was a bit high in neutral at 900ish)
when vacuum is reconnected timing is about 45 at idle
when vacuum disconnected and rpm at 2000 timing was 20, I did not check with vacuum reconnected
no pinging and great pick up in the sierra foothills, of course as some of you have learnt, my speedo is totally off by 10 mph... but still the engine went up at 60mph on some of the steep inclines, because I assumed that the speedo was correct and 50mph on the steepest climb because I did not challenge the skinny pedal
OK. I'll give it shot. and no need to apologize. There's tons of conjecture and not many that have a way to get to the science and engineering.
Manifold vacuum is useful indicator of load when in gear.
When at idle in gear (automatic of course) the manifold vacuum can also be used as an indicator of engine efficiency since its under some load.
This use of the manifold vacuum is not related to setting initial timing.
Whether vacuum advance should be used at idle generally has most to do with the mechanical curve and base timing.
In cases where the mechanical curve doesn't provide enough timing at idle, then sometimes vacuum advance is used. Sometimes its used conditionally (for example to reduce engine temperatures if its running hot).
Setting the initial is always done with the vacuum advance disconnected and the port plugged (or golf tee in the hose). One reason is the mechanics of the distributor. Setting the initial timing with zero advance insures the mechanical timing will be correct. If the vacuum advance is adding even a little bit, then we don't know where the mechanical timing is, or how much the vacuum is adding.
In terms of engine tuning, the timing is usually set for initial to be for idle rpm. Then the mechanical advance increases timing as the rpms go up to literally provide more time for combustion to match the increasing speed of the pistons. The end result is maximum pressure is developed through the angles of maximum leverage on the crank.
The specifics of the initial and the advance vary from engine to engine but all develop some increased combustion efficiency and speed as rpms increase. This may seem like getting off subject, but its important to see where the initial and the mechanical advance come from.
Yes denser mixtures burn quicker so need less timing.
The answer to your question is that idle mixtures are relatively rich. On engines where emissions were of no consideration, hot idle in the range of 13.0 to 13.5 AFR were typical because it was the most efficient. So generally, very little or no vacuum advance is needed at idle, because the mixtures are not lean.
Now here's the kicker. Going from closed throttle to part throttle the fuel air mixture gets leaner. It's supposed to. The accelerator pump shot is to cover the temporary delay in the idle system response. The AFR only needs to go richer if the throttle is floored or nearly floored.
I'm going to be lazy here, go here for a summary and some textbook and industry graphs of load vs fuel/air mixtures.
If not, maybe this will be of some helpo.
and this one on early (pre-cat) emissions controls by chrysler.
Using your example, the PV is a single stage that opens at vacuum under 7.5 "Hg.
And the vacuum advance goes away complete below 8 "Hg.
So the timing at wide open throttle will be initial plus mechanical advance.
If the initial was 10* at 600 rpm, then when the throttle is floored at idle, the timing will be 10*.
Fuel mixture will whatever the idle circuit provides plus the pump shot until the air flow has enough velocity past the boosters.
The pump shot will probably last at least a second. I have some logs from Holleys4150 but not a 4010/11 or a Motorcraft.
By the time the engine is above torque converter stall speed, the fuel is being supplied by the main system. Fuel will flow through both the main jets and the power valve restrictions.
The simplest approach with a hopped up engine is to go by a general rules of thumb if you can't find a non-smogged similar engine.
Look at camshaft duration and displacement, then use the suggested initial.
For example take a look at the advance curves for our jeeps from a '79 shop manual.
Those are advance curve specs in engine degrees and rpm.
To get timing, add the initial degrees BTDC to the advance.
If its attached to the timed vacuum port, then it will add no more than a couple degrees. Usually zero.
I'm a little confuse about your definition of perfect. Mechanical advance plus initial is the timing for wide open throttle, or around 100% load.
At reduced loads, less dense mixtures produce more power and efficiency. vacuum advance insures these have enough time to develop pressure through the range of maximum leverage on the crank.
Ported vacuum for the distributor advance has the added benefit of the ignition timing remaining steady regardless of how the engine is loaded at idle. So long as the throttle is shut, the ignition timing will stay the same.
Not so on manifold vacuum. The engine idling in neutral makes a lot of manifold vacuum and advances the timing, raising the RPM. Put the transmission in gear though and it loads the engine, which reduces the manifold vacuum, retarding the timing, which slows the engine down more, which reduces the vacuum more...
Engines that idle high in neutral but fall on their face in drive are often engines using manifold vacuum for the distributor advance.
Been away from a computer for a week (was pretty awesome to be away from a computer for a week!) - am just seeing the new replies now. THANK YOU for all the info and bearing with me! I have some reading to do and will be working my ignition curve this weekend. You guys are the best!
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