Ported vacuum advance or manifold. Let's debate!

[hunt2elk]

I was a Carpenter...your graph probably makes sense to office guys but I don't understand it.

Those pushing the manifold vacuum option haven't explained it in a way that makes sense to me either.
Here is how I see it, feel free to tell me if I am wrong.
Example #1:
Your initial timing is at 18 degrees BTDC, you have ported vacuum that adds NOTHING at idle. The centrifugal advance (mechanical) adds 18 degrees for a total mechanical of 36 degrees. You crack the throttle from an idle and it responds normally as the mechanical advance adds timing.
At WOT, the vacuum source adds nothing to the timing. At part throttle, it adds 20 degrees. You roll down the road at 50 mph with about 56 degrees of advance at light throttle and all seems well.

Example #2:
Your initial is set to 6 degrees BTDC, you have manifold vacuum that adds 20 degrees like the above distributor. You crack the throttle from an idle and the timing retards.
How could this NOT cause a stumble or hiccup in performance? You go from 26 degrees of timing to some lesser and unknown amount.

What I would question about your explanation, is why would you drop the initial timing in example #2 to 6? In the one car that I am using mv, I left my initial at 20. Hooked to manifold, the can adds 10. So it starts at 20 with no starter kickback when hot. Idles at 30. This is where mv shines, that is if your motor likes it. Every motor responds differently to the 2 manifold port options imo.

 

[Kern Dog]

In my red car, I have a pretty big cam....probably bigger than I need. It idles pretty good though at 17 degrees initial and 14 degrees of advance. It has no bad manners using ported vacuum. I actually got 15.1 mpg on a road trip with a smaller cam.

 

[Geoff 2]

Kern dog,
Post #133. 40+ yrs of doing something proves you have 40 yrs of experience at it. And apparently you have never heard of 'colloquial expressions'.....such as driven off the showroom floor....

This is where 40 yrs of experience allows me to answer your question that using MVA to 'crank up the timing' doesn't make sense. It doesn't make sense to you because you do not understand what is happening at idle/low speeds.
Think about this: stock engine idles smooth at 500 rpm; increase idle rpm to 800 rpm & it still idles smooth, right? Put a long duration cam in the engine; engine now will NOT idle lower than 800 rpm & idle is rough, not smooth. What happened? It is all tied together....
At idle & cruise, the engine is throttled, ie, the air entering the engine is less dense; greater distance between molecules. Air takes longer to burn [ it is the air combusting that creates the expansion to push on the piston; [ the petrol is merely the catalyst to get the fire going ]. The longer burn time requires more timing to produce optimal hp. There is also residual exh in the chamber that slows down the burn time. With longer duration cams, there is more overlap & a greater amount of exh gas stays in the chamber to contaminate the fresh incoming charge. Result is uneven, rough combustion....& rough idle. The contaminated mixture also takes longer to optimally burn, so hence the requirement of more timing at idle & cruise.

Under heavy load, the air is more dense, mixture burns quicker, so less timing needed. This is why [a] a lot of timing at idle is needed but it needs to drop off under load.

Bob Bolles got it right when he said the only reason the engine needs low timing is so that it will crank easy, see below...

 

[Kern Dog]

I do understand what happens at idle, I do understand the need for more initial timing with more aggressive cam timing.

"Colloquial expressions" ? You mean bullshit lies to describe something?
I look at matters in literal sense. If someone says new off the showroom floor when they bought the car and drove it from the lot, that isn't true. It is a lie. It doesn't impress me.
The owner of FBO ignitions was a huge proponent of manifold vacuum but came across as a zealot and fanatic when asked to elaborate. I had an email exchange with him 10 years ago and he was curt and rude. There are numerous ways to get your point across without looking like an asshole.
He wasn't the only one with this problem. I don't need to be coddled or have my *** kissed but I sure don't need to be insulted. I don't need a condescending tone either.

 

[Geoff 2]

66 Sat,
Post #140. If you are happy with your car idling at 12*, all well & good. Others, like me, want optimum idle quality & tip in response.

 

[Geoff 2]

And this.....
Improved throttle response with more idle timing...

 

[66 Sat]

Geoff, I used a magnifying glass to read what you posted (and what someone else wrote).

You still haven't answered the questions I asked above.

1. Is the vacuum advance identical in MV and PV once the throttle is part-opened?
2. If so, how does this help tip in throttle response?
3. The article on improved throttle response doesn't mention MV or PV. They recurved the distributor with lighter weights and weaker springs (from what I can read) for total timing to come in by 2,000rpm. It's also all about tweaking the Holley carb (you favorite).

Thanks

 

[Geoff 2]

Kern Dog,
You obviously do NOT understand what happens at idle; read the last sentence of YOUR 133 post.

There was nothing condescending in my tone; I did not appreciate the belittling remark, purposely aimed at me, that somebody who has 40+ years of experience 'means nothing'.
And maybe you need to be a little less 'literal' because you are the only person not to get the 'driven off the showroom floor' remark...

 

[66 Sat]

I did drive my new car "off the showroom floor".
It was 2005 and I bought a brand new Ford Falcon XR6. The dealer in Brisbane had it on the main display when I went to collect it and they opened these big glass doors and I drove it right outta there. They said they did it for all the new cars. I wish I had a photo but it was before smartphones and I didn't have my camera.

 

[Kern Dog]

That is great. Here in America, people use that phrase all too often. It is similar to guys that say "Posi" when referring to Sure Grip differentials. My Dad sold cars from 1968 to 1995. I sold them for just one year. If some customer ever actually did drive the car out of the building, it was not standard practice. People say things to make a story sound better. The chick was a 10, the casino asked me to leave since I was on a winning streak, the fish I caught was THIS big, etc...
Regarding 40 years of experience....Sure, you can have 40 years, 3 years, 50 years and your skill level can still be all over the map. Stating your many years in a trade is done to impress others or to add credence to your opinions.

My hesitation to jump on the Manifold vacuum bandwagon is based on thinking of the radical timing change that would occur once the throttle is suddenly opened from a dead stop. I have not tried it to prove my suspicions, I'm just reluctant to accept it.
I started this thread 7 months ago just out of curiosity and to see what others think.
I may try this just to see for myself.

 

[Geoff 2]

66 Sat,
Sorry about the poor print quality. Scanned at 400 dpi; went to 1200 on my printer & it was hardly any better.

Happy to answer your questions, in order post #147:
1. MVA & PVA. Yes identical once the t/blade is pulling in PVA at the PVA port.
2. Tip in response improvement. It simply does with MVA & I have cured many tip in flat spots just by using MVA to increase timing at idle & just off idle. We know that extra timing at idle, whether it be from MVA, locked dist etc, the engine is making more hp [ because the rpm goes up ]. At the instant the throttle is opened slightly to take the increased load, the hp is already 'there' to take the load. It does not need to be generated.
3. They went from 15* to 26* at idle. It might not say they used MVA to get the 26*, but MVA could be used to get it. The point is that whether the 26* came from MVA or locked dist, there was 26* there which you cannot get at idle with PVA.
It is just another example of the benefit of more IDLE timing. The engine doesn't care how it gets the idle timing, as long as it gets it...MVA is just one method, but it has the added of advantage of being load sensitive & drops way under load....which is what is needed.

Hope that answers your questions.

 

[66 Sat]

Thanks Geoff.
I'm not disagreeing on anything, just trying to understand it and learn a bit. I pulled my distributor a few weeks back when I was repainting the engine. I thought I'd put it back in the same place. I started the car cold to take it out to warm it up so I could set the timing properly. It started up like a new car, and felt like it was already warm, just drove off up the road instead of having to nurse it for a few minutes. I knew I had more initial advance from the way it was driving and it started pinging like crazy on my short drive home (I drove him gently in 2nd to avoid it). I had about 30 degrees of initial. It got me thinking, how to get this feeling at start up, but really limit the vacuum advance. I looked into the Progression Distributor - might be something for the future.
My car's going to a local specialist next week who races his classic Mustang and is well respected for his carburetor and distributor work. He has a chassis dyno and is going to tune my car properly instead of my backyard efforts. I'm going to ask him about switching to MV and see what he thinks.

 

[Geoff 2]

Sat,
Unfortunately,
This [ MVA & benefits ] is a most misunderstood topic. I have a mechanic friend, 50 yrs in the business & he doesn't get it or want to try it.

I am assuming the 30* of initial was NOT from MVA, just where the dizzy ended up after re-installing? Then I would expect pinging because you probably had 30* + some of the centri curve coming in.
This the beauty of MVA. You can keep, say 12*, init & add 18* with MVA. You will still have 30* for idle/tip in. I am in the midst of fine tuning a 440 for a bloke. The spring in the adj VA unit of the Summit Chrys dist was too strong, even on the softest setting, to fully deploy at idle [ biggish cam ]. I swapped a softer spring from an adj HEI VA unit I had & it works perfectly now.
The worst thing you can do is use a VA unit whose spring is too stiff to work with reduced vac, such as you get with big cams. The timing will be erratic. This is probably the #1 cause of the comment 'MVA didn't work for me'. Not the MVA at all, but how it was set up.
It does take time & experimentation to get this 'right', but the result is worth it.
If one is not prepared to do this, then MVA may not be for you.

 

[66 Sat]

Geoff, I want to try the Manifold Vacuum myself before I take my car to the tuner.
I'm currently running 14° initial, 36° all in, not sure how much the vac advance adds.
Aside from plugging the vacuum line to the manifold port what else do I need to do for this trial?

 

[Photon440]

I haven't seen anyone drive a car off the showroom floor either, but I do know quite a few people who said that they bought the car off the showroom floor, which is in line with what the car dealer was hoping would happen.

 

[Geoff 2]

Photon,
As I said earlier, a colloquial expression, tongue in cheek.

 

[Geoff 2]

66 Sat.
Sent you a PM with my ph #.

 

[66 Sat]

Well I have tried Manifold Vacuum instead of Ported and I'm a believer! All I did was move the vacuum line to the lower port and took it for a drive.



It definitely had better throttle response off idle and once warm I had to back off the idle speed screw significantly (maybe 1.5 turns) - shows how much difference it makes.

Only problem now is I have part-throttle pinging - even though in theory the vacuum drops once the throttle is opened, in practice (and without any other adjustments) it's adding more vacuum advance at light throttle than with Ported. I don't think my vacuum canister is adjustable so i may have to go back to Ported until next week when I take it to the local guru.

 

[Geoff 2]

Happy days!

 

[HawkRod]

Having manifold vacuum artificially crank up the timing, then a press on the accelerator pulls out 90% of it doesn't seem to make sense.

Example #1:
...

Example #2:
...
How could this NOT cause a stumble or hiccup in performance? You go from 26 degrees of timing to some lesser and unknown amount.

I have not tried it to prove my suspicions, I'm just reluctant to accept it.

Kern,

I think @Geoff 2 has explained it pretty well, but I'll try it as well to see if it hits home. Maybe a slightly different way will help reinforce what is going on.

You need to get out of your head that more timing is good and less timing is bad. For optimum power, an engine needs the right amount of timing - not too much and not too little. There are two main things that need to change timing:
1) As engine speed goes up you need to ignite the mixture sooner so the main force of the flame wave hits just beyond TDC
2) As the air/ fuel mixture goes down (lean), you need to ignite the mixture sooner as well.
The two forces above are really independent. More RPM needs more timing, and lean mixture needs more timing. But remember the reverse is also true, so stating the obvious: rich mixtures need less timing.

The big question you seem to have is that when you mash the throttle with MV that the timing drops: How could that not screw up performance???

At idle, the mixture is lean and the air/fuel mixture burns slow. An engine likes a lot of timing to idle smoothly. With MV, if a car has 12* static/ initial and 18* Vacuum Advance, it will be happy idling at 30* timing. Now you mash the throttle. Yes, timing will drop from 30* to 12*. But remember that a lot of other things are happening too. The carb accelerator pump is dumping lots of raw fuel into the incoming air, so all of the sudden your air/fuel mixture has changed dramatically - it will now burn quicker. Rich mixtures need less timing. If you still had 30* of timing in it you car likely would have detonation. Because the air/ fuel mixture has changed, your timing needs to change with it.

Now as the engine accelerates and has a higher RPM, it starts to need more timing. So now your centrifugal advance adds timing in based on engine speed, but vacuum advance is doing nothing. Now you reach cruising speed and you reduce throttle to maintain speed. You air/ fuel mixture goes lean so again, more timing is needed even on top of what centrifugal advance has added.

Not sure if this helps - hopefully it does...