increased vacuum with increased ig timing

General engine tech -- Drag Racing to Circle Track

Moderator: Team

Belgian1979
Guru
Guru
Posts: 4576
Joined: Sat Mar 26, 2011 11:34 am
Location: Belgium - Koersel

increased vacuum with increased ig timing

Post by Belgian1979 »

Can someone explain to me what causes the engine to pull more vacuum when ignition timing is increased ?

Thanks
naukkis79
Pro
Pro
Posts: 383
Joined: Tue Jul 29, 2014 5:14 am
Location:

Re: increased vacuum with increased ig timing

Post by naukkis79 »

Engine gets more power from air it's getting so it will rev higher at same throttle opening.
User avatar
John Wallace
Guru
Guru
Posts: 1511
Joined: Tue Jan 04, 2005 10:20 am
Location: was Central Illinois - Now in Sunny Florida!
Contact:

Re: increased vacuum with increased ig timing

Post by John Wallace »

Put the engine idle rpm back to what it was with previously.
Is the vacuum still higher?

:?:
John Wallace
Pontiac Power RULES !
www.wallaceracing.com
GARY C
HotPass
HotPass
Posts: 6301
Joined: Tue May 14, 2013 10:58 pm
Location:

Re: increased vacuum with increased ig timing

Post by GARY C »

Belgian1979 wrote: Sun Dec 10, 2017 8:47 am Can someone explain to me what causes the engine to pull more vacuum when ignition timing is increased ?

Thanks
At low rpm you have very little air for optimum fuel burn so if you ignite it sooner then you get a more complete burn so your intake system functions as it should...In that the chamber is clear to draw in the next intake charge at low rpm.
Please Note!
THE ABOVE POST IN NO WAY REFLECTS THE VIEWS OF SPEED TALK OR IT'S MEMBERS AND SHOULD BE VIEWED AS ENTERTAINMENT ONLY...Thanks, The Management!
GARY C
HotPass
HotPass
Posts: 6301
Joined: Tue May 14, 2013 10:58 pm
Location:

Re: increased vacuum with increased ig timing

Post by GARY C »

John Wallace wrote: Sun Dec 10, 2017 12:46 pm Put the engine idle rpm back to what it was with previously.
Is the vacuum still higher?

:?:
Yes, until you reach it's limit.
Please Note!
THE ABOVE POST IN NO WAY REFLECTS THE VIEWS OF SPEED TALK OR IT'S MEMBERS AND SHOULD BE VIEWED AS ENTERTAINMENT ONLY...Thanks, The Management!
PackardV8
Guru
Guru
Posts: 7619
Joined: Sun Jul 30, 2006 2:03 pm
Location: Spokane, WA

Re: increased vacuum with increased ig timing

Post by PackardV8 »

GARY C wrote: Sun Dec 10, 2017 12:52 pm
John Wallace wrote: Sun Dec 10, 2017 12:46 pm Put the engine idle rpm back to what it was with previously.
Is the vacuum still higher? :?:
Yes, until you reach it's limit.
And at that advance limit, with some engines, idle is all it's good for. The timing will be so far advanced, in a vehicle with any weight and/or tall gears, the engine will ping trying to pull off idle. It takes a good computer controlled EFI/ignition/knock sensor to use max advance at idle and cruise and then immediately recognize when more power is signaled and pull back the advance. Distributors and carbs aren't usually able to handle that reliably.
Jack Vines
Studebaker-Packard V8 Limited
Obsolete Engineering
GARY C
HotPass
HotPass
Posts: 6301
Joined: Tue May 14, 2013 10:58 pm
Location:

Re: increased vacuum with increased ig timing

Post by GARY C »

PackardV8 wrote: Sun Dec 10, 2017 12:58 pm
GARY C wrote: Sun Dec 10, 2017 12:52 pm
John Wallace wrote: Sun Dec 10, 2017 12:46 pm Put the engine idle rpm back to what it was with previously.
Is the vacuum still higher? :?:
Yes, until you reach it's limit.
And at that advance limit, with some engines, idle is all it's good for. The timing will be so far advanced, in a vehicle with any weight and/or tall gears, the engine will ping trying to pull off idle. It takes a good computer controlled EFI/ignition/knock sensor to use max advance at idle and cruise and then immediately recognize when more power is signaled and pull back the advance. Distributors and carbs aren't usually able to handle that reliably.
I disagree and have done it with carb, HEI and adjustable vac advance (actually becomes a vac retard at load when hooked to full vac port), you end up with a curve that matches that of say a modern LS engine.
Please Note!
THE ABOVE POST IN NO WAY REFLECTS THE VIEWS OF SPEED TALK OR IT'S MEMBERS AND SHOULD BE VIEWED AS ENTERTAINMENT ONLY...Thanks, The Management!
Dan Timberlake
Guru
Guru
Posts: 1739
Joined: Tue Jan 15, 2013 8:10 pm
Location:

Re: increased vacuum with increased ig timing

Post by Dan Timberlake »

"actually becomes a vac retard at load when hooked to full vac port."

Once the throttle plates are open a little bit, I'd figure the vac advance function is the same whether ported or manifold. More load = more open thottle = lower vacuum = less advance.

Something like this,
https://assets.hemmings.com/story_image ... .jpg?rev=2

Here is a link to some presumably stock Buick distributor curve, showing limits of cent plus vac advance under varying rpm and load. The lack of vac advance below 1000 rpm suggest ported vacuum is used.
The mpg plotted along with rpm suggests chugging around at pretty low speeds in high gear was an expected operating condition for a 1950 Buick.
GARY C
HotPass
HotPass
Posts: 6301
Joined: Tue May 14, 2013 10:58 pm
Location:

Re: increased vacuum with increased ig timing

Post by GARY C »

Dan Timberlake wrote: Sun Dec 10, 2017 1:45 pm "actually becomes a vac retard at load when hooked to full vac port."

Once the throttle plates are open a little bit, I'd figure the vac advance function is the same whether ported or manifold. More load = more open thottle = lower vacuum = less advance.

Something like this,
https://assets.hemmings.com/story_image ... .jpg?rev=2

Here is a link to some presumably stock Buick distributor curve, showing limits of cent plus vac advance under varying rpm and load. The lack of vac advance below 1000 rpm suggest ported vacuum is used.
The mpg plotted along with rpm suggests chugging around at pretty low speeds in high gear was an expected operating condition for a 1950 Buick.
Hook up two vac gauges on your engine, one to ported and one to full and see what happens?
If you plot the timing curve that can be created in the manner I presented vs the timing curve on a modern FI engine you will see the similarity.
It does require the correct spring and weight package along with an adjustable vac port and some actual work to get them in sink.
Please Note!
THE ABOVE POST IN NO WAY REFLECTS THE VIEWS OF SPEED TALK OR IT'S MEMBERS AND SHOULD BE VIEWED AS ENTERTAINMENT ONLY...Thanks, The Management!
Belgian1979
Guru
Guru
Posts: 4576
Joined: Sat Mar 26, 2011 11:34 am
Location: Belgium - Koersel

Re: increased vacuum with increased ig timing

Post by Belgian1979 »

I maybe should explain a little more. I have tried several settings on my engine. It's an EFI engine equiped with maf sensors, ignition and fuel controlled by the ECU (fuel is ego controlled) and the rpm at idle is controlled by a closed loop IAC.

What I see is that when using more timing, vacuum goes up, while airflow through the mafs goes down. Power is up by the feel of it. In essence I would not have expected this effect. I was guessing at the reasons, but I was thinking that exhaust speed was up due to more pressure left in the cylinder when the exhaust valve open (in itself due to a better more complete burn and higher power stroke pressure) and this would scavenge the cyls better.
GARY C
HotPass
HotPass
Posts: 6301
Joined: Tue May 14, 2013 10:58 pm
Location:

Re: increased vacuum with increased ig timing

Post by GARY C »

Belgian1979 wrote: Sun Dec 10, 2017 3:06 pm I maybe should explain a little more. I have tried several settings on my engine. It's an EFI engine equiped with maf sensors, ignition and fuel controlled by the ECU (fuel is ego controlled) and the rpm at idle is controlled by a closed loop IAC.

What I see is that when using more timing, vacuum goes up, while airflow through the mafs goes down. Power is up by the feel of it. In essence I would not have expected this effect. I was guessing at the reasons, but I was thinking that exhaust speed was up due to more pressure left in the cylinder when the exhaust valve open (in itself due to a better more complete burn and higher power stroke pressure) and this would scavenge the cyls better.
All of the above, you see the same on a good ignition tune on a factory EFI, they are tuned for economy, emissions and smoothness not performance, a big part of what creates a performance tune is the timing curve to burn the fuel.
Please Note!
THE ABOVE POST IN NO WAY REFLECTS THE VIEWS OF SPEED TALK OR IT'S MEMBERS AND SHOULD BE VIEWED AS ENTERTAINMENT ONLY...Thanks, The Management!
user-23911

Re: increased vacuum with increased ig timing

Post by user-23911 »

Belgian1979 wrote: Sun Dec 10, 2017 8:47 am Can someone explain to me what causes the engine to pull more vacuum when ignition timing is increased ?

Thanks

You're improving the efficiency going from insufficient timing to the correct timing.
Better efficiency means more work out and less heat.
That means more RPM from the same fuel which then means you can close the throttle to maintain original RPM from less throttle opening.
Belgian1979
Guru
Guru
Posts: 4576
Joined: Sat Mar 26, 2011 11:34 am
Location: Belgium - Koersel

Re: increased vacuum with increased ig timing

Post by Belgian1979 »

OK, and how do you know when you've gone too far or are at a correct timing ? I mean, if vacuum keeps dropping this hardly looks like the factor to determine the correct ignition timing.
GARY C
HotPass
HotPass
Posts: 6301
Joined: Tue May 14, 2013 10:58 pm
Location:

Re: increased vacuum with increased ig timing

Post by GARY C »

Belgian1979 wrote: Sun Dec 10, 2017 3:45 pm OK, and how do you know when you've gone too far or are at a correct timing ? I mean, if vacuum keeps dropping this hardly looks like the factor to determine the correct ignition timing.
Dropping? you OP said increasing?
Can someone explain to me what causes the engine to pull more vacuum when ignition timing is increased ?
Please Note!
THE ABOVE POST IN NO WAY REFLECTS THE VIEWS OF SPEED TALK OR IT'S MEMBERS AND SHOULD BE VIEWED AS ENTERTAINMENT ONLY...Thanks, The Management!
Belgian1979
Guru
Guru
Posts: 4576
Joined: Sat Mar 26, 2011 11:34 am
Location: Belgium - Koersel

Re: increased vacuum with increased ig timing

Post by Belgian1979 »

GARY C wrote: Sun Dec 10, 2017 3:49 pm
Belgian1979 wrote: Sun Dec 10, 2017 3:45 pm OK, and how do you know when you've gone too far or are at a correct timing ? I mean, if vacuum keeps dropping this hardly looks like the factor to determine the correct ignition timing.
Dropping? you OP said increasing?
Can someone explain to me what causes the engine to pull more vacuum when ignition timing is increased ?
Sorry, map drops (expressed in kPa) but this means that vacuum increases...my fault.
Post Reply