| [GMCnet] TIMING AND VACUUM ADVANCE 101 [message #255485] |
Tue, 15 July 2014 18:11  |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
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Someone, whom I can't now recall, posted the link below for the subject
article. I highly recommend it.:
http://www.camaros.net/forums/showthread.php?t=150344
It's certain to generate some interesting controversy since it makes a
flat statement, with good rationale, for using ONLY manifold vacuum, NOT
ported, for timing advance. What it does not do is explain in detail the
changes in centrifugal advance that should be made to compensate for
changing the GMCMH from ported to manifold vacuum. But it does explain why
some changes may be necessary.
Ken H.
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Ken Henderson
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www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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| Re: [GMCnet] TIMING AND VACUUM ADVANCE 101 [message #255487 is a reply to message #255485] |
Tue, 15 July 2014 18:31  |
bwevers
Messages: 597
Registered: October 2010
Location: San Jose
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I wonder how they can control the idle speed without a ported vacuum?
What happens when the A/C is turned on, the vacuum drops and the timing advance is lost?
Maybe they're working with radical camshaft timing and no A/C.
Regards,
Bill
Bill Wevers GMC49ers, GMC Western States
1975 Glenbrook - Manny Powerdrive, OneTon
455 F Block, G heads
San Jose
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| Re: [GMCnet] TIMING AND VACUUM ADVANCE 101 [message #255488 is a reply to message #255485] |
Tue, 15 July 2014 18:37 |
bwevers
Messages: 597
Registered: October 2010
Location: San Jose
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He also makes this statement:
"At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in..."
As I recall the idle mixture is very rich, not lean at all.
Bill Wevers GMC49ers, GMC Western States
1975 Glenbrook - Manny Powerdrive, OneTon
455 F Block, G heads
San Jose
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| Re: [GMCnet] TIMING AND VACUUM ADVANCE 101 [message #255491 is a reply to message #255485] |
Tue, 15 July 2014 18:58 |
Bullitthead
Messages: 1411
Registered: November 2013
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Ken,
I've used manifold vacuum for the advance in all my vehicles since I figured out the operation when I got my first car. Never figured out why some insisted on ported vacuum, but they were all impressed with the increase in takeoff torque when we hooked it up that way. It does require a different tuning setup, but it has always given me more power and better mileage. Nice to see that someone agrees with me.
Terry Kelpien
ASE Master Technician
73 Glacier 260
Smithfield, Va.
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| Re: [GMCnet] TIMING AND VACUUM ADVANCE 101 [message #255496 is a reply to message #255485] |
Tue, 15 July 2014 19:22 |
bwevers
Messages: 597
Registered: October 2010
Location: San Jose
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Many years ago I did some experimentation on my Ford with 289 and ported vs manifold vacuum advance.
I noticed that the tailpipe emissions on our garage tester were much higher without the ported advance.
And the engine would not idle properly at a consistent RPM.
I figured that Ford must have designed it that way for a reason....
Bill Wevers GMC49ers, GMC Western States
1975 Glenbrook - Manny Powerdrive, OneTon
455 F Block, G heads
San Jose
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| Re: [GMCnet] TIMING AND VACUUM ADVANCE 101 [message #255497 is a reply to message #255487] |
Tue, 15 July 2014 19:22 |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
Karma: 1
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bwevers wrote on Tue, 15 July 2014 17:31I wonder how they can control the idle speed without a ported vacuum?
What happens when the A/C is turned on, the vacuum drops and the timing advance is lost?
Maybe they're working with radical camshaft timing and no A/C.
Regards,
Bill
I have run manifold vacuum with no negative issues whatsoever for at least 10 years now. Only positive results.
Bob de Kruyff
78 Eleganza
Chandler, AZ
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| Re: [GMCnet] TIMING AND VACUUM ADVANCE 101 [message #255498 is a reply to message #255488] |
Tue, 15 July 2014 19:24 |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
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bwevers wrote on Tue, 15 July 2014 17:37He also makes this statement:
"At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in..."
As I recall the idle mixture is very rich, not lean at all.
It is very lean unless you have opened up the idle screws as almost everyone has by now.
Bob de Kruyff
78 Eleganza
Chandler, AZ
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| Re: [GMCnet] TIMING AND VACUUM ADVANCE 101 [message #255499 is a reply to message #255485] |
Tue, 15 July 2014 19:29 |
bwevers
Messages: 597
Registered: October 2010
Location: San Jose
Karma: 5
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My 455 vacuum advance adds 15 degrees to my timing.
The idle RPM increases a lot with this.
Then when the A/C clutch engages the RPM drops and vacuum drops, idle rpm too low.
Ported vacuum helps this situation.
Bill Wevers GMC49ers, GMC Western States
1975 Glenbrook - Manny Powerdrive, OneTon
455 F Block, G heads
San Jose
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| Re: [GMCnet] TIMING AND VACUUM ADVANCE 101 [message #255502 is a reply to message #255485] |
Tue, 15 July 2014 19:32 |
bwevers
Messages: 597
Registered: October 2010
Location: San Jose
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Funny how A/C and idle quality is more important to me than when I was younger.
Bill Wevers GMC49ers, GMC Western States
1975 Glenbrook - Manny Powerdrive, OneTon
455 F Block, G heads
San Jose
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| Re: [GMCnet] TIMING AND VACUUM ADVANCE 101 [message #255503 is a reply to message #255499] |
Tue, 15 July 2014 19:36 |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
Karma: 1
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bwevers wrote on Tue, 15 July 2014 18:29My 455 vacuum advance adds 15 degrees to my timing.
The idle RPM increases a lot with this.
Then when the A/C clutch engages the RPM drops and vacuum drops, idle rpm too low.
Ported vacuum helps this situation.
The only way I can tell when my AC engages is by the click and the noise of the compressor.
Bob de Kruyff
78 Eleganza
Chandler, AZ
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| Re: [GMCnet] TIMING AND VACUUM ADVANCE 101 [message #255514 is a reply to message #255485] |
Tue, 15 July 2014 21:04 |
kwharland
Messages: 246
Registered: November 2005
Location: Central Florida
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My GMC is presetly my only vehicle that started out as ported and changing it over was one of the first things I did. If you follow accepted guidelines for selecting correct vacuum cans, you'll never have a problem when your A/C engages.
1978 Eleganza II
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| Re: [GMCnet] TIMING AND VACUUM ADVANCE 101 [message #255525 is a reply to message #255508] |
Tue, 15 July 2014 22:54 |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
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Matt Colie wrote on Tue, 15 July 2014 19:11Bob de Kruyff wrote on Tue, 15 July 2014 20:22I have run manifold vacuum with no negative issues whatsoever for at least 10 years now. Only positive results.
Bob,
Are you running the stock vacuum pot?
Where are you setting the base timing (Vacuum plugged off)?
I have done all kinds of tuning on dyno engines, but never been concerned with other than WOT.
Matt
Everything stock and 14 degrees base timing on a 403. I think the 403's have a higher base timing than 455's.
Bob de Kruyff
78 Eleganza
Chandler, AZ
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| Re: [GMCnet] TIMING AND VACUUM ADVANCE 101 [message #255526 is a reply to message #255509] |
Tue, 15 July 2014 22:56 |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
Karma: 1
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Bullitthead wrote on Tue, 15 July 2014 19:12"The only way I can tell when my AC engages is by the click and the noise of the compressor."
And that is because Bob figured out the slight tuning and timing adjustments needed to use the manifold vacuum advance.
Bravo Bob!
Do you also have people asking you how your stuff runs smoothly, reliably, and gets decent gas mileage without pinging?
And then they tell you that your setup is wrong 
Oh yes !!
Bob de Kruyff
78 Eleganza
Chandler, AZ
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| Re: [GMCnet] TIMING AND VACUUM ADVANCE 101 [message #255675 is a reply to message #255612] |
Wed, 16 July 2014 22:58 |
mrgmc3
Messages: 210
Registered: September 2013
Location: W Washington
Karma: 2
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The author's premise for his discussion of vacuum advance is fatally flawed. He claims that light load mixtures are "lean" and heavy load are "rich". While this is true to a small extent in a well-dialed carb, it is untrue with modern FI (that runs at 14.7:1 except very high loads). What he means to say is the mixture is very dilute at light loads. At idle and other high vacuum points there is little fresh air and fuel. Most of the combustion chamber has spent gases from the previous cycle. As loads increase (vacuum decreases) the fresh mixture will crowd out ( or scavenge ) the burned gases, up to wide open throttle where the cylinder is nearly filled with fresh charge (100 % filled if you are lucky - greater than 100% with turbo or supercharging). With that in mind the timing is advanced at light loads because the mixture is dilute, not lean. There is a fundamental difference here that should be understood. At low loads you have very dilute cylinder fill but can have either lean or rich mixtures, as A/F only refers to the RATIO of fuel and air, not of fuel air and uncombustable gases from a prior cycle. High cylinder fill, as at high throttle openings (low vacuum) require less advance, as a chamber full of fresh fuel and air burns more quickly.
Chris Geils - Twin Cities / W Wa
1978 26' Kingsley w/ very few mods; PD9040, aux trans cooler, one repaint in stock colors, R134a, Al rad, Alcoas, 54k mi
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| Re: [GMCnet] TIMING AND VACUUM ADVANCE 101 [message #255677 is a reply to message #255612] |
Wed, 16 July 2014 23:35 |
Bullitthead
Messages: 1411
Registered: November 2013
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Matt,
I usually start off at 12 degrees and adjust it by ear from there. Put as much load as you can on the engine in top gear (WOT up a hill if you can)and listen for any pinging, and back off the timing if it does ping or increase the timing if it doesn't. Set the timing right below the ping point. Usually ends up being between 10 and 14, but have had some require it to be dropped to 8 or 9 to avoid the ping under load. Most of the time that is from carbon build up or hot spark plugs. The distributor advance springs can make a big difference in the driveability and I understand that Dick Paterson is the distributor guru for the GMCs and probably most other distributor ignition engines. A high compression ratio or a lean mixture is also going to limit the amount of timing advance that can be used without preignition, so they will require a higher octane fuel or even water/alcohol injection. None of that has anything to do with the vacuum advance source just like the article explained. It's just old school mechanical knowledge and experience.
Terry Kelpien
ASE Master Technician
73 Glacier 260
Smithfield, Va.
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