Vacuum Advance or not?
September 4th, 2018, 01:32 PM
#1
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Vacuum Advance or not?
Hi All, I’m still very confused about setting total timing with or without vacuum advance. After reading as Many of the threads as I could, I decided to set total timing at 36 with vacuum advance disconnected, then reconnect the vacuum advance and call it good. I have a 455 with a Delco HEI and a newly installed Edelbrock 750 cfm (#1411). I’m still not happy about the performance of the motor and want to rule out the vacuum advance as being an issue. I do see a lot of folks say they don’t run vacuum advance so now I’m thinking it is optional or at least needs to be factored in when dialing in the timing. Any suggestions?
September 4th, 2018, 03:46 PM
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Vacuum Advance
performance issues aside, I want to make sure I understand the importance of vacuum advance. Seems like a lot of folks don’t run vacuum advance and maybe there’s a good reason for it. I set total timing at 36 degrees with vacuum advance disconnected and both vacuum ports on carburetor plugged. Idles and runs very smoothly but wot power is severely lacking so searching for answers.
September 4th, 2018, 03:49 PM
#5
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First of all, vacuum advance only operates when there is manifold vacuum. At wide open throttle, there is essentially no manifold vacuum and thus no vacuum advance. The whole point of vacuum advance is to provide better response, performance, and economy at small throttle openings like part throttle cruise. If you have a performance issue at wide open throttle it cannot be related to vacuum advance. As for the "lot of folks" who don't run vacuum advance, they are either clueless or are running extremely radical cams that don't produce enough vacuum at idle. In the latter case, part throttle cruise is irrelevant - it will suck anyway with a cam like that.
September 4th, 2018, 04:00 PM
#6
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Here is a good read
http://www.crankshaftcoalition.com/w...ance_Specs.pdf
http://www.crankshaftcoalition.com/w...ance_Specs.pdf
September 4th, 2018, 04:41 PM
#7
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Here is a good read
http://www.crankshaftcoalition.com/w...ance_Specs.pdf
http://www.crankshaftcoalition.com/w...ance_Specs.pdf
September 4th, 2018, 04:47 PM
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September 4th, 2018, 05:07 PM
#9
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I’ll look into a wide band sensor for sure. I bought the car at a much lower elevation and when I got back to Colorado Springs it ran terribly with the QJet. Since it needed a lot of attention, and the fuel connection was stripped when I removed it, I hit the easy button and bought a new E’brock with the correct jets/rods. Even though it runs great, I feel the wot power is not there. I know I give up about 30% at 7500 feet elevation but geez it is slow.
September 4th, 2018, 05:52 PM
#11
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Mechanical weights play a role, check this out.https://www.speedwaymotors.com/GM-HE...g-Kit,740.html
September 4th, 2018, 05:54 PM
#12
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The need for part throttle vacuum advance was explained to me like this; at part throttle, the throttle plates aren’t open far enough for a complete fill of the cylinder. To ensure best combustion, the spark needs to begin sooner to allow complete flame front across the entire cylinder burn. To quotaJoe, the people who say don’t run it are either to stubborn, ignorant, or arrogant to want to learn how to make it work best for a certain application
September 4th, 2018, 06:49 PM
#13
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Vacuum Advance
Throttle opens almost all the way, could use a little adjustment to get the last little bit. Engine revs beautifully with no load but driving it seems like there is very little to no difference between 70% throttle and wot. I will adjust the linkage and recheck.
September 4th, 2018, 08:06 PM
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September 4th, 2018, 08:22 PM
#15
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September 6th, 2018, 09:07 PM
#16
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I live here right next to Bandimere Drag Strip in Lakewood/Morrison area of Colorado. There are two things we are fighting here; one, the raw elevation calculation for leaning out the fuel curve with the recommended rods and jets; two, the Density Altitude can change here where 6,000 feet raw elevation can turn into 8,500+ Density Altitude.
I have a handheld Innovate Wideband that I can sell to you for cheap or I can loan it to you as well.
As Cutlassefi said, you won't know what's really going on until you check it with a wideband. Rods and Jets may not be the only issue, there is also the fuel enrichment (step-up) springs which need to be calibrated to your engine and air/fuel ratio(step-up springs basically act like a Holley power valve by lifting the fuel rods out of the jets when the engine needs enrichment under lower vacuum/ drop and increased load).
Your fuel curve may be way off, but you won't know until you get a wideband on it.
I would also check your mechanical timing curve to see how fast (or slow) your mechanical advance is coming in. Using a dial back timing light is a good way to check this at different RPM's.
Before you do any more timing adjustments, make sure you verify that your ZERO on your balancer and pointer is at true Top dead center on #1.
I have a handheld Innovate Wideband that I can sell to you for cheap or I can loan it to you as well.
As Cutlassefi said, you won't know what's really going on until you check it with a wideband. Rods and Jets may not be the only issue, there is also the fuel enrichment (step-up) springs which need to be calibrated to your engine and air/fuel ratio(step-up springs basically act like a Holley power valve by lifting the fuel rods out of the jets when the engine needs enrichment under lower vacuum/ drop and increased load).
Your fuel curve may be way off, but you won't know until you get a wideband on it.
I would also check your mechanical timing curve to see how fast (or slow) your mechanical advance is coming in. Using a dial back timing light is a good way to check this at different RPM's.
Before you do any more timing adjustments, make sure you verify that your ZERO on your balancer and pointer is at true Top dead center on #1.
September 6th, 2018, 10:08 PM
#17
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@Battenrunner thanks for the info. I am mechanically inclined and do most of my own work but have to admit I am getting intimidated the more I learn about carburetors. Not sure I would know how to effectively use a wide band sensor if I had one. Do you need to install an O2 sensor to use it? Also, I can see where the mechanical timing is coming on but I honestly don’t know what it should look like other than about 36 Degrees all in.
September 6th, 2018, 10:58 PM
#18
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I would be happy to stop by sometime when on my way to work or coming back, as I go to Colorado Springs about once a week on average.
To measure air/fuel ratio, you either need an O2 bung welded into your exhaust pipe roughly at the header collector or welded in the same area on a manifold down pipe. These bungs are affordable and obtainable via the internet or possibly a local speed shop. Ideally, you want the bung mounted above horizontal for exhaust moisture exposure reasons. The wideband gauge/controller unit then runs the wideband sensor in the exhaust pipe, which in turn shows the air/fuel on a gauge/screen.
As for an ideal fuel curve, you would want about 14:1 at idle, 12-12.5:1 upon acceleration, and around 12.5-13:1 at wide open throttle. Running between 14.4:1 and 15:1 at low throttle cruise.
I would make a simple rpm vs. timing advance table and start at idle, all with the vacuum advance can unhooked and the vacuum source plugged. If your mechanical advance curve ( the rate of advance) is very slow ( higher rpm) to arrive at 36 degrees, that will also badly hinder performance, even though the total timing is correct.
For Example:
Rpm Observed advance
700 rpm - 8 degrees btdc
1000 rpm - 12 degrees btdc
1500 rpm - 22 degrees btdc
2000 rpm - 28 degrees btdc
2500 rpm - 32 degrees btdc
3000 rpm - 36 degrees btdc
3500 rpm - 36 degrees btdc
4000 rpm - 36 degrees btdc
4500 rpm - 36 degrees btdc
also, listen and feel for missing or weak fuel system, another thing that the wideband will show. You may have the right tune on the carb, and not enough fuel pump to feed it and it lays down and goes way lean.
what converter stall speed do you have and what gears?
To measure air/fuel ratio, you either need an O2 bung welded into your exhaust pipe roughly at the header collector or welded in the same area on a manifold down pipe. These bungs are affordable and obtainable via the internet or possibly a local speed shop. Ideally, you want the bung mounted above horizontal for exhaust moisture exposure reasons. The wideband gauge/controller unit then runs the wideband sensor in the exhaust pipe, which in turn shows the air/fuel on a gauge/screen.
As for an ideal fuel curve, you would want about 14:1 at idle, 12-12.5:1 upon acceleration, and around 12.5-13:1 at wide open throttle. Running between 14.4:1 and 15:1 at low throttle cruise.
I would make a simple rpm vs. timing advance table and start at idle, all with the vacuum advance can unhooked and the vacuum source plugged. If your mechanical advance curve ( the rate of advance) is very slow ( higher rpm) to arrive at 36 degrees, that will also badly hinder performance, even though the total timing is correct.
For Example:
Rpm Observed advance
700 rpm - 8 degrees btdc
1000 rpm - 12 degrees btdc
1500 rpm - 22 degrees btdc
2000 rpm - 28 degrees btdc
2500 rpm - 32 degrees btdc
3000 rpm - 36 degrees btdc
3500 rpm - 36 degrees btdc
4000 rpm - 36 degrees btdc
4500 rpm - 36 degrees btdc
also, listen and feel for missing or weak fuel system, another thing that the wideband will show. You may have the right tune on the carb, and not enough fuel pump to feed it and it lays down and goes way lean.
what converter stall speed do you have and what gears?
Last edited by Battenrunner; September 6th, 2018 at 11:08 PM.
September 7th, 2018, 07:33 AM
#19
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Battenrunner that is a generous offer and I may have to take you up on it. I would need to get an O2 bung installed first but that shouldn't be a big deal. I am sending PM so you have my contact info. Archery season is open so I've got to make time for both!
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