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#26 | |
Registered User
Join Date: Jan 2007
Location: Puyallup, WA
Posts: 1,662
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Re: Questions about using a timing light
Quote:
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72 C10 lwb fleetside -stock 350/350 combo Last edited by 68 P.O.S.; 07-18-2018 at 11:53 AM. |
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#27 | |
Registered User
Join Date: Jan 2003
Location: Safford,AZ
Posts: 3,612
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Re: Questions about using a timing light
Info-
Quote:
http://67-72chevytrucks.com/vboard/s...d.php?t=486908 |
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#28 |
Registered User
Join Date: Jan 2007
Location: Puyallup, WA
Posts: 1,662
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Re: Questions about using a timing light
There you go Warren, I was too lazy to search for that thread haha! Dave is the HEI master. The Crane adjustable vac can has every GM vac can setting built right in
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72 C10 lwb fleetside -stock 350/350 combo |
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#29 |
Registered User
Join Date: Aug 2006
Location: Redmond, WA
Posts: 6,332
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Re: Questions about using a timing light
Here's my theory, take it for what it's worth.
1) Vacuum advance should be ported 2) Lots of people have poorly tuned motors with vacuum leaks, etc 3) More timing crutches those symptoms 4) Running vacuum advance to manifold gives you a bunch of initial timing So yeah, plugging the vacuum advance into manifold vacuum will give you a bunch of initial timing that will make it idle better, but it's not the right way to do it. There are SOME things (like TCS - Transmission Controlled Spark) that are just nonsense emissions crap that was a bad idea even then. But vacuum advance is a big deal for driveability (and mileage if you care).
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1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible |
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#30 |
Registered User
Join Date: Jan 2007
Location: Puyallup, WA
Posts: 1,662
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Re: Questions about using a timing light
You only use ported if your engine is running an EGR valve for emissions. Straight from the GM developmental horse’s mouth: ““Ported Vacuum” was strictly an early pre-converter crude emissions strategy and nothing more. Don’t believe anyone who tells you that ported vacuum is a good thing for performance and drivability – it’s not. Anyone with a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy…”. Call me weird or call me crazy, but I'll believe the GM engineer who helped develop the system.
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72 C10 lwb fleetside -stock 350/350 combo |
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#31 | |
Senior Member
![]() Join Date: Apr 2016
Location: Cypress, TX
Posts: 3,704
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Re: Questions about using a timing light
I would like to thank everybody for your input on this thread. I think it is getting pretty interesting and I'm learning.
Quote:
I have a GM HEI distributor that a friend gave me. I don't know what vehicle it came out of. I've been looking for a part number stamp on it but don't see it. Perhaps it is inside. I will look some more. The vacuum advance can was damaged and it didn't work. Another friend shade tree mechanic put the current vacuum advance on it, and I don't know how it was selected, I just know it was new from O'Reilly Auto Parts. He set the timing, had trouble figuring out the timing light, and the engine was working OK but pinging sometimes while driving. Then I picked up from there and started this thread late last year. I am getting more involved with the mechanics of this truck. Now you guys are getting me interested in tuning. I don't have a vacuum gauge but now I'm interested in getting one. I don't know how much mechanical advance the distributor has. I don't know what type of heads the engine has. The casting number is 10147898. After reading all the great info on this forum that you guys offer up, I have recently moved my vacuum advance from ported to manifold vacuum. When I brought this thread back to life the other day I was interested in knowing the technical details about: if most total advance you can go with your timing without causing detonation is desirable, why is higher amount of vacuum advance not better than smaller amount of vacuum advance? Please tell me if I figured out the answer from input from you guys and the links provided that I have been reading. I want the maximum total advance you can get at wide open throttle and heavy loads without causing detonation. If I have a lot of vacuum advance, and adjust the initial timing as advanced as possible to avoid detonation at idle and mid-throttle, it will be the appropriate amount of total advance at idle and mid-throttle. But the total timing will drop more than I want at wide open throttle, because all of the vacuum advance goes away at wide open throttle. On my engine, since I have 23 degrees of vacuum advance, I will lose 23 degrees of advance from my total at wide open throttle. Optimum performance may be realized if I "lost" a lower number of degrees. Do that by lowering vacuum advance and increasing initial advance by about the same amount. The engine with stock parts was designed for minimum warranty period repairs, pretty much without regard for performance, and severely limiting advance under load reduces the chances of my engine having any detonation problem, regardless of quality of fuel. Reducing the amount of vacuum advance is part of hotrodding the distributor. |
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#32 |
Registered User
Join Date: Jun 2018
Location: Fuquay-Varina, NC (Just outside of Raleigh)
Posts: 237
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Re: Questions about using a timing light
As you are well aware, I am a novice too. But I am understanding it the same way I think.
It sounds like your total advance is giving you good performance at idle/cruise right now. But perhaps not during acceleration or WOT (due to so much advance loss, vac advance). Solution would be to change vac advance with different canister either adjustable or fixed that provides less advance. Then adjusting timing to achieve same total advance for idle/cruise, but less would be lost during acceleration or WOT. I'm learning too, but I think I am interpreting this the same as you are. |
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#33 | |
Registered User
Join Date: Aug 2006
Location: Redmond, WA
Posts: 6,332
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Re: Questions about using a timing light
Quote:
Obviously they had vacuum advanced for at least 10 years before EGR! What I will grant is that in later years, particularly once they started to care about idle emissions and ESPECIALLY once you have EGR and cats then they're doing all kinds of goofy things like running ZERO initial advance and no vacuum to keep the chamber temps where they need to be. But you tell me why vacuum advance is ported in 1967 then, if not for driveability. I'm sure there were people who thought PCV was evil and road draft tubes were good enough too....
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1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible Last edited by davepl; 07-19-2018 at 09:42 AM. |
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#34 | |
Registered User
Join Date: Jan 2007
Location: Puyallup, WA
Posts: 1,662
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Re: Questions about using a timing light
Quote:
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72 C10 lwb fleetside -stock 350/350 combo Last edited by 68 P.O.S.; 07-21-2018 at 04:44 PM. |
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#35 | |
Registered User
Join Date: Jan 2007
Location: Puyallup, WA
Posts: 1,662
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Re: Questions about using a timing light
Quote:
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72 C10 lwb fleetside -stock 350/350 combo Last edited by 68 P.O.S.; 07-20-2018 at 12:43 PM. |
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#36 |
Registered User
Join Date: Jan 2007
Location: Puyallup, WA
Posts: 1,662
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Re: Questions about using a timing light
Dmj, you ever get this figured out?
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72 C10 lwb fleetside -stock 350/350 combo |
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#37 |
Senior Member
![]() Join Date: Apr 2016
Location: Cypress, TX
Posts: 3,704
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Re: Questions about using a timing light
Hi, yes, from information gathering and learning standpoint, I believe I have it. Thanks very much for the education.
I have a stock distributor with a stock amount of vacuum advance (a lot). The initial timing is set more advanced than stock, to the highest BTDC I can go while preventing knocking. This allows for efficient and easy starting with no kickback, and adequate idle and mid-throttle economy and performance. At wide open throttle, my timing is not sufficiently advanced for high performance and efficiency. This is because the timing is retarded by the vacuum advance can. The timing curve is set up for conservative stock performance and maximum engine safety. I have the vacuum advance running on manifold vacuum. My engine works equally well on ported or manifold vacuum, it doesn't seem to make any significant difference except when switching from one port to the other I need to adjust idle speed. And it only affects the engine at idle. So, I'm not convinced it makes any big difference which to use. My logic leads me to choose manifold because it just burns more of the gas in the cylinder before sending it to the exhaust, and therefore must be more efficient. I don't have an A.I.R. system or EGR, so nothing is going to happen with any unburned gas on my engine, except going out the tailpipe. Therefore, I can't find a reason to retard the timing at idle by using ported vacuum. I will probably get into hot rodding this HEI at some point, or at least mapping the curve so I know what I have for future reference. I can't tell when because I have my interior cab work to do and A/C to get going, shock installation and other maintenance to work on. In 1969 there was no starter brace, and I don't want to start breaking things, so I want to add the brace before messing with the timing curve. The guy who put in my exhaust put in glass packs, so I want to work on correcting that annoyance fairly soon, too. He asked what kind of tone I want, and I told him "no tone at all, I don't want to be able to tell the engine is running." Apparently to him that means glass packs welded in. I was mad, but just wanted to get the truck out of there and back to my side of Texas. I've now had a couple of years of listening to it every day, so it is getting old. Like everything on my truck I need to do it twice, that is my curse. That kind of stuff slows down my progress. |
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#38 |
Who Changed This?
![]() Join Date: Jan 2013
Location: Simi Valley, CA
Posts: 10,397
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Re: Questions about using a timing light
When I put mufflers on my truck, it's going to be a pair of those babies that go on the 500 CI Cadillacs. They obviously flow well, since they usually only get one for the whole 500 cubes, and they're plenty quiet. I'll also have an H-Pipe.
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~Steven '70 Chevy 3/4T Longhorn CST 402/400/3.56 Custom Camper Simi Valley, CA |
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