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Old 06-10-2007, 10:02 AM
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Ignition Timing (again....)

I have been doing some research on ignition timing for a few reasons. First and foremost, I have a problem. Second, its always been a bit of a mystery. Third, everone believes something different. I located a thread from Rick W (timing and vacuum advance 101) which is worth its weight in gold. Thanks for the detail. Many will benefit. As usual, its created a couple of questions...

I have a '77 3/4 ton p/u hauling a camper and pulling a boat (sometimes a small boat, sometimes a big boat). It has a 350, 4 speed, 410's and 31" tires. The engine is +.030 and fresh. It is running a "stump puller" cam (don't remember which one), the original cast iron intake and rochester with a Proform HEI.

The vacuum advance is plugged into a ported source (offerring 0 advance at idle). Initial timing is 6 degrees and total is 26 degrees all in at 2250 with the vacuum disconnected. Total with the vacuum connected is 56 degrees.

So if i read the correctly, I'm getting 20 degrees from my weights and 30 from vacuum. Am I correct so far

At first blush the truck seems to run fine. Clean start, clean shutdown, no pinging. However, after long hauls (when the engine gets hot) a few things appear. I get stumbling at WOT at about 4000 and pre-detonation during part throttle acceleration.

I brought the initial down to 3 and both problems seem to have disapeared (although its hard to duplicate the environment without spending a couple of hours in the truck). I hear no pinging at part throttle and I easily revved to 4500. Overall, the engine seems happier and still starts/shuts clean.

I would ordinarily stop here. But.....after reading Ricks post I wonder if theres more to gain?

I have a non-ported vacuum source at the carb. The 26 degree total is well short of the 34-36 mark defined in ricks article. If I move the vacuum source and add 10 degrees of initial (and readjust the idle speed) will I gain noticable improvements in milage and performance?

Would this change also put my total vacuum at about 65 degrees (well above the 52 degree mark specified in part two)? I have an adjustable vacuum pot but my understanding is it allows me to adjust WHEN it activates not how much advance it throws.

Have I already found the sweet spot or should I continue?

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Old 06-10-2007, 11:39 AM
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I would definitely read the article by Damon Nickles referenced in timing 101. I've set my 350 up this way and it's darn near perfect. Actually, it IS perfect!
Also when towing, after your long haul are you gaining altitude???? You'll be running richer if so. It sounds like you're close .
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Old 06-10-2007, 12:44 PM
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You are correct on your mechanical and vacuum but it's totally goofed up. If you read the article you are short on mechanical. On a 350 Chevy you want base around 10 to 12 BTDC, 36 to 37 total. I'd dial in all the timing in by 3000 RPM.

For the vacuum advance you want it hooked to manifold vacuum and 8 degrees of advance added due to the vacuum advance can.

Bottom line, if your readings are correct, you don't have enough total timing (way too short) and you have way, way, way too much vacuum advance.
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Old 06-10-2007, 01:39 PM
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Quote:
Originally Posted by Rick WI
You are correct on your mechanical and vacuum but it's totally goofed up. If you read the article you are short on mechanical. On a 350 Chevy you want base around 10 to 12 BTDC, 36 to 37 total. I'd dial in all the timing in by 3000 RPM.

For the vacuum advance you want it hooked to manifold vacuum and 8 degrees of advance added due to the vacuum advance can.

Bottom line, if your readings are correct, you don't have enough total timing (way too short) and you have way, way, way too much vacuum advance.
Hi Rick,
Would you mind posting a link to this "timing 101" thread? I just did a keyword search but didn't stumble onto it.
Thanks, Don

Driveline specs:
66 GMC pickup, +.030 454, reconditioned stock 781 heads, roller rockers and stiffer springs, Forged Flat-tops, camshaft 224/232, .527/.553 114 LOBE SPEAD, RPM Air Gap, brand new 750 CFM Holley 3310 (Too small, I think), Hedman 69111 Headers, TH400, 2400 stall, 3.73 gears, reman (stock) HEI, Holley Blue pump.

I have done engine break-in only, and haven't yet driven it. Engine will idle @ 600-700 rpm but it's happier at about 800. Initial @ 8 ... no idea on anything else yet ... but I don't think it's quite right. It seems to fall on it's face at around 2500 when you blip the throttle. (power valve ?)

Last edited by 66GMC; 06-10-2007 at 02:03 PM.
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Old 06-10-2007, 02:06 PM
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I myself don't know what the title that post was under but I think they stuck it in the knowledge base somewhere. I'll do a quick search until the funny cars come on ESPN.
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Old 06-10-2007, 09:55 PM
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My experience with those adjustable vacum pots is they are adjustable for amount of vacume it takes for movement but not for travel, I make a small sheetmetal limiter that bolts on with one of the screws that holds the pot on, the limiter is slotted so I can adjust the amount of advance then use the allen wrench to fine tune what it needs for activation. Tune your initial for idle quality and response then dial in your curve for 38ish total by 3000 rpm, then add vacume in small amounts for part throttle cruize mileage. Heavy vehicles will like a slower curve.
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Old 06-10-2007, 10:04 PM
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Here's a pic of a vaccum advance limiter I made. The picture is not very good but its basically a rectangular piece of sheet metal with a slot in it, and one of the canister's mounting bolts goes through the slot. It can be adjusted for any amount of vacuum advance.

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Old 06-11-2007, 12:02 AM
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66 GMC, for the link, go in the knowledge base, go to general engine info/ articles.....general engine info.....find the search bar and type in timing 101.
It should take you there. I think it might be wise here to mention that the vacuum advance hose may be routed to the wrong port. (too much vacuum). This is why I hate quadrajet carbs! I had 4 possible locations on my old one!! Sorry, I'm a Holley nut. No guesswork on my new carb.
Bringing the initial timing down to 3 btc seems to me to be compensating for too much vacuum. My initial is set at 12 btc, 24 at 3500 from mechanical, the rest is vac which you can adjust, I believe you said your vacuum was adjustable. Follow the info in the link and I think you will be OK. BTW, are you gaining altitude when you tow and have this problem??? You'll be running rich and loading up your plugs if you are. I only say this because I live at 5000 ft and go to work down the hill at almost sea level. It's recommended to go 1 jet size smaller for every 2000 ft. I choose to be in the middle as you have to find some happy medium or change jets every day
Man, It's gettin late. I hope I've said something that makes a difference for you. Let us know how you're progressing. John
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Old 06-11-2007, 01:28 PM
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Thanks for all the interest.....

No, altitude is not a major factor here. I have read the timing 101 (parts 1-3) which is what really prompted these questions.

I agree, at first blush my distributor seems way wrong. But somewhere in the 101 post was mentioned distrubutors with initial advance around 4 degrees, less mechanical advance and lots of vacuum advance connected to a timed vacuum port in an attempt to meet some [trumped up] federal mandate. Is it possible this is the setup I have?

I have the same distributor in my (fairly built) '57 nomad. That engine doesn't really seem to "pop" either, although it runs extremely well (I'll need to map the curves to find out if I have an identical situation).

Assuming this is the case, what I'm hearing here is I should set initial about 10-12 with a full vacuum port (disconnected) which would render about 30-32 (or maybe change the stops to give 36) total mechanical, swap the vacuum pot and adjust accordingly to get 52 total?

Is this the general feeling? Any idea why the engine runs so well if the distributor is so far out of whack?
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Old 06-11-2007, 04:57 PM
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All I'll say is we do a ton of dyno work and a ton of EFI tuning and installs. Nothing close to that curve strikes a bell on "what worked well" for a 350 or 400 small block Chevy. A year and a half ago we tried a couple of the cheap ProForm distributors on street stock circle track cars. Since then we have converted them to small cap MSD's with external coils. They were not the best performers for those applications.

Absent that, here is as much thought as I think warrents your situation. In other words not turing this into a mental masturbation thought process as it's pretty straight forward.

Small blocks with stock heads and mild cams like yours will run very well with a timing curve of 12 degrees of initial advance at 800 or so RPM (your idle speed), 36 degrees of advance by 3000 RPM and 8 degrees of vacuum advance. If you were cruising at 3000 RPM then the advance would be 36+8 = 44. The 44 is not total timing it's total plus vacuum advance. That's PLENTY of advance for cruise/light load operation.

My experience tells me you are way underadvanced on mechanical and way overadvanced on vacuum advance. 56 total plus vacuum is way too much.
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Old 06-11-2007, 05:29 PM
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Thanks for the candor.....not sure wether I'm to be insulted here or not. Either way, I appreciate the subtle mention as to your opinion of ProForm. I'll verify the curves on this, and the Nomads, distrubutors and decide where to go from there (probably re-map the nomad first as a test). Thanks again....
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Old 06-11-2007, 05:44 PM
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Quote:
Originally Posted by speedydude
66 GMC, for the link, go in the knowledge base, go to general engine info/ articles.....general engine info.....find the search bar and type in timing 101.
Thanks Speedydude.
Here's a direct link if anyone else is interested.
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Old 06-11-2007, 07:53 PM
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It's hard on the internet to say things in a nice way but matter of fact. Don't take it as an insult at all, I really was trying to be nice. I wanted to put the basic theory of dialing in the mechanical and the vacuum advance in as few words as possible. To me, after doing this for 25 to 30 years and doing dyno work for over 10 it's very straight forward.

I didn't want to rip the ProForm stuff as I know a few folks have run them and so far been happy with them. For us, the MSD small cap with external ballast has by far been the best performer both from a power standpoint as well as reliability. BUT, all these distributors require dialing in.
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Old 06-12-2007, 06:53 AM
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I understand....thanks for the clarification. I dont think I'll bail on the ProForms quite yet (primarily 'cause I have two) but I do think I'll order a curve kit and the "correct" vacuum pot and invest some time getting this correct. Again, I really do appreciate the input.
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Old 06-12-2007, 07:31 AM
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Crane makes an adjustable vacuum advance. Summit sells them.

Rick is right. Too much vacuum advance and not enough mechanical. However, 50 to 55 degrees of timing during very light throttle is good for mpg.
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