timing?? tuning carb - Team Camaro Tech
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post #1 of 25 (permalink) Old Jul 4th, 02, 03:46 PM Thread Starter
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hey just broke in my new 454. it is bored .30 over,hyperutectic flattop pistons,moly rings,69 rect port heads,427(435 hp) heads,10.25-1 comp,comp magnum 294s cam,.595 lift,248 dur @.50,solid cam,dart single plane intake,bg 850 carb, hei dist, all in a 68 camaro convert.car is not painted so it has not been driven.i broke engine in for 35 mins at 1800-3000 rpm, everything went well never went over 200, had at least 60 psi oil pressure at all times. i did not re adjust lifters yet, i changed oil, i am running 20w-50.i brought all idle screws to 1.5 turns out seems to start nice and idle, however i started it today (first time since tues and i had some popping in exhaust pipes????is that a lean condition? or timing?? only lasted 1 or 2 mins then ran great, once warm it wound up to 5000 smooth.occasionaly i will get a back fire through carb and i did not have any vaccuum hose to plug port on carb so i could time it. how much would the vaccuum advance, advance timing at 900 rpm idle, my timing light says like 18-20'btdc, if i try to bring it down to like 12-14 it does not like it at all, i don't have much experience tuning carbs and high hp engines, any help would be great, i have reccomended valve springs and stainless valves.please some insight as i have tomorrow off, thanks
Jake
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post #2 of 25 (permalink) Old Jul 5th, 02, 01:42 AM
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Plug the vacuum leak with something and leave the vacuum advance can disconnected. Mark the balancer at 36 degrees to the right of the factory mark or buy a dial back timing light. Rev the engine up till it stops advancing and set the zero on the tab to line up with the mark at 36 degrees and lock it down. Re-install the vacuum advance hose and set the idle speed. Adjust the mixture screws for best vacuum or for best idle if you do not have a gauge. Reset the idle speed if needed and repeat the mixture screw adjustment as required. Drive it!

Hope this helps.

-Mark.

I realy need to type this up and save it because it gets asked a few times a week. We are usualy firing up a mild to heavily modified engines here and the original specs go out the window. I probably should include a more detailed explanation as to "why" we should do it this way as well. I always found instructions without the "why" to be much less helpful.

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post #3 of 25 (permalink) Old Jul 5th, 02, 08:50 AM
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Hey Mark,

I hate to disagree with you but I will try to be nice about it.

You are correct in setting the timing at 36* on most big blocks. You said after doing this to reconnect the vacuum to the dist. In doing this if the vac adavance unit is a 18* unit this will bring the total timing to 54*. If you are setting total timing the vac advance should be included in the setting process, do you agree?

Larry www.lnjstreetrods.com

[This message has been edited by lnjstreetrods (edited 07-05-2002).]
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post #4 of 25 (permalink) Old Jul 5th, 02, 09:48 AM
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Nope.

-Mark

Politely no.
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post #5 of 25 (permalink) Old Jul 5th, 02, 12:50 PM
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Now for a more serious answer...

Since this does come up so often, I have put this together for anyone that wants to read up on it. I need to make up a fresh engine startup posting that either includes this stuff or maybe just references it.

Here are some thoughts on vacuum advance and distributor advance curves that you might find interesting. Thanks and credit goes to Lars Grimsrud of SVE Automotive Restoration, Musclecar, Collector & Exotic Auto Repair & Restoration Broomfield, CO

I have added a few suggestions of my own here as well.

The factory did not follow the same tune up method that most all of us learned as beginner mechanics. On the test firing line they reved the engine up till the mechanical advance was all in and set the timing at 36 degrees and locked it down. The initial spec could fall where ever it was supposed to fall based on the curve engineered inside the distributor. If all was well inside the distributor, it would be set within "book" spec.

The initial advance "book" spec is what we have traditionaly used at tune up time but once the engine gets modified, we are lost as to what it should be set to. A fresh high performance engine combination will require a different advance curve as compared to a stock smog engineered vehicle application. High performance advance curves are typicaly much faster than a typical stock smog legal curve found in fatory spec disributors. Most smog era engines use "ported" vacuum advance while engines up till the late 60's were set up with "manifold" vacuum at the distributor. If you do not need the emissions calibration you should set up your distributor for best performance, economy and drivabillity as was done before emissions.

The vacuum advance control unit on the distributor is intended to advance the ignition timing above and beyond the limits of the mechanical advance (mechanical advance consists of the initial timing plus the centrifugal advance that the distributor adds as rpm comes up) under light to medium throttle settings. When the load on the engine is light or moderate, the timing can be advanced to improve fuel economy and throttle response. Once the engine load increases, this "over-advance" condition must be eliminated to produce peak power and to eliminate the possibility of detonation ("engine knock"). A control unit that responds to engine vacuum performs this job remarkably well.

Most GM V8 engines (not including "fast-burn" style heads), and specifically Chevys, will produce peak torque and power at wide-open throttle with a total timing advance of 36 degrees (some will take 38). Also, a GM V8 engine, under light load and steady-state cruise, will accept a maximum timing advance of about 52 degrees. Some will take up to 54 degrees advance under these conditions. Once you advance the timing beyond this, the engine/car will start to "chug" or "jerk" at cruise due to the over-advanced timing condition. Anything less than 52 degrees produces less than optimum fuel economy at cruise speed.

The additional timing produced by the vacuum advance control unit must be tailored and matched to the engine and the distributor's mechanical advance curve. The following considerations must be made when selecting a vacuum advance spec:

How much engine vacuum is produced at cruise? If max vacuum at cruise, on a car with a radical cam, is only 15 inches Hg, a vacuum advance control unit that needs 18 inches to peg out would be a poor selection. You want the manifold vacuum advance to be maxed out at idle if possible. Otherwise you could find your idle unstable due to the advance "hunting" as the idle vacuum varried.

How much centrifugal advance ("total timing") is in effect at cruise rpm? If the distributor has very stiff centrifugal advance springs in it that allow maximum timing to only come in near red-line rpm, the vacuum advance control unit can be allowed to pull in more advance without the risk of exceeding the 52-degree maximum limit. If the engine has an advance curve that allows a full 36-degree mechanical advance at cruise rpm, the vacuum advance unit can only be allowed to pull in 16 more degrees of advance.

Are you using "ported" or "manifold" vacuum to the distributor? "Ported" vacuum allows little or no vacuum to the distributor at idle. "Manifold" vacuum allows actual manifold vacuum to the distributor at all times.

Does your engine require additional timing advance at idle in order to idle properly? Radical cams will often require over 16 degrees of timing advance at idle in order to produce acceptable idle characteristics. If all of this initial advance is created by advancing the mechanical timing, the total mechanical advance may exceed the 36-degree limit by a significant margin. An appropriately selected vacuum advance unit, plugged into manifold vacuum, can provide the needed extra timing at idle to allow a fair idle, while maintaining maximum mechanical timing at 36. YOUR CAR WILL LOVE ADDITIONAL ADVANCE AT IDLE. It will even idle cooler. You do not want to just run more initial mechanical advance as it will make cranking difficult especialy with high compression ratios. The manifold vacuum advance only comes on after the engine is already started.

Thus, we see that there are many variables in the selection of an appropriate control unit. Yet, we should keep in mind that the control unit is somewhat of a "finesse" or "final tuning" aid to obtain a final, refined state of tune; we use it to just "tweak" the car a little bit to provide that last little bit of optimization for drivability and economy. The vacuum advance unit is not used for primary tuning, not does it have an effect on power or performance at wide open throttle.

-Mark ET AL
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post #6 of 25 (permalink) Old Jul 5th, 02, 03:44 PM Thread Starter
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hey thanks alot, tomorrow, i will plug vaccuum adavance and carb and set timing at 3000-3500 to 36 btdc i have a dial back timing light so i will adjust that to 36 and line mark up with xero and tighten, now the vaccuum advance is stock, is this a 18' unit?? also i put the lightest springs in my mr.gasket timing curve kit, i have read that in hi perf engines you want all your timing in at or around 3000 rpm. also do i just forget about intial timing and let it be for whatever it falls at when total is set at 36'?? i will do this then wait till its painted and final tune it then, also when checking for most vaccuum will one of the 4 corner idle circuits make a diff ??or do i have to change all at 1/2 turn in or out to see where it makes most vaccuum??? thanks alot
Jake
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post #7 of 25 (permalink) Old Jul 5th, 02, 07:22 PM
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Now that you have taken time to make a complete statement things are clear. You are right about this timing thing being an on going thing. This is a good and complete statment, I totally agree. The other post was not clear as to where the vacuum was being connected. If to ported vac my statment would be true, and to manifold vac it would be correct as you say. I am not arguing here, I say again I agree with your total statment.

Larry www.lnjstreetrods.com
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post #8 of 25 (permalink) Old Jul 6th, 02, 02:38 AM
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Larry, no problem at all.

BBCamaro,
As for the canister, it has 2 sets of numbers on it if it is factory. The 3 digit number is the last 3 digits of the part number. The 2 digit number is the amount of advance in crank degrees available in the canister. I like 16 degrees but this is realy getting down to optimization rather than troubleshooting. The canisters are available from several sources but I have numbers for Echlin which are sold at NAPA. They are about $8.99 or so.

I will try to do a cut and paste here but I think the part numbers at the bottom will not be legible when posted. If anyone wants a MS word doc, just ask. BBCamaro, you will get mail.

-Mark.

********************************************

Part Number
There are many different sources for these control units. Borg Warner, Echlin, Wells, and others all sell them in their own boxes and with their own part numbers. Actually, there are very few manufacturers of the actual units: Dana Engine Controls in Connecticut manufactures the units for all three of the brands just mentioned, so it doesn’t make much difference who you buy from: They’re made by the same manufacturer. The part numbers I have listed here are the NAPA/Echlin part numbers, simply because they are available in any part of the country.

ID#
Every vacuum advance control unit built by Dana, and sold under virtually any brand name (including GM), has a stamped ID number right on top of the mounting plate extension. This ID, cross referenced below, will give you all specifications for the unit. So now, when you’re shopping in a junkyard, you’ll be able to quickly identify the “good” vs. the “bad” control units.

Starts @ “Hg
Vacuum is measured in “inches of Mercury.” Mercury has the chemical symbol “Hg.” Thus, manifold vacuum is measured and referred to as “Hg. The “Start” spec for the control unit is a range of the minimum vacuum required to get the control unit to just barely start moving. When selecting this specification, consideration should be made to the amount of vacuum that a given engine produces, and what the load is on the engine at this specification. For example, an engine with a very radical cam may be under very light load at 7 inches Hg, and can tolerate a little vacuum advance at this load level. Your mom’s Caprice, on the other hand, has such a mild cam that you don’t want the vacuum to start coming in until 9 – 10 inches Hg. For most street driven vehicle performance applications, starting the vacuum advance at about 8” Hg produces good results.

Max Advance
Since the vacuum advance control unit is a part of the distributor, the number of degrees of vacuum advance is specified in DISTRIBUTOR degrees – NOT crankshaft degrees. When talking about these control units, it is important that you know whether the person you’re talking to is referring to the distributor degrees, or if he’s talking crankshaft degrees. All of the listing show in the following chart, and in any shop manual & technical spec sheet, will refer to distributor degrees of vacuum advance. You must DOUBLE this number to obtain crankshaft degrees (which is what you “see” with your timing light). Thus, a vacuum advance control unit with 8 degrees of maximum advance produces 16 degrees of ignition advance in relationship to the crankshaft. When selecting a unit for max advance spec, the total centrifugal timing at cruise must be considered. Thus, a car set up to produce 36 degrees of total mechanical advance at 2500 rpm needs a vacuum advance control unit producing 16 degrees of crankshaft advance. This would be an 8-degree vacuum advance control unit.

Max Advance @ “Hg
This is the range of manifold vacuum at which the maximum vacuum advance is pegged out. In selecting this specification, you must consider the vacuum produced at cruise speed and light throttle application. If your engine never produces 20” Hg, you better not select a control unit requiring 21” Hg to work.

The following listing is as follows: The first two part number listings are the two numbers that are most commonly used in a Chevrolet performance application. The “B1” can is the most versatile and user-friendly unit for a good performance street engine. As you can see, it was selected by GM for use in most high performance engines due to its ideal specs. The “B28” can was used on fuel injected engines and a few select engines that produced very poor vacuum at idle. The advance comes in very quick on this unit – too quick for most performance engines. Do not use this very quick unit unless you have a cam/engine combination that really needs an advance like this. It can be used as a tuning aid for problem engines that do not respond well to other timing combinations.

After this, the listing is by Echlin part number. The Chevrolet applications are listed first by application, followed by a complete listing of all of the units used on any GM product (all GM units are interchangeable, so you can use a Cadillac or GMC Truck unit on your Vette, if that’s what you want to do).

P/N ID# Application Starts @ “Hg Max Adv (Distr. Degrees @ “Hg.)

VC680 B1 1959 – 63 All Chevrolet 8-11 8 @ 16-18
1964 Corvette exc. FI
1964 Impala, Chevy II
1965 396 High Perf.
1965-67 283, 409
1966-68 327 exc. Powerglide
1967-68 All 396
1969 Corvette 427 High Perf.
1969 396 Exc. High Perf.
1969 Corvette 350 TI
1969-70 302 Camaro
1970 400 4-bbl
1970 396 High Perf.
1970 Corvette 350 High Perf.
1973-74 454 Exc. HEI

VC1810 B28 1965 409 High Perf. 3-5 8 @ 5.75-8
1965 327 High Perf.
1966 327 High Perf.
1964-67 Corvette High Perf. FI

VC1605 B9 1965 impala 396 Exc. High Perf. 7-9 10.3 @ 16-18
1965 327 All Exc. FI
1969 327 Camaro, Chevelle, Impala
1969-70 Corvette 350 Exc. High Perf.
1969-70 350 4-bbl Premium Fuel
1970 350 Camaro, Chevelle, Impala High Perf.
1971-72 350 2-bbl AT
1971-72 307 All

VC1675 B13 1968 327 Camaro Powerglide 9-11 8 @ 16-18
1968 327 Impala AT
1968 307 AT
1968 302, 307, 327, 350 Camaro, Chevy II
1970 350 Camaro, Chevelle Exc. High Perf.

VC1760 B19 1969 350 Camaro, Chevelle, Impala 4-bbl 5.5-8 12 @ 14-18
1969-70 350 2-bbl

VC1765 B20 1965 396 Impala High Perf 5-7 8 @ 11-13
1966-67 Corvette Exc. High Perf.
1966-67 Impala 427 Exc. High Perf.
1966-68 327 Powerglide Exc. High Perf.
1969 307 All
1969-70 396, 427 Camaro, Chevelle High Perf.
1970 400 2-bbl
1970 307 MT
1973 Camaro 350 High Perf.

VC1801 B21 1971 350 2-bbl 7-9 10 @ 16-18
1971-72 400, 402
1971-72 307 AT

VC1802 B22 1971-72 350 4-bbl 7-9 8 @ 14-16


Other Part Numbers & Specs:

VC700 B3 8-10 11.5 @ 19-21
VC1415 M1 6-8 10 @ 13-15
VC1420 M2 5-7 11 @ 16-17
VC1650 B12 8-10 10 @ 15-17
VC1725 B18 8-10 12 @ 13-16
VC1740 A5 6-8 12 @ 15-17.5
VC1755 A7 8-10 12.5 @ 18-20.5
VC1804 B24 6.5-8.5 10 @ 12-14
VC1805 M13 6-8 12 @ 14.5-15.5
VC1807 B25 5-7 8 @ 13-15
VC1808 B26 5-7 8 @ 11-13
VC1809 B27 5-7 9 @ 10-12
VC1812 B30 5-7 12 @ 11.75-14

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post #9 of 25 (permalink) Old Jul 7th, 02, 01:36 PM Thread Starter
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thanks alot stingray, got total mechanical at 36' at 3000 the light springs and new arms work great, much smoother coming on, thanks again for the help and info
Jake

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post #10 of 25 (permalink) Old Jul 8th, 02, 08:45 AM
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Mark has it nailed - good post; total timing is initial plus centrifugal only, vacuum advance is added and reduced based on engine load.

For most performance applications, especially with non-stock cams, the Echlin #VC-1810/B28 is ideal, as it is fully deployed at 8" Hg, which will provide a stable idle (with manifold vacuum on it). most of the cans with "stock" calibrations aren't fully deployed until 12-16" Hg, which means they are "dithering" around at any less vacuum than that, contributing to unstable idle conditions.

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post #11 of 25 (permalink) Old Jul 8th, 02, 02:35 PM
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STINGR69 & JOHNZ ..Which one would you guys recommend for my car??It pulls 6" of vacuum at idle(1000 RPM) and 4" of vacuum in drive(600 RPM)..I have a big cam and I know my vacuum advance isn't working at idle or in drive..I can disconnect it at idle and doesn't change a thing..I am at my limit on base timing 20*..Hoping to help my idle and rpm drop in drive...Thanks in advance

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post #12 of 25 (permalink) Old Jul 8th, 02, 04:16 PM
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badcaiman, If you have any hope at all of running a vacuum advance at idle you will probably need to run an adjustable vacuum advance. Crane sells a kit for it. Even the B28 can would not work that well for you given those vacuum readings. I hope the rest of the engine is in tune. Those vacuum numbers sound pretty low. The gauge is suspect to me but that is only a guess. Be sure to use manifold vacuum to the canister and not ported vacuum. The tap you use needs to have vacuum with the car idling. Good luck and keep us informed of your progress.

-Mark.
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post #13 of 25 (permalink) Old Jul 8th, 02, 04:39 PM
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STINGR69..The car actually runs great now..Vacuum is solid at 6" at idle(1000rpm) and 4" in drive(600rpm)..I am running a large [email protected] tells me I shouldn't expect any more than 6" at 1000rpm on a 355 SBC..But would like some vacuum advance at idle..So I can turn back the base timing..

I thought about the crane adjustable vacuum can..Just wasn't sure if it would work with just 6" of vacuum(Summit tech couldn't help me out)...Also should I be looking for something that reacts with 4" or 6" of vacuum...Thanks again

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[This message has been edited by Badcaiman (edited 07-08-2002).]
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post #14 of 25 (permalink) Old Jul 8th, 02, 04:51 PM Thread Starter
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hey guys thanks for all the help, also like badcaimen i have a 248 dur @.50 have not put vaccuum gauge onit yet, but suspect vaccuum will be down around 8-12inch pounds, i will get a vaccuum can, but also like badcaimen when i unplug vaccuum advance it does nothing to the idle quality as i am around 16-20 'btdc, will the echlin part work for me, the cam is .595 lift,248 [email protected] single pattern cam, if the echlin will work i will order one tomorrow at napa, also will this help lower idle? my motor seems to like 900-950, maybe a tad more, but again i won't be able to drive it till its back together, thanks so much
Jake

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post #15 of 25 (permalink) Old Jul 8th, 02, 07:05 PM
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Hey guys,

MSD has a couple of different ways to advance, and retard timing electronicly. They use to call one a "Ping Control". The other is a timing retard box used when using nitrous, it has 3 stages of retard plus a wire that goes to the starter that retards timing 20* while starter is ingagued. The "Ping Control" has a knob on the dash to adjust the timing.

Just a thought, Larry
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