Ignition 101 - Chevelle Tech
 8Likes
Reply
 
LinkBack Thread Tools Search this Thread Display Modes
post #1 of 102 (permalink) Old Aug 17th, 07, 2:24 PM Thread Starter
Senior Tech Team
Steve
 
Join Date: Jan 2005
Location: Granite Bay, Ca.
Posts: 4,414
Ignition 101

A common problem that comes up here on TC is ignition setup. Much of the discussion devolves into "this works for me" without supporting data. These can be misleading and sometimes just wrong. What follows is some very helpful information about how ignition works that I have compiled from various sources and have found very useful.

Part of my point in posting this is to put into one place a good background piece that we can point folks to, as in "search for ignition 101". Some of this may be slightly duplicated, but again I am attempting to create a one-stop shop. So following in successive responses...

BTW if you have additional reference data to add feel free. Links to advance limiters would probably be a good addition for example.

Steve
1968 El Camino, 402 with TKO

Last edited by vrooom3440; Aug 17th, 07 at 2:40 PM.
vrooom3440 is offline  
Sponsored Links
Advertisement
 
post #2 of 102 (permalink) Old Aug 17th, 07, 2:28 PM Thread Starter
Senior Tech Team
Steve
 
Join Date: Jan 2005
Location: Granite Bay, Ca.
Posts: 4,414
Re: Ignition 101

Distributor Tuning and Theory - Part 1

There appears to be a lot of conversation on timing, vac advance, distributor tuning and, in general, hacking up a perfectly good ignition system to compensate for a poorly tuned carburetor.

from
JohnZ
Tech Team

posted 03-23-2003 03:41 PM

The following two articles review the basics of distributor tuning quite well and have worked for years and years and are based on sound engineering principals. I thought it would be helpful for some to review these prior to hacking up their distributors. Hacking up your distributor to compensate for a poorly tuned, misapplied or defective carburetor is not very sound engineering, for a street application or otherwise.

Here's an interesting article on vacuum advance written by a GM engineer:

As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.

TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.
Bow_Tied and WillyKJr like this.

Steve
1968 El Camino, 402 with TKO
vrooom3440 is offline  
post #3 of 102 (permalink) Old Aug 17th, 07, 2:34 PM Thread Starter
Senior Tech Team
Steve
 
Join Date: Jan 2005
Location: Granite Bay, Ca.
Posts: 4,414
Re: Ignition 101

Part 2

Distributor Vacuum Advance Control units
Specs and facts for GM Distributors

by Lars Grimsrud
SVE Automotive Restoration
Musclecar, Collector & Exotic Auto Repair & Restoration
Broomfield, CO Rev. B 8-19-02


I’ve been seeing a lot of discussion and questions regarding distributor vacuum advance control units; what do they do, which ones are best, what was used on what, etc., etc. To clarify some of this, I thought I’d summarize a few facts and definitions, and provide a complete part number and specification listing for all vacuum advance control units used by Chevrolet on the points-style distributors. I’m also providing a listing of the specs for all other GM (non-Chevrolet) control units, but without the specific application listed for each (it would take me a bit too much time to research each part number by application across each of the GM Motor Divisions – it took me long enough to compile just the Chevy stuff…!). This latest revision to this paper also includes the HEI listings (the HEI distributors use a longer control unit, so the non-HEI and HEI vacuum advance control units CANNOT be interchanged).

As always, I’m going to include the disclaimer that many of these are my own comments and opinions based on my personal tuning experience. Others may have differing opinions & tuning techniques from those presented here. I have made every attempt to present factual, technically accurate data wherever possible. If you find factual errors in this information, please let me know so I can correct it.

Background
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.

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. A tuning note on this: If you choose to run straight manifold vacuum to your vacuum advance in order to gain the additional timing advance at idle, you must select a vacuum advance control unit that pulls in all of the advance at a vacuum level 2” below (numerically less than) the manifold vacuum present at idle. If the vacuum advance control unit is not fully pulled in at idle, it will be somewhere in its mid-range, and it will fluctuate and vary the timing while the engine is idling. This will cause erratic timing with associated unstable idle rpm. A second tuning note on this: Advancing the timing at idle can assist in lowering engine temperatures. If you have an overheating problem at idle, and you have verified proper operation of your cooling system components, you can try running manifold vacuum to an appropriately selected vacuum advance unit as noted above. This will lower engine temps, but it will also increase hydrocarbon emissions on emission-controlled vehicles.

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, nor does it have an effect on power or performance at wide open throttle.

With these general (and a little bit vague, I know…) concepts in mind, let’s review a few concepts and terms. Then it’s on to the master listing of specs and parts…..:

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 listings shown 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 (HEI) is as follows: The first four part number listings are the 4 numbers that are most commonly used in a Chevrolet performance application. The “AR12” can is the most versatile and user-friendly unit for a good performance street engine. The AR 15 and AR23 are almost identical, with only slight variations in their “start-stop” specs. The “AR31” can is the HEI equivalent to the “B28” Hi-Perf can used on the early engines: The advance comes in very quick on this unit – too quick for many 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, and can be successfully used in applications where direct manifold vacuum is applied to the can (see paragraph and discussion on this above)

After this, the listing is by Echlin part number. All GM HEI vacuum advance units are interchangeable, so you can use a Cadillac or GMC Truck unit on your Vette, if that’s what you want to do.

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

VC1838 AR12 1975 350 Buick 7-9 7 @ 10-12

VC1843 AR15 1977 305 All Exc. Hi Alt, Exc, Calif. 3-5 7.5 @ 9-11
1974 400 All w/2-bbl
1977 305 El Camino
1976 262 Monza Exc. Calif
1976 350 Vette Hi Perf, Incl. Calif
1975 350 Z-28
1977 305 Buick Skylark

VC1853 AR23 1976 350 All Calif. 5-7 7.5 @ 11-12.5
1976 350 Vette Calif., Exc. Hi Perf
1976 400 All, Exc. Calif
1975 350 4-bbl
1974 350 All w/1112528 Distr.
1978 350/400 Heavy Duty Truck, Exc. Calif, Exc. Hi Alt.

VC1862 AR31 2-4 8 @ 6-8
----------------------------------------------------------------------------------------------------------------
VC1703 N/A 1978-79 Vette Special Hi Perf N/A N/A
1979 305 El Camino Calif.
1978-79 350 Blazer & Suburban
1979 Buick 305/350

VC1825 AR1 1976 454 Caprice, Impala 3-5 9 @ 6-8
1975 454 Caprice, Chevelle, Monte, Suburban

VC1826 AR2 5-7 12 @ 10-13

VC1827 AR3 5-7 9 @ 9-11

VC1828 AR4 1975-76 350 Buick & Olds 6-9 10 @ 12-14
1976 350 Pontiac

VC1831 AR7 6-8 12 @ 14-16

VC1832 AR8 1975-76 455 Buick Electra 4-6 12 @ 12-14

VC1833 AS1 1975-76 500 Cadillac Exc. Calif. 4-6 14 @ 15-16

VC1834 AR9 4-6 13 @ 13-16

VC1835 AS2 1975-76 350 Olds 5.5-7.5 12 @ 15-17

VC1836 AR10 1977 305 All Hi Alt, Exc. Calif. 3-5 9 @ 11-13
1977 350 All exc. Calif.
1977 350 Vette Exc. Calif, Exc. Hi Perf
1976 305 All Exc. Calif
1976 350 All Exc. Vette, Exc. Calif
1976 350 Vette Exc. Calif., Exc. Hi Perf
1975 262, 350 All w/2-bbl carb
1975 350 All 4-bbl w/ 1112880 & 1112888 Distr.
1977 305 Chev Truck Light Duty
1975-76 350 El Camino 2-bbl

VC1837 AR11 1976 305 Blazer, Exc. Calif 6-8 12.5 @ 10.5-13.5
1976 350/400/455 Pontiac 4-bbl

VC1839 AR13 4-6 12 @ 11-13

VC1840 AR14 1975-76 350/400/455 Pontiac Firebird 6-8 10 @ 9-12

VC1841 AS3 1975-76 500 Cadillac Calif. 5-7 10 @ 13-14

VC1842 AS4 1976 350 Olds Cutlass 5-7 12 @ 13-15

VC1844 AR16 3-5 12 @ 13.5-15.5

VC1845 AS5 1978-79 425 Cadillac w/F.I. 4-6 14 @ 14-16
1977 425 Cadillac

VC1846 AR17 1977 301 Buick Skylark 3-6 13 @ 10-13
1977 301 Pontiac

VC1847 AS6 1978 403 Motor Home 4-6 12 @ 12-14
1977-79 350/403 Buick LeSabre Hi Alt, Riviera, Olds
1977-79 350/403 Pontiac Hi Alt

VC1848 AR18 4-6 12 @ 9-12

VC1849 AR19 4-6 12 @ 7-10

VC1850 AR20 1977 350/400 Pontiac 4-6 10 @ 8-11

VC1851 AR21 1977-79 350 Buick LeSabre, Century 5-7 12 @ 11-13
1978-79 350 Pontiac

VC1852 AR22 77-78 305/350/400 Chev Truck, Heavy Duty 7-9 5 @ 12-14
1975-76 350/400 Chev Truck Heavy Duty

VC1854 AR24 3-5 13 @ 10-13

VC1855 AS7 1977-79 260 Olds Cutlass 3-5 15 @ 10-12

VC1856 AR25 3-6 15 @ 10-14

VC1857 AR26 3-6 12 @ 13-16
VC1858 AR27 1978-79 305 All 3-6 9 @ 11-13
1978 350 Camaro
1978 305 Chev Truck, M/T, Light Duty
1978 350 Chev Truck Hi Alt
1978 305/350 Buick & Olds
1978-79 305 Pontiac

VC1859 AR28 1979 350 Vette Exc Hi Perf 3-6 10 @ 9-12
1978-79 305 w/1103282 Distr., Incl. El Camino A/T
1979 350 Camaro, Impala, Nova, Malibu, Monte
1979 350 Suburban
1979 350 Buick Century
1978 305/350 Buick & Olds
1978-79 305 Pontiac Hi Alt.

VC1860 AR29 3-6 12 @ 10-13

VC1861 AR30 1978-79 301Buick 3-5 13 @ 11-13
1979 301 Olds
1978-79 301 Pontiac

VC1863 AR32 2-4 10 @ 11-13

VC1864 AR33 1978 305 Chev Truck, A/T, Light Duty 4.5-6.5 13 @ 11-13

VC1865 AR34 1973-74 350 Vette Special Hi Perf 3-5 15 @ 8.5-11.5

VC1866 AS8 1978-79 425 Cadillac w/carb 3-5 14 @ 13-15

VC1867 AS9 2-4 10 @ 8-10

VC1868 AR35 1979 305 Chev Truck & El Camino 2-4 10 @ 6-9
1979 305 Buick & Olds
1979 305 Pontiac A/T

VC1869 AS10 2-4 12 @ 8-11

ed: There is a similiar list for point-type distributors which I have left out as I see little point running points with their increased maintenance requirement on a performance engine.
Bow_Tied likes this.

Steve
1968 El Camino, 402 with TKO
vrooom3440 is offline  
 
post #4 of 102 (permalink) Old Aug 17th, 07, 2:38 PM Thread Starter
Senior Tech Team
Steve
 
Join Date: Jan 2005
Location: Granite Bay, Ca.
Posts: 4,414
Re: Ignition 101

ed: The following is a somewhat opinionated discussion by someone who went by a screen name of "cuda66273", but the examples are illustrative and useful.

Let's set the paramiters of this discussion board:
1. This is the HOTRODDERS Board
2. We need to assume that everyone here is driving a Hot Rod
3. We are not discussing timing events for anything other than naturally aspirated engines.
4. Emmission control devices are not in the equation

So now we have established the bases of the discussion let me try and simplify the timing control systems.

Your engine needs lot's of timing at idle and part throttle, I think we've established this from previous comments from qualified technicians.

This is achieved by a vacuum advance unit hooked to MANIFOLD vacuum.

As you load the engine the timing needs to be decreased, this is where the vacuum timing hooked to Manifold drops the timing off and relays the responsibility to the MECHANICAL Advance.

As the engine RPM increases it requires more timing, so the mechanical advance moves the timing up as RPM increases.

This is not an opinion but a fact, this is how they're designed to work.

So let's now look at more details and at some typical issues with distributors.

First off, your stock or aftermarket distributor is not ready to install in your performance engine right out of the box, NO WAY, NEVER, EVER will it not work right. It's no different than a carb or a set of roller rockers, the distributor must be phased, curved and set up to YOUR engine.

90% of carburetor problems are IGNITION related, pick up any basic tuning book and it will read "Ignition First then Carburetor"

I'll use a fairly simple example here:
350 Chevy in a 72 Nova

Example #1

224 @ 50 cam
9.0:1 compression
Iron heads
Headers
3.90 gear
2200 stall convertor
575 Speed Demon
Performer manifold
HEI stock ignition
91 Octane California premium fuel

This car should have a good aftermarket coil, initial timing at about 18* and vacuum timing of 20* at 16" for total timing of 38* at idle and light throttle cruise. The mechnical timing of 18* should come in at about 3000 RPM for a total under power timing number of 36*

Example #2:

Same car
244 @ 50 Cam
10.5:1 compression
Aluminum heads
Headers
4.10 gear
3200 stall convertor
650 Mighty Demon
Air Gap manifold
Pro Billet MSD Distributor
6 AL Box
91 Octane California premium fuel

So now you've deleted the vacuum control (Which is probably hurting you) and the car won't idle, loads up, stinky exhaust, over heats, stumbles...but it runs like a raped ape at WOT.

This car will take 22-24 inital timing (At idle) the mechanical advance should be set to come all in just before the convertor hits so around 3000 would be fine if the heads are efficient, spark plug heat range is right, A/F ratios correct...basically a good tune-up on the thing.

So you read the instructions from MSD and they supply a bushing that will alow you to limit the total mechanical advance to 18* so you add in the 24* of initial and your at a total of 42*???? Detonation City!

So you get out the timing light and back the initial down to 18* for a total of 36* the detonations gone but now it's back to loading up and that fine fragrance of un-burned un-leaded fuel.....sound familiar?.....are you frustrated yet?

You can call Barry Grant techline or MSD and tell them your story, they've heard it 20,000 times and if you get a good tech they'll tell you to call us. We machine bigger bushings to give you 20 and 24* of initial and 16/12* mechanical to stay at 36* total and make it idle.

Let's step back and consider what a vacuum distributor would do for engine #2...

The motor will start at 18* if everything else is correct so the black bushing in your kit will work fine. The vacuum can can be adjusted to give you another 10-15* at 10-12" of vacuum which will bump the idle timing up to about 28-30*. This will give you enough timing to properly adjust the carb to clean up the idle an get rid of that skunk in your tail pipe.

Remember your hooked to MANIFOLD vacuum so as soon as you touch the loud peddle the vacuum timing will drop away and mechanical will take over the job of advancing the timing as RPM increases. The rate (RPM) at which it advances is known as the curve as is set-up with the various springs supplied with the distributor, the total amount of the advance is controlled by the bushings or stops in your kit.

The truth is most of you don't have the machinery or knowledge to properly set up a distributor for your car that will deliver the correct Initial timing, Vacuum timing, and mechanical timing at various loads and engine RPM. We have a data base of over 2500 engine types and combinations from work boats to 8 second Pro street cars, between Jim and I we have over 80 years of experience with rebuilding and curving performance distributors, we've seen it all, we've tried every possible combination, tested every component and aftermarket VooDoo distributor ever made and it really comes down to simply setting it up correctly, curving it right and using good quality parts.

Some advice from a Fat Bald Old guy....

GM Owners-90% of the cars we see with aftermarket distributors will do no more than drain your wallet. A well built stock HEI distributor can be set up to work flawlessly on most applications for 1/2 the money.

Mopar Owners-Your distributors have difficulties with the vacuum advance system. An all mechanical distributor with a REAL ECU will usually give you 25-75 HP. There's only one ECU that really works correctly, you can find it by searching "Mopar Ignition" in google, pay attention to the Fat Bald Old guy.

Ford Owners- Your distributors need help search "Ford Distributor Curving"

Final words- Don't buy a Race distributor for a Street car, don't call a Proctologist to fix your tooth ache so don't call a Carburetor company for Ignition advice, bright and shiny usually only attracts crows and doesn't do anything for performance.

"Ready to Run" is the biggest lie in the performance industry.

Steve
1968 El Camino, 402 with TKO

Last edited by vrooom3440; Aug 17th, 07 at 3:01 PM. Reason: I really gotta learn to spell...
vrooom3440 is offline  
post #5 of 102 (permalink) Old Aug 17th, 07, 2:46 PM
 
Join Date: Jul 2007
Posts: 101
Re: Ignition 101

Steve - you are my new posting hero - this will be extrordinarily helpful to a lot of folks out there This is a lot of great info and you did a fantastic job laying it out!! For those who get overwhelmed by this new found guidance, and run directly out to get there car running well (possibly for the first time ever) Thank you.
bntyhntr00 is offline  
post #6 of 102 (permalink) Old Aug 17th, 07, 3:09 PM
Senior Tech Team
Vince
 
Join Date: Jun 2003
Location: Thornton, Pa
Posts: 13,557
Re: Ignition 101

Steve, why would one not chose a vac can that fully deploys at say 1-3" of vacuum vs. getting one that deploat at 2" BELOW your lowest vacuum?

For the record, my GM can deploys at 1-3", and adds 20*, but I trimmed it back to 14*.

*69 SS 502 EFI /224* HR/9.8:1/T56/3.90/ 4000lbs RW 11.5/124mph
*11 C6, 6.2, 6spd
*96 Stang 6.0 turbo 10.5/135mph
*2011 Mustang GT premium
*61" Scag Kohler 22hp 7mph
69-CHVL is offline  
post #7 of 102 (permalink) Old Aug 17th, 07, 3:11 PM
Senior Tech Team
 
Join Date: Jan 2001
Location: Fairfield, CA
Posts: 4,651
Re: Ignition 101

Quote:
Originally Posted by 69-CHVL View Post
Steve, why would one not chose a vac can that fully deploys at say 1-3" of vacuum vs. getting one that deploat at 2" BELOW your lowest vacuum?

For the record, my GM can deploys at 1-3", and adds 20*, but I trimmed it back to 14*.
Which GM can do you have?
onovakind67 is offline  
post #8 of 102 (permalink) Old Aug 17th, 07, 3:19 PM
Senior Tech Team
Vince
 
Join Date: Jun 2003
Location: Thornton, Pa
Posts: 13,557
Re: Ignition 101

AR35. Looks like it starts at 2-4 and is pegged 6-9

*69 SS 502 EFI /224* HR/9.8:1/T56/3.90/ 4000lbs RW 11.5/124mph
*11 C6, 6.2, 6spd
*96 Stang 6.0 turbo 10.5/135mph
*2011 Mustang GT premium
*61" Scag Kohler 22hp 7mph
69-CHVL is offline  
post #9 of 102 (permalink) Old Aug 17th, 07, 4:09 PM
Senior Tech Team
 
Join Date: Jan 2001
Location: Fairfield, CA
Posts: 4,651
Re: Ignition 101

One problem you might run into is the vacuum advance coming in at high rpm's if you develop much manifold vacuum up there. If you're into 2-bbl carbs this becomes a real problem, or if your VS carb doesn't work well.
onovakind67 is offline  
post #10 of 102 (permalink) Old Aug 17th, 07, 7:21 PM
Senior Tech Team
Ken
 
Join Date: Sep 2005
Location: Missouri
Posts: 2,562
Re: Ignition 101

Great collection of part #'s. I copied it and am putting in the compartment of my distributor machine.

When I tell people they need over 50 degrees advance, they think I'm nuts. I have seen advance specs up to 60 degrees.

Another part of ignitions that many overlook is proper alignment of the rotor and cap contacts when the spark fires. This can be checked by grinding a bunch of holes by the contacts in an old dist cap, and checking the alignment with a timing light. This is really important with CD ignition boxes, as fast rise time will allow sparks to jump to many more places than you think they would. Getting the spark in the right place sometimes necessitates modifying the vacuum canister rod. That's why I'm mentioning it here. When the contacts are properly aligned, everything just feels better.

Don't forget to clean those fingerprints off the cap and rotor. Does anybody still make non-e-gap rotors?

kirkwoodken
406 SB, Original Bill Thomas '63 Rochester FI
AFR 210, Lunati 501C2LUN, 255/[email protected]", .628"
10:1, TH400, 3.31's, 4400 SS

"Life is too short to not run a solid roller cam."
"Nothing is impossible; if you don't know what you're talking about."
Kirkwoodken

Last edited by kirkwoodken; Aug 17th, 07 at 7:33 PM.
kirkwoodken is offline  
post #11 of 102 (permalink) Old Aug 17th, 07, 8:53 PM
Lifetime Premium Member
Todd
 
Join Date: Dec 2006
Location: San Martin,CA
Posts: 1,368
Re: Ignition 101

Does anybody make a vacuum can for the MSD pro billet that starts at a lower vacuum rate than the one it comes with?

I tried hooking up my vac advance to full vac and couldn't get the idle to stabilize due to low vacuum at idle (8") Also, is adding vacuum advance sometjhing you would try on a 12.5:1 BB??

Great post!!!
496blaze is offline  
post #12 of 102 (permalink) Old Aug 17th, 07, 10:34 PM
Senior Tech Team
Ken
 
Join Date: Sep 2005
Location: Missouri
Posts: 2,562
Talking Re: Ignition 101

Quote:
Originally Posted by 496blaze View Post
Does anybody make a vacuum can for the MSD pro billet that starts at a lower vacuum rate than the one it comes with?

I tried hooking up my vac advance to full vac and couldn't get the idle to stabilize due to low vacuum at idle (8") Also, is adding vacuum advance sometjhing you would try on a 12.5:1 BB??

Great post!!!


Re-read entire post!

kirkwoodken
406 SB, Original Bill Thomas '63 Rochester FI
AFR 210, Lunati 501C2LUN, 255/[email protected]", .628"
10:1, TH400, 3.31's, 4400 SS

"Life is too short to not run a solid roller cam."
"Nothing is impossible; if you don't know what you're talking about."
Kirkwoodken
kirkwoodken is offline  
post #13 of 102 (permalink) Old Aug 17th, 07, 10:58 PM
Senior Tech Team
 
Join Date: Aug 2002
Location: Up North
Posts: 1,288
Re: Ignition 101

O.k. but what about folks with the new HP series of Holley carb WITHOUT any hook up for manifold vaccum to run a vacuum canister?

Could my ''skunk in the tail pipe'' be from the HP carb and non vacuum can MSD distributor?

Man that stuff sure cost a lot (HP carbs and race dizzies)! But as mentioned, it does run like a raped ape at WOT!

Last edited by Chris_69_SS; Aug 17th, 07 at 11:11 PM.
Chris_69_SS is offline  
post #14 of 102 (permalink) Old Aug 17th, 07, 11:23 PM
Lifetime Premium Member
Todd
 
Join Date: Dec 2006
Location: San Martin,CA
Posts: 1,368
Re: Ignition 101

Quote:
Originally Posted by kirkwoodken View Post
Re-read entire post!
I re-read the post and still don't know if the msd vac advance is interchangeable with a GM unit.
496blaze is offline  
post #15 of 102 (permalink) Old Aug 17th, 07, 11:34 PM
Senior Tech Team
Ken
 
Join Date: Sep 2005
Location: Missouri
Posts: 2,562
Re: Ignition 101

Quote:
Originally Posted by Chris_69_SS View Post
O.k. but what about folks with the new HP series of Holley carb WITHOUT any hook up for manifold vaccum to run a vacuum canister?

Could my ''skunk in the tail pipe'' be from the HP carb and non vacuum can MSD distributor?

Man that stuff sure cost a lot (HP carbs and race dizzies)! But as mentioned, it does run like a raped ape at WOT!
Again: Re-read entire post. If you have a car, any car, with any engine, and you drive that car at cruise speeds at part throttle, you need vacuum advance. How much simpler can this be stated. ALL engines running at part throttle under light load need vacuum advance. It's not about compression. It's not about the camshaft. It's not about the carb size. It's not about the weight of your car. It's about running at part throttle when mixtures are leaner than at wide open throttle.

Let's say it another way. If you do not drive your car with your foot to the floor 100% of the time, you need vacuum advance.

Does your car have power brakes? Is there a pipe plug anywhere on your intake manifold?

kirkwoodken
406 SB, Original Bill Thomas '63 Rochester FI
AFR 210, Lunati 501C2LUN, 255/[email protected]", .628"
10:1, TH400, 3.31's, 4400 SS

"Life is too short to not run a solid roller cam."
"Nothing is impossible; if you don't know what you're talking about."
Kirkwoodken
kirkwoodken is offline  
Sponsored Links
Advertisement
 
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Chevelle Tech forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address. Note, you will be sent a confirmation request to this address.

Email Address:
OR

Log-in









Old Thread Warning
This Thread is more than 979 days old. It is very likely that it does not need any further discussion and thus bumping it serves no purpose.
If you still feel it is necessary to make a new reply, you can still do so though.

Thread Tools Search this Thread
Show Printable Version Show Printable Version
Email this Page Email this Page
Search this Thread:

Advanced Search
Display Modes
Linear Mode Linear Mode



Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome