verifying your real advance curve

Discussion in 'Ignitions & starters and electrically related comp' started by grumpyvette, Mar 29, 2011.

  1. grumpyvette

    grumpyvette Administrator Staff Member

    you need to start by verifying TDC on the timing tab and damper actually show TRUE TDC.
    IF that base line (TDC on CRANK AND TIMING TAB) is not correct everything done after that is based on that TDC, will be wrong.

    every engine combo will require a slightly different ignition advance curve to produce the ideal potential results, yet you'll find most will need rather similar and predictable requirements and (yes the SBC AND BBC both tend to require similar ignition advance curves and use the same basic distributor design).
    youll find a good average advance curve too stat with to set up your advance curve, pictured below, plot your existing advance curve then make the required changes to get close to that standard as a start point.
    step one, VERIFY your REAL CURRENT ignition advance curve, once you know what your dealing with then, you can start testing if changes YOU document help or hurt, but making random changes and guessing is rather counter productive, the OBJECT of changing the combustion, ignition point, is to maximize the effective pressure curve over the piston,thus maximizing the potential torque the engine produces, AS ITS THE PRESSURE OVER THE PISTON, PAST TDC, that produces useable power, yet youll need too prevent your engine from getting into detonation.
    you need a timing tape or marked damper ,timing tab indicating TDC and both a TACH and a TIMING LIGHT, you need some graph paper and an understanding of what your doing, which is basically recording on paper where the timing light indicates the ignition timing falls every 200 rpm or so from idle to 4000rpm , BTW more radical cams tend to like more initial advance and a slower advance curve, a fairly radical solid lifter cam may run better with your starting at 12-14 degrees and running up to 36 degrees by 3200rpm, youll simply make a graph, along the base line mark the rpms, along the vertical mark degrees of advance, then plot the curve as you change engine rpms at each 200 rpm level
    on a basically stock engine you would set the timing at about 6 degrees BTDC , at 800rpm , then raise the rpm in 200 rpm, stages, marking the ignition advance to plot the advance curve, until the ignition stops advancing, (usually at about 3500rpm on a stock ignition, once you plot the curve you make changes then re-plot the resulting ignition advance. youll need a decent tach,.
    ( having a friend to help keep the engine rpms steady while you plot the graph,sure won,t hurt) and a sheet of graph paper, and a well marked damper , and an accurate timing light, won,t hurt either.

    at lower engine rpms less ignition advance is needed because theres more time available, between ignition and cylinder pressure building , over the piston ,as the flame crosses the cylinder, so most of the pressure occurs after the cranks rod journal passes TDC, at lower rpms this burn & pressure build can take 50 thousands of a second, as rpms increase the time available is much shorter requiring a longer lead time or a greater "ADVANCE" but as rpms further increase ,turbulence caused by rapid compression increasingly speeds burn times












    many dial back timing lights have a well deserved reputation for minor in-accuracy.


    The ignition control module in the distributor is another item that normally fails when hot, that needs to be replaced is you suspect its defective
    first step is verifying TDC, and having a marked damper you know to be correct , its important to know how fast and how far the ignition curve advances, generally 34-38 degrees max advance is the limit but knowing where your spark occurs as the rpms increase is important to consistent performance, and engine heat and
    watch video ... r_embedded
    summit racing and a few other places sell clear distributor caps which can be useful


    keep in mind as rpms increase so do port speeds and volumetric efficiency UP TO A POINT, WHERE THE TIME LIMITATION TO FILL AND SCAVENGE the cylinder limits power, this increase in mixture motion above the piston, in part due to greatly increased exhaust scavenging by about 3000rpm-3500rpm in most engines means the ignition advance need no longer increase as the burn efficiency tends to keep increasing the burn speed as the rpms increase
    KEEP IN MIND that theres TWO totally different damper and timing tab locations that are correct on the SBC engines and you must use matched components for almost all years the ting tabs and dampers show TDC to be at about 2 o,clock, but theres a few applications that used a 12 o,clock timing tab and damper combo and you can,t mix&match the two types
    the object to selecting a cam, and setting your ignition advance, selecting the correct headers etc. is to maximize the engines cylinders volumetric efficiency in the intended power range and thus maximize the engines torque curve,in the intended power band, your torque is basically related to how effectively you build and use cylinder pressure and your engines displacement.
    as long as most of the cylinder pressure builds after the crank throw passes TDC, the pressure in the cylinder is used to push the piston down on the power stroke and make power, any pressure built before the crank rotation reaches TDC reduces power as it resists rotation, keep in mind at low rpms it takes about 30-50 thousands of a second to burn a cylinder of f/a mix, as the rpms increase the time required to burn the compressed mix decreases due to several factors like squish and turbulence, but at low rpms you don,t need a great deal of ignition advance because at 1000rpm your only getting 500 power strokes per minute per cylinder, or about 8 power strokes every second, so 6-8 degrees advance allows plenty of time to build pressure above the piston, as it reaches and passes TDC, and have most of that pressure build after tdc in crank rotation.
    BUT if you increase the rpms to 3000rpm and youve cut the available burn time into less than a third, so ignition needs to occur sooner in the rotation, thus the need to advance the ignition point in relation to piston movement compressing the fuel/air mix, but as rpms continue to increase the flame pattern advance due to constantly being compressed faster and more violently, decreases the need for further increased ignition advance at some point, usually at about 3200rpm, where your ignitions usually fully advanced







    viewtopic.php?f=70&t=1809 ... urve.shtml

    viewtopic.php?f=70&t=1015 ... g-148.html

    Its really very simple, just use a .045 gap and ignition wire with LESS THAN 500 ohms of resistance per foot, good grounds and name brand single electrode ground plugs, a Fuel/Air ratio of about 12.7:1-14.7:1 , verify your ignition advance and timing, and if thats not working its not the GAP, its something ELSE in the ignition system that requires repair, modification or replacement
    I have no proof these are better or WORSE, than the other options but the price looks good so I have to wonder if they are junk or everyone else just marks the similar distributor up considerably more once its re-boxed and re-labeled

    read thru this


    this is an area where relying on a mechanic rather than doing it yourself is not always the ideal route to follow, especially if you have not verified TDC on the damper and timing tab,and your ignition advance.
    its very common for guys to check the ignition timing at idle, and assume thats all thats required,you need to verify the ignition advance and total advance and it helps a great deal to have a marked damper and a shop manual to give you a detailed procedure on timing the car as a base line to start from and guys frequently either forget to follow instructions and disconnect this connector during the process,
    or to be clueless about how to verify the timing advance curve,
    either can result in not having the timing correct

    now Ive pointed out several times that you need to know how your ignition advance curve works so you can adjust it if required, but I guess I never went into detail about how you go about verifying or modifying that advance curve. (an obvious mistake that was recently brought to my attention)
    you'll generally need to know what your dealing with before you can make any logical changes , so you start by verifying where you are currently, for that you'll want a clip board , a timing light a tachometer, timing tape and a buddy you can trust too help you if possible.
    if the timing advance comes in to quickly or advances too far you build excess pressure before the piston reaches TDC and your much more likely to get into engine destroying DETONATION
    if the timing advance comes in to slowly or advances too little, you build less pressure both before and after the piston reaches TDC and your much more likely to get into excessive exhaust heat that can burn exhaust valves as the fuel/air mix partly burns as the exhaust is open

    obviously you need to have a consistent base line advance curve to work with,
    on most Chevy v8 engines that run cams designed for street/strip use Ive generally found a advance that goes from about 8-12 degrees at idle speed (800-900rpm in most cases) and smoothly advances the ignition to about 36 degrees or about 28 degrees advance from where it started at to reach 36 degrees at about 3200rpm , is generally a good place to start, or about 82 rpm increase per degree of ignition advance , up to about 3200rpm,and above where the constant increase in turbulence and squish in the mixture as its compressed tends to speed the burn process, you can then play with the engine and determine what changes MIGHT be required.

    ok lets drop back to basics
    you need to start by finding and marking TDC
    then getting that correctly marked and get a timing light and timing tape on the damper so you can see whats going on.
    once youve got the timing tab and damper marked verify the timing so you know what your dealing with before making changes.
    your total ignition advance should generally be consistent and look like the chart below, weights and springs in the distributor control that to some extent, your total advance is more critical to engine operation than the at idle setting.
    obviously each engine will require tweaking that curve and the rpm points to maximize the power curve.


    if your experiencing detonation issues that are cured by swapping to higher octane rated fuel, and you would prefer to use the lower octane , less expensive fuel, you should adjust your cars ignition advance combination , so that its advance curve has either less initial timing, or delaying the mechanical advance vs. rpm with some stiffer springs, or a combination of both might reduce the pinging under load at 2500-3500 rpm where its most commonly seen,. Does this detonation or pinging, only occur at WOT? If not, limiting the vacuum advance with a stop, or using an adjustable vacuum advance unit and raising the amount of vacuum required vs. the amount of vacuum advance might be warranted also and installing a lower temp rated t-stat and adjusting the engine fuel/air ratio a bit richer may also help..

    the amount in degrees the ignition advances with changes in vacuum and RPMS can be CHANGED

    your at a huge disadvantage if you don,t have an accurately marked damper and timing tab, there are both timing tape and damper covers that can be used



    some graph paper, or the blank rpm graph I posted (PRINT IT OUT) that you can use to document the true results.TDC (TOP DEAD CENTER)
    on your damper and verifying the engines timing tabs do indicate that true TDC.
    heres related threads on those aspects






    viewtopic.php?f=55&t=109 ... index.html


    once youve located TRUE TDC, you either install timing tape on your current damper, or a marked cover
    or ideally you have a damper thats both correctly marked

    and an adjustable timing tab

    your obviously going to need a tachometer and a timing light

    first youll want to verify theres no vacuum leaks,youll use a un-lighted propane torch not any flammable sprays as its far more precise, as you can stick the torch tip exactly where you suspect leaks,and it less messy and leaves no residual crap on the engine.
    ITS important to understand how and why things work,IF you don,t have a timing light, generally you use a vacuum gauge on a STOCK engine to set timing by hooking it up to plenum vacuum , set the idle to normal idle speed (usually 670rpm-850rpm)and adjust timing to max vacuum reading, this DOESN'T,T ALWAYS result in the best possible timing but it will generally be fairly close on a DEAD STOCK ENGINE

    [​IMG] ... 95670.html

    you'll then want to get a friend to hold the engine rpm level steady at 200rpm steps so you can use the tach and timing light on the timing tab and damper tape to record where the ignition advance moves at each rpm level, and place a dot on the clip boards graph sheet so you can document the timing at each step or 200rpm, level, then graph out the resulting advance curve

    you should eventually find you get something similar to this


    standard vacuum connections

    manifold to power brake booster
    manifold to distrib vacuum advance
    pvc to carb or air cleaner assembly

    clearer example
    THANKS Indycars

    clearer example THANKS Indycars




    Last edited by a moderator: Jun 12, 2018
  2. Indycars

    Indycars Administrator Staff Member

    Attached Files:

    Last edited by a moderator: Jul 22, 2016
  3. Indycars

    Indycars Administrator Staff Member

    Looks like the fingers weren't cooperating when you got to "or about 82 degrees advance per hundred rpm".
  4. grumpyvette

    grumpyvette Administrator Staff Member

    first, thank you for the great job on the printable form to use for documenting the advance curve

    and yes your correct , it was late,
    start at lets say 900rpm, ending at lets say 3200rpm=2300rpm in the effected rpm band ,divided by 28 degrees, total advance= about 82 rpm per degree of advance in the ignition advance degrees

    heres some info for the MSD advance curve kit

    BTW HERES A OPTIONAL ROUTE THAT MIGHT BE OF INTEREST TO SOME, as Ive stated many times theres a huge amount of info in the sub links ... Timing.pdf


    there are weight and spring kits available from Accel also
    yes you'll want too open and read the links and quotes
    Here's a list of resistance values for anyone interested in making their own MSD chips. 1/4 watt resisters work just fine.

    RPM Kilo-OHMS
    1400 = .491
    1500 = .639
    1600 = .819
    1800 = .947
    2000 = 1.161
    2200 = 1.364
    2400 = 1.50
    2500 = 1.58
    2600 = 1.70
    2800 = 1.78
    3000 = 1.906
    3200 = 2.062
    3400 = 2.222
    3600 = 2.385
    RPM Kilo-OHMS

    3800 = 2.551
    5000 = 3.63
    5200 = 3.82
    5400 = 4.02
    5600 = 4.20
    5800 = 4.41
    6000 = 4.62
    6200 = 4.82
    6400 = 5.04
    6600 = 5.26
    6800 = 5.46
    7000 = 5.72
    8000 = 6.96

    To make an adjustable RPM switch get a 6 position rotory switch with detents from an electronics store. Then solder in the resistors for the desired RPM levels. Use it in conjuction with a two step to control launch RPM. It works great. I found that when you launch the car 1,000 RPM below converter stall the car hits real hard as you take advatage of the flashing.

    Below is the resistance ratings for Autometer chips for their shift lights.

    • RPM kohms
    • 3000 = 1.897
    • 5600 = 4.22
    • 5800 = 4.44
    • 6000 = 4.64
    • 6200 = 4.85
    • 6400 = 5.08
    Last edited by a moderator: Aug 5, 2017
  5. Indycars

    Indycars Administrator Staff Member

    At first I thought you meant 8.2 degrees/100 RPM, but when I did the math it came out something like 1.2 degrees /100 RPM. It's just SO DAMN easy when you are the only one looking at some numbers, to flip something around. :cool:
  6. grumpyvette

    grumpyvette Administrator Staff Member

    can you post a clear picture of that great attachment link that guys can simply print , rather than download, exactly like that link, as I know some guys will have trouble using it other wise

    AdvanceCurve01.pdf [113.12 KiB]
  7. Indycars

    Indycars Administrator Staff Member

    That's easy, I'm not sure what size it will be when they print it. This way they will have a choice though.

    Attached Files:

  8. grumpyvette

    grumpyvette Administrator Staff Member

    ok, first you need to verify that your TRUE TDC and the timing tabs and the marks on the damper do reflect reality,
    then you need to do a bit of research and graph out your true actual ignition advance curve by raising the rpms from idle to the rpm range where the ignition stops advancing, Id suggest graphing the info every 300 rpm and printing out a custom graph indicating the true ignition advance curve your dealing with rather than guessing. now that you can see the actual true advance curve you have a chance at dealing with the facts vs what you might think is the issue. pinging or detonation will quickly destroy an engine so obviously its something you need to avoid, a fairly tight .038-.042 quench usually helps.
    detonation will limit your power and looking at that cam, your dynamic compressions fairly high, my calculations show about a 9.1:1 effective compression which will require at least a higher than pump octane fuel, high test pump octane fuel rarely exceeds 93 octane you need at least 98 octane with that combo.. the detonation you've experienced has most likely already damaged your pistons leading to the excess oil burning










  9. bytor

    bytor Well-Known Member

    I just dialed my advance in on my build. I discovered my setup likes a lot of initial 23*. So I had to limit my mechanical advance with a bushing that would only allow 10*. I’m running a Mallory Powermaster distributor with no vac advance. Here's what the engine seems to be happy with. The blue line is the actual measured advance. The red line is what Dynosim calculated for MBT (mean best torque) and the orange dashed line is what the Mallory spring numbers calculated out to with 2 heavy black springs installs and the 10* advance bushing. Very pleased with the throttle response. No ping or overheating issues.

    Attached Files:

  10. grumpyvette

    grumpyvette Administrator Staff Member


  11. Indycars

    Indycars Administrator Staff Member

    How did you determine what the engine needed for ignition timing?

    How easy does it turn over when you hit the starter? Wonder if it will change during cold weather?

  12. 87vette81big

    87vette81big Guest

    Bytor's SBC static ignition timing is unusual.
    More like for an Oldsmobile V8 with factory iron heads.
    1976-79 Olds 403 engines had base ignition timing set at 24 degrees BTDC.
    No starter kickback ever.
    Fast advance curve all in by 1100 RPMS And total timing at 32 minimum with 87 octane pee water pump gas.
    With 93 octane on a cool day of 55-60. F set for total advance of 36-39 BTDC.
    No detonation issues.
    Static compression of 8.5 :1.
    Comp Cams 268H grind used.
    Ran it for 85,000 mies.s
    Then another 20,000 miles in my '63 GP.
    750CFMQ-jet recalibrated by me.
    Olds V8's since 1964 use a daul quench combustion chamber.
    Wierd but it works with 6 degree valve inclination to deck surface iron heads.

    Bytor are you using Brodix 18 degee or 15 or 12 degree heads??
    Only times I have witnessed lots of Base timing or initial advance tolerated.
    Common Brodix cylinder head hardware on 410 - 434 ci 750HP To 950 HP normaly aspirated dirt track race engines I have been around
  13. 87vette81big

    87vette81big Guest

    We ran 14.0:1 to 18.0:1 static compression ratios.
    $9 per gallon 114 motor octane engines.
  14. bytor

    bytor Well-Known Member

    Basically just trying different initial settings and advance rates until the car felt right. I started at 14* initial and the car was sluggish off the line and it seemed like it took a lot of gas petal to get the car moving. Also at 14* it had bad idle quality. I then went lower with my initial and the symptoms got worse and the car ran a bit hotter. So, I went the other way up to 23*. If I go any higher on the initial, the engine is hard to turnover when hot. Once I figured 23* was a good spot, I just limited the mechanical advance to 10 * for a total of 34*

    I have no issues starting the car at 23*. It starts right up. Throttle response is crisp and it now takes very little gas petal movement to get the car moving. One thing I noticed as well is the car seems to be quieter. I assume I was getting some additional exhaust noise at the lower initial settings. Major seat of the pants improvement. The tires are chirping all the time now when I take of at a red light. :D So, I think its close.
  15. Indycars

    Indycars Administrator Staff Member

    Thanks Bytor for the answer! What's next is on your agenda???

  16. grumpyvette

    grumpyvette Administrator Staff Member

    Introduction by: David Vizard

    This article is about advance curves and HEI style Distributors. As such I would have to say that Steve Davis, the boss at Performance Distributors, is one of the most knowledgeable people in the country on this subject. Personally I have counted on a Performance Distributor to get the job done, without any drama, for so many article project engines that I have truly lost count. One of the painfully few advantages of getting old is that you get to see who’s product performs consistently well over the years – and whose does not. On that score it’s worth throwing in a few words about Performance Distributors to put things into prospective.

    Performance Distributors was started about 1974 by Steve’s dad, Kelly Davis. I met Kelly and Steve first in 1980 when I was on a visit to Memphis to see the guys at Comp Cams and RHS. Kelly was good enough to spend time showing me the ropes so to speak as far as HEI distributors went although that is far from all the types they handle. At the time I was so heavy into small block Chevy’s that the HEI was all I was interested in. Kelly dialed me in to the HEI’s short comings as a high rpm race orientated ignition system.

    It could be argued that any shortcomings in an ignition system would be a turn off for duty as a race unit but the basic premise of a one wire hook-up had me well and truly hooked-up on HEI’s even this far back. Here I thought it was the ultimate in simplicity – now if we can make this go the distance rpm wise life as a small block Chevy builder would suddenly get simpler – at least in the spark department. Well Kelly assured me he had the answers to the problems so later on that year I tried a Performance Distributor in one of my project engines which I believe, appeared in Super Chevy magazine.

    I gave Kelly the engine spec so he could build a suitable set of advance characteristics for both the centrifugal and vacuum advance. Although Performance Distributors was in Memphis and I was in California I figured I could do what ever fine tuning was needed at my shop once the engine was on the dyno. I was pleasantly surprised to find that for this hot street motor the advance curves were about as close as can be had as any change in timing anywhere in the rpm range from 2200 to 6500 resulted in a drop in output. Since those days I have relied on spark power from Performance Distributors for some 50 or more project engines. Just a small sample of these from just the last 8 years are scattered throughout this feature. The sheer number should be a clear indication of the confidence I have in this company’s products.

    It’s now 28 years later and Kelly is no longer with us – passing in 2000. Steve now runs the company with his mom Pat doing as she has done for many years, organizing trade shows handling company personnel and accounts payable. Although no longer with us the legacy of Kelly’s original work and later (from 85 on) Steve and Kelly’s joint efforts is still very much evident. I tend to think of Performance Distributors in terms of HEI’s because I just love those systems - but that’s far from the whole picture here. Performance Distributors is in the spark generation business in very much a heavy duty fashion and as such has developed a variety of ignition systems and support equipment such as alternators and batteries for race and high performance applications. They obviously cover the big three domestic brands but they also cover popular imports, sport compacts and various others.

    What have I learned from my 28 years experience dealing with this family business? Other than polite courteous service and timely delivery they have totally satisfied the ignition needs of all of my project engines without one single hassle or failure of any kind. In short all the units I have had from Performance Distributors have done exactly what they were supposed to do. I wish I could say the same about all the companies whose parts I have tested or had to work with – but unfortunately life does not work that way!
    Spark Advance Strategies By Steve Davis

    I thought hard about an opening sentence here and decided that in this day and age it’s not just power we are looking for, as fuel efficiency is taking an even footing. Let me tell you, this story is as much about mileage as it is about power, from 200 mph Cup Cars to street cars.

    The whole concept of ignition advance is centered on charge burn time and burn time can be broken down into two categories. These are ignition delay and the bulk burn rate. The goal we are trying to achieve is to get peak cylinder pressure to occur at about 15 degrees past TDC on the power stroke. If we make that goal then the three main issues of torque, hp and fuel economy will, from the ignition stand point at least, be maximized. What we are not trying to do is run as much advance as possible. I throw that in because in the past I have read features by even some quite well known tech writers that certain mods allowed the engine builder to pile in more advance as if advance was, in itself, good for more output. That’s not the case! If an engine needs a lot of advance to make it’s best output, then that could be a sign of a problem rather than an acceptable solution. In a healthy, well spec'd engine, you should run the minimum amount of advance to get the job done. Any more and negative scenario’s will, sooner or later, start to play out. Since so many factors affect the overall speed of combustion, let’s start at square one and look at these before we get into what an engine may need in terms of advance curves and the like.

    Ignition Delay.

    There are essentially two components that make up the burn time as a whole. These are ignition delay and the bulk burn rate. Ignition delay is the time between the delivery of the spark into the charge and the point where an identifiable flame kernel can (if the means to do so exists) be seen. During this phase there is no cylinder pressure rise due to any combustion, as this event involves only a miniscule amount of the charge mass to be burned. The delay time varies from one fuel to another and can be anything from about 4 degrees to as much as 12 degrees of crank rotation.

    The biggest difference I see is the delay time between leaded race fuels and non-leaded fuels, race or otherwise. Although not universally so, you can usually count on a leaded race fuels needing about 4 to as much as 6 degrees more timing anywhere in the operating envelope of the engine. The longer delay time of a hard core race fuel is not dependant on the octane as it is the lead content. Remember adding enough Tetra-ethyl-lead (TEL) can boost an 87 octane fuel to 100 but such a fuel will usually need far more advance for optimum results than will a non-leaded 100 race fuel.

    Between various brands of service station pump fuels the delay rate changes little so there is little need to jump in under the hood and retime the ignition accordingly. The same cannot be said if you make the change from unleaded to leaded fuels. If such a change is made a re-evaluation of the entire timing envelope will be needed. The most critical area of operation here will be the total timing under Wide Open Throttle (WOT) operation. For most practical purposes the only move necessary will be to adjust the initial timing so that the whole envelope is moved forward or back as required.

    Although the ignition delay is a characteristic of the fuel it is affected by what’s going on in the cylinder and the spark energy, temperature and duration. This is a complex area and I don’t claim to have all the answers but there again, unless I’m seriously delusional, no one else does either. Here’s my take on it at the present time. At part throttle low rpm operation increasing spark energy, temperature and duration all seem to reduce ignition delay time. Just how much they do so appears to depend on the starting conditions. High mixture turbulence and optimal fuel atomization/vaporization almost always means the charge is easier to light off where as a poor quality air/fuel mix with little mixture motion is not. In the first instance spark energy and temperature play a role toward a quicker light off, but spark duration, for the most part, seems less important. In the second instance where the ignition has to overcome inadequacies in the mixture preparation, spark duration can play a more significant role where longer is better.

    Bulk Burn Rates.

    Notice I am using the term ‘burn rates’. That’s with good reason as the charge does not explode as is so often implied. On the other hand detonation is an explosion and that can wreak havoc on an engines internals. Detonation occurs when a combination of temperature and pressure brought about by the charge so far burned, causes the remaining unburned charge to spontaneously ignite. The sudden heat release and the extreme rise in cylinder pressure will quickly destroy the pistons. I may as well blow another myth here while I am at it – detonation does not occur when two flame fronts collide. Here is a short description of what happens when two approaching flame fronts meet – next to nothing! Detonation is in fact a complex event but normal combustion is probably even more so. If you want to visually see, in step-by-step fashion, how combustion progresses, check out the illustrations in David Vizard’s article at Turbulence and Combustion Dynamics.

    Three primary factors affect the rate of burn within the cylinder. These, in the general order of descending influence, are Pressure, Temperature and Mixture Motion (or PTMM). A charge at low pressure, temperature and with zero mixture motion, burns at little more than a walking pace. That’s hardly fast enough to allow us to turn a motor over at idle let alone 9,000 rpm. By increasing the pressure, temperature and mixture motion at the time of combustion the speed at which the flame front progresses can be increased dramatically. The peak speed of flame progression in a Sprint Cup car engine at 9000 rpm is well over 200 mph. Getting the charge burned too good effect means having about 90% of it burned in little under one thousandth of a second. Since the time scales are so small it should be evident that there is even less margin of error for any errors in timing.

    Mechanical Advance

    There are two types of advance – vacuum is one and rpm related advance (usually referred to as mechanical) being the other. Let’s talk mechanical advance first.

    What the engine needs in the way of advance is solely dependant on the PTMM factors just mentioned. In practice we find that at low speed there is time to get the charge burn going such as to achieve peak pressure at about 15 degrees after TDC. But as rpm goes up, the time to get the burn process done in time, gets less. To compensate here we need to advance the ignition as the rpm goes up. Countering this, and speeding up the burn rate, is the fact that mixture motion increases as rpm increases. Also, especially when a bigger cam is used, cylinder filling and hence compression pressure will increase until the breathing capacity of the induction system is reached. On a power curve you can see approximately where this happens as it closely coincides with peak torque. So with an increasing rpm we need to put in more advance but as the mixture motion and the compression pressures come into play so the burn rate speeds up thus cutting the amount of advance required. At first RPM wins out, but as the engine gets to 3000-4000 rpm we find the pressure and agitation of the charge speeds up the burn at just the rate to counter the effects of increasing rpm. At this point the timing can be fixed from there on up. What this means is, at first the timing needs to advance with increasing rpm but as the rpm goes up so the amount of extra timing drops off until the engine reaches a point where no additional timing is needed from that point on up. The maximum timing needed by an engine is commonly called the ‘total timing’.

    The biggest factor affecting the required advance curve in the distributor is the camshaft. The longer the duration the lower the cylinder pressure is at lower rpm. When swapping out a stock cam for a bigger cam we find that the advance needs to come on sooner and faster. The next factor to consider when determining what curve is likely to be needed is the compression ratio. When the compression goes up the compression pressure at the end of the compression stroke is higher so the rate at which the advance needs to come on goes down. In other words it works around the other way to the effect a bigger cam has.

    Just for the record when a bigger cam is installed in a normally aspirated engine, the compression ratio should be increased to get best results. By having a better match of CR to cam duration the loss of low speed output from a bigger cam is almost negated until cams of over about 280 or so advertised degrees are involved. In other words pairing off compression and a bigger cam means more top end without loss of bottom end. For the amount the compression would be typically raise though, the effect of the bigger cam will still be the principle control factor.

    The key toward making a big cammed motor run sweeter under low rpm WOT conditions is to have the timing advance curve that the new parts combo really wants. But that’s far from the whole story. The fact of the matter is, that by adding a vacuum advance function to the distributor it is possible to run a cam of at least 5 degrees more duration before idle quality drops below a certain limit. That extra 5 degrees of duration can easily mean 10 extra horses. Since it is a power factor let’s look at vacuum advance in a little more detail.

    Vacuum Advance.

    Neither the canister on the distributor nor the MAP sensor, if you have an EFI engine, is there for decoration. Too many street rodders see that the pro racers use distributors without vacuum advance and figure if it’s good for the winning car it’s good for them. Unfortunately this is not so. Although a drag race car may not realize much in the way of an advantage, none of it performance related, it’s about the only type of race car that falls into this category. Here’s how the puzzle fits together.

    When you back out of the throttle the manifold vacuum goes up. This means the compression pressure at the end of the compression stroke is way lower. When a spark timed for a normally fast burning charge (as it is at WOT) fires under these circumstances it is going to occur far too late for the pressure to be at it’s peak at 15 degrees after TDC. This means that less than optimal use of the energy content of the fuel has been made. That in turn means burning more fuel to get the level of power being demanded by the driver at that particular moment. By having vacuum advance pull in appropriately more timing you can let out of the throttle more and consequently cruise on less fuel.

    Also you may have based a cam selection on the basis of minimal negative idle impact. This may have limited your choice of cam to one of say a 280 degrees (advertised). If this was to be paired with a distributor with no vacuum advanced as most competition ones are then rest assured you have probably given away about 10-15 hp. If a vacuum advance is used the ignition timing at idle can be optimized where-as without vacuum advance it cannot. Assuming a realist street engine idle speed a ‘mechanical advance only’ distributor is barley pulling in any advance. What with say 12 degrees of initial and at most 5 degrees mechanical the total timing at idle would be only 17 degrees. In practice we find that a big cammed V8 needs about 50 degrees for best idle along with the lowest idle fuel consumption. I have seen some engines require as much as 55 degrees yet these engines – with cams as big as 260 to 270 degrees at 0.050, would idle tolerably well at 1000 rpm – sometimes less. The same goes for cruise. The vacuum advance can pull in the desired advance which at about 2500 rpm and a quarter throttles can be as much as 50 degrees. Normally an engine with mechanical advance only would have no more then about 25 to 30 degrees max. It does not take a diploma in rocket science to see that cruise fuel consumption is going to be much higher with the timing as much as 25 degrees out!

    So where am I going with all this talk of vacuum cans and the like? Simple, if you want your street/strip car to return of it’s best in mileage then best you order your distributor with a vacuum advance can. Not only are you likely to get in the order of 2- 4 more mpg from a typical hopped up Detroit V8 not so equipped but also the motor will idle far smoother and have less tendency to foul up plugs. This is because it will idle on a leaner mixture so there is less overly wet charge passing through the system.

    This brings up the subject of ported vacuum versus manifold vacuum. Ported vacuum is sourced just above the carb butterflies. At idle there is no vacuum signal to the distributor so no vacuum advance is pulled in. This means at idle the timing is way retarded over what is otherwise needed. It is only when the throttle is opened slightly that the butterfly goes past the vacuum port. At this point the port is now connected to manifold vacuum and advance is pulled in.

    Manifold vacuum taps into the intake manifold below the carb throttle plates so actually sees manifold vacuum. If you are looking for the best idle it’s manifold vacuum all the way. The ported vacuum was a means toward cleaning up emissions at idle. By having the engine demand more air (and consequently more fuel) at idle the hydrocarbon parts per million could be reduced. To get the idle speed down the timing used was retarded in as much as no idle vacuum advance was present.

    For you circle track racers here is something I want you to think about. If your car’s engine had vacuum advance it would get about 20% more mileage on the yellow flag laps. Just how many times have we seen a lead car run out of fuel with just half a lap to go?

    In the business, vacuum cans are often designated ‘hard’ or ‘soft’. For most street applications where the cam is short and the amount of manifold vacuum seen is about 12 to as much as 20 inches of mercury, we find that the vacuum needs to start coming in at about 5 - 7 inches or so and pull in about 15 to as much as 20 degrees of advance by the time the vacuum has reached 12 -15 inches.

    For an engine with a racier cam the can needs to be soft and advance needs to start almost as soon as any vacuum is seen. This is not always practical from the hardware standpoint but for a big cammed engine we will typically use a can that comes in at 5 inches and pulls in 15 or more degrees by the time 10 inches of vacuum is seen.

    Hopefully this primer on advance curves has answered some of your questions. Down the road we will look at other aspects of ignition which contribute to more power and better mileage. However do not loose sight of the fact that your first goal is to have the spark occur at the correct moment for all engine operating conditions.
    Written by: Steve Davis and David Vizard,
  17. Indycars

    Indycars Administrator Staff Member

    I can remember back in the 70's that the SBC timing was always 38° to 42° BTDC. Now speed ahead to today and the timing is 34° to 36° BTDC. I figured that it was the better combustion chamber design we have today, but as I see from the article above it's more about the removal of lead from gasoline requiring less timing by 4° to 6°.

    One might think that high octane racing gasoline would not produce as much power, since the there would be more negative work with the greater timing advance. But since this occurs during the period of what's called "Ignition Delay" then there would no increase in negative work by the engine during this period.

    Ignition Delay Definition:

    Ignition delay is the time between the delivery of the spark into the charge and the point where an identifiable flame kernel can (if the means to do so exists) be seen. During this phase there is no cylinder pressure rise due to any combustion, as this event involves only a miniscule amount of the charge mass to be burned.

    I've always wondered why the ignition timing needed to be all in by 3000 to 4000 RPM. The time available to burn the mixture continues to get shorter as RPM increases, so why stop advancing the timing. The light came on when I read the following paragraph in the article above.

    What the engine needs in the way of advance is solely dependant on the PTMM factors just mentioned. In practice we find that at low speed there is time to get the charge burn going such as to achieve peak pressure at about 15 degrees after TDC. But as rpm goes up, the time to get the burn process done in time, gets less. To compensate here we need to advance the ignition as the rpm goes up. Countering this, and speeding up the burn rate, is the fact that mixture motion increases as rpm increases. Also, especially when a bigger cam is used, cylinder filling and hence compression pressure will increase until the breathing capacity of the induction system is reached. On a power curve you can see approximately where this happens as it closely coincides with peak torque. So with an increasing rpm we need to put in more advance but as the mixture motion and the compression pressures come into play so the burn rate speeds up thus cutting the amount of advance required. At first RPM wins out, but as the engine gets to 3000-4000 rpm we find the pressure and agitation of the charge speeds up the burn at just the rate to counter the effects of increasing rpm. At this point the timing can be fixed from there on up. What this means is, at first the timing needs to advance with increasing rpm but as the rpm goes up so the amount of extra timing drops off until the engine reaches a point where no additional timing is needed from that point on up. The maximum timing needed by an engine is commonly called the ‘total timing’.

    Well anyway, that's what I came away with after reading this article, what did you learn?

  18. grumpyvette

    grumpyvette Administrator Staff Member


    you might also find these useful


    viewtopic.php?f=50&t=9816&p=37278#p37278 ... _notes.htm


    Last edited by a moderator: Nov 21, 2015
  19. bytor

    bytor Well-Known Member

  20. Indycars

    Indycars Administrator Staff Member

    Thanks Mr. Bytor !

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