Ignition Timing with Modern Fuels

Getting the ignition timing of your classic's engine right is a critical part of getting it to run properly. Get it wrong and you can adversely affect performance, economy and even engine longevity. Get it right and you can enjoy smooth, responsive and practical classic motoring. 

But now that the leaded fuels our cars were originally designed to run on have been withdrawn from garage forecourts and replaced with chemically different unleaded petrol, vehicle owners handbooks can no longer be relied upon to provide accurate ignition timing figures. 
The following tutorial explains how to set up a classic engine to make the most of modern fuels and minimize the risk of damage to your engine.

In order to achieve peak fuel efficiency, the air/fuel mixture in an engine's cylinder should be at the peak point of its burn at around 20° ATDC. Knowing the rate at which fuel burns allows manufacturers to 'work backwards' and define the ignition timing for an engine at idle, typically around 4-6° BTDC.
However, modern unleaded combusts at a notably slower rate than the older leaded fuels, and ignition timing must be adjusted to compensate for this.
To further complicate the matter, many classic sports and performance engines have higher compression ratios (say 10:1 compared to a more standard 8:1) to take advantage of 5 star fuel - a 100 RON octane rated petrol phased out during the late 1970s. Modern Super Unleaded rarely has an octane rating higher than 97 RON, and so significant adjustment to engine timing is required to set these engines up properly for modern use.

1
Begin by setting the ignition timing to the figure given in the vehicle owner's handbook, and mark the distributor at the base to record its original position before commencing adjustment. Set the carburettor(s) to achieve a reasonably smooth idle. Remove the distributor vacuum advance and plug the end of the hose to prevent weak running. It is a wise idea to use a gas tester or Colortune kit to ensure that the optimum amount of fuel is entering the cylinders at this point. 

2
When satisfied that the carburettor(s) are in reasonably good order, slacken the pinch-bolt and rotate the distributor body in a clockwise direction. It is useful to have a strobe-timing light for this job. Advance the timing by 1 degree and mark the distributor body again in its new position. The two marks side-by-side provide a visual scale denoting one increment. This may be useful at a later stage during fine tuning.

3
Advance the ignition further. As the timing is advanced, the engine should settle down to a smoother idle. At this stage, it may be necessary to increase the engine idle speed at the carburettor(s) to prevent stalling or labouring: 800-900 rpm is an ideal speed for discerning a change in engine smoothness. After advancing the ignition far enough, the engine should become less smooth again. At this point, mark the distributor body once again. The three marks will show the range over which the engine's timing may be advanced whilst maintaining a smooth idle, including one increment of that scale.

4
When this scale is ascertained, rotate the distributor back until the timing is set at midway between the two extremes of the scale

5
Check the richness of the carburettor(s) again with a Colortune or gas tester kit, and take the car on a test run. Accelerate through all of the gears, and assess the engine's willingness to pull the car forwards. If performance is sluggish, it is indicative that the timing may be advanced further. Slow the car down in each gear until the engine speed is around 1000rpm. Accelerate firmly without flooring the throttle and listen for any metallic 'ringing' or 'pinging' noises coming for the engine. Built the road speed up, and accelerate from around 2000rpm in top gear, listening for the same.
This metallic ringing, often referred to as 'pinking', is a sound caused by burning fuel pushing back against the head of the piston before it has reached the top of its stroke, and is indicative that the timing is too far advanced. Pinking can cause wear to piston rings, valves and big-end bearings. Over a sustained period, it may lead to compression loss, and if left unrectified can lead to engine failure.

6
If no problems occur and pinking cannot be heard, advance the ignition further by one increment (or one degree if a strobing light is at hand) and repeat the test. After advancing the timing one degree at a time and repeating the test a number of times, pinking will eventually become detectable under hard acceleration. At this point, retard the timing by one degree. Run the engine for a week or two in all road conditions, testing for performance, economy and smoothness. If pinking is detectable at any point, or the engine seems to be noticeably down on power (and it is clear that this is not due to carburetion), retard the ignition by a further degree and run for another week.

7
When satisfied with the engine's performance, make a note of the timing position for future reference and set the carburettor(s) up properly, following the tutorial in the Step-by-Step guide.

It is worth noting that once set, the engine must be run on the same grade of fuel used during the test thereafter. Modern, computer-controlled engines are capable of making dynamic adjustments to engine timing to compensate for higher or lower grade fuels, but these adjustments must be made manually in an older vehicle.
If you are running standard 95 RON unleaded, and decide to change to 97-98 RON super unleaded, the ignition timing should be advanced to achieve peak performance. If you are running super unleaded and wish to change to standard, the timing MUST be retarded to avoid damaging pinking.

10 comments:

  1. This one is my favorite post. I think it will help me a lot in my further studies and research. Very well written I appreciate & must say good job..

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  2. Really that’s great article, these points are very helpful, thanks for sharing with us.

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  3. Any idea by how many degrees you should typically expect to need to advance the timing by changing to 98RON over 95RON?

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    1. As far as I know it is best to do it the way described above. It really is about running the car, get it to pink and back off. Different cars have different advance retard requirements anyway. Don't be fixated on degrees, fixate on the running of the engine. A timing gun gets you somewhere handy but fine tune as above. If you fit electronic ignition, you need to really whack the distributor surprising forward in my Triumph. So marking on side of distributor became useless and needed new marks. Strobe, to correct advance 8ish degrees on mine and took her out for a thrill and edged it forward to pink and then backed off. Some Lucas distributors have a little wheel on the side which can take if forwards and back by 1/10 of a degree. These engines are old, a bit slack in tolerances. Hopefully your ears aren't. But getting electronic ignition on, gives a better spark and is said to be more accurate (although some will debate, most accept). Hence the ear approach I found best on mine. It won't turn into a turbo or anything but when it is right it feels right.

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  4. Confused here - I always understood that ignition timing was RETARDED for unleaded petrol as the lead tended to smooth out the burn. Wasn't this why some cars had 'pinking' problems on changing to unleaded?
    Dave S

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    1. I think this was because from leaded to unleaded there was a drop in octane. Putting richer fuel you need to advance. Super unleaded is near the same as leaded, however, doesn't make enough difference to warrant the price. If you put in unleaded instead of leaded (such timed) the engine needs retarding as essentially the engine is too advanced for unleaded fuel.

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  5. i ttthink i am even more confussssed now ...

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  8. I am running a fully restored 1969 TR6 and have dumped the Lumenition electronic ignition for a modern electronic ignition system plus Lucas sports coil. Huge improvement. Now onto the fuels and timing. The CP (pre '73 150bhp model) 2.5 PI engine states 11-degrees before top-dead-centre timing in the original Triumph handbook. This needs to be done with idling lowered a bit to avoid any action from the distributor's centrifugal weights coming into play, but normal 'acceptable' idling is around 850rpm, so lower this with the handy air intake idle-adjustment thumb-screw at the end of the throttle linkage/airbox region to about 600rpm for accurately checking the timing via a strobe lamp. Whilst all the above that has been said is most relevant, if running an engine like this, (which was supposed to run on 5-star 100 RON fuel originally, with 4-star 97 RON acceptable too), then today's fuels need to be explained because there's much that is not known by everybody. It should be noted that if wanting to fully enjoy these cars, then it really pays to use the super unleaded variant with it's higher octane rating. However, there is now only one brand of petrolium acceptable for our older cars that does not harm the rubber and petrol tank linings. This is Esso. All other brands of petrol (of either grade) use a bio-fuel additive called Ethanol, which is a harmful constituent of their fuel to old cars. This is an attempt by Western Governments to lower the use of fossil fuels with a percentage of sustainable bio-fuel in the mix. At the moment we have anything from 5-10% known as E5 and E10. Using Esso super unleaded in my TR6, (free of Ethanol at the moment and not likely to be added in the foreseeable according to Esso who have made a statement to this effect in the last year), with the 11-degrees TDC timing adjustment, (no vacuum advance on the early CP cars to worry about), there is still detonation or pinking at mid-range hard acceleration. This is because even the 97 RON Esso fuel does not have the composition of the original 4-star fuels of the early 1990's and before. I am going to try a half-tank of Shell's super unleaded V-Power or whatever it is called now, but only to see if their 99-RON fuel improves/removes this pinking aspect. I cannot endorse running my car on this long-term though as it does have the dreaded Ethanol within it's mix and the problems are very real with rubbber and fuel tank linings' deterioration, - as widely commented on by other classic car users. On-line though, I do remember finding, at considerable expense it must be said, an outlet that sold petroleum of various RON ratings that went to over 100 RON. I have temporarily forgotten the operation's name, but it would be worth a try because my TR6 is not anything like my milder-camshaft 1972 Triumph 2.5 PI saloon's 132bhp performance, which I had between 1989 and 2000 and was fully restored, running on Piranha electronic ignition. The lack of power on the TR as compared with the 2.5 PI saloon being purely down to the fuel used as everything else has been done: metering unit calibration, compression check on establishing the engines were running to spec., etc. A fully working early TR6 should turn in superior performance to the heavier and lower bhp 2.5 PI saloon, which in the day was blindingly quick, - even by today's standards, with stats I dare not quote on-line!

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