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Elsewhere on this forum I made the observation that in my experience,
if an engine has the mixture and timing set up during cold weather, it will
be more likely to pink when the weather warms up.

I also made the observation that if any of my engines are going to pink,
they are more likely to do this during hot weather and less likely to do
this during cold weather.

Now, theoretically, the mixture would become richer during the hotter
conditions, so again, one would normally imagine that this should reduce
the likelihood of pinking. This is assumed to be that the excess fuel will
act as some kind of cooling agent. My experience told me otherwise...

I needed to come up with a theory on this subject that would explain this.

I considered the concepts of "leaner" and "richer" as opposed to "lean"
and "rich". This I felt was key to getting the answer. It is generally
recognised that when tuning an engine, the mixture is chosen and dealt
with first, then the timing is adjusted to light the chosen mixture. Thus, we
can choose a richer mixture for more power, and a leaner mixture for more
fuel economy. We then sort the timing to suit.

It is also a given that this will be done on either an engine dyno, or a
rolling road facility. It would also be quite obvious that any decent
dyno/RR facility will have mixture analysis gear amongst its equipment.

What I have read on this subject has told me that a leaner mixture takes
longer to burn and so needs lighting earlier to achieve good results. The
engine would need more advance than an engine running a richer mixture.

Consequently, I figured that this would be a valid argument why I have always
found that an increase in temperature will increase the likelihood of pinking.

The mixture on the hotter day will be richer, therefore, the mixture on the
cooler day would have been comparatively leaner. That's leaner, not lean...
The timing would have been set to cope with the cool conditions at the time.

When the temperature of the engine and/or ambient air rises, the engine
does not now need as much advance as it has been provided with, which
was configured for a cooler environment. As a consequence it will be more
likely to pink.

Discuss:
1) Does a rise in temperature increase the likelihood of pinking?
2) Please provide a reason for your answer.

I am open to any suggestions on this matter and will be quite happy to
change my view should a logical explanation be forthcoming.

Ian

Ian,

Any increase in the air inlet temperature will cause an increase in pinking - all other things being equal.

Even allowing for mixture strength, air density, etc temperature will have the dominant effect. Reason for this is that pinking (detonation) occurs when fuel auto-ignites in the combustion chamber due to a combination of temperature and pressure.

Carbon,

Thank you for your response.

Ian

carbon wrote:Ian,

Any increase in the air inlet temperature will cause an increase in pinking - all other things being equal.

Even allowing for mixture strength, air density, etc temperature will have the dominant effect. Reason for this is that pinking (detonation) occurs when fuel auto-ignites in the combustion chamber due to a combination of temperature and pressure.

I was pretty much going to say the same thing.

Here's a post I made in another forum a while ago, bear with it as it is some basic 101 stuff, but I feel has relevance;-

"The distributors have an automatic advance mechanism that advances the ignition timing with increasing engine speed (although on some non-moke engines there are a few that require a bit of retard at high speed) and some have vacuum advance.

The burning of the fuel mixture in the cylinders does not happen instantly although it is fast. At some point in that burning process, it will reach it highest or peak pressure.

Ideally, we'd like that peak pressure to happen at about 14 degrees after the piston reaches top dead center (ATDC) (that is on it's way down, which makes sense).

So if we had a fixed (no auto advance) distributor, we can easily set the timing so that the peak pressure occurs at just the right moment (14 degrees ATDC) at say 2000 rpm (or at say 60 kmh in top gear).

When the engine speed is increased, the time it takes for the piston to move is also faster. Since it does take time for the fuel mixture to burn, the peak pressure will occur later and the most efficient use of that cycle has not been taken advantage of, ie, efficiency is lost. At even higher speeds the exhaust valve will in fact begin to open while that fuel mixture is still burning so even more efficiency is lost as the pressure that would otherwise be driving the piston down the cylinder is lost out the exhaust! Good for getting the exhaust gases out of the cylinder but doesn't do much for moving the car! Under these circumstances, the ignition timing is said to be retarded.

OK, so now if we set our fixed distributor to suit say 4000 rpm (or say 100 kmh because we all like going away!), then at slower engine speeds the burning cycle will start too early and if it occurs too close to top dead center and so the piston will tend to drive in to the bearing rather that turn the crankshaft. Again, the best efficiency is lost as the engine is trying to compress the con rods or squash it's bearings. This is said to be over advanced.

Usually, but not always, over advance ignition timing can be heard, it can sound like someones poured a packet of gravel in to the engine or there is an aluminum can stuck inside it. I think it fair to say most of us here have heard this and is known as pinking, although there are many other causes of this, not just ignition timing.

Who remembers riding their push bike? At what point when pushing the pedals did you get the most out of that push? It wasn't when the pedal was up, it was probably when the pedal was around the 20 to 30 degree mark. What happened when you did try to push to pedal when it's too close to the top? You went red in the face and not much happened, sometimes the pedals would even go backwards and more often than not, you'd hurt yourself. Yeah, I remember that. What happens in an engine is not a lot different.

So to reduce those effects and gain the best efficiency from the engine, the ignition timing advances automatically with increasing engine speed. There are many factors, mostly inside the combustion chamber of the engine that affect the speed that the fuel burns at and of course the actual mixture ratio (weak or rich) will have a big bearing on this too. Engines are very sensitive to this and it is for this and so the amount of advance is rarely linear or a straight line but more of a curve. It is for this reason that a certain distributor that suits one type of engine will not suit another. Also, to further complicate this, different Octane fuels burn at different rate, high octane burning slower (no that is not a typo) hence why you need more advance with higher octane fuels. Compression ratio too has a bearing on ignition timing requirements.

Here's a comparison of the 1275 and 1100 Distributors (data taken from the factory manual)



So, for example, if we put a 1100 distributor in a 1275 engine, it would over advance and likely ruin the pistons (seen that more than once).

So far, this has been about the Automatic or Centrifugal Advance in the Distributor.

Some also have a Vacuum Advance as well. When you accelerate the cylinders fill reasonably, but when cruising down the road, they don't. Under these conditions, there is little mixture in the cylinders and again, it take even longer to burn. Also on cruise or part throttle, the manifold has a high vacuum. This vacuum is used to operate another mechanism on these distributors to further advance the ignition timing although this has little to do with avoiding engine damage, it is more about getting even better efficiency from the engine. In most cases, it could be disconnected and won't do any harm, other than use a little bit more fuel.

Some problems that can occur if the ignition timing gets too far outside the sweet spot - other than lost efficiency - are;-

Over Advanced;-
Damaged Engine Bearings,
Damaged Pistons - the top of the piston can squash down on the top piston ring and jam it,
Burnt holes through pistons (though, this is extreme),
Engine overheating

Retarded;-
Burnt Exhaust Valves (very common),
Cracked Cylinder Head (it will crack between the inlet and exhaust valve seats),
Cracked Exhaust Manifold,
Engine overheating"

Great post!

Spider, thank you for your input.

mk1mini - I agree, it's a great post... and very helpful!

hi sirs,
it is very useful post. thank you all.

by the way, do you have the method to set the correct timing? (fix distributor and with auto advance)?
for example, my engine is set at 10 degree after top dead (fixed distributor on 1100cc engine). is it correct ignition timing?

warm regards
ink

Thanks guys for the kind words, though, it's probably verging on OT in regards to the original question.

matic wrote:hi sirs,
it is very useful post. thank you all.

by the way, do you have the method to set the correct timing? (fix distributor and with auto advance)?
for example, my engine is set at 10 degree after top dead (fixed distributor on 1100cc engine). is it correct ignition timing?

warm regards
ink

I actually think in regards to this question, my post is anything but helpful!

Is your engine standard, other than say an over-bore? If it is, is it fitted with the factory fitted distributor and is it in good order? If so, then setting it as per workshop manual will be quite close, however, in these cases, I usually set them by road testing, Hard accelerating and gradually advancing it until the on-set of pinking can be heard or there is a noticeable loss in performance, the retard the dissy about 2 - 3 degrees.

The factory dissys for stock engines are really not optimal with the fuels we have available these days, however, in the absence of a rolling road session, these would still be the safest way to go.

If your engine has been modified, then the only way you can be sure of i) getting the right dissy curve and ii) setting the correct ignition timing is with a dyno session. You can try it by trial and error with road testing, but will take a long time and you cannot be sure of getting it right. Or maybe just ask Smiffy

Here's just one example of one that got it wrong;-



That's an exhaust valve. The engine was an 1100 HC Unit, but fitted with a 998 dissy. At normal cruising speeds, the timing was retarded.

edit:- to insert a missing word!

Spider wrote:Thanks guys for the kind words, though, it's probably verging on OT in regards to the original question.

matic wrote:hi sirs,
it is very useful post. thank you all.

by the way, do you have the method to set the correct timing? (fix distributor and with auto advance)?
for example, my engine is set at 10 degree after top dead (fixed distributor on 1100cc engine). is it correct ignition timing?

warm regards
ink

I actually think in regards to this question, my post is anything but helpful!

Is your engine standard, other than say an over-bore? If it is, is it fitted with the factory fitted distributor and is it in good order? If so, then setting it as per workshop manual will be quite close, however, in these cases, I usually set them by road testing, Hard accelerating and gradually advancing it until the on-set of pinking can be heard or there is a noticeable loss in performance, the retard the dissy about 2 - 3 degrees.

The factory dissys for stock engines are really optimal with the fuels we have available these days, however, in the absence of a rolling road session, these would still be the safest way to go.

If your engine has been modified, then the only way you can be sure of i) getting the right dissy curve and ii) setting the correct ignition timing is with a dyno session. You can try it by trial and error with road testing, but will take a long time and you cannot be sure of getting it right. Or maybe just ask Smiffy

Here's just one example of one that got it wrong;-



That's an exhaust valve. The engine was an 1100 HC Unit, but fitted with a 998 dissy. At normal cruising speeds, the timing was retarded.

Love it more Plug Chop jealousy

All posts are welcome, this is intended to be an adult discussion...

No-one will be dismissed as an idiot if their views do not match
100% those of others.

Spider, although possibly "off-topic", any background information
that adds context to the point you are trying to make is welcome.

Your chart with the two curves is quite handy in this respect...
First, bear in mind that the chart is dizzy degrees, for crankshaft
degrees, multiply by 2.

Say, for example, we have an 1100 engine, a bog standard one,
which needs the curve supplied by the dizzy shown. Imagine now
that we only have the 1275 dizzy available, again as shown.

Looking at the chart, we could quite easily advance the 1275
dizzy by about 3 degrees, and it would then match the 1100
curve from 2000 dizzy rpm upwards, ie where the curve is flat.
This is great, we can get maximum power...

However, advancing the timing will lift the WHOLE line on the chart
exactly vertically: Up to about 500drpm, there will be far more timing
than required, the engine will probably pink, between that and 2000drpm,
there will be nowhere near enough and the engine will run poorly.

The engine is a complicated device, and one size certainly does
not fit all. Sometimes even tiny differences in an engine spec can
make a considerable difference to its set up requirements, as I
have said elsewhere...

POLITE REQUEST:
If using the "Quote" facility to reply to posts on this thread,
unless it's a short quote, PLEASE TRIM THE MESSAGE to show the points
that you wish to highlight or agree/disagree with as we would not want
to end up with page upon page of duplication.

Best wishes,

Ian

I noticed the mention of max pressure taking place about 14 degrees atdc. My 1000 cc Insight motors come with the crank bores offset to the cylinder bores so that the rods are straight in the bore at 14 degrees atdc. Honda found that this arrangement reduced ring and bore wear and improved power/economy. Toyota now does this with the Prius as well. I wonder why all motors aren't this way.

Carbon, you mentioned pinking and detonation as though they are same thing. Which they most definitely are not!
-Though it is very common for the terms to be confused / interchanged.

Pinking (pinging in USA) is when the mixture ignites at more than one point in the combustion chamber (often due to carbon build-up / hot-spots, etc)

Detonation is the almost instant ignition of the whole mixture -effectively exploding, rather than "burning in a controlled manner".

Engines can usually tolerate some pinking for a period of time (a very general statement!), but they can't tolerate detonation (I've seen race engines hole a piston in less than 4 miles, during practice...).



I notice that no mention has been made of humidity. -The amount of moisture in the air can significantly affect the mixture burn rate. -It tends to slow it down, hence the use of water injection in some turbo engines etc.
Of course the other causes mentioned are still relevant and possible...

mk1mini wrote:Great post!

Perhaps re-visiting an old subject, but it has been said here to use
a particular amount of timing, at an exact amount of engine revs,
even without knowing the specification of an engine.

Say, for example, we look at 1200rpm engine speed, which will be
600rpm distributor revs. The difference between the two lines on
Spider's advance curve charts is quite a bit different. Bear in mind
also that this figure will also have to be ADDED to the amount of static
timing that the distributor is installed with.

This very nicely demonstrates that different engines need varying
amounts of timing, even amongst standard units.

An engine that I built recently needed 15 degrees at 1500rpm, so
if I'd have blindly gone with, for example, 16 degrees at 1200rpm,
this would have been far too much...

olddad wrote: crank [sic] bores offset to the cylinder bores

Yamaha motorcycles are also doing this with some of their latest units.
They call it a "Desaxe" layout...

andy1071 wrote: pinking and detonation

Thanks, that's great clarification, we've probably all said about an engine
detonating at some point, whereas one that really has "detonated" is
unlikely to run again.

Anyone tried "un-detonating" a bomb?

andy1071 wrote: I notice that no mention has been made of humidity.

Your point is very valid, but I did not dare mention this as well
in case I got reprimanded for "Changing the Conditions" or simply
providing too much for discussion...

Thanks for all the contributions to this thread so far.

Does anyone have the opposite view, ie that a colder engine
is more likely to pink?

Ian

ianh1968 wrote:Does anyone have the opposite view, ie that a colder engine is more likely to pink?
Ian

Reminds me of a great album, by Marillion