Regular Unleaded and Ignition Timing

[captaintact]

I use regular Unleaded instead of the super stuff. As i'm not interested in total performance i don't feel it's worth the extra money especially as the car can run on either. My question is, does the ignition timing need to be changed?

[bsdnazz]

captaintact said:

I use regular Unleaded instead of the super stuff. As i'm not interested in total performance i don't feel it's worth the extra money especially as the car can run on either. My question is, does the ignition timing need to be changed?

An important aspect of the octane rating is that it affects the fuel's knock resistance.

Knocking is when the fuel ignites spontaneously due to the heat of compression (like a diesel) and not because the spark plug has sparked. Knocking causes engine damage.

The use of a high octane fuel enables higher compression ratios (which will heat the fuel) with out the fuel spontaneously igniting. It also allows the spark to be sparked later in the compression stroke safe in the knowledge that knocking won't occur.

So, the ignition timing does not need to be changed but the compression ratio might!

If a lower octane fuel burns more slowly then you probably do want to fire the spark a little earlier or you'll loose power.

[bsdnazz]

Wikiedia says this about the effects of the octane rating.

[captaintact]

OK. As my Chim is a 4.0HC version then the HC (High Compression) means that pre-detonation may be more of a problem? I must admit, i have not noticed any difference when using either reg or super (having allowed time to flush a tankful through. I suppose the fact that the Standard Lucas ECU does NOT control the ignition/ignition timing then the ignition cannot be retarded when sensing potential knocking?

My original question did presuppose possible pre-detonation hence asking if the timing needed changed (retarded). Perhaps if a fully mapped system was installed (as many seem to have done) then either reg or super can be used?

[captaintact]

Slight sideways step here. My early (pre-serp - how can i tell?) Chim runs at about 70deg on the motorway and the fan comes in at and maintains at 90deg when in traffic. The engine warms quickly. I mention this as in page 110 of "the bible" it is suggested that one uses an 82deg thermostat so my car must have the cooler 74deg one? Should i change it? the bible suggested this change in order to help prevent pre-detonation and smoother running. Running at around 80deg (normal for most cars) may mean the ECU map is more efficient and perhaps better running and better economy?

Any thoughts?

[bsdnazz]

captaintact said:

My original question did presuppose possible pre-detonation hence asking if the timing needed changed (retarded). Perhaps if a fully mapped system was installed (as many seem to have done) then either reg or super can be used?

Changing the timing won't stop knocking unless the spark sparks very early. However, chances are the early sparking would it's self cause damage as the piston would still be in the compression phase.

[captaintact]

bsdnazz said:

Changing the timing won't stop knocking unless the spark sparks very early

I was under the impression retarding the timing was exactly what a modern ECU does in co-operation with a knock sensor?

[shpub]

1. HC is NOT high compression. It stands for High lift Cam. The HC engines typically have the same or lower compression than the non HC engines (depending on which of the many figures TVR gave as right).
2. 70 degrees temp on the motorway is more a problem of the air flow causing localised cooling on the temp guage sensor.
3. The cooling system has a bypass so that the thermostat is not a complete on off switch.
4. At 70 degrees any temperature compensation should be removed anyway.
5. Yes you should retard the ignition timing. See chapter 4 in the bible or the unleaded FAQ on my website www.shpub.co.uk

[shpub]

I think you have litres and gallons mixed up.

6000 miles at 4 miles per litre (about 18 mpg) is 1500 litres of fuel. At 15p a litre difference, that's £225 a year.

A lot does depend on the price differential and how easy it is to get super.

Edited by shpub on Tuesday 8th February 08:31

[Dr Mike Oxgreen]

bsdnazz said:

Knocking is when the fuel ignites spontaneously due to the heat of compression (like a diesel) and not because the spark plug has sparked.

Hmmm... I'm guessing you were told that by someone who thought they knew what they were talking about... but I'm afraid that's simply wrong.

"Knocking", "pinking", or detonation (which are all the same thing, although strictly speaking "knocking/pinking" is the sound caused by detonation) is not spontaneous combustion of the mixture. Detonation is when the spark ignites the mixture, and the mixture then "explodes" rather than burning in a progressive way. The result is that the cylinder pressure rises much too quickly and reaches its peak too early. Detonation is caused or exacerbated by: high compression, lean mixture, advanced timing. A higher octane fuel is better at resisting detonation; in fact that is the definition of the octane rating. Severe detonation will damage an engine if you let it continue happening for a while, but light to moderate detonation is only a mild problem. In fact, light detonation can even be beneficial in keeping the combustion chambers clean.

What you have described above is more like pre-ignition, which is an entirely different thing - although people will often try to tell you that it's the same thing as detonation/knocking/pinking. Pre-ignition usually happens as a result of something in the cylinder getting very very hot (e.g. carbon deposits, or a piece of metal swarf from a damaged spark plug or valve). This causes the fuel mixture to ignite before the spark - often a long time before. The result is that the cylinder pressure rises to its peak much, much too early - resulting in huge pressure and temperature. Pre-ignition is always a major problem; it will destroy your engine in minutes or even seconds, usually by burning holes in the piston crowns.

Pre-ignition is never caused by using fuel of the wrong octane rating, so pre-ignition is not really relevant to the question of whether to use super or normal unleaded.

There is no such thing as "pre-detonation". That's simply a confused amalgamation of two bits of terminology. Remember: detonation and pre-ignition are different beasts.

The following is a really good explanation of what detonation and pre-ignition really are - it's well worth reading all 8 pages of it:

http://www.streetrodstuff.com/Articles/Engine/Deto...

bsdnazz said:

The use of a high octane fuel .... also allows the spark to be sparked later in the compression stroke safe in the knowledge that knocking won't occur.

I'm sure that's just a typo - it should say "allows the spark to be sparked earlier in the compression stroke".

captaintact said:

I was under the impression retarding the timing was exactly what a modern ECU does in co-operation with a knock sensor?

That's correct. The key thing is preventing the peak cylinder pressure from being too high - which may cause the rest of the mixture to suddenly detonate. Loosely speaking, the cylinder pressure is governed by two things: the pressure due to combustion, and the pressure due to the position of the piston in the cylinder. If the spark is earlier, then the combustion pressure reaches its peak earlier, when the piston is still very high up in the cylinder. Hence the total cylinder pressure will be much higher. By retarding the spark, the combustion pressure peaks when the piston is further down the cylinder, hence the total pressure is lower.

Edited by Dr Mike Oxgreen on Tuesday 8th February 09:47

[bsdnazz]

Dr Mike Oxgreen said:

Detonation is when the spark ignites the mixture, and the mixture then "explodes" rather than burning in a progressive way.

Thank you for the correction. Now I can see why changing the ignition timing would have an effect.

Yes, the other thing was a typo!

[captaintact]

shpub said:

1. HC is NOT high compression. It stands for High lift Cam. The HC engines typically have the same or lower compression than the non HC engines (depending on which of the many figures TVR gave as right).
2. 70 degrees temp on the motorway is more a problem of the air flow causing localised cooling on the temp guage sensor.
3. The cooling system has a bypass so that the thermostat is not a complete on off switch.
4. At 70 degrees any temperature compensation should be removed anyway.
5. Yes you should retard the ignition timing. See chapter 4 in the bible or the unleaded FAQ on my website www.shpub.co.uk

Thanks for the corrections.
Re 2/ I understand what you say and indeed this morning is was very cold outside and the temp read 55deg on the motorway! However, there was no heat from the internal heater. I understand local cold air could affect the sensor but surely the thermostat should hold the water temp much nearer 74 or 82 depending on stat? I recently had to change the stat on my Alfa 156 for the same reason and all was solved? It is a "Serp" engine. Should i fit an 82deg stat?

[shpub]

Unless the stat is faulty and staying open, I can't see the point of changing it. I suspect that you have an 82 one anyway.

You have checked for an air lock and that the coolent levels are OK?

[captaintact]

Coolant levels fine Steve. As no heat from heater and all the symptoms i have observed with several various cars in the past when i've had a faulty stat i'll change both stat and otter switch. Parts are cheap and easy enough to do. Will perhaps borrow and infra red gun gun to check first though?