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Aug 20, 2012 9:25 am
Aug 20, 2012 9:25 am
Subject: Ignition timing ("be a little retarded")
Here are some interesting reflections received from David Hine. Seems logical to me but I never thought about it before. Worth trying on your engine. Would be nice to have a rolling road around the corner..
Be a little retarded ! (Written by David Hine)
This article is intended as a thought for the day and not strictly accurate technical advice !
The very words “advance” and “retard” when related to ignition timing can be misleading because of other connotations relating to the development of our brains.
I studied petroleum refining at college so I know a little about how petrol is made but that was back in 1962 and technology has moved on a lot since then.
What I am now certain of, is that the timing marks that are beautifully engraved on your flywheel are probably in the wrong place.
When our cars were built the engine designers had to cater for the petrol available which, on the whole, was pretty awful stuff. It was distilled from crude oil and contained a huge mixture of hydrocarbon elements ranging from paraffin type through to light ether types. Things got even worse during hostilities when the more desirable elements were boiled off for use in aircraft engines leaving the residue as “pool” fuel for the ordinary motorist.
You may notice the hot spot designs on some of our cars where the fuel mixture is heated by the exhaust gas to help it evaporate. However the most common design feature was the very low compression ratio as the mixture was compressed into the relatively large combustion chamber. Ratios as low as 5 and 6 to 1 were the norm.
Any higher compression ratio and the fuel mixture heats it up too fast and it starts to burn and explode far too soon.
Pure Iso Octane, which is an eight carbon / hydrogen compound was considered the gold standard by which fuel quality was measured. Octane was not commercially viable but could stand compression ratios of up to 9 : 1 without compression ignition occurring.
Then there was the serendipitous discovery of tetra ethyl lead. A unique product which when added to the mix tended to eliminate compression ignition and even the crude fuel mixes with “lead” produced petrol which performed almost as well as octane.
The compression ratio has the major effect on the timing of the spark. Fuel of any kind does not burn instantaneously so it need to be ignited before the piston reaches top dead centre where the big bang is required to drive the piston back down again. With compression ratio of say 5: 1 the spark needs to be advanced to 45 degrees before TDC. With compression ratio of 6 : 1 the fuel burns faster so only 35 degrees of advance is required.
With the advent of “leaded” petrol we were all encouraged to plane our cylinder heads or fit domed pistons. This higher compression gave a faster burn so we could retard our ignition and get more power. Unless we had a rolling road facility, the finding of the optimum ignition advance was a matter of trial and error. Those who have cars fitted with an ignition lever could experiment under load. The advice is to advance the ignition until the knocking, thumping or “pinking” is felt and then retard the ignition until it just disappears when accelerating hard.
Did you actually register what I just wrote ! I said “retard the ignition and get more power”. This is a difficult concept when we have always seemed to do the manly thing and advance the ignition to get more power.
Sure, if you retard the ignition too much you will get sooty plugs and a very hot exhaust manifold. This is because the fuel is still burning as it is exhausted from the cylinder.. However if the ignition is too far advanced the fuel will be ignited too soon and the explosion will fight the piston as it is still rising to top dead centre and power will be lost.
The petrol now available to us is often much maligned and considered to be the cause of many engine malfunctions. However as an elderly technologist I am most impressed at the consistency obtained from pump to pump and the fact that it has been made possible to eliminate “lead” from the fuel by sophisticated petrochemistry.
However there is no gain without pain and the location of our petrol pumps under the hot bonnet is now likely to give rise to vapour lock on a hot sunny day. For high performance cars it is necessary to fit the petrol pump below the level of the petrol tank towards the rear of the car. With my touring car I have fitted a booster pump into the reserve petrol line below the seats and still pump through the existing pumps as well.
The modern fuel also burns much faster after ignition which means that the original timing marks on our flywheel are probably unnecessarily and undesirably too advanced .
It is difficult to be specific because every engine is different. But, for example, my M45 which is standard apart from flat top pistons and 100 thou” machined off the cylinder head, now runs at 22 degrees fully advanced.
My son Nick’s LG 45 also has the same raised compression ratio ( approx 8 : 1 )but , in addition, a high lift camshaft. This means it ingests more fuel mixture which in turn gives a bigger bang. We have retarded the ignition to 18 degrees for optimum high revolutions performance.
The M45 was arrived at by trial and error but Nick’s car was done using a rolling road and running the engine at 4000 rpm under full load. I had to have the twin Scintilla Vertex magnetos modified so the they do not advance more that 12 degrees as the engine speeds up.
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Here are some interesting reflections received from David Hine. Seems logical to me but I never thought about it before. Worth trying on your engine. Would be nice to have a rolling road around the corner..
Be a little retarded ! (Written by David Hine)
This article is intended as a thought for the day and not strictly accurate technical advice !
The very words “advance” and “retard” when related to ignition timing can be misleading because of other connotations relating to the development of our brains.
I studied petroleum refining at college so I know a little about how petrol is made but that was back in 1962 and technology has moved on a lot since then.
What I am now certain of, is that the timing marks that are beautifully engraved on your flywheel are probably in the wrong place.
When our cars were built the engine designers had to cater for the petrol available which, on the whole, was pretty awful stuff. It was distilled from crude oil and contained a huge mixture of hydrocarbon elements ranging from paraffin type through to light ether types. Things got even worse during hostilities when the more desirable elements were boiled off for use in aircraft engines leaving the residue as “pool” fuel for the ordinary motorist.
You may notice the hot spot designs on some of our cars where the fuel mixture is heated by the exhaust gas to help it evaporate. However the most common design feature was the very low compression ratio as the mixture was compressed into the relatively large combustion chamber. Ratios as low as 5 and 6 to 1 were the norm.
Any higher compression ratio and the fuel mixture heats it up too fast and it starts to burn and explode far too soon.
Pure Iso Octane, which is an eight carbon / hydrogen compound was considered the gold standard by which fuel quality was measured. Octane was not commercially viable but could stand compression ratios of up to 9 : 1 without compression ignition occurring.
Then there was the serendipitous discovery of tetra ethyl lead. A unique product which when added to the mix tended to eliminate compression ignition and even the crude fuel mixes with “lead” produced petrol which performed almost as well as octane.
The compression ratio has the major effect on the timing of the spark. Fuel of any kind does not burn instantaneously so it need to be ignited before the piston reaches top dead centre where the big bang is required to drive the piston back down again. With compression ratio of say 5: 1 the spark needs to be advanced to 45 degrees before TDC. With compression ratio of 6 : 1 the fuel burns faster so only 35 degrees of advance is required.
With the advent of “leaded” petrol we were all encouraged to plane our cylinder heads or fit domed pistons. This higher compression gave a faster burn so we could retard our ignition and get more power. Unless we had a rolling road facility, the finding of the optimum ignition advance was a matter of trial and error. Those who have cars fitted with an ignition lever could experiment under load. The advice is to advance the ignition until the knocking, thumping or “pinking” is felt and then retard the ignition until it just disappears when accelerating hard.
Did you actually register what I just wrote ! I said “retard the ignition and get more power”. This is a difficult concept when we have always seemed to do the manly thing and advance the ignition to get more power.
Sure, if you retard the ignition too much you will get sooty plugs and a very hot exhaust manifold. This is because the fuel is still burning as it is exhausted from the cylinder.. However if the ignition is too far advanced the fuel will be ignited too soon and the explosion will fight the piston as it is still rising to top dead centre and power will be lost.
The petrol now available to us is often much maligned and considered to be the cause of many engine malfunctions. However as an elderly technologist I am most impressed at the consistency obtained from pump to pump and the fact that it has been made possible to eliminate “lead” from the fuel by sophisticated petrochemistry.
However there is no gain without pain and the location of our petrol pumps under the hot bonnet is now likely to give rise to vapour lock on a hot sunny day. For high performance cars it is necessary to fit the petrol pump below the level of the petrol tank towards the rear of the car. With my touring car I have fitted a booster pump into the reserve petrol line below the seats and still pump through the existing pumps as well.
The modern fuel also burns much faster after ignition which means that the original timing marks on our flywheel are probably unnecessarily and undesirably too advanced .
It is difficult to be specific because every engine is different. But, for example, my M45 which is standard apart from flat top pistons and 100 thou” machined off the cylinder head, now runs at 22 degrees fully advanced.
My son Nick’s LG 45 also has the same raised compression ratio ( approx 8 : 1 )but , in addition, a high lift camshaft. This means it ingests more fuel mixture which in turn gives a bigger bang. We have retarded the ignition to 18 degrees for optimum high revolutions performance.
The M45 was arrived at by trial and error but Nick’s car was done using a rolling road and running the engine at 4000 rpm under full load. I had to have the twin Scintilla Vertex magnetos modified so the they do not advance more that 12 degrees as the engine speeds up.
Attachments: