Wrong octane petrol for the
compression ratio. Higher octane petrol is more stable, so less inclined
to explode spontaneously. 96 octane will work up to 10:1 with ease if
everything else is right, even up to 12:1 has been possible with
modified squish and very careful set-up.Benz corteco oil seals.
Wrong advance rate (too much advance too soon) – with higher compression or lower octane fuel the engine will need less advance around 1500 – 2500 RPM, with about the same total advance at higher revs.
Too much squish for the compression ratio:
Squish (the squish of fuel/air trapped between the piston top and the bit of the head face which overlaps the bore and ejected with great speed into the combustion chamber as the piston gets to the top) is a design feature to inject the right amount of turbulence to get proper burning.Benz corteco oil seals.
Higher compression also promotes turbulence, so the general rule is that as the compression goes up the squish should go down. If you have an MGA1500 head on an MGB for example, the chamber volume is smaller (=higher compression) giving at the same time more squish due to the overlapping head face area being larger. Squish is further increased due to the bigger bore size. Benz corteco oil seals.So the result of combining all these will be most likely combustion irregularities
To limited extent, too much squish can be ameliorated by changes to the ignition timing, but the ideal solution is to reduce the squish whilst retaining the compression and then re-profiling the distributor.Benz corteco oil seals.
Inadequate cooling of the head, esp round the spark plugs. Check head gaskets for obscuring the coolant holes and that the hole pattern is the same in block and head. Check the casting in the water jacket around the spark plug holes for featuers which might reduce heat transmission to the water jacket.Benz corteco oil seals.
An alloy head works better in this respect but it would be an expensive way of curing detonation, and may not solve it at that – it depends where the problem is.
Wrong heat range spark plug. N9Y or equivalent is standard for 9:1 on 98 octane (what the engines were designed for). Cooler plugs may be needed for lower octane and/or higher compression. N8Y is good to use, above 10:1 it may be necessary to use N7Y.Benz corteco oil seals.
There is one remaining peculiar circumstance in which pinking can occur. If the engine has the wrong vacuum unit there is a distinctive symptom of pinking on part throttle which disappears or diminishes on full throttle. I have yet to find any other cause of this symptom (though there is always a first time!). (Refer to the page on vacuum units for a full explanation and diagnosis.)
Hot spots on the head from carbon or small machining dags (eg when the head is faced but the chamber edge is not radiused off where it meets the newly machined head face)
Even lean mixture can be a cause.Benz corteco oil seals.
Wrong advance rate (too much advance too soon) – with higher compression or lower octane fuel the engine will need less advance around 1500 – 2500 RPM, with about the same total advance at higher revs.
Too much squish for the compression ratio:
Squish (the squish of fuel/air trapped between the piston top and the bit of the head face which overlaps the bore and ejected with great speed into the combustion chamber as the piston gets to the top) is a design feature to inject the right amount of turbulence to get proper burning.Benz corteco oil seals.
Higher compression also promotes turbulence, so the general rule is that as the compression goes up the squish should go down. If you have an MGA1500 head on an MGB for example, the chamber volume is smaller (=higher compression) giving at the same time more squish due to the overlapping head face area being larger. Squish is further increased due to the bigger bore size. Benz corteco oil seals.So the result of combining all these will be most likely combustion irregularities
To limited extent, too much squish can be ameliorated by changes to the ignition timing, but the ideal solution is to reduce the squish whilst retaining the compression and then re-profiling the distributor.Benz corteco oil seals.
Inadequate cooling of the head, esp round the spark plugs. Check head gaskets for obscuring the coolant holes and that the hole pattern is the same in block and head. Check the casting in the water jacket around the spark plug holes for featuers which might reduce heat transmission to the water jacket.Benz corteco oil seals.
An alloy head works better in this respect but it would be an expensive way of curing detonation, and may not solve it at that – it depends where the problem is.
Wrong heat range spark plug. N9Y or equivalent is standard for 9:1 on 98 octane (what the engines were designed for). Cooler plugs may be needed for lower octane and/or higher compression. N8Y is good to use, above 10:1 it may be necessary to use N7Y.Benz corteco oil seals.
There is one remaining peculiar circumstance in which pinking can occur. If the engine has the wrong vacuum unit there is a distinctive symptom of pinking on part throttle which disappears or diminishes on full throttle. I have yet to find any other cause of this symptom (though there is always a first time!). (Refer to the page on vacuum units for a full explanation and diagnosis.)
Hot spots on the head from carbon or small machining dags (eg when the head is faced but the chamber edge is not radiused off where it meets the newly machined head face)
Even lean mixture can be a cause.Benz corteco oil seals.
No comments:
Post a Comment