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(or any other engine for that matter).
There are two general phenomena which are loosely described as detonation - spontaneous combustion which occurs independently of ignition spark (diesel action), and pre-ignition, brought about by the spark
occurring too soon. Technically they are quite different phenomena, but sometimes they are linked. Some of the causes are common to both.
For simplicity, the following doesn’t differentiate between the two, as to the untrained
ear it can be hard to tell the difference.
Causes of detonation are:
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Wrong octane petrol for the compression ratio. 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.
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Wrong advance rate (too much advance too soon) - with higher compression the engine will need less advance around 1500 - 2500 RPM, with about the same total advance at higher revs.
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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. 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 camber 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.
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.
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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.
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.
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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
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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.)
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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)
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Even lean mixture can be a cause.
The order for checking detonation causes is:
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Check spark plugs are right heat range - examine for signs of overheating
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Check carb needles are correct spec & not badly worn at the thick end and that the mixture is not wildly out at idle. If the needle is worn at the thick end, richen it up 2-3 flats on the adjuster and see if that makes any difference to the detonation. If it does, buy new needles.
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Check compression ratio approximately using a
compression tester. Roughly, every 20psi is 1 ratio. (Not 15psi as you'd expect since the process is adiabatic not isothermal - the air heats as it is compressed - and so the simple form of Boyles Law does not apply.) Now you know what you are working with.
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Check ignition advance rate (see notes below) and adjust if necessary.
I'd check where the advance is before doing anything. Re-profiling the advance curve very easy, and a lot easier and cheaper than removing the head, let alone replacing it. I'd always regard checking the advance curve as the first step All that's involved is measuring with a timing light what the timing is doing now (vacuum disconnected) and then tightening up the advance springs in the distributor a bit until it does what you want. In higher performance engines a spring change may be necessary, but not for what you are doing. Rough rule of thumb: 9:1 compression look for 25 deg total advance at 2000RPM and 30 deg after 2500 RPM, 10:1 compression no more than 20 deg total advance at 2000RPM reaching 30 deg after 3500 RPM.
Refer also the page on distributors.
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If compression higher than standard, try one grade lower in spark plugs.
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That's the end of the simple checks, after that it's a case of remove the head, check the squish area/chamber size/head type/cooling holes/hot spots etc
The later MGB heads had the same chamber capacity as the early MGB/MGA1622, but had a shallower chamber with less squish area; most had the same valve sizes, but some had bigger inlets so one of these would be the one to get if you could. Refer my web Technical pages site for notes on the various MGB heads.
I'd be reluctant to give away compression ratio as a cure to detonation - compression increases both power and economy. The B series engine functions perfectly happily on 9:1 to 9.5:1. 10:1 is very viable with a little care and setting the advance curve correctly and even 11:1 is reasonably comfortable on 96 octane if everything else is optimised.
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