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Thread: Sparking plug washers.

  1. #11
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    Sparking plug washers.

    Hi Peter,

    I did say everything else being equal meaning the only thing that has changed is less power. I consider if its producing less power it may be expected that its producing less heat and the components are under less load which in my opinion should result in longer life.

    The torque curves I was referring to is the ones issued by Rotax for both the single and twin spark engine of 503. To be honest I view both just a guide at best.

    I was making the point that I thought moving the spark position down by 1mm would make no material difference to the engine , what is your opinion on this?

    Sorry if I was not clear in my statements.

  2. #12
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    Sparking plug washers.

    Mick,

    When you said you've tried different plugs, did you include iridium in that? I've put iridiums in my 462's and they definitely do improve starting. Because the 462 is a points engine I had to gap them down from the .040 they came with to around .020 or less, but with the fatter spark iridiums give the engines now start easier than they did with the normal NGK's I used to use. As for the electronic ignition 447 in my minimax, I haven't decided yet whether to bother with them or whether to leave well alone.

    Best regards,

  3. #13
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    Sparking plug washers.

    Hi Bob,

    Iridium plugs are good and if the mixture is good will last some time.

    I think you are getting better starting because of the closer gap reducing the requirement to get a spark, this in theory will have a small knock on effect at speed but a spark is a spark and petrol/air being what it is you will end up with a bang.

    If its working why change ;-)

  4. #14
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    Sparking plug washers.

    Hi all,

    As someone who's spent a lifetime tinkering with 2 stroke motorcycles and having read several manuals on tuning them, I can vouch for the fact that a single spark plug is more efficient in terms of retaining combustion chamber shape and the correct swirl characteristics. Having two spark plug heads and their electrodes poking into the combustion chamber doesn't do the flame propagation any favours, but of course from the point of view of reliability (notably in aviation engines), having two plugs firing from independent ignition circuits is invaluable and to be honest we're looking for reliability and not the last ounce of BHP. ALL aviation engines should be under-stressed for obvious reasons.

    I swear by Iridium plugs. They provide a really fat spark and to be honest I don't think they have as long a life as the standard type, but the increase in performance and ease of starting is well worth the extra sponduliks to buy them and change them when the maintenance schedule demands. The difference in my Yamaha RD 350 LC in particular was remarkable after fitting them.....much harder acceleration and the exhaust really crackled. I'm not sure if the economy improved - maybe it did but I didn't care! On reflection, if the engine is working more efficiently, less throttle would be required to produce the same power so I suppose economy should be improved along with performance.

    As for the washer question - even if combining the washer with the CHT ring DOES withdraw the plug by 1mm the performance loss would be absolutely minimal - the compression ratio being very low anyway. Fit Iridium plugs and in my humble opinion it'll make up the difference tenfold!

    Happy landings,

    Phil.

  5. #15
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    Sparking plug washers.

    The voltage required to jump a gap is the same for the same gap when considering a spherical spark gap. The discharge is affected by the more pointed iridium electrode, causing it to fire at a lower voltage and therefore advancing the spark a very (probably immeasurably)small amount. That ought to improve starting.

  6. #16
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    Sparking plug washers.

    Mick Broom wrote: Hi Peter,

    I did say everything else being equal meaning the only thing that has changed is less power. I consider if its producing less power it may be expected that its producing less heat and the components are under less load which in my opinion should result in longer life.

    The torque curves I was referring to is the ones issued by Rotax for both the single and twin spark engine of 503. To be honest I view both just a guide at best.

    I was making the point that I thought moving the spark position down by 1mm would make no material difference to the engine , what is your opinion on this?

    Sorry if I was not clear in my statements.
    Hi Mick and apologies to Tom- I appreciate we are drifting off topic and this aspect of the discussion is somewhat academic.

    As an example to illustrate my point: If an engine has it's ignition retarded, it may consume the same quantity of fuel but make less power. The fuel is still burned and its energy released as heat. Less of the heat is converted to power at the crankshaft (i.e. absorbed into the expansion of gas) so it must leave the engine via the exhaust and the cooling system (higher EGT and burnt piston!).
    Poor ignition may well lead to slow combustion, giving effects similar to retarded ignition.

    With regard to spark plug position; I would not be concerned about the small change in compression ratio. The ideal location for the spark would be at the centre of the combustion chamber volume. Realistically, it has to be located where there is metal to mount it. Moving the spark further from the centre of the chamber leads to slower combustion. I could not comment on the extent of that difference in this engine.

    Back to the original question: The plug seal washer, if it were retained, would only seal to the CHT washer. The CHT washer would still need to seal to the head. If the CHT washer is not soft enough to create a seal to the head, then you have a leak. If it does create a seal at the head, it would also be expected to create a seal to the plug.
    It would be logical to remove the seal washer when using a CHT washer.

  7. #17
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    Sparking plug washers.

    Indeed, Peter. It is not about compression but about optimising the flame front. That is why I want the gap in the right place. My cht washers are aluminium. I think I shall remove the crush washers.

  8. #18

    Sparking plug washers.

    I machined copper CHT washers* that were 1.6mm thick, so removing the fitted plug washer was essential. The rather crude CHT sensors that are thin plate with the thermocouple crushed in a projecting tab aren't so accurate, depending where the tab ends up in the cross head cooling air flow. Biggest issue with a plug not precisely filling the head thread is build up of carbon on either the protruding plug, or the unfilled head threads. In either case you run the risk of damaging threads in the future. On a Rotax two-stroke, there's no chance of pistons touching a slightly protruding plug.
    Dave
    *I also added a stainless pin projecting out from the washer. This sits between cooling fins and stops the washer rotating with the plug when fitting or removing and damaging the thermocouple wires.

  9. #19
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    Sparking plug washers.

    Thanks Dave.

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