OK I did see Roy last night and the truth is somewhere in between what has been discussed above. If I have understood Roy correctly then he did modify the heads but not by taking out the bathtub shape. He machined the head at both sides, roughly at 90 degrees to the line joining the valve centres and made an inwardly inclined face which could be interfaced to the modified top of the piston. This was to create a squish band at each side. I do not expect you to understand this as I do not understand it properly but will get Roy to draw it for me when we meet up again and then I can report back.
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Pazon Sure Fire
- Thread starter Howard
- Start date
OK I did see Roy last night and the truth is somewhere in between what has been discussed above. If I have understood Roy correctly then he did modify the heads but not by taking out the bathtub shape. He machined the head at both sides, roughly at 90 degrees to the line joining the valve centres and made an inwardly inclined face which could be interfaced to the modified top of the piston. This was to create a squish band at each side. I do not expect you to understand this as I do not understand it properly but will get Roy to draw it for me when we meet up again and then I can report back.
I take it that Roy has twin plug heads, or has he found a way of installing a central 10mm plug?
I look forward to the drawing.
Thanks.
That suggests that they work. My doubts are less over twin plugs per se, as twin plugs on Vincents. My original Manx Norton head (replaced by a Summerfield "bathtub / Ilmor" replica) had been converted to twin plugs, then converted back again to single plugs. I assumed there turned out to be no advantage. Nor can I think of any modern engine fitted with twin plugs as original equipment. I'm open to correction.
i´ve found on my D that with twin plugs, it´s very easy to start, and low speed running (town) is much improved. Can´t say it has made any difference to the top end performance though (hardly surprising with the 7.2:1 pistons i fitted the same time as the twin plug conversion).
The drawback of the coils i am using is that should one plug gum up, the other continues firing. This i assume, means that i´d be firing late on that pot. (currently at 28 BTDC but still playing around)
The best performance i had from the bike was at 34 BTDC on single plugs and 10:1 pistons (1 3/16 monoblocks and 320 mains). It still pulled strongly at an indicated 110
Roy's bike has twin plug heads. Compression ratio is about 10:1 and ignition timing has been varied between 18 and 24 BTDC on the bored and stroked engine which is about 1275ccs. Both the twin plugs and the squish bands required more retard. Exact figures are for Roy to divulge.
Just a couple of comments, Boyers have been modified so that they no longer revert to full advance in low battery voltage conditions. I have used Boyers on various bike since 1972, without any problems, twin plugging will give you the ability to run high compression ratios on modern fuels without problems, and as a side effect give smoother running and easier starting.Quite a few modern engines use twin plugs such as the the BMW 1050 range of twins, together with the Alfa Romeo range of 4 cylinder petrol engines. Done for emissions reasons, possibly because 500cc cylinders can be difficult to burn properly.Charles