Misc: Everything Else Standard Comet Special

vibrac

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There are also the theories and claims of the Indian (continent) tuner who uses grooves in the heads to propogate the flame
Look up Somender Singh squish zone grooves
 

davidd

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I think the "Singh slots" work best on side valves. There is not a lot of need for them in the tight quarters of the Vincent combustion chamber.

David
 

vibrac

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A good question would be the pros and cons on a Vincent head if you were to sacrifice the second plug to increase the squish area
I never found any increase in power with twin plugs on the methanol racer not that thats conclusive
 
D

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Tests last year with a twin plugged, two valve, pushrod operated 500 cc single cylinder racer (not a Comet) gave the same performance, whether the two plugs, or either single plug alternative positions were fired, and all with the same ignition timing.
Combustion efficiency is dependent on many small details, and how they are combined, before one gets around to the spark.
 

timetraveller

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That is very interesting. In the world of racing Vincents the general feeling is that one has to retard the ignition when two plugs are use. Tests with people I know suggest that as much as 8 to 10 degree more retard might be required when two plugs are used. Did the test engine you referred to have a lot of squish built in?
 
D

Deleted member 3831

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It depends what you mean by a lot of squish.
In terms of mean squish area, this is in no way excessive. In terms of mean squish velocity it is of a magnitude that might be considered reasonably excessive.
For a combustion chamber that requires 8 to 10 degrees more ignition advance with one plug than with two, it's telling a lot about the rate of the spread of the flame front across the chamber.
 

timetraveller

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Hmmm. That is what I suspected and why I asked the question. In the world of Vincents quite a lot of people have tried various forms of squish, many trying to work with relatively unchanged piston crowns. Most of us are playing with picking up information from various sources and it sounds as though you would know what shape the head and piston crown could/should be. Terry Prince in Australia is a man who has put his money where his mouth is and made new head shapes which seem to be a major improvement over the originals. Doesn't mean to say that they could not be improved with professional input. Over to you.
 

greg brillus

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Of the two 600 singles, both Vincent's I have just done, one using twin plugs the other with a single plug, the one using twin plugs definitely had a stronger idle and very crisp response as you crack the throttle. By comparison the single is fine, and probably very normal as we know them. They both run on normal pump fuel, the twin plug set at 26 degrees full advance and the single on 34 full advance. Seat of the pants performance of both seamed very good, although the gearing for both bikes was quite different, the twin plug being a stock Comet and the single plug in a Slimline Featherbed geared much taller, similar to a twin. My take on twin plugs is that the larger the bore size the more effect it would have, plus having a piston with a very tall dome with the plug over to one side, running two plugs must help. I have recently also had some discussions with a very experienced engine tuner of classic japanese in line four race engines. I would say that high reving smaller bore engines like these suffer from a poor "Burn" in the combustion chamber possibly because of the high revs they run. By using a tapered squish band on the piston and combustion chamber and twin plugs, the burn improves very much, and he is getting very good HP figures from these engines now. This is mostly on Methanol fuels for both solo's and outfits.
 

Oldhaven

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This is very good information and thank you. Do you know at what rpm's the max advance happens with these two examples? With an ignition with a preprogrammed fixed curve there is not much choice but for a programmable ignition there is a whole new variable to consider. If we can find the right machines to test I hope to do some dyno work this winter. Oldbritts has done this for Nortons with the Power Arc ignition and supply it with a suitable curve.
 
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