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Misc Standard Comet Special

Cyborg

Well Known and Active Website User
VOC Member
#41
Actually the O-ring is a better idea than the flange nut and washer. Not sure why that didn't dawn on me. I put one on the B twin shift shaft. Tool complements of Dan Smith.
Oring reamer2.jpg
 

Oldhaven

Well Known and Active Website User
VOC Member
#42
In another thread that I don't want to hijack Greg Brillus wrote:

"If any of you are running the squish band top end kits with twin plugs you need to be absolutely sure of what ignition system you are running, if it is not right you can be sure of a piston failure. There is a "lot more" involved with some of these bolt on improvements than one might at first think."

Since I am using one of these big bore kits this concerns me of course. I have already learned that I should use piston skirt clearances of 5-6 thou rather than the 3.5 recommended. Even though I live in Maine, the new climate we seem to be experiencing means a few weeks every summer when we see temps above 33C.

I will be using a Power Arc programmable ignition we are developing that allows me to design my own advance curve. Bob Larmour built the housing. I have been running an alpha prototype successfully on my B Rapide for a year, but we needed to get a few changes made, and are in the beta stage development now.

IMG_1023.jpg IMG_1043.jpg

As delivered from PA and modified for Vincent engines it has four curves available. and is a multi spark system with three sparks and adjustable dwell and rev limit

vincent 1.0 first.jpg



Vincent 4th curve.jpg
The top curve is the most advanced and is used when the two sensor wires are grounded. the bottom curve is the least advanced and results from ungrounding the two sensor wires. the two middle curves are intermediate and will allow selecting an advance that fits a standard single plug road going Comet or twin. I intend to use a Voes so under load I could automatically drop down the advance to something like the last curve here.

And here is a "playing in the sandbox" curve that is at the extreme low end for a Comet dual plug squish engine. Completely unscientific and the result of 5 minutes playing around with the curve. I know some dyno time may be warranted, and we are planning on it, but this shows what is possible.

comet dual plug .jpg

As you can see, the curve is programmable all the way up to the rev limit. Davidd has found that a curve as low as 19 or the low 20's may be useful, so we will see. All still in the developmental stage now.



Finally, other recommendations from the top end fitting instructions:

Ring Gaps

TOP CHROME 0.015”-0.020”

SECOND 0.010”- 0.020”


Oil hole is optional with forged CP pistons

Before finally fitting of cylinder muff and piston the squish

clearance must be checked, it should be .050”to .055”

Any thoughts on whether these figures are OK? Especially ring gaps. I know Greg recommends using the oil hole, though others have said it is not really necessary

Thanks
 
Last edited by a moderator:

timetraveller

Well Known and Active Website User
VOC Member
#43
There is one Vincent Comet racer using up to 8,000 rpm. You might want to think about that for the very fast boys. Also earlier experiments with other systems showed that some of them were not fully retarded if the engine was stalled, rather than being switched off. This meant that when it was restarted it might try to start fully advanced with poor consequences for the French electric start mechanics. A system built in, which ensures that the engine can only be started fully retarded, would be a useful function. Thank you for your efforts.
 

Oldhaven

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VOC Member
#44
The rev limit is programmable for those that dare go there. I am not sure I want to do that myself, but a programming cable is available that can be used while the ignition is installed, and the software is easy to use and available. I will check the stall advance locked possibility, but don't think that it is a problem. Thanks for the heads up.
 

davidd

Well Known and Active Website User
VOC Member
#45
Ron,

Unless the manufacturer or the rings says differently, I use:

.004"/inch of bore as a gap for chrome compression rings,

.003"/in of bore for chrome oil control rings and

.003"/inch of bore gap for unplated compression or oil control rings.

Pat spoke with Terry last night and told him that there were a lot of seizures reported on the forum. He was surprised and concerned. I think he might be reconsidering his advice for the singles.

I don't use an oil hole in the liner, but if you do make sure it is in the correct position. I use aluminum liners made from 4032 that are plated. However, I was talking to Millenium Technologies recently and they mentioned that many owners put the oil hole in their drawings, so many owners use the oil hole with Nicasil liners.

I shoot for .030" for squish or quench. I am pretty sure it does not work at .055". It depends a lot on how high you rev and if it is a titanium rod (due to the stretching). For street, I would think .030" to .045" would be good. I don't think that .055 will damage the performance too much, after all, you're not racing the bike, but if the piston is too far from the head it won't transfer much heat from the piston to the head, which is the "quenching" that helps performance.

Talk to John Healy as he has experience with squish on many engines and maybe .050" is OK for the street.

David
 

greg brillus

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VOC Member
#46
Sorry Guy's been away on a local bike rally this weekend........ Ok, the way I see it with ignitions is this...........They are either too basic, or too complicated. However, only very few are capable of driving a twin plug head successfully that will not only create power but be easy to start and not damage the engine as well. You need around 4 to 5 degrees of timing for easy starting and a good strong idle, with no more than 26 to 28 degrees full advance, and even then a fuel of around 91 octane may still cause detonation. The 600 Norvin I just finished ran very well on one plug, BTH mag set at 34 degrees, with a 36 mm Mikuni carb, and a factory spec Mk 2 cam. My understanding of a squish band is that they actually work better if the band tapers from the outside inward and if the angle is about 30 degrees angled upward on the piston crown and the outer area of the combustion chamber. This gives a much better power output than just a flat band that we have with these heads. My thoughts on the use of the oil feed to the rear.........As the crank and rod assembly rotate in a clockwise direction from the timing side, it makes sense that the oil flung from the bigend is aiming at the forward wall of the liner, this is not the thrust face which is at the rear. It is obvious that it must still get lubrication if the feed is blanked off, but i still maintain that the thrust face at the rear is where the lubrication needs to be.
 

BigEd

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VOC Member
#47
Sorry Guy's been away on a local bike rally this weekend........ My thoughts on the use of the oil feed to the rear.........As the crank and rod assembly rotate in a clockwise direction from the timing side, it makes sense that the oil flung from the bigend is aiming at the forward wall of the liner, this is not the thrust face which is at the rear. It is obvious that it must still get lubrication if the feed is blanked off, but I still maintain that the thrust face at the rear is where the lubrication needs to be.
Dear Greg,
I trust that the rally was enjoyable. Could you expand a little on your thrust face comment?
 

greg brillus

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VOC Member
#48
Hi there Eddy yes a good rally interesting mix of classic bikes..........I am always weary riding in such groups, they and their bikes can be unpredictable. Anyway, well on most old singles the oil feeds are at the rear of the cylinder, this being the thrust face of the piston skirt. I know there are members on here who don't use it, just like some like to blank off the oil tank breather, relying on the pin hole in the cap to vent the tank........It's just not my view .......Manufacturers tend to do things for a reason, and I know people have their reasons for altering things. There may not be a right or wrong answer, everyone has to work out what seems right for them. I personally do not like "Come backs" and so I do the best for my and others bikes for what I feel will work best. It is one of the reasons I like to see what comments appear on this forum..............:).
 

davidd

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VOC Member
#49
My understanding of a squish band is that they actually work better if the band tapers from the outside inward and if the angle is about 30 degrees angled upward on the piston crown and the outer area of the combustion chamber. This gives a much better power output than just a flat band that we have with these heads.
My understanding of the squish band angle is the 90 degrees gives the largest gain in performance based on turbulence. However, it is more prone to detonation than the 30 degree squish band due to the sharper edges. I stuck with 90 degrees because it was well tested by Ian Hamilton and was easy to do. I never had any detonation issues.

I have not researched this in the last couple of years, so the technology opinions may have changed. Often with these performance mods you end up locked in to the decisions you made years ago, because so few folks are doing new performance design on Vincents and to change the head design and piston design gets expensive for a privateer.

Here is a 90 degree squish:
DD04.jpg
Here is a 30 degree squish. It is Stuart Hooper's Velocette. It is a little difficult to see the angles, but it is tapered up as Greg says.
IMG_0175.JPG

David
 

Oldhaven

Well Known and Active Website User
VOC Member
#50
David suggests

"I shoot for .030" for squish or quench. I am pretty sure it does not work at .055". It depends a lot on how high you rev and if it is a titanium rod (due to the stretching). For street, I would think .030" to .045" would be good. I don't think that .055 will damage the performance too much, after all, you're not racing the bike, but if the piston is too far from the head it won't transfer much heat from the piston to the head, which is the "quenching" that helps performance."

I am pretty much going with whatever squish is built in, since I will not be machining the head or liner, but as you mention, I will not be racing so the extra capacity is going to be used for a little more top end or pulling power. If anything, the band will be made bigger if I need to use a thicker cylinder base basket. The piston top edge is supposed to be flush with the liner top at TDC.
I will be taking Greg's advice and grinding the liner flange to head joint. I can see the tool marks he mentioned and due to the increased bore and the fact that the liner flange has to be slightly relieved to clear the 10mm bolt diameter, the sealing area is not very wide.
 

vibrac

Well Known and Active Website User
VOC Member
#51
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

Well Known and Active Website User
VOC Member
#52
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

Well Known and Active Website User
VOC Member
#53
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
 

Grey One

Well Known and Active Website User
Non-VOC Member
#54
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

Well Known and Active Website User
VOC Member
#55
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?
 

Grey One

Well Known and Active Website User
Non-VOC Member
#56
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

Well Known and Active Website User
VOC Member
#58
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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VOC Member
#59
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

Well Known and Active Website User
VOC Member
#60
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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