Forum

[Alan R]

Hi guys ----------- and well done lads for putting across simply what to some people is a "Black Art" of fuels and combustion...How often do you hear someone say that "The fuel explodes and---------------- etc, etc"..It actually BURNS and takes a measurable time in so doing..........Ideally we would want the fuel/air mixture to be ignited spontaneously and equally within the combustion chamber------but, to date this hasn't been achieved, as far as I'm aware that is, so the nearer we can get to this ideal the better.....Now remember TREVORCH's reference to start-up checks on the twin magnetos??--- and NICO with the FORMULA 1 3-plug idea ?? It's actually the EFFICIENCY of the combustion process that's being improved.. ie getting as near as we can to burning ALL the fuel/air mix within that combustion chamber with absolutely NO unburnt HYDROCARBON fuel particles left over whatsoever...........Those of you who have changed from the standard CB points ignition to an electronic system will probably had to lower the idling speed ?? Same thing being seen from a different standpoint........With regard to the fuel/air mixture it's somewhere in the approx. ratio of 13:1 (can't find on my Apprentice Tech. notes at the moment for a more accurate figure)... This means that we need 13 weight units of air to 1 weight unit of fuel (which now has a 5% Ethanol stirred in !!) to give a mixture that SHOULD give complete combustion in IDEAL conditions........... Have a look at this }-------- http://www.engineeringtoolbox.com/air-c ... d_212.html -------------- notice that Oxygen is approx 20% whilst Nitrogen is just under 80% !!! It's the Oxygen content that we are after as it increases the RATE of combustion ( but does not burn by itself ie it's not a hydro-carbon )whilst Nitrogen is INERT..doesn't react with Oxygen, and just passes straight through..In other words it just gets in the bloomin way and has a free ride on top of the piston in the process....... That's enough for now 'cos my bonce is aching !---- Has that helped a bit ??...............I hope so ( ps, have a look at High-speed diesel injection systems, dual-stage injection process and bowl-in piston crown aligned to hemi-spherical cylinder heads )if you're a glutton for punishment !!!!!!!!!!!!

[ChrisD]

Johnb
All I have has been emailed to you - including photos. Most of what I got came from this website. I went for the twin plugging for two reasons, to add a bit more oomph (see comment by Paul Henshaw on the racing Bullets) (another 1-2HP is a heck of a lot if you only have 22 to start with) and from others for cooling. Both aspects worked OK and I can now cruise at 100kph on a bit less than half throttle without the bike getting too hot. So it was well worth the effort. Don't worry about naysayers - try it, that way you'll know the reality.
Cheers, ChrisD

[Nico]

Hi ChrisD,
what did you do to the ignition advance.
As I have stated before, with the twin plugs you sholud be able to reduce advance by several degrees or vice versa run into knock if you run the same advance as before.
On the other side, if you are running the original 6.5:1 knock is not really an issue anyway.
Cheers
Nico

[Bullet Whisperer]

Hi all,
Seeing as my name has been mentioned here, I'd better chip in. Both of the race Enfields, 350 and 500 have twin spark ignitions, also the ex-race 500, now back on the road, this latter runs on petrol, the first two on methanol. The methanol bikes run 15 and 14 to 1 compressions respectively and the petrol machine is at around 10:1 these days, The twin spark ignitions were fitted for accuracy and reliability over the magneto type ignition, one of which showed an 11 degree variation on timing when checked by strobe. Once, while on a dyno, I removed one plug lead [the left one] and connected it to another loose plug and earthed it, so the engine fired on one plug only. The engine lost about one quarter of a horsepower and the line on the dyno graph screen became a little more ragged, other than that, no change. That said, if I built another race engine, it would have twin sparks, as every little helps and the starting and reliability has been very good, although numerous coils and cdi boxes have been replaced since 2006, to cure some misfires etc we have had along the way.
Is twin spark worth it? That's up to the individual !
Cheers, Paul.

[ChrisD]

Nico, I had previously had the barrel and spigot skimmed by a couple of mm so raising compression with OEM ‘hand grenade’ piston to ~7.5:1. Then added a 8.5:1 forged piston and the knock was unavoidable even with barely 26deg advance. Our best fuel is 95 octane here in Cape Town. Then I added a JP 7:1 piston when I found the OEM handgrenade had gone off (collapsed and pinched the rings). Now with 130psi cold compression (~8.5:1) with the nearly flat topped JP piston I am still searching for the best ignition timing. Experimenting with JB Weld in the A/R slot and gradually filing it out to find the best position.
Cheers,ChrisD