Are two spark plugs better ?
Discussion
I've heard lots of different opinions on this one.
There's a box called MSD 6A(L) which gives 2 sparks with one spark plug, claiming to give better combustion.
I've been looking into one of these because the MSD 6AL includes a 'soft touch' rev limiter, which I might need...
Anybody got more info?
There's a box called MSD 6A(L) which gives 2 sparks with one spark plug, claiming to give better combustion.
I've been looking into one of these because the MSD 6AL includes a 'soft touch' rev limiter, which I might need...
Anybody got more info?
When you come to having it serviced it costs twice as much for spark plugs.
Originally the twin spark was used to allow two very large valves to be used and thus increase the rate at which charge could be inducted and exhausted and thus increase power. This forced the location of the plug to the side of the combustion chamber which reduced combustion efficiency and so another plug was used on the opposite side of the chamber to minimise this effect. This allowed the head to flow as much gas as a sixteen valve head (almost) whilst using half the number of valves and associated components.
However, the trend toward four valve per cylinder heads was followed (some would say unfortunately) by alfa. The primary plug can therefore be placed in the space between the four vales, allowing an even spread of combustion. The additional plug is now used more to 'clean' the combustion cycle but has a much reduced benefit and is agrguably more about keeping the 'Twin Spark' branding viable.
I would suggest Twin Spark used to make a hell of a lot of difference and now hardly any.
All IMHO of course.
Originally the twin spark was used to allow two very large valves to be used and thus increase the rate at which charge could be inducted and exhausted and thus increase power. This forced the location of the plug to the side of the combustion chamber which reduced combustion efficiency and so another plug was used on the opposite side of the chamber to minimise this effect. This allowed the head to flow as much gas as a sixteen valve head (almost) whilst using half the number of valves and associated components.
However, the trend toward four valve per cylinder heads was followed (some would say unfortunately) by alfa. The primary plug can therefore be placed in the space between the four vales, allowing an even spread of combustion. The additional plug is now used more to 'clean' the combustion cycle but has a much reduced benefit and is agrguably more about keeping the 'Twin Spark' branding viable.
I would suggest Twin Spark used to make a hell of a lot of difference and now hardly any.
All IMHO of course.
Not the same thing. The MSD etc delivers multiple sparks until 3000rpm, after which, just a single spark.
But, at the end of the day, if the first spark ignites the mixture, which it should, then the rest are rather pointless anyway.
Multiple sparks, and having 2 spark plugs are 2 entirely different things.
Your analogy of lighting one match or two near petrol isnt very realistic.
Ignition timing advance is required becasue the mixture takes a certain time to burn. Ideally you would have no advance, and the mixture would ignite instantaneously, and produce max torque/combustion pressure.
Because this doesnt happen, we start the burn, way before we want peak pressure to occur.
What having 2 spark plugs does, is effectively make the combustion chamber smaller, and a much faster burn, requiring much less spark advance. This is good. It will make more power, emissions will be better, and run less risk of detonation, as we have more control over the burn.
A better analogy with the matches would be, light one, at one end. It takes a while to burn. Consider the match itself to be the combustion chamber.
Then light a match at both ends, it burns much faster.
Actually designing a setup that will benefit from 2 plugs is much harder, as packaging with valves etc is difficult.
But, at the end of the day, if the first spark ignites the mixture, which it should, then the rest are rather pointless anyway.
Multiple sparks, and having 2 spark plugs are 2 entirely different things.
Your analogy of lighting one match or two near petrol isnt very realistic.
Ignition timing advance is required becasue the mixture takes a certain time to burn. Ideally you would have no advance, and the mixture would ignite instantaneously, and produce max torque/combustion pressure.
Because this doesnt happen, we start the burn, way before we want peak pressure to occur.
What having 2 spark plugs does, is effectively make the combustion chamber smaller, and a much faster burn, requiring much less spark advance. This is good. It will make more power, emissions will be better, and run less risk of detonation, as we have more control over the burn.
A better analogy with the matches would be, light one, at one end. It takes a while to burn. Consider the match itself to be the combustion chamber.
Then light a match at both ends, it burns much faster.
Actually designing a setup that will benefit from 2 plugs is much harder, as packaging with valves etc is difficult.
stevieturbo said:
What having 2 spark plugs does, is effectively make the combustion chamber smaller, and a much faster burn, requiring much less spark advance. This is good. It will make more power, emissions will be better, and run less risk of detonation, as we have more control over the burn.
I'd agree that is what was happening with the old 2 valve system but I think the new 4 valve twin-sparks have delay between the sparks. IIRC the first spark ignites the charge to kick off the power stroke. The second is then used to ignite any remaining uncombusted charge fractionally later. The intention being to release a little more energy and clear up any half burnt hydrocarbons remaining. I know it doesn't sound terribly realistic or efficient and my gut feel is that it isn't. I don't believe the 4 valve set up is primarily about power as the primary plug is ideally located in the centre of the chamber and so the burn is even and there would be little advantage (power wise) in starting another fire at the edge of the chamber (where the second plug is). Thus my, perhaps overly cynical, belief that the main reason for the current version of twin-spark is more marketing than engineering lead.
rustybin said:
Originally the twin spark was used to allow two very large valves to be used and thus increase the rate at which charge could be inducted and exhausted and thus increase power... This allowed the head to flow as much gas as a sixteen valve head (almost) whilst using half the number of valves and associated components.
... The additional plug is now used more to 'clean' the combustion cycle...
I suspect that at the time Alfa couldn't afford to develop a 16-valve head, and the twin-spark route was cheaper. A two-valve layout with big valves might flow as much air as a four-valve with smaller valves, but the four-valve will have higher gas speed which can help with fuel atomisation and swirl, giving you more power and better emissions.
stevieturbo said:
Actually designing a setup that will benefit from 2 plugs is much harder, as packaging with valves etc is difficult.
30 years ago BMW had a four-valve F2 engine with three plugs per cylinder... not that it was any good.
Alfa originally developed the 8 valve Twinspark because the old 80 degree valve angle (or single plug) heads were renowned for their low and mid range torque. Alfa decided not to relinquish this virtue and hence the 8v Twinspark.
It was not until the advent of variable valve timing (which I believe Alfa were the first to commercialise) that 16 valve engines were able to virtually match the higher torque of the 8 valve engines. The Phase 2 16 valve units also have variable inlet valve tract length which reduces the otherwise naturally cammy effect of these higher tuned engines.
The 16 valve engine is otherwise just a regular 16 valve. There are no features which specifically boost maximum power or torque outputs. They are just well tuned engines with VIVT and variable tract length to improve driveability and lower emissions.
Incidentally, Toyota took the VIVT one step further and produced the VVTLI (variable inlet timing and lift) whereby a secondary cam follower locks solid at 6200rpm, and the valve then follows the characteristic of this secondary cam so that the valve is opened with more lift and duration.
Ford now have variable valve timing on both inlet and exhaust camshafts, but like the Alfa system, this benefits mainly with a flatter torque curve and reduced emissions rather than creating more power.
To further answer the original question, the 8 valve Twinspark had 18 BHP, and 6 lbft more than its equivelent 1962cc single plug counterpart.
>> Edited by Alfa Mad on Tuesday 28th September 23:03
It was not until the advent of variable valve timing (which I believe Alfa were the first to commercialise) that 16 valve engines were able to virtually match the higher torque of the 8 valve engines. The Phase 2 16 valve units also have variable inlet valve tract length which reduces the otherwise naturally cammy effect of these higher tuned engines.
The 16 valve engine is otherwise just a regular 16 valve. There are no features which specifically boost maximum power or torque outputs. They are just well tuned engines with VIVT and variable tract length to improve driveability and lower emissions.
Incidentally, Toyota took the VIVT one step further and produced the VVTLI (variable inlet timing and lift) whereby a secondary cam follower locks solid at 6200rpm, and the valve then follows the characteristic of this secondary cam so that the valve is opened with more lift and duration.
Ford now have variable valve timing on both inlet and exhaust camshafts, but like the Alfa system, this benefits mainly with a flatter torque curve and reduced emissions rather than creating more power.
To further answer the original question, the 8 valve Twinspark had 18 BHP, and 6 lbft more than its equivelent 1962cc single plug counterpart.
>> Edited by Alfa Mad on Tuesday 28th September 23:03
OK. This thread's got me wanting to twin-spark my MZ, which should be easy, by drilling/tapping a hole for a second plug in an ES250 head. Only trouble is spark plugs have a really weird thread (at least no nut in the junk box will ever fit a spark plug 
). Anyone know what the thread officially is, for a spark plug with 21mm flats?
). Anyone know what the thread officially is, for a spark plug with 21mm flats? Pigeon said:
OK. This thread's got me wanting to twin-spark my MZ, which should be easy, by drilling/tapping a hole for a second plug in an ES250 head. Only trouble is spark plugs have a really weird thread (at least no nut in the junk box will ever fit a spark plug
M14 * 1 I think
Pigeon said:
OK. This thread's got me wanting to twin-spark my MZ, which should be easy, by drilling/tapping a hole for a second plug in an ES250 head. Only trouble is spark plugs have a really weird thread (at least no nut in the junk box will ever fit a spark plug
Interesting project - could you do a before and after rolling road to give an idea of power benefits?
P.S. shouldn't you fix the exhaust first
stevieturbo said:
But, at the end of the day, if the first spark ignites the mixture, which it should, then the rest are rather pointless anyway.
I agree, that was the point I was trying to make in my analogy - I wasn't referring to twin plugs.
My dad used to do lots of conversions on motor bikes to twin plugs, takes a bit of confidence/experience to stick a cylinder head on the milling machine and drill a hole through the side of it!
Eliot.
Fatboy said:
OK. This thread's got me wanting to twin-spark my MZ, which should be easy, by drilling/tapping a hole for a second plug in an ES250 head. Only trouble is spark plugs have a really weird thread (at least no nut in the junk box will ever fit a spark plug
Interesting project - could you do a before and after rolling road to give an idea of power benefits?
Access to a rolling road would be brilliant, but I love to fiddle, and if I had to pay I'd end up spending many times what the bike is worth
I make do with before and after rides down the same piece of road and compare the performances. Very subjective, but works if I don't do too much at once. Fatboy said:
P.S. shouldn't you fix the exhaust first
Thanks for reminding me. I fixed it a while ago, and have now updated my profile!
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