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400 Four detonation

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K2-K6:
I guess it's  not of mainstream interest but I'm fascinated by this stuff.

I like seeing things others have built up or modified running well. Some of the renovated bikes or specials I see out at a bike meet sound so well put together and run so sweetly,  they obviously know what they are doing.

neilg:

--- Quote from: Trigger on January 11, 2018, 09:57:15 AM ---This was one of the problems with the old Triumph twins. With the dome shape piston and the spark plug to the side, detonation was never getting to the opposite side to the plug in the combustion chambers . That is why when you change the oil on a triumph twin, you always can smell fuel in the oil, which hasn't been burnt off.
On the Triumph race engines I always blocked off the original plug hole and relocated it to the top of the engine, this gives a better detonation and power ;)

--- End quote ---
  I had similar but different, (if you get my drift), with my old Air Head BMW. When we lost leaded fuel the pre ignition was so bad I seriously worried about what damage was being done. I had already converted the valve seats but never envisaged this would happen at the time. I ended up twin plugging the heads and fitting Delorto pumper carbs to give it an extra squirt on acceleration. Also the timing unit was altered to give a different advance curve. The carbs were set up on a dyno and since then no problems in the last 15 years or so. (I.m looking for some wood to touch)

MrDavo:

--- Quote ---On the Triumph race engines I always blocked off the original plug hole and relocated it to the top of the engine, this gives a better detonation and power
--- End quote ---

I had a T110 with centre plugs, and was never really sure why they'd bothered until I read this.


--- Quote ---It's funny that they seem to live in mortal fear of higher rpm,  guess that's from brit bike experience.
--- End quote ---

Yes because they happily blew themselves to bits when you revved them hard. Been there done that, still got some of the wreckage. Also the resulting vibration from running at high revs would make any number of random cycle parts break in sympathy, just for a laugh. If I went over 85mph at night on my A7SS, the vibration would blow the headlamp bulb, plunging me into instant and terrifying sudden darkness.

BSA A10 conrods had a hole drilled through on one side, to provide oil to the main bearing from the big end (or maybe to oil the big end, I don't remember now). If you went fast enough you soon found out what could possibly go wrong with that, in a big way.



I think you're right it just feels wrong to rev the potatoes off any four stroke after an upbringing like that, whereas to someone accustomed to two strokes (don't rev them and they foul the plugs) and small jap bikes it would be second nature.

Drew400:
Thanks for this interesting thread K2-K6,

I have very limited technical knowledge but understand the main principles being discussed.

Does having Boyer Bransden electronic ignition have any bearing, good or bad, on your assertions?

Ta, D.

K2-K6:
Good question Drew, I was thinking about that too.

It's something that's not claimed generally by most types of system. They normally go with fatter / hotter spark, improved starting, improved high speed accuracy etc, making it difficult to see if they have considered more in depth issues and completed a design to address something very specific. They appear, in this field generally,  to cover this sort of stuff with words like "improved driveability" which could mean any number of things.

The Boyer has always seemed a very competent system but ultimately it may be worth posing a specific question to them to understand, in their view, how they approach something like this.

It's complicated by the Honda original, in my view, being a very good mechanical design with some clearly very subtle qualities that are very hard to improve without very specific knowledge.
The area that would seem in the original fit to be most influential is the length of spark,  ie duration of the spark event. Honda appear to have designed in a specific length that would benefit their combustion chamber requirements. The bigger is better school of thought may trip up on something like this.

This is pulling together what I view as happening to produce error;  with a still relatively low piston speed at around 2/4000rpm the ignition reaches its maximum advance. The time of piston travelling during the period of spark initiation until piston reaches tdc,  if it's more time than spark duration,  then the spark will have completed prior to tdc. It looks to me that Honda are trying to cover this area of interest by designing in a "long" spark.
If a replacement system shortens the spark,  it looks like this area is the most vulnerable to complications. So anyone claiming a shorter ignition duration will appear suspicious.
It's notable ( I think it's correct)  that Honda delay the max advance rpm point for the 400 Four above those of the 500 / 750 so they are also massaging this point as well to avoid the detonation as I view it. I think the Boyer doesn't change this spec but also uses it for the whole range of these four cylinder engines.
If you then add in that mixture curve that we can see has the possibility of going very lean (peaks about 15.7 parts air to fuel ratio) it all meets at this critical point,  making it very marginal to get a clean combustion event completed.

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