SOHC.co.uk Forums > CB350/400

400 Four detonation

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Mr-Gear:
What taysidedragon said.

Good technical content. Thanks for Posting. I'm sure it will have a subconscious affect on the way I ride my 400 from now.

K2-K6:
I re-read an initial road test published for the 400 when introduced to the UK,  they just seem obsessed in some older journalism of performance at low rpm,  ie lugging it around. They often talk of how the engine will accept top gear at whatever rpm is deemed impressive,  it's no wonder sometimes that these are ridden originally in such a way.
It's funny that they seem to live in mortal fear of higher rpm,  guess that's from brit bike experience. Although oddly they comment that they don't think it needs a sixth gear.
To get to 10,000rpm, the carbs are sized relatively large for the engine capacity to get that max flow,  so they need to be used with a little moreover finesse to make sure you don't drop into that lean hole at low rpm.

Cars have always coped with this effect by using composite ignition advance to avoid detonation. Swing weights form the initial advance,  plus vacuum from inlet manifold pulls the total up to optimum spec. But when you tread on the throttle at low rpm the vacuum drops away and so does the supplementary advance. This happens until the throttle starts to close again or the engine catches rpm catches up with throttle position,  at which point vacuum builds again. So part throttle cruising at high vac gives max advance and best economy,  but the risk of detonation is avoided automatically.

The bikes of this era,  it's all down to the rider. The 750 has enough torque to not get bogged down at lower rpm. The 500  obviously less but appears to be able to avoid it as well.
The 400,  if you got it in 5th or 6th perhaps with a passenger on a long uphill will most likely sit there not being able to gain any significant rpm. So the risk is much greater.
Being sensitive to not getting any rpm increase when winding open the throttle is just part of riding them I guess.

K2-K6:
Some more detail in this link that expands what happens and the timescale over which an ignition spark occurs

http://www.crypton.co.za/Tto%20know/Ignition/burn%20time.html

The devisions going across the illustration are in milliseconds.  Contrary to many views that a spark should happen in one big powerful chunk,  it's interesting to see how long the process is extended and that different systems will have a bias to give the engine designer the characteristics that are needed to run a particular combustion chamber design.

The Honda system appears to favour an extension of spark event to match the range in which it works and may be important in mitigating against early detonation tendency.

K2-K6:
To accompany the above post,  another link that I feel gives a good account of what the burn event in a combustion chamber really looks like.

http://www.sdsefi.com/techcomb.htm

This era of Honda combustion chambers have most similarities with the "hemi" type referenced in American V8 engines,  albeit in miniature. One of the areas of interest is just how long the combustion takes in terms of crankshaft degrees. The may be smaller bore but they run more outright rpm than a car engine so have very similar advance requirements to get the combustion completed in the time available.

Trigger:
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 ;) 

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