Author Topic: 400 Four detonation  (Read 369 times)

Offline K2-K6

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400 Four detonation
« on: December 05, 2017, 09:05:04 AM »
Alright,  I know I " go on about it" but been looking at why I feel detonation can easily occur on this engine given the right / wrong conditions.

As others pointed out you normally associate lean mixture with something like this,  which is not obviously the case if jetted properly,  you'd have thought?
A characteristic of slide carbs is that you can create it with how you ride,  even if setup is correct.
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Above should be a dyno trace that Jensen has kindly shared of his 400 Four running in 4th gear with throttle rolled open in that one gear to build the power curve you see in top traces.

The really interesting bit is the lower trace,  left scale is mixture with rpm along the bottom. It's obvious from that peak leading up to 4000rpm that it's running toward an over lean condition. Complete burn is considered to be 14.7 parts air to 1 part fuel. Most engines won't get that close. During acceleration you'd expect closer to 12 to 1 to make it move,  with cruising state in between the two.
It's not a wrong setting on here,  just characteristic of running from low rpm to high in one gear which is needed for the dyno plot.

What it does is the vacuum the engine draws at low rpm with throttle wide open cannot match the flow the carb venturi is making,  resulting in loss of vac on the main jet,  and so less fuel than ideal is pulled from float chambers ( it's this bit that PD carbs fill in with accelerator pump,  clever a?)  you can't jet your way out of it as the carb and main jet is almost fully open. It's really a failure of slide carbs used in this way and what would be termed "lugging " the motor in high gear.

In addition,  the ignition advance reaches max around 3000rpm and quite early for full load application ( more suited to cruising on light throttle to give good economy)  so in that lean peak area you've also got probably too much advance for a short period.

Put those conditions together,  perhaps riding along into a headwind,  fith or sixth gear,  trying to keep up speed at lowered rpm and you've got classic detonation. Riders with experience would recognise it's not pulling properly,  but if the circumstances are held like that without the engine accelerating through that, it will most likely detonate. It'll only take a few minutes like that to start the destruction of the pistons and rings.

You can't jet it differently,  it's just how you ride it. They need to be revved if you want them to go.
You've either got to back down the throttle or drop gears and spin it to higher revs.

Offline ka-ja

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Re: 400 Four detonation
« Reply #1 on: December 05, 2017, 09:13:46 AM »
Even when relatively a new bike (got mine in 1979) you had to use them above 5000/6000rpm to gain decent performance.
nice bike,nothing in the bank

Offline Nurse Julie

  • 1973 CB750 K2.1977 CB550/4 project. 1978 CB400/4.
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Re: 400 Four detonation
« Reply #2 on: December 05, 2017, 09:32:49 AM »
That's really interesting Nigel. So, going back to the mess on my pistons / rings on my engine strip thread, it would make sense to me how my engine got in such a state. If a PO had not ridden it as the bike likes to be ridden for optimum air / fuel mixture ie, high revs and the engine 'pulling' a majority of the time then it makes total sense how the damage had occurred. I have said many times that the CB400/4 likes to be ridden like a 2 stroke, high revs and minimal lugging and I can see how riding it for long periods in the wrong gear / too lower revs can start a chain of events leading to damage.
Can I ask you to put a copy of this on my engine thread as I think it is a very good explanation of how the chain of events could have lead to the damage on mine.
« Last Edit: December 05, 2017, 09:34:49 AM by Nurse Julie »
Im a Nurse, trust me, this won't hurt.....much !!!

LINK TO MY CB400/4 ENGINE REBUILD
http://www.sohc.co.uk/index.php/topic,14049.msg112691/topicseen.html#new
LINK TO THE CB400/4 DETONATION THREAD
http://www.sohc.co.uk/index.php/topic,14609.msg122071/topicseen.html#new

Offline K2-K6

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Re: 400 Four detonation
« Reply #3 on: December 05, 2017, 10:46:33 AM »
Transferred to your post Julie, it's a bit awkward for cut and paste on the tablet I use but think it's done the job ok.

There's something with these engines as they are so refined and benign ticking over and just tootling around that it convinces some that they don't like revs,  but they were built to do just that,  as you point out. Honda did a remarkable job in combining such a range of response in them,  and such tough little things to survive the effects you had in yours.

Larger engines will generally have enough torque to pull it through sub optimal sections like this,  but the smaller one's have to be driven much more like a small racer really. Properly oiled they don't have any real problems but people convince themselves that revs are damaging.

Nigel.

Offline taysidedragon

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Re: 400 Four detonation
« Reply #4 on: December 05, 2017, 08:34:40 PM »
I'd never really thought that the way I ride would affect the fuel/air ratio. Very enlightening, thanks.  :)
2010 Street TripleR
1977 CB400F

Offline Binman180

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Re: 400 Four detonation
« Reply #5 on: December 20, 2017, 12:29:07 PM »
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.

Offline K2-K6

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Re: 400 Four detonation
« Reply #6 on: December 20, 2017, 03:17:22 PM »
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.

Offline K2-K6

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Re: 400 Four detonation
« Reply #7 on: January 11, 2018, 09:28:23 AM »
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.

Offline K2-K6

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Re: 400 Four detonation
« Reply #8 on: January 11, 2018, 09:35:37 AM »
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.

Offline Trigger

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Re: 400 Four detonation
« Reply #9 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 ;) 

Offline K2-K6

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Re: 400 Four detonation
« Reply #10 on: January 11, 2018, 10:14:06 PM »
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.

Offline neilg

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Re: 400 Four detonation
« Reply #11 on: January 12, 2018, 01:04:28 PM »
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 ;)
  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)