SOHC.co.uk Forums > CB350/400
Before I took it apart.....
Nurse Julie:
--- Quote from: K2-K6 on February 05, 2018, 10:37:49 PM ---
--- Quote from: Nurse Julie on February 02, 2018, 10:03:45 PM ---Good looking bike there Arfa. Bloody strange how, by your description, your engine damage is the same as mine and Craig CB400's, pistons 1,2, and 3 all damaged but No.4 unaffected. I wonder just how many other CB400's have had the same damage that we are unaware of?.
--- End quote ---
Julie, it's interesting (as you've pointed out) that more have turned up with that same 3 affected and the same one intact cylinders. Certainly starting to look more of a consistent pattern. As an initial thought, one clear difference that exists is the exhaust header pipes differ in length which would mean they have a different rpm point at which they are effective / less active on a cylinder by cylinder basis. The longer the pipe, the lower the rpm at which the scavenge affect on the cylinder is most effective. If the number four pipe being the shortest is coincidentally at its best when the other cylinders are in trouble, then it could makes the difference. Generally it's best practice to make them all the same, this series of Hondas all have the odd lengths though.
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I agree with what you are saying Nigel. With my very limited knowledge base of all things technical, I assume you can't counteract the difference in the activity of each cylinder to compensate for the header pipe lengths by tuning each carb differently, if you get my drift ?
hairygit:
Not the first time I've heard that about exhausts. Back in the early 80's a mate had a full Yoshi tuned 460/4, and I questioned why the exhaust took such a seemingly tortuous route, his answer was that Yoshi aimed to keep all downpipes the same length for performance and reliability reasons. Never thought any more about it until now....
taysidedragon:
I believe some manufacturers have used downpipes of slightly different diameters to equalise back pressure where they couldn't equalise the lengths.
K2-K6:
If you follow the exhaust pulse down the pipe you can visualise the affect it has.
This is how I understand it; the exhaust valve opens and the pulse sets off down the pipe, when it reaches a significant change in volume (enters the collector junction) it expands which makes a pulse volume change. This can't be achieved without an opposite effect, which is a negative pressure pulse going back up the header toward the engine.
It's this that someone once explained to me as the following ; imagine that pulse as a miniature swimmer doing breast stroke, as he swims up the pipe his arms are pulling the still present exhaust gasses past him and assisting in it's progress away from the cylinder. That's one effect that's general.
The further it reaches up the pipe and into the engine the more intricate it gets. The camshaft timing to let the engine rev freely to ten thousand rpm has an overlap of both the exhaust and inlet valves being open, for a small period of time, together. This is used at elevated rpm in a dynamic flow to increase engine efficiency and hence more power.
But at low rpm it will effectively cause a problem and drop in torque. Now this is the clever bit of good primary pipe design, if you arrange that pipe design to deliver the reverse pulse at exactly the point in the rpm range you are trying to improve, then that pulse passes right through the combustion chamber and continues up the inlet, though the carb and out to the end of the bell mouth. In effect it improves the flow right through the whole combustion system
As the cylinders, cam, carbs etc are all the same, then it follows that the primaries also need to follow suit to impart their effect on all cylinders at the same rpm point.
On these they are obviously different so by default must show a variance of best effect at differing rpm across the cylinders. Because no1 has longest pipe, the cam overlap will be best utilised by the pulse at the lowest rpm of all cylinders. Obviously with no4 being the last / highest rpm point as it has the shortest route for the pulse to get back to the cylinder.
As Tayside says, if you changed the pipe volume by altering the bore it could potentially be equalised, even with different lengths within reason.
It's this affect that is generally referred to as "back pressure"
Renault F1 Engine: http://youtu.be/y2iBbwocYZw
This clip is interesting because it partly shows the standoff created ( engine is running with airbox removed) and the amount of flow influence that can exist outside of the inlet bell mouth. The spray bars contain the fuel injectors and you can see the flow even above the injector point so it shows you how far out the engine can influence the air with its pulses.
Notice those matched primaries on the F1 Engine, very large bore for maximum flow at about 18,000rpm and very short to place the best pulse effect also high up, probably close to where the engine rpm drops to when going up a gear to maximise the torque at each up change.
K2-K6:
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I agree with what you are saying Nigel. With my very limited knowledge base of all things technical, I assume you can't counteract the difference in the activity of each cylinder to compensate for the header pipe lengths by tuning each carb differently, if you get my drift ?
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Cor, you don't arf ask some awkward questions Julie ;) ;D
I kept thinking about that but couldn't come up with anything that would stand up to realistic scrutiny and in any competent way compensate if there's a defined difference there because of header length.
But then, by trying to do that, realised that some conventionally accepted setup routines may be doing exactly the opposite, ie adding to any error that could be there.
It can be demonstrated that the standard pipes enhance the carburation at tickover and lowest rev range by the effect when a "performance" system is fitted. They have often required idle jet replacement to bring this range back to operating correctly. It's mistaken to believe the engine needs more fuel in this scenario, it's because the new system looses some of the vacuum effect through intake and responds at a different rpm point. Because the vac is reduced, less fuel is pulled from the jets so you have to reset just to get back to the original fuelling balance.
The designer of the original system has to place the response somewhere, complicated by the design with different pipe lengths spreading it over a much wider rpm scale but also differing across the cylinders.
So if you place the #1 pipe response down near tickover, then let the successively shorter pipes cascade across a rising rpm scale you'd make best use of a technically wrong pipe. Now that won't make a massive difference to running the engine, assuming everything else is equal.
But if you now examine setup, then it leads you to produce a biased setup that is wrong. Potentially.
If you have everything setup equal across all cylinders, then when you run it at tickover the #1 cylinder should produce the most vacuum (you can see where this is going from that statement) because of the header pipe interaction. This should produce a vac gradient decreasing incrementally across the cylinders to #4.
Now that setup above IS correct. But if you put gauges on it then it will force you to raise the #1 throttle slide to drop the vacuum and match number 4 and less so the 2/3 cylinders. You've now got an error across the engine in throttle slide height, which is wrong.
Although this may not make a huge difference when it's running normally, when it gets to that lean run wide open throttle peak from the dyno, then number 4 will be running richest with the others getting successively leaner across to number 1. Giving you a variance of the detonation point onset.
It looks like it should have the carbs equalled as the Bryanj method describes, then left alone. Tickover smooth running needs to be adjusted by individual setting of the idle jets for each cylinder. This would maintain carb parity across the rev range and help to avoid the onset of detonation.
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