Why is tickover erratic with more aggressive cams ?
Discussion
What is it that makes the engine erratic at low revs with more aggressive cams? I understand that it is easier to overcome the problem if mapable ignition timing as well as fuelling is available. Also heard a comment that to get power delivery at the top end requires cam timing that compromises low speed performance. The cam I have fitted now seems to pull cleanly from very low revs provided it is at a large throttle opening, below 1500rpm with a light throttle it gives the effect of a misfire. Just would be interested to understand what the cause is. Even at low revs the “suck squeeze bang blow” timing must be, relative to each action the same, so what is going on ?
Overlap,performance cams have more of this,usually with more lift also.Overlap is the time the inlet and exhaust valves are open together,this helps make power further up the rev range,but at low revs,and a much lower throughput,it makes the tickover lumpy.Someone who really knows what theyre on about wil be along shortly to explain better
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.Inertia is what's happening. The gases flowing into and out of the cylinder can't start and stop flowing instantly, and the valve timing takes advantage of that.
Say you have a piston descending with the intake valve open. Air/fuel mixture is drawn in. At BDC the piston changes direction and starts rising. Let's assume the intake valve stays open. If the air/fuel mixture had no inertia, it would instantly flow back into the intake.
Actually what happens is that air/fuel mixture keeps flowing into the cylinder because it has inertia, ie takes a while to slow down and stop. If the valve timing is correct for the engine speed, the valve closes just at the point where the maximum intake charge has flowed in, and that will be some time after the piston has started to rise.
The same applies to the exhaust side, and to the interaction between the two which occurs during the 'overlap' period when intake and exhaust valves are open. You can only optimise the valve timing for one engine speed, at which all these inertia effects work out perfectly (unless you have some sort of variable timing).
Say you have a piston descending with the intake valve open. Air/fuel mixture is drawn in. At BDC the piston changes direction and starts rising. Let's assume the intake valve stays open. If the air/fuel mixture had no inertia, it would instantly flow back into the intake.
Actually what happens is that air/fuel mixture keeps flowing into the cylinder because it has inertia, ie takes a while to slow down and stop. If the valve timing is correct for the engine speed, the valve closes just at the point where the maximum intake charge has flowed in, and that will be some time after the piston has started to rise.
The same applies to the exhaust side, and to the interaction between the two which occurs during the 'overlap' period when intake and exhaust valves are open. You can only optimise the valve timing for one engine speed, at which all these inertia effects work out perfectly (unless you have some sort of variable timing).
To answer this correctly would take about 20 pages of a4 paper so a real short version!! firstly as mentioned earlier it is the overlap that kills idle quality and slow running, there are several reasons for this one that andrew mentioned on inertia/scavenging, take it on idle lets say a cylinder is 1ltr, so if its a v8 its an 8ltr, and you have a static compresion ratio of 10-1, at idle with the gas speed being so slow you are probably going to only fill this cylinder a half full, to make matters even worse on the overlap period when the exhaust valve is closing and the inlet opening, again because the air speed out of the exhaust is slow so there can be no scavenging or inetia to pull all the burnt gasses out the cylinder a lot goes back into the intake manifold and pollutes the incoming charge, so what you do pull in is not 100% fresh air and fuel, eventually you are probably only filling your cylinder a third full, effectively you have ended up with a 2.6 ltr v8!! this also means your compresion ratio has dropped from 10-1 to 3.3-1 with only having a third of the cylinder full, the benefit of fitting mapped ignition is to basicaly be able to retard the ignition to make less power, then increase the power back up by opening the throttle more, what this effictively does is increase the gas speed through the engine and help with better cylinder filling
Very good explanations that even I can understand, thank you. Can understand that the engine produces a lot less power with a less complete charge of nice clean air and fuel. But still don’t understand why it is so lumpy. Even if it gets only half the ideal charge you would think it would get the same half charge each time and just be way down on power rather than act like it is intermittently missing. Unless the gas flow at low speeds is so chaotic by nature that no two cycles are the same.
Is that the purpose of the vacuum advance retard on the Dizzy then ? does it retard the sparks at low revs ?
Oddly I don’t know what cam is fitted. The “ tuner” that fitted it selected what he thought best for the result I wanted. He did say but I can’t remember and can’t seem to get hold of him today. Don’t think it is a 404, he didn’t seem keen on that because of valve piston clearance.
Is that the purpose of the vacuum advance retard on the Dizzy then ? does it retard the sparks at low revs ?
Oddly I don’t know what cam is fitted. The “ tuner” that fitted it selected what he thought best for the result I wanted. He did say but I can’t remember and can’t seem to get hold of him today. Don’t think it is a 404, he didn’t seem keen on that because of valve piston clearance.
steve-v8s said:
... Unless the gas flow at low speeds is so chaotic by nature that no two cycles are the same.
Spot on. Because the valve events are mistimed the gas flows are unreliable and, as you say, chaotic, so the engine stumbles from one cycle to the next.
steve-v8s said:
Is that the purpose of the vacuum advance retard on the Dizzy then ? does it retard the sparks at low revs ?
Generally the engine needs more advance at part-throttle, so the vacuum system retards the ignition at idle and at full throttle.
Had another thought why the engine stumbles at low RPM. May be with all the problems explained above there is not always an adequate charge of anything combustible at all around the plug to actually ignite. As result the next cycle would have a nice clean charge not contaminated with left over exhaust gas resulting in a nice clean burn. If this is the case then I guess it may actually fire every other time, so what feels like a misfire may actually be that. If that is the case I guess the MOT man wouldn’t be that happy about all the un burnt fuel coming out the exhaust.
steve-v8s said:
May be with all the problems explained above there is not always an adequate charge of anything combustible at all around the plug to actually ignite. As result the next cycle would have a nice clean charge not contaminated with left over exhaust gas resulting in a nice clean burn.
That's exactly what happens. The better one cylinder burns, the greater the effect of exhaust gas recirculation on the next firing.
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