If this is happening it would only be up/down and probably not even measurable as there would be zero load side to side and the rod is centered on the crank due to oil film. Even up down load in this test is only piston/ring friction against the cylinder wall, so almost nothing. I agree with this. So establishing TDC Ferrari's way would result in cam timing to match as designed by them. Finding TDC with a piston stop would give you 1/2 deg retarded valve events all around.
Haha To the first part, he did state "neglecting any oil film". But that' can't be ignored, as it is there.
You have between 0.05 and 0.097 mm clearance according to the WSM. From TDC you would need to rotate the crank more that 2 degrees off TDC to close the 0.05mm gap. If there is some oil film, sure, a little less. But there is no oil pressure when doing this; no centering force. And if the crank journal didn't move in the rod there would be no dwell. And that's not even including wrist pin clearances. Now actual dwell and measured dwell will depend on how accurate you can measure piston position which translates into how well you can find TDC.
I tend to feel (need to lay it out) that the dwell is a result of the crank journal diameter making contact with the rod bearing and almost being flat at the very top. Meaning it's all side to side movement in the very small zone at the top. I'm probably not saying what I mean very well.
Here is what we need to do: With a crankshaft on V blocks, place an indicator on the rod journal, rotate the crank. According to John, we would have no dwell then. Is this correct, John? This is something I can easily layout in CAD if I have the dimensions. I'll look at the WSM. EDIT: actually, I just need the crank offset and then it's a matter of looking at the chord height of 1 degree of the circle the journal follows. In theory, it will show zero dwell but the delta will probably be very small.
No, they will bot give the same if done correctly. Dwell or no dwell, the angle at which the piston first stops as you turn toward TDC is at TDC. The piston stop method find TDC by measuring the angles at which the piston hits the stops in both rotation directions. That angle will be exactly symmetric. And, the measurement is made in a region where the piston motion is much more sensitive to crank angle, thus the angle can be measured very accurately if you have the tools. TDC will be exactly 1/2 between them plus the tolerance of the angle measurement. But as I said to Dave, with the Ferrari method there is a limit no matter how accurate your measurement system is. The reason is simple. As you incrementally rotate the crank and watch the dial gage you don't know if the piston has stopped until you move one addition increment and the dial gage doesn't change. Thus the best you can say is that TDC is somewhere between that and the previous crank position.
Can't measure dwell on the crank, have to consider the length of the rod just swinging at tdc. Longer the rod the more dwell (imagine a 3ft long rod vs a 3in long rod) Yes no oil pressure but the oil is still there. Doesn't take much to fill a .002in gap. The journal remains centered in the rod unless it pushed all the oil out, which it doesn't, and even then only up/down as there's no side/side resistance other than swinging its own weight.
If Ferrari says tdc is when piston initially reaches its highest point (start of dwell) how is that the same as piston stop method (center of dwell .5 deg later)
I just laid this out. The stroke is 3.03". The crank Rod journal diameter is 1.7176. So, the motion circle is 4.7476. The vertical change in height thru 1 degree of motion at the top is .00009039" That's pretty darn small.
Woke up to some interesting thought... What if Ferrari translated wrong in the WSM and was talking about spark TDC? On all those race bikes I played with we used 2 TDC's for spark and centerline both setting different operations. Now time for coffee going to pick avocados today
Dave, I don't totally understand your following post, but for this what you are implying is that the vertical motion is so slight that you can not measure it or see it on the dial gage. But as with the picture I emailed you, if the journal doesn't move with respect to the rod bearing, the movement around TDC is like this: Image Unavailable, Please Login Now, there is a difference between measurement accuracy, or the ability to see small movement on the dial gage and dwell. One is resolution, not dwell. Dwell, real dwell, can only come from the clearances where the crank can move but the piston doesn't. The picture is for F355 specs and would be for the case where the crank journal remains centered in the con rod bearing..
Abut a decade ago I build a CFD model of the F355 engine in the Lotus engine simulator. This model allowed one to see what would happen if you simply retimed the valves. The following picture indicates ABOUT what would happen using standard F355 valves and simply adding more (or less) overlap period. The different lines represent 2 degree changes in overlap: Image Unavailable, Please Login There are very minor differences up top (above 6,000 RPMs) in the RPM band used on race tracks. I am not surprised Ernie did not find anything on the stop watch. What is not evident in the graph (except indirectly) is that as overlap increases the tune to the headers becomes much more sensitive, sensitive to the point where one might want to consider a header with different lengths and diameters.
Piston stop method is not center of dwell. Piston stop method measure the angles at which you hit the stop rotating the crank clockwise and then counter clockwise. The motion of the piston rising in the cylinder is symmetric. So, 1/2 way between the two resulting angles is exactly TDC to with in the measurement accuracy. It's like I said. If you want to define start of the dwell at TDC, and the end as where the piston start to move down, then whether you consider clearances of not, if you start with the piston at TDC and measure the dwell rotating clockwise you will get the same result if you were to rotate the motor counter clockwise.
How is piston stop not center of dwell, John? Damn - you are going to make me mount a head and try this dead stop!
Dave, I don't know hat you did here, but the motion circle is just the stroke/2. The change in height is (stroke/2) x (1-cos (1)) = 0.0023"
John - on the road now. I will post layout later. I posted what an indicator would read in vertical travel going from .5 degrees away from 0 or the top
What was the purpose of this thread? We know from RD the Ferrari way. Why stress about it just do it. Why worry about dwell? You move crank to highest number you get at how many degrees. Rotate back retake up slack and see if you get it again at same max. You verify that number a couple of times...done! Dwell isn't important unless you want to piston stop TDC. When cam timing finding TDC is the 1st step. When you make tiny percentage mistake at the beginning of a multi-step process you magnify the error all the way to your final result. If you make a tiny error at the end of a process the error is still tiny. Mitch you confirmed on paper what I found on the racetrack. Cool to find the same conclusion from 2 very different ways.
If dwell is measured at 1 crankshaft deg piston stop method centers it within that degree. Ferrari method puts it .5deg earlier (start of dwell). Could not be any simpler. Longer rod = more measured dwell, fact.
