The Ica is the Intake closing Angle and is the thing you are measuring.
You cannot measure the Lobe centerline; you have to infer it.
For your cam,Hughes lists the intake opening point as 14* and then you rotate the engine until the valve begins to close and you find that point is 44 ABDC. Between TDC and BDC is 180* the distance you rotated.
Ok so add that all together;
14+180+44=238* the intake duration measured at .050 tappet rise.
So now to find the centerline you do math to infer it. The halfway point of 238 degrees is119 the centerline of the lobe. But where is it relative to TDC? Well if you did it right, Hughes says 14* of those degrees are on the otherside of TDC, the Before side,so subtract 14 from the 119 and you have the inferred centerline of 105* After TDC. Since if the cam was installed at straight up, the centerline would be at 108*, this 105* must be 3* advanced.
The only number you need to know and manipulate,on a known cam is the ICA, the intake opening angle usually taken at .050 tappet rise. But to calculate the approximate cylinder pressure, you also need the advertised Ica which is usually measured at .008 tappet rise.
To calculate all the valve events, you need exactly 4 numbers; the durations of the lobes, the Lobe separation angle,and the amount of advance or retard
To compare one cam to another, you need to specify the lift at which the durations were calculated at. You cannot compare two unknown cams that have different starting points like one is measured at .008 tappet lift and another at .004 tappet lift . This is where the manufactures tend to confuse us. could be on purpose, IDK.
Back in the day you could buy a dozen 268 cams and they would all run different in your engine and all perform different. Then, in the 70s I think it was, they at least standardized the .050 measuring point. So now you can buy a half a dozen 235* same LSa cams and chances are that altho they might all idle differently, chances are they will all make similar power.
Ok off topic I know.
So say you have a 268/276/110 cam. from one manufacture this might be a 222/
[email protected] So now we have 3 numbers, The fourth is where to install it, and most will want it 4* Advanced. So there are the 4 numbers you need, written thus
222/230/110/+4
So lets manipulate the numbers
Ok the number we want is the ICA, and this is how we get it. First we need to know the installed centerline which is 4 degrees ahead of the 110 LSA, so 106 is the target .
Next you take the intake duration and divide it by 2 to get the center point which is 222/2=111.
Finally you have to synchronize them. So 111 is 5* from the 106 we want, so we turn the cam so that 5* go to the otherside of TDC, into Before TDC, leaving 222less 5=217 on this side, After TDC.
Next from TDC to BDC is 180*, and we have 217 left so 180-217=
37* ABDC ..This is the ICA , the intake closing point, at .050 tappet rise.
Easy peasy.
Now you do the same for the exhaust if you want to know.
If you want to get an idea of how it will idle and generate an Ica for calculating cylinder pressure, then you repeat the exercise using the advertised numbers.
So using this same cam;268/276/110+4
The Installed Centerline is still 110 less 4=106*
Next 268/2=134 the lobe center point, still inferred not measured. To synchronize them to the manufacturer spec, we have to move the cam from 134 to 106 which is 28* so we slide those over to the BTDC side leaving us 240 on this side After TDC
And 180 less 240=
60*After BDC, your ICA at advertised Duration. This is the number you would plug into the Dcr calculator.
I know clear as mud. After you do a few of these you'll get the hang of it.
I found it much easier after I put the events on a circle representing 720 degrees or two revolutions, representing the 4-stroke cycle. . Each quarter circle then represents 180*. I overlaid a Vee-notch at the top to represent overlap. After you do a few of these exercises on this circle thingy you will get a much clearer picture of what the cam does inside your engine. Or at least I did.
