Engine build on the brain
Don’t let AJ hear you say that lol
hah
This number does not look correct when used. Some clarification would be great
You're on the right track;
Heres how it goes;
First you have to determine the installed position, and back-calculate where the intake opens.
Next you add those degrees to one revolution or 360 degrees.
Next you subtract the intake duration from the previously calculated 360+intake opening.
This will result in a number that is left over(*1) to be the compression degrees. This number you then subtract from a half turn or 180 degrees to get the ICA,Intake closing Angle.
Ok so, lets take an example of a popular cam, the
268/276/114+4 Mopar cam
Mopar wants it in at 4* advanced so 110.
then; 268/2 less 110 is 24degrees to the overlap cycle. Adding that to 360 equals the total available degrees of 384, for intake plus compression.
From that, we subtract the intake duration of 268 to get 116* dedicated to compressing the mixture.
Backing up from TDC, this is 180-116=64* Ica. which is 64 ABDC, same as 116 BTDC.cuz from bottom to top is 180*.
here it is in longhand;
180-{[(360 +(268/2-110)]-268}=64
This helps you nothing, except that you can use that Ica in other calculations to see what your cranking cylinder pressure might be. And knowing that, you can get an idea of how strong or weak the bottom end might be. And knowing that, helps you select a stall-rpm and rear gears.
But one thing you always have to bear in mind; this number is a calculated number, based on an advertised number , that is not exactly when the intake closes. Most of the advertised numbers that you will find, are at some other than zero-lift number; such as at .008 tappet rise, or .006 tappet rise , or in some cases .001 or .002.
What this means is that the number that the calculator spits out, is not altogether very accurate, because the valve might take some unknown number of degrees to actually close and seal. This number could be just a few degrees or it could be a lotta more degrees, and so to find it, you'll have to install it and measure it.
However, knowing this now, allows you to cheat.
If the calculator says that a certain combo will make 165psi, which is generally the ceiling for best pumpgas,
but the number you inputted to the calculator was a result of a .008 spec, You know you have several degrees to play with as compared to a .006 spec'd cam. But even better is that if you know the cam has really long slow ramps, you might ultimately be able to run this combo on LESS than best pump gas. Now, if you're paying attention, this also gives you a chance to cheat some more and build to an even higher cylinder pressure, knowing that your pressure is gonna be real-world be less than what the Calculator spits out. Hmmmmmmmmmmmmmmmmmmmm
Of course if you get greedy and it doesn't work out, it's best to have a back-up plan.
Mine was the .028 gasket. All calcs were done with the .028. So then I would have the option of adding gasket, to reduce pressure. But this too comes with a caveat; I wanted a tight quench, so zero-decked the pistons. So, what I read was to stay out of the .050 to .080 range,which they said was prone to detonation. PLus the scuttlebut was that quench over .050 was no Q at all. so from .028 to .050 was a pretty limited range.
To get away from the nervous cheating, I opted for aluminum heads which are not nearly as sensitive to this kindof trickiness.
(*1) left over;
the bigger the cam, the greater the intake duration will be, and you gotta take those degrees from somewhere,cuz you only have so many to spread around. So some will go into the overlap, and some will be stolen from compression.... because the total amount of degrees is fixed by 720 plus overlap. This cannot be changed.
For instance; on a 292/292/108 cam the overlap is 76* so the total number of degrees available is 720+76= 796... but on a 262/268/112 cam, the overlap is 41, so the total available for distribution is 720+41= 761.
It is for this exact reason why your static compression ratio has to be increased to maintain cylinder pressure, because a drop in cylinder pressure always, always, always, affects the low-rpm torque..... in a negative way.
So if you follow that thinking, then an increase in cylinder pressure, always affects the low-rpm torque in a positive way........until you have so much that you either run into the dreaded detonation wall, or the engine just can't help but spin the tires every time you lay a lil throttle to her.... lol.
For Street,I like the 360 cubic inch engine, cuz you can move the numbers around fairly economically anywhere in between . Smaller engine have to be biased one way or the other pretty hard, and bigger engines end up with a preponderance of low-rpm torque that is difficult to impossible to get rid of.
My yardstick is 10 cubes per pound of chassis, so 360x10+3600 pound vehicle; car and driver. A 318 would be 318x10=3180........ and so on. My car is 3650 me in it, and the engine was bored to make 367 cid.
Depending on your chosen parts, the 360 can be built between 8/1 and 11/1 with very little machining. which gives you a lotta lotta adjustability to fit just about any suitable for street, cam.
