Thread: intake duration

Hi,

Me and a friend of mine are interested in calculating the ideal length (L) of the intake at a given rpm.

We need to make sure 1 variable, which is intake duration.

The formula:

L=(Vg * ((A/360)*(1/N/60))/2)

A: crank angle intake valve open
Vg: speed of sound (365m/s)
N: revolution RPM.

So looking in FSM, page 379 for the SR20DE highport Nx2000 1991-1993 we get this drawing:



But the numbers stated there don't make much sense to us.

Looking at the picture, we know TDC must be 0 and BDC must be 180.

So lets add up C+A+D and F+B+E

14+248+54= 316
60+248+8= 316

How should we read this diagram?

A is the degrees that the exhaust is open for. Similarially, B is the degrees the intake is open for. D and F are simply the degree difference from BDC that the intake and exhaust are closed, respectively. Same goes for C and E for TDC. Don't forget, this is all based on a 2-rotation system. 720 total degrees.

From the way I'm reading it, the degrees listed are simply a reference for where during rotation certain events occur. They will not add to anything in particular. They will only tell you at what rotational degree opening and closing of valves will occur, and using A and B's measures, you can figure the percentage of rotation the intake or exhaust is open, and how long compression and other events occurs.

As you read around, you can see that at the end, or top, of the diagram, the intake opens before the exhaust closes, for a degree duration of C+E, or 18deg on a DE. It then resets, closing the exhaust and goes past BDC where then the intake closes. You then have a compression stroke, and at a degree of F before BDC after ignition (almost 360deg from the intake closing), the exhaust opens, and remains open until TCD+E, and intake opens just before that at TDC-C. This is the full engine cycle.

Ok, so I took the SR20DET figures by mistake, I'm interested in the SR20DE intake duration.

What I understand from your words, just to get the intake duration, because I still don't really get the diagram, the intake duration = B - 180 = 248 - 180 = 68

Or I just do C + D = 9 + 59 = 68?

Is 68 the correct answer for intake duration? Did I mess up something?

Intake duration is B.
Exhaust duration is A.

C, D, E, and F are reference numbers. They don't add in for duration. They simply let you know how far from TDC or BDC the valves will open or close.

The only thing you'd need to add would be to figure valve overlap. For instance, Intake and Exhaust overlap is C+E. Degrees the valves are all closed is 360-(D+F).

Ok, I'm officially lost here.

Intake duration of 248 would be a bit long, I indeed did think of the overlap and didn't know what to do with it so I just ignored it.

But then again, something like this drawing is way more readable:



What am I missing in the FSM here? I still stick with 68 for intake duration but I'm totally not aware why, pretty uncertain and

Well, to be honest, it's straight-forward, especially considering the FSM directly states the numbers. You're not missing anything, you're over-thinking it.

You're formula to get 68 as a duration is only adding degrees from one side of the circle. You're forgetting the other 180deg on the other side. Notice C and D are on the left of the circle. You need to add 180 for the entire right side between them. 68+180=248, which is what the FSM shows as the intake duration. You were halfway there.

As far as 248 being too long, consider this. First off, the diagram you show has a degree of 232 for the intake. Thats only 16deg difference from the FSM's diagram. BC Stage 4 Race Cams have an intake degree of 288. Heck, their Stage 1's for street use are even 264. While these measures might seem big, they work because of the 720deg system, and intake/exhaust overlap.

Exhaust duration is often identical to intake, or at least close. But let's stick to the FSM's diagram and numbers. Add the intake and exhaust durations (A+B), we have 488deg so far. According to the FSM diagram, compression and ignition after intake closes and before exhaust opens is 360-(D+F), or 250deg. That brings us to 738deg. Take the overlap into account (C+E), which is 18deg, and we're at an even 720deg.

Also, as you're no doubt noticing by now, different cams and different engine types will greatly affect your formula, so if you're planning on making the "ideal length", it will be application-specific to the engine and it's cams. By no means am I looking to deter you, just inform you. Unfortunately, there isn't a "one design fits all" option on this one, based on where you seem to be going with it.

Agree, it's a formula, it's for instance not taking in corporation bends, diameter and how well the surface ''flows'' and intake temperature. Also the speed of sound seems a bit akward to me.