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degrees to close valves

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gotbl4d3

10+ Year Contributor
78
0
Feb 14, 2011
billings, Montana
Could somebody help me how to find the degrees to where my intake valaves are closed. I looked at the diagram on the boost pro thread and i dont quite understand could somebody help me out with this thanks
 
The valves in what cylinder? You will never have all 8 intake valves closed at one time, there will always be a set open in one cylinder.
 
Alright then what do people mean by having all your intake valves closed for boost leak test. I dont understand the diagram on the boost net thread for it to be at a certain degree to do a boost leak test
 
Alright then what do people mean by having all your intake valves closed for boost leak test. I dont understand the diagram on the boost net thread for it to be at a certain degree to do a boost leak test

It's not about having the intake valves closed, you need to have the engine so that none of the cylinders have the intake and exhaust valves open at the same timg (overlap).
 
Ohh alright i underatand now. But could somebody explain to me what degree i have to be at and also help me out how to find that diaagram on the turboboost tester thread is really confusing me.
 
Yes i i know this thread i have read through it many times. My question is about the diagram on the bottom i am having trouble underatanding it.
 
Yes i i know this thread i have read through it many times. My question is about the diagram on the bottom i am having trouble underatanding it.
Simply put, the diagram is just for showing the valve overlap area. Technically (and a little hard to understand) the diagram shows when any intake or exhaust valve is open, only on the strokes when they are open, relative to the crank angle for any cylinder over two revolutions. Don't get confused thinking it shows what happens in only one revolution (or where valves are for all cylinders at one instant in time). The exhaust circle happens on the next crank revolution after intake stroke (starting near end of power stroke and going through exhaust stroke). The intake circle starts slightly before the exhaust circle ends (overlap) on the same revolution. This valve overlap is what you must avoid when performing a boost leak test (since both intake and exhaust valves are partially open at the same time).

Remember 4 cycle engines have 4 strokes: intake, compression, power, exhaust. Two pistons go up/down together in each pair (1&4 or 2&3). While one of the pair is on one stroke (eg. cyl 1 intake), the other is on 2 strokes later (eg. cyl 4 power). So the diagram shows the intake open circle (cyl 1). The other of the pair (cyl 4) being on the power stroke does not have the intake valves open (nor exhaust) over the same crank angle so none is shown.

For the exhaust circle, while one of the pair is on one stroke (eg. cyl 1 exhaust), the other is on 2 strokes later (eg. cyl 4 compression). So the diagram shows the exhaust open circle (cyl 1). The other of the pair (cyl 4) being on the compression stroke does not have the exhaust valves open (nor intake) over the same crank angle so none is shown.

If you are still confused with the diagram, try viewing it this way: The exhaust valve opens near the end of the power stroke (at 57* BBDC) and stays open for the entire exhaust stroke. When nearing the end of the exhaust stroke (at 21* BTDC ) the intake valve opens and stays open for the entire intake stroke. Near the beginning of the intake stroke (at 15* ATDC) the exhaust valve closes (and stays closed until it's near the end of the power stroke again). Near the beginning of the compression stroke (at 51* ABDC) the intake valve closes (and stays closed until it's near the end of the exhaust stroke again).

So to answer your original question of what degree to set the crank at for a boost leak test, you must avoid the valve overlap area. A good choice (as you can see by the diagram) would be approximately either 30* ATDC or 40* BTDC (the exact value is not critical as long as you're far enough away from the valve overlap area). [ie. You must not be in the TDC or BDC area as some cyl will have both intake and exhaust valves open there (valve overlap)].
 
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