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View Full Version : Cam Timing - Technical Question

BillWelter
08-15-2002, 09:06 AM
If a cam is spec'ed at (for instance) at 270 degrees duration, and has an intake lobe center of 106 degrees, then that means that valve will start opening at 106 degrees minus 1/2 of 270 degrees, and will close at 106 degrees +1/2 of 270 degrees. So it starts opening at 29 degrees before TDC and closes completely at 61 degrees after BDC (241). The 29 degrees BTDC sounds right, but the 61 degrees ABDC sounds like a bit much. If you use a bit of triginometry, you can compute that the piston is well into it's compression stroke by the time the valve closes. My math indicates that for that cam, the engine will only get about 75% of its full stroke after the valve closes completely. Does that sound right? Thanks.

Gary T
08-15-2002, 07:10 PM
It may be right, and an illustration of the real-world effects of gas flow. Intake valves open during the end of the exhaust stroke, with the piston still moving up. It's tempting to think that this would push stuff into the intake manifold, but it doesn't--the exhaust gasses flowing out induce the intake gasses to follow. In a similar manner, the intake flow continues even when the piston is moving back up again. It's non-intuitive.

In the early days of automobile engines, these elements of cam timing were figured out by trial and error, seeing what worked in practice. I'd be curious to find out if nowadays it can be calculated, or do they still have to pretty much try it and see.

zwede
08-15-2002, 08:54 PM
The numbers sound correct. Keep in mind that the "270 dgr" cam is what is known as "advertised duration". This is the duration when the valve just begins to open. Very little air flows during the first 20 degrees or so. A better indication is the 0.050" duration, which starts when the valve is open 0.050" and ends when it goes back to 0.050". A 270 advertised is likely to have around 220 dgr @050.

BTW, there are some very sophisticated computer programs that let engine designers model engines. These programs look at the whole engine as a system, starting at the aircleaner and all the way to the tailpipe. Each part has its own fluid dynamic properties with flows, resonances and so forth.

Markus

AHunter3
08-15-2002, 10:30 PM
Yeah, air is compressible, which is a very important consideration at the rate at which pistons sweep their volume. The still-open exhaust port doesn't let the air all leak out because it doesn't compress evenly (the air closest to the rising piston is more compressed) and the exhaust port is only nominally open by that time anyway.

It's a fluid, and therefore has latency time.

David Simmons
08-16-2002, 02:05 AM
Originally posted by AHunter3
Yeah, air is compressible, which is a very important consideration at the rate at which pistons sweep their volume. The still-open exhaust port doesn't let the air all leak out because it doesn't compress evenly (the air closest to the rising piston is more compressed) and the exhaust port is only nominally open by that time anyway.

It's a fluid, and therefore has latency time.

Correct. Air has momentum. The part of the air in the incoming column of air that is close to the piston compresses as the piston starts to rise, but the air further away still has incoming momentum so it keeps on coming until the pressure wave from the rising reaches it.