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My friends call me Phish
 
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Explanation of Left and Right Cam Shaft Lobes

I can't figure out why the L and R cam have completely different lobe configs, open V and a narrow V.

Someone please explain how it works.

Thanks.
Old 11-06-2010, 04:02 PM
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They both turn the same direction. Its the way the lobes run on the rocker surface. One side starts on one end of the rocker the other side starts on the other side of the rocker.
Old 11-06-2010, 05:27 PM
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The distance in separation of the cams lobes allow opening and closing of the valves of each cylinder in firing order sequence, as the cams rotate in relation the crank. The cams rotate twice for every rotation of the cam. If you were to watch the inner workings of the engine animated in slow motion the lobes would open the valves at the right time to let air in or out. The lobes are where they need to be to allow the exchange in timing with air, fuel and spark.
Old 11-06-2010, 05:40 PM
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I understand how the lobes work ,what I don't understand is that the cams aren't identical, like typical engine cams.

The right cam has the lobes for intake and exhaust (estimated) at 30 degrees from each other while the left cam has (estimated) 120 degrees from each other.

So, why aren't the cams symmetrical?
Old 11-06-2010, 08:51 PM
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I slept on it last night and this is what I might of figured out, but I can't explain it very well.

The reason why the R and L cams are different lobe angles is because they both spin counter-clockwise at the same time and since the cylinders are mirrored from each other (all pistons facing outward), having the same cam profile wouldn't work, it would counter act each other.

Makes sense in my head just hard to describe it.

Let me know if I am on the right track.

Last edited by pemz0r; 11-07-2010 at 06:07 AM..
Old 11-07-2010, 06:04 AM
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It's pretty simple...

Since the heads are identical on the LHS and on the RHS, the orientation of the intake and exhaust valves is different on each side of the engine. On one bank, the intake valve is located closer to the flywheel than the exhaust valve and on the other bank it's the opposite. The orientation of the cam lobes on the LHS and RHS have to be different to accommodate this.
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Old 11-07-2010, 06:48 AM
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Still confused.

The lobes determine the duration right, so why is the right side duration (lobes closer together) quicker then the left side (further apart)?

I guess I am just going to have to watch it work :/

Last edited by pemz0r; 11-07-2010 at 06:03 PM..
Old 11-07-2010, 05:53 PM
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Duration is the period, measure in crankshaft rotation, that the intake and exhaust valves stay open. It has nothing to do with the relative position of the intake and exhaust cam lobes. The valve overlap or lobe center is a measure of the opening and closing of the intake and exhaust valves relative to one another. The position of the intake and exhaust cam lobes relative to each other sets the overlap and lobe center.

This video shows it reasonably well

YouTube - Porsche 911 valve train Pro Engineer
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Old 11-07-2010, 10:02 PM
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I understand how the duration of valves work. What I am not understanding is the that the cams aren't identical with the angle of the lobes.

I watched the video, and the cam that is being used would be the right side, correct? Because the intake/exhaust lobes have a small "V" profile. The video shows the intake and exhaust almost simultaneously.

Now, if they had a left side cam in the video the lobes would have a wider "V" profile, correct? In turn the duration would be longer because of the wider "V" profile, therefore the intake and exhaust would happen at slower rate of opening and closing.


So, what I am not understanding is that the durations are different on each side because of the wide "V" and narrow "V"
Old 11-07-2010, 11:17 PM
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Duration is only dictated but the shape of the lobes. The angle of the V between the different lobes is a matter of timing. The heads are on opposite sides, and operate 120 crank rotation from each other.
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Old 11-08-2010, 07:03 AM
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Eureeeeeka

Thanks Everyone !
Old 11-08-2010, 10:04 AM
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Lots of answers - I'll throw mine in for further comment......

The intake and exhaust lobes being of different orientation to each other from right to left has nothing to do with which cylinder they are firing, their orientation to the flywheel, the duration, lobe shape or compression for that matter. The original question was why the left cam is identified by wide apart (think the letter "L") pairs of lobes and the right by close (rabbit ears) lobes.

The simple reason that the Left lobes point further apart than the Right is that:

On one side the cam lobes effect the exhaust valve before the intake and on the other side the opposite occurs. Because we want the left lobes to open the valves they encounter in the opposite order of the right the lobes point wider on the left.

Therefore the angle between the lobes can not be the same. Valve angle, rocker reach, mirrored heads and counter-clockwise rotation all figure in. An engine could use different rockers on one side to make up for this but that wouldn't be Porsche simple. BTW - The crankshaft turns twice for each rotation of the cams.
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Last edited by notmytarga; 11-12-2010 at 10:18 AM.. Reason: Goofs clarity humility
Old 11-12-2010, 09:03 AM
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