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Author Topic: Gamma Camshaft  (Read 478 times)
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Brian Long
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« on: 07 August, 2017, 08:02:11 AM »

I have been watching a friend of mine rebuild a Gamma engine. One camshaft seemed to have unusual inlet and exhaust lobe positions, i.e.on the RH camshaft they looked 'normal' i.e. the inlet and exhaust lobes had the usual looking timing positions for both cylinders.
However, the LH camshaft front cylinder lobes, I think they were, inlet and exhaust lobes were positioned diametrically opposite each other. The other cylinder's lobes on that camshaft looked 'normal'.
Can anyone shed any light on this apparent anomaly?
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Brian Long
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Charles
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« Reply #1 on: 05 September, 2017, 01:09:02 PM »

A very interesting observation Brian.  The picture shows that the inlet and exhaust lobes for both cylinders on the nearside (no1 & no3) are both almost diametrically opposed whereas those on the offside are close together.  The reason is that the Gamma is a boxer engine and so the plane of one bank of cylinders is 180 degrees away from the other in terms of rotation so the lobes of one cam are also rotated 180 degrees in relation to the other.  One thing that I had not noticed before, despite being a Gamma owner for 20 years is that the position of the inlet and exhaust lobes are swapped on the cams.  Those who look closely will see that the lobes are not symetrical, they have a sharply lifting side and a more gently lifting side.  On the n/side camshaft the inlet lobes are in the middle with the exhausts at each end.  You can see that the inlet cam lobes snap the valves open very quickly and then close them more gently whereas the exhaust lobes do the opposite. Close examination will reveal that the lobes on the o/side cam are the other way around i.e. inlets at each end with the exhausts in the middle.


* Gamma camshafts.JPG (144.76 KB, 1920x1080 - viewed 19 times.)
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frankxhv773t
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« Reply #2 on: 05 September, 2017, 06:38:47 PM »

An interesting explanation but it doesn't seem to explain why the lobes on one cam act on the valves in quick succession whereas on the other cam they operate half a turn apart. The answer seems to be that the rocker arrangement on one bank of cylinders is completely different to the other bank.


* Gamma engine 4 (Small).jpg (67.53 KB, 466x400 - viewed 84 times.)
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ben
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« Reply #3 on: 05 September, 2017, 08:49:13 PM »

The reason for the difference in the relative positions of the lobes on the l/h and r/h camshafts of the Gamma engine is as Charles says due to the 180 degree rotation of the plane of the alternate cylinders. However such a rotation on its own would not require any change to the relative positions of the lobes were it not for the fact that at the same time the relative positions of the inlet and exhaust valves are reversed so that the inlet port stays on the top of the engine and the exhaust port at the bottom.

For any particular cylinder, observation of the relative positions of the in and ex lobes on the camshaft does not give a direct indication of the relative timing of the valve openings unless the valves are in line(as is the case for many common engines of course).
For the Gamma with its hemispherical combustion chambers and inclined valves the valve actuation is via rockers which are pivoted on opposite sides of the camshaft.The relative positions of the cam lobes has to take account of this.
For such a design if we imagine the cylinder to be upright the relative cam lobe positions would be like the Gamma L/H camshaft if the exhaust was on the L/H side of the engine and the inlets on the right.However if the exhaust was on the R/H side of such an engine ie if the crossflow goes the other way across, then the relative cam lobe positions would be like the R/H Gamma camshaft.

With reference to Brians original observation if he thinks the R/H camshaft lobes look "normal" that suggests he is familiar with crossflow engines with the inlets on the R/H side and the exhausts on the left!
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frankxhv773t
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« Reply #4 on: 05 September, 2017, 09:17:43 PM »

Thanks Ben. That makes sense to me now. You live and learn.
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