Centaur Valve Timing

[HopperParticipant @hopper]

Posted by MichaelR on 18/06/2022 10:58:32:

Some time ago I built the Wyvern and remember having to do a lot of adjustments to get the cams set, I finally got them set and the engine ran and started OK.

Wyvern cams and timing

MichaelR

So the Wyvern has a similar discrepancy between the cam specs and the timing chart.

The cams drawn at 120 and 130 degrees would give a duration in crankshaft degrees of 240 Inlet and 260 Exhaust.

The timing diagram shows 235 and 250 respectively. Different by 5 and 10 degrees each.

But he does say below that diagram "Set cams by trial" so I guess that is the answer.

First thing I would do is set them with zero overlap so exhaust closes and inlet opens at exactly TDC and see how that goes. If it doesnt, and the problem is definitely not spark or fuel or other factors, then I would try increasing overlap by 5 degrees per valve at a time.

Trouble is with a new engine that won't start, you can't know for sure if the sparkplug is sparking strongly enough under compression, or if the fuel mixture is just right, so now you have a third variable, cam timing.

It would certainly pay to look around at stationary engines by other designers and see what their timing diagrams look like.

I suppose you could put a little grub screw in each cam lobe so you have infinite adjustment that can be done quickly and easily then drill and pin it finally when the best combo is found.

Edited By Hopper on 18/06/2022 11:23:43

[MichaelRParticipant @michaelr]

Hopper, As you say I put grub screws in the cam boss and adjusted for running position from what I remember there is no lap, I time the ignition to fire at TDC.

The Wyvern running here Link

MichaelR

[HopperParticipant @hopper]

Another lovely looking engine there and running beautifully on your setting of zero overlap.

[HopperParticipant @hopper]

Clive, have you looked at the article in ME3549 by RR May? See Here

He says he could not get his Centaur started and remade the cams to match ETW's timing chart, but still could not get it to start, until he made a starting pulley wirh a length of cord wrapped around it to spin the crank over. I guess these days we would just hook up the cordless drill to the flywheel and let it rip.

But even then he says it ran poorly until he first upped the voltage to his ignition system for a stronger spark and then limited the movement of the governor to stabilise the throttle movement.

 

Edited By Hopper on 24/06/2022 09:59:16

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