Cam timing 101 (2011)

acadian said:

Digging this thread up as I'm getting ready to set up a Web312A cam.

Web indicates the following specs for this cam:

Valve Lash (Inch):.006"
Valve Lift (Inch):0.390
Advertised Duration:310°
Duration @ 0.050": 270°
Lobe Center:104°
Intake Opens 31°Before TDC
Exhaust Opens 59°Before BDC
Intake Closes 59° After BDC
Exhaust Closes 31° After TDC

Valve Timing Is Checked With Zero Valve Lash @ 0.050 Inches Of Valve Lift.

Though they provide duration at 0.050, they don't specify what the actual lift should be here. Also, in their general set up instructions (not specific to any cam), they instruct to measure lift at the valve retainer on pushrod engines (not at follower).

So, after I've confirmed/adjusted lobe centers as close to 104 degrees as possible, I'm assuming the main value I should be concerned with is duration @ 0.050?

Anyone with experience degreeing webcams please chime in, did you take all the measurements from the retainer?

Cheers!

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You should ping Mr. Comstock, @comnoz, as he has a wealth of experience with these.

jseng1 said:

When you're done racking your brains out with the number crunching - check your work by turning the crank to TDC and check the lift on the intake and on the exhaust. With a symmetrical cam you want the intake to have .010" or so more lift than the exhaust. You can visually check by peeking into the exhaust port with a tiny flashlight. Once you verify the in/ex lift difference for a particular cam - that's all you have to do next time. All symmetrical cams (intake and exhaust lobes are identical) seem to come out with about .010" to .020" more lift on the intake. If the lift is the same - its timed "straight up" or "even split".

Acadian - the specs Web provided are "straight up" or "even split". But it should run better with a little bit of intake lead (approx .010" more intake lift than on the exhaust at TDC).

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Thanks for the tip Jim

With Manx Nortons, the cam timing is usually set by measuring the valve lift at TDC. - work with the timings that lift begins to occur and finishes, you have something to work with when you are optimising the timings to suit your exhaust and inlet system. Pretty obvious that when developing the manx, Norton got results and specifying the valve lift came from what actually worked in practice. They probably changed the timings on the cam they ended up with, then measured the valve lift at TDC, so that it's performance could be replicated in other motors. Does anybody know what the lifts at TDC should be with the PW3 cam or the 2S cam ?

One of the advantages four strokes have over most two strokes, is asymmetric valve (or port) timing. Kadency effects work with both types of motor. With a commando, would you use the same cam timings for straight pipes as you would when using megaphones ? - I would not.

Update:

I received the new cam sprocket from RGM, will spare you all the mind numbing calculations, but at the stock timing marks the Web312A was slightly retarded with the intake at 106 degrees and exhaust at 103 degrees. Moving the cam sprocket to the +3 advance keyway brought me to 100 on the intake and 109 on the exhaust (Web specs for this cam show a straight up split with lobe centers at 104 degrees). I expected the #3 slot to bring me closer to specified split on the cam rather than move it 6 degrees, I didn't bother trying the #6 slot. So it appears that the keyways may not be as precisely placed as described, but I'm happy with where it got me regardless, and I didn't bother adjusting the timing pinion at all.

Total lift was less than published specs by an average of .015, and because of the 4" radius applied to the lifters I couldn't use the opening and closing specs published by Web, so I opted to time the cam by lobe centers, and attempt to tweak it as best I could with the multi keyway sprocket, I aimed for a slight advance (based on suggestions from Jim, Jim and others), and at TDC the intake has a few thou more lift than the exhaust.