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Basically correct John, changing the valve clearance will affect the duration (how long it is open for) the max valve lift point won't change, & that is what you are setting.

Bob.

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I have been trying to do some more checking of this cam, it is installed in the engine, so not that easy to do checks, I have a degree tap stuck to the pulley, and rocker gear in.

I have set the inlet valve to 0.010", inlet opens, pushrod just going tight at 30 degrees BTDC, and closes at 260 degrees from TDC, pushrod going free, or 80 degrees ABDC.

The exhaust valve is set to 0.014" and opens, pushrod just going tight at 110 degrees after TDC, and closes pushrod just free at 20 degrees ATDC. 

This still doesn't seem to match the original Newman spec of  38/78/78/38. or the inlet clearance of 0.014", and 0.016" for the exhaust which if set would retard the opening of the inlet and exhaust.

I may be a couple of degrees out either side of my measurements, But I think good enough for this exercise, but should I leave as is,  or open again and see if I can get the timing better? Am I still doing something wrong?

If I do open it again should I use the #6 valves on equal lift and the crank at TDC, would that improve things?

John

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7 hours ago, John L said:

I have been trying to do some more checking of this cam, it is installed in the engine, so not that easy to do checks, I have a degree tap stuck to the pulley, and rocker gear in.

I have set the inlet valve to 0.010", inlet opens, pushrod just going tight at 30 degrees BTDC, and closes at 260 degrees from TDC, pushrod going free, or 80 degrees ABDC.

The exhaust valve is set to 0.014" and opens, pushrod just going tight at 110 degrees after TDC, and closes pushrod just free at 20 degrees ATDC. 

This still doesn't seem to match the original Newman spec of  38/78/78/38. or the inlet clearance of 0.014", and 0.016" for the exhaust which if set would retard the opening of the inlet and exhaust.

I may be a couple of degrees out either side of my measurements, But I think good enough for this exercise, but should I leave as is,  or open again and see if I can get the timing better? Am I still doing something wrong?

If I do open it again should I use the #6 valves on equal lift and the crank at TDC, would that improve things?

John

As @Lebro has said, clearance has nothing to do with cam timing, only duration. You're correct to say that opening up the clearance will retard the opening of a valve, but it also advances the closing. Result: no change in timing of the point of max lift (which is what we mean by timing), but a reduced duration.

You have inlet lobe CL timing of 115 ATDC, so 5 degrees retarded vs the calcs Bob provided. This will bring power in later (i.e. higher RPM) than correct timing. Unless you're looking for max bhp at the expense of torque, you're better off having the cam advanced than retarded. BTW I think @Andy Moltu mixed up his advance/retard when he said "with any given cam the more you advance the timing, the higher in the rev range the peak power comes". I'd go for half a tooth advance from where you are.

Finally, I still don't know where you get your "38/78/78/38" from. As Bob has said, the spec sheet, rubbish though it is, says 28/68/68/28 and duration of 276, which is internally consistent.

If you decide to re-time, you really don't need to start from scratch. Just be very careful not to rotate anything by accident. Remove the camshaft chainwheel and rotate by just over 90 degrees and re-engage with the chain. You know where this is because the bolt holes in the chain wheel will be slightly out of alignment with (and in advance of) those in the camshaft. Refit, ensuring that the cam has been rotated a tiny bit clockwise to allow the bolt holes to line up. Hopefully you've read elsewhere that the two pairs of bolt holes in the chain wheel are not at 90 degrees to each other. They are 1/2 tooth offset. Then check everything to see what timing you've ended up with. As @Tom Fremont has pointed out elsewhere, the play in the bolt holes allows a couple of degrees fine adjustment.

Hope all this makes sense. If I've got anything wrong, those I've tagged will certainly put me right :D

John

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On 1/6/2021 at 5:20 PM, John L said:

Gosh this is getting.........

As the engine is together at the moment, If I turn the crank to 110 degrees after TDC, compression on #6,  number #1 inlet should be stationary, fully open and any movement on the crank either side of the 110 degrees the inlet should show movement. This is my logic now, correct? 

I suppose this timing should really be be done with a DTI direct on the cam follower, taking out any valve clearance and ratio of the rocker?

Surely now depending on what tappet setting is used the valve timing will be a bit advanced or retarded to what figure I'm not sure?

John

  

No 1 TDC is the same whether you are on a firing stroke or induction - that's determined by the cam. Find tdc. use timing whee to move on 110degrees.

Move your dial guage to No1 inlet and ascertain max lift.  Without moving anything fit the sprockets. If they don't line up perfectly with no slack on the opposite side to the tensioner, (you can advance the crank by a degree or two rather than retard it as the chain will stretch over time). Once you have done that, dont move cam or crank until the chain is on and the bolts on the cam sprocket are in.

There is an element of adjustability of the cam cog - flipping over or round may give you half a tooth of adjustment, although a vernier sprocket is easier.

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4 hours ago, ntc said:

As I said with a up rated cam you find full lift and then introduce large valve clearance you are not going to get the best out of it 

I can understand how clearance makes a timing difference on an asymmetric cam, but if the cam is symmetrical, does it make a difference?

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Surely unless you are trying to get the last possible bit of power out of the engine & that means high revs, you are best to stick to the cam manufacurers timing & clearance recomendations, which is what has been outlined more than once in this thread.  

Bob.

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