Cam timing CP

[John L]

I'm trying to do some cam timing checking, but there is one item I still don't quite understand, perhaps some kind gent could put me straight.

When it lists cam opening of 35,65,65,35 for the std PI on a cp motor, is this the point that the camshaft starts to lift the pushrod, or when the valve actually commences opening using up the tappet clearance with the std valve clearance?

 

I did establish that with equal opening of valves on #6, #1 should be on TDC, but the openings of the valves don't line up with the 35, BTDC. I'm not totally sure now that I have genuine std cp cam.

 

I have checked the cam lift at the pushrod, without the rocker shaft fitted, inlet and exhaust are mostly at 0.252", but 2 inlets on #1 and #6 are 0.238" 0.197" so it looks like I have to change the camshaft now.

 

I have seen that the Moss's CP cam is a new Newman's, but a much better price than from Newman direct seems strange? Will follow info from forum about fitting the Nitrided followers.

 

John

[TriumphV8]

Hi John,

the confusions comes from sellers that also do not understand what is what.

Normally the opening is quoted as the real curve of the cam profile.

That profile on the camshaft is led in and out by a ramp to compensate the valve clearance.

That is why Americans quote their cams timing with a much larger valve clearance.

In the area of the ramps and the starting of the profile results will be very bad.

The reason is that a lot of degrees turning the cam make only little lift.

You will fail to time the cam with these data.

Also some quote these data with normal valve clearance

some do that with special valve clearance.

-A lot of data that confuse instead of help.-

 

Best is to measure where the lobe is very steep.

That is around half lift.

There small differences in lift give big differences in degree.

That is what we want for precision.

 

What can we do with that?

The cam is quoted with a spread, that is how much the lobe is turned away from TDC.

That is what we can find and set very precise

 

So we go like that to get good results:

 

We have to find true TDC

 

We place a degree wheel.

 

We measure the inlet lobe.

We estimate a cam lift of 5mm or something like that

at the lifter with a gauge.

Let us say the degree wheel says 80 degrees at that point.

Than we turn, get more lift and after top it decreases.

After some time we again have 5mm at the lowering side

and again read the degree wheel with 130 degrees

 

Than in the middle is the top of the lobe and the spread what is 210/2 = 105 degrees.

We can not measure that directly because again at the top of the lobe a lot of degrees

make only little change in lift what gives bad results.

 

The chain must be pushed on the not load side to get precise results.

 

That is the best way to get a clean result.

Hope it helps!

[John L]

Having reread my post, I think I'm sure now I don't have a true CP cam, If I checked the equal overlap on cylinder #6 with an inlet lift of only 0.197" I'm sure it cannot be right? But then again perhaps it is if I set the tappet adjustment the same for both valves on the back of the cam.

But I still don't quite understand Andreas's post.

If TR state that the inlet opens at 35 degrees BTDC is that when the valve is actually opening, having taken up the slack of the tappet adjustment, or is it when the cam lobe just starts to move the push rod....Perhaps I still missed this in his comment? Help me here please.

John

[Tom Fremont]

 

If TR state that the inlet opens at 35 degrees BTDC is that when the valve is actually opening, having taken up the slack of the tappet adjustment,

John

Yes. Been running a CP cam that way for 85K miles and counting. When the pushrod just doesn't turn easily you're there.

 

It is up to you if you want to advance it a little to compensate for chain slack which develops in due course. I set mine ahead 3 degrees @ crank , as 6 degrees of slack can develop eventually.

 

I believe 0.040" clearance is enough to check the timing via the symmetry method ( but this should be seconded ).

 

Cheers,

Tom

[Rob Salisbury]

Hi John,

Google "David Vizzard's book Tuning Triumphs over 1300cc" , chapter 14, pages 95 to 102 gives a very concise description of cam fitment and timing (worth printing off and keeping in the workshop), but yes the cams figures refer to the point at which the clearance ramp has been fully taken up and the tappet/pushrod begins to rise rapidly, use of a DTI is the best method for determining this point.

Cheers Rob

[John L]

As I'm going now to replace the cam, Newman have recommended their PH2 cam, does anybody have any thoughts or recommendations/have used this one or should I stick with the std 307689 one.

Its a CP engine with a 516816 head at 9.5cr, and a phoenix manifold and straight thru exhaust.

John